# A New Electrohydraulic Hybrid Energy-Saving System Used on the Reversible Rolling Mills

Authors: Rui Du; Zhiqi Liu; Lei Ge; Jinyuan Cui; Lei He
Journal: Advances in Materials Science and Engineering
 (2023)

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## Abstract

In the face of the global energy crisis, the use of hybrid technology is an important way to achieve efficient energy storage and utilization. Hybrid systems have been successfully applied to automobiles and construction machinery, but no research study has been conducted on the design and application of energy recovery systems for reversible rolling mills. This article proposes a new type of noncoaxial parallel electrohydraulic hybrid drive system configuration for reversible rolling mills to save energy. It is proposed to collect the braking energy of the previous pass of the reversible rolling mill roll and the surplus power of the original primary drive motor and then start or accelerate the process for the next roll pass. A test experiment was established and the energy-saving efficiency of the proposed system was verified. By rolling 14 passes, 255.892 kJ of energy can be recovered. The overall energy efficiency rate is around 71.547%. The system will significantly promote energy saving, low carbon, green, and other aspects of the reversible rolling mills.


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## Body

## 1. Introduction

In the face of the global energy crisis, environmental pollution, increasing stringent emission standards, and the “double carbon” (carbon peak and carbon neutral) vision, the promotion of energy-efficient technologies is one of the initiatives to promote energy saving and emission reduction continuously and to reach carbon peak and carbon neutrality. Regarding energy consumption and carbon emissions, China is currently the world’s largest carbon emitter and consumer. Among the 31 sectors of the manufacturing industry, the steel industry accounts for around 15% of the country’s carbon emissions, making it the second largest emitter of carbon dioxide after the power industry. To achieve carbon neutrality in the steel industry, energy-saving and energy-reducing designs are adopted in the manufacturing process [1].Approximately 90% of China’s steel is rolled into the material. Steel rolling mills are the equipment to complete the whole rolling process [2]. Some small- and medium-sized mills with constant loads and some large openers and slab mills are equipped with flywheels, which are designed to reduce the main motor spike load when rolling milling, increase the main motor load at no load, and uniformly distribute the load in order to improve the transmission’s stability and overload capacity.However, for pure rolling times with a long gap time, no spike load mill and reversible rolling mills should be installed on the flywheel because, in this case, the flywheel storage and energy release role is not significant, and the flywheel itself consumes a certain amount of energy as a load. New mills rarely use flywheels. It is also possible to feedback excess power from the mill’s main drive motor to the grid through power generation devices. In this method, additional generators and rectifiers are required, as well as inverters and other corresponding supporting equipment.Generally, reversible rolling mills are driven directly by motor rolls to achieve reversible drive [2]. The frequent starting and braking of the drive motor dissipate kinetic braking energy in the form of heat, which contributes to energy loss and a reduction in component service life. When the reversible rolling mills are equipped with an additional energy recovery system, the reversible rolling mills’ roll braking energy and surplus power from the original primary drive motor are collected and used to start or accelerate the roll in the next pass until the entire rolling process is completed. The main drive motor of the mill can be reduced in installed power as well as the heat generated during motor braking is reduced and the motor’s service life is extended. A dual-drive model combining the main and auxiliary drives is also created. An energy recovery system can also be offset by energy savings or increased steel production.Storage of energy requires having a high energy density (can provide an endless supply of energy), a high power density (can absorb and release high power quickly), a high number of charge and discharge cycles, a high energy conversion efficiency, and several other characteristics. In Table1, different energy storage components are compared based on their performance parameters [3–9]. Electric drives offer zero emissions, low noise, high efficiency, strong overload capacity, large starting torques, fast torque responses, ample charging and discharging currents, high-speed regulation efficiency, etc. However, they also suffer from a low power density and a low braking energy recovery efficiency. The hydraulic drive is particularly suitable for instantaneous high power demand conditions because of its high power density and low cost [10]. In order to reduce emissions and energy consumption, energy-regenerative electrohydraulic hybrid drive technologies based on reversible rolling mills are the best options.Table 1
Performance parameters of different energy storage elements [3–10].
ParametersMechanical energy storageElectrical energy storageHydraulic energy storageEnergy storage unitFlywheelLead-acid batteryNickel metal hydride batteryLithium batterySupercapacitorHydraulic accumulatorEnergy density per unit mass (W·h/kg)5∼20030∼6530∼110100∼25010∼302.4∼6.5Power density (W/kg)5000∼1000090∼500250∼12003550∼4000500∼50002000∼19000Number of cycles∼107300∼80025002000∼100001053–5 yearsEfficiency≈96≈80≈90≈95≈95≈90Impact on the environmentHardlyLargeLargeLargeSmallSmallThe reversible rolling mills, however, generally roll at a maximum speed of 2 to 7 m/s [2], according to relevant literature. Roll braking produces a small amount of kinetic energy, and the energy recovery device contributes a small amount of this energy back to the central drive system, which presents a challenge in designing the electrohydraulic hybrid drive system configuration for reversible rolling mills. It has neither reported how electrohydraulic hybrid drive systems are used in reversible rolling mills nor the relevant aspect been applied. In this article, we propose a configuration that reduces both peak and valley power of the main drive motor of the reversible rolling mill, as well as recovering kinetic energy from the braking of the rolls. As a result of the incorporated energy recovery system, the reversible rolling mill’s braking cycle can be shortened, thereby increasing production yield and reducing rolling cycle time. Presented in this article is the first proposal for an electrohydraulic hybrid drive for reversible rolling mills. By using a motor control strategy, a hydraulic pump displacement control, and other techniques, the auxiliary power source can be improved for better drive efficiency [11].This article is organized as follows: first, the mechanical system and operating conditions of a reversible rolling mill are illustrated in Section2. The proposed system is described in Section 3. The system parameter matching is developed in Section 4. Simulation modeling, analysis, and calculation are carried out in Section 5. Conclusions are drawn in Section 6.


## 2. Composition of the Reversible Rolling Mills and Their Operating Conditions

### 2.1. Components of the Reversible Rolling Mill

Generally, steel rolling machines consist of three components: the main drive motor, a primary transmission device, and a working machine [2]. In order to roll metal, the motor’s output torque is transmitted to the work roll, which rotates at a certain speed. As shown in Figure 1, most steel rolling machinery uses a reducer, gear seat, connecting shaft, and coupling as its main drive device. To compensate for the motor’s frequent starting, acceleration, and braking, a low-speed, high-torque motor must directly drive the work roll in the reversible rolling mill for medium-thickness plates, as shown in Figure 2. Due to the absence of a gear seat and reducer, frictional losses can be reduced and mill productivity can be increased. At the same time, motor flywheel torque decreases significantly due to this solution.Figure 1
Structure of primary drive device of general rolling mills: (1) main motor; (2) motor coupling; (3) reducer; (4) main coupling; (5) gear seat; (6) universal joint shaft; (7) rolls.Figure 2
Structure of primary drive device of the reversible rolling mills: (a) with reducer and (b) without reducer.
(a)(b)


### 2.2. The Operation Mode of Reversible Rolling Mills

Rollers are rotated by the motor of reversible rolling mills. A low speed is used to bite into the bar and then a higher speed is used to increase its speed. This process is repeated until the end of the rolling process, where the speed of the motor is reduced, the roll is thrown out at low speed, then its direction of rotation is changed, and then the reverse rolling is carried out. There are two types of velocity diagrams for reversible rolling mills: the triangular velocity diagram and the trapezoidal velocity diagram. Triangular velocity diagrams do not have a constant speed rolling phase such as trapezoidal velocity diagrams. Reversible rolling mills have six stages of speed and torque relationship, which can be seen in the trapezoidal speed diagram in Figure 3: 0, idling constant speed period; 1, idling acceleration period; 2, accelerated rolling period; 3, constant speed rolling period; 4, slow rolling period; 5, idle deceleration period. Rolling speed and torque for the reversible rolling mills are shown in Figure3, and the speed and torque from t0 to t5 stage in Figure 3 are described in Table 2.Figure 3
Rolling speed diagram and load diagram of the reversible rolling mill.Table 2
Description of speed and torque of one pass of the reversible rolling mills.
StagesOperating modeSpeedSpeed descriptionTorqueValue0 (0-t0)Idling constant speed periodn0The speedn0 does not changeMD0Mkon1 (t0-t1)Idling acceleration periodn0⟶nyny is the rotation speed of the biteMD1Mkon + Mdon2 (t1-t2)Accelerated rolling periodny⟶nwnw is the speed of stable operationMD2Mj + Mdon3 (t2-t3)Constant speed rolling periodnwThe rotational speednw remains unchangedMD3Mj4 (t3-t4)Slow rolling periodnw⟶npnp is the rotation speed when the rolled piece is thrown outMD4Mj − Mdon5 (t4-t5)Idling deceleration periodnp⟶0The rotation speed is reduced fromnp to 0MD5Mkon − Mdon


## 2.1. Components of the Reversible Rolling Mill

Generally, steel rolling machines consist of three components: the main drive motor, a primary transmission device, and a working machine [2]. In order to roll metal, the motor’s output torque is transmitted to the work roll, which rotates at a certain speed. As shown in Figure 1, most steel rolling machinery uses a reducer, gear seat, connecting shaft, and coupling as its main drive device. To compensate for the motor’s frequent starting, acceleration, and braking, a low-speed, high-torque motor must directly drive the work roll in the reversible rolling mill for medium-thickness plates, as shown in Figure 2. Due to the absence of a gear seat and reducer, frictional losses can be reduced and mill productivity can be increased. At the same time, motor flywheel torque decreases significantly due to this solution.Figure 1
Structure of primary drive device of general rolling mills: (1) main motor; (2) motor coupling; (3) reducer; (4) main coupling; (5) gear seat; (6) universal joint shaft; (7) rolls.Figure 2
Structure of primary drive device of the reversible rolling mills: (a) with reducer and (b) without reducer.
(a)(b)


## 2.2. The Operation Mode of Reversible Rolling Mills

Rollers are rotated by the motor of reversible rolling mills. A low speed is used to bite into the bar and then a higher speed is used to increase its speed. This process is repeated until the end of the rolling process, where the speed of the motor is reduced, the roll is thrown out at low speed, then its direction of rotation is changed, and then the reverse rolling is carried out. There are two types of velocity diagrams for reversible rolling mills: the triangular velocity diagram and the trapezoidal velocity diagram. Triangular velocity diagrams do not have a constant speed rolling phase such as trapezoidal velocity diagrams. Reversible rolling mills have six stages of speed and torque relationship, which can be seen in the trapezoidal speed diagram in Figure 3: 0, idling constant speed period; 1, idling acceleration period; 2, accelerated rolling period; 3, constant speed rolling period; 4, slow rolling period; 5, idle deceleration period. Rolling speed and torque for the reversible rolling mills are shown in Figure3, and the speed and torque from t0 to t5 stage in Figure 3 are described in Table 2.Figure 3
Rolling speed diagram and load diagram of the reversible rolling mill.Table 2
Description of speed and torque of one pass of the reversible rolling mills.
StagesOperating modeSpeedSpeed descriptionTorqueValue0 (0-t0)Idling constant speed periodn0The speedn0 does not changeMD0Mkon1 (t0-t1)Idling acceleration periodn0⟶nyny is the rotation speed of the biteMD1Mkon + Mdon2 (t1-t2)Accelerated rolling periodny⟶nwnw is the speed of stable operationMD2Mj + Mdon3 (t2-t3)Constant speed rolling periodnwThe rotational speednw remains unchangedMD3Mj4 (t3-t4)Slow rolling periodnw⟶npnp is the rotation speed when the rolled piece is thrown outMD4Mj − Mdon5 (t4-t5)Idling deceleration periodnp⟶0The rotation speed is reduced fromnp to 0MD5Mkon − Mdon


## 3. Energy Recovery System Configuration and System Structure Scheme Based on the Reversible Rolling Mills

### 3.1. Configuration of Energy Recovery System Based on the Reversible Rolling Mills

Due to the low speed of the reversible mill and the effect of the speed of the recovery pump/motor on recovery efficiency, a recovery scheme is proposed with high-speed gearing at roll speed and reduced motor at roll speed. Table3 shows the configuration of the proposed schemes. Series construction has the advantage that there is no mechanical connection between the motor and the load and that the motor will not be affected by fluctuating loads, but there are more energy conversion links and more changes to the original primary drive. With a parallel structure, the hydraulic pump/motor and motor can be driven together or separately to move the load. In comparison to a non-coaxial parallel structure, the control of the coaxial parallel structure is more complex and requires more changes to the original central drive system.Table 3
Configuration of energy recovery system based on the reversible rolling mills.
ConfigurationRoll-speed increasing gear-high-speed motorRoll-low-speed motorSeriesCoaxial parallelNoncoaxial parallel


### 3.2. Structural Scheme and Principle of Energy Recovery System Based on the Reversible Rolling Mills

The previous analysis indicates that large amounts of recoverable energy generated during frequent braking of the reversible rolling mills, which is converted into thermal energy in the original system by the heat generated by motor resistance, results in energy waste and reduced motor life. As shown in Table3, the noncoaxial parallel structure does not require significant changes to be made in the primary drive system, and the load can be driven by both the motor and the hydraulic pump/motor separately, so we focus on this configuration in this article, while the other configuration options are only discussed as options. A hydraulic energy recovery system consisting of a hydraulic recovery pump/motor, motor, and hydraulic accumulator is introduced based on the rotary hydraulic energy recovery system of construction machinery. Hydraulic recovery pump/motor added accumulators are adopted as energy recovery modes and electrohydraulic hybrid drives are adopted as energy utilization modes (mainly electric drive, supplemented by hydraulic drive) [12]. Figure 4 illustrates the electrohydraulic hybrid drive rotary energy recovery system based on large reversible rolling mills.Figure 4
Schematic diagram of electrohydraulic hybrid drive energy recovery system based on the reversible rolling mills: 1, main drive motor M1; 2, electromagnetic clutch YC1; 3, work roll; 4, torque speed sensor; 5, electromagnetic clutch YC2; 6, variable displacement pump/motor; 7.1 and 7.2, 21, 24-relief valves; 8.1–8.4, electromagnetic ball valves; 9.1 and 9.2, 22-one way valve; 10, hydraulic reversing valve; 11, 25-pressure sensor; 12.1–12.4, safety ball valve; 13.1 and 13.2, high-pressure accumulator; 14, temperature sensor; 15, liquid level gauge; 16, cooler; 17, high-pressure oil filter; 18, oil pump motor unit (M2 is the drive motor for the oil refill pump); 19, flowmeter; 20, oil suction filter; 23.1 and 23.2, low-pressure accumulator; 26, shut-off valves.As a principle of energy recovery and utilization, the auxiliary system starts the hydraulic motor M2, filling the low-pressure accumulators with oil. The pressure sensor 25 measures the low-pressure accumulator’s filling pressure and changes in pressure. Once the low-pressure accumulator filling pressure reaches the set value, the oil pump motor unit 18 stops and the filling is complete.The electromagnetic clutch 2 is combined and the main drive motor 1 drives the rolls clockwise when the variable displacement pump/motor 6 rotates clockwise. Once the speed has been reached, the electromagnetic clutch 2 is disengaged, allowing the rolls to spin under inertia, and the variable pump/motor 6 brakes the rolls with the electromagnetic clutch 5 combined.Therefore, solenoid ball valves 8.3 and 8.4 operate in the lower position when the roller decelerates or brakes counterclockwise. The hydraulic pump/motor 6 continues to rotate due to the large inertia of the rolls. There are two ports on the hydraulic pump/motor 6: one on the high-pressure side and one on the low-pressure side. Currently, the hydraulic pump/motor 6 is in pump mode, with the low-pressure port on the A port. Through the upper position of the reversing valve 10, the B port of the hydraulic motor draws oil from the low-pressure accumulator 23.1 and 23.2. The A port of the hydraulic motor produces high-pressure oil, which enters the high-pressure accumulator 13.1 and 13.2 through the solenoid ball valve 8.3 at the lower position so that the roller braking process energy can be recovered, as shown in Figure5. RED arrows are in the direction of high-pressure oil, and BLUE arrows are in the demand of low-pressure oil.Figure 5
Deceleration or braking of the roll.The main drive motor becomes the primary power source when the roll rotation is started by electromagnetic clutches 2 and 5. The high-pressure accumulator is used as an auxiliary power source.When the roll accelerates counterclockwise or starts, the solenoid ball valve 8.4 gets the signal and opens, while the solenoid ball valve 8.3 closes. High-pressure oil is connected to port B and low-pressure oil is connected to port A of the hydraulic motor. The hydraulic reversing valve ten is in the on position after setting it to the lower position. With the upper position of the solenoid ball valve 8.4, the high-pressure accumulators 13.1 and 13.2 drive the rolls of the hydraulic motor. In contrast, the low-pressure oil from the A port is stored in the low-pressure accumulators 23.1 and 23.2 through the lower position of the reversing valve 10. Figure6 illustrates how the system reuses energy in this way. RED arrows are in the direction of high-pressure oil, and BLUE arrows are in the demand of low-pressure oil.Figure 6
Acceleration or starting of the roll.


## 3.1. Configuration of Energy Recovery System Based on the Reversible Rolling Mills

Due to the low speed of the reversible mill and the effect of the speed of the recovery pump/motor on recovery efficiency, a recovery scheme is proposed with high-speed gearing at roll speed and reduced motor at roll speed. Table3 shows the configuration of the proposed schemes. Series construction has the advantage that there is no mechanical connection between the motor and the load and that the motor will not be affected by fluctuating loads, but there are more energy conversion links and more changes to the original primary drive. With a parallel structure, the hydraulic pump/motor and motor can be driven together or separately to move the load. In comparison to a non-coaxial parallel structure, the control of the coaxial parallel structure is more complex and requires more changes to the original central drive system.Table 3
Configuration of energy recovery system based on the reversible rolling mills.
ConfigurationRoll-speed increasing gear-high-speed motorRoll-low-speed motorSeriesCoaxial parallelNoncoaxial parallel


## 3.2. Structural Scheme and Principle of Energy Recovery System Based on the Reversible Rolling Mills

The previous analysis indicates that large amounts of recoverable energy generated during frequent braking of the reversible rolling mills, which is converted into thermal energy in the original system by the heat generated by motor resistance, results in energy waste and reduced motor life. As shown in Table3, the noncoaxial parallel structure does not require significant changes to be made in the primary drive system, and the load can be driven by both the motor and the hydraulic pump/motor separately, so we focus on this configuration in this article, while the other configuration options are only discussed as options. A hydraulic energy recovery system consisting of a hydraulic recovery pump/motor, motor, and hydraulic accumulator is introduced based on the rotary hydraulic energy recovery system of construction machinery. Hydraulic recovery pump/motor added accumulators are adopted as energy recovery modes and electrohydraulic hybrid drives are adopted as energy utilization modes (mainly electric drive, supplemented by hydraulic drive) [12]. Figure 4 illustrates the electrohydraulic hybrid drive rotary energy recovery system based on large reversible rolling mills.Figure 4
Schematic diagram of electrohydraulic hybrid drive energy recovery system based on the reversible rolling mills: 1, main drive motor M1; 2, electromagnetic clutch YC1; 3, work roll; 4, torque speed sensor; 5, electromagnetic clutch YC2; 6, variable displacement pump/motor; 7.1 and 7.2, 21, 24-relief valves; 8.1–8.4, electromagnetic ball valves; 9.1 and 9.2, 22-one way valve; 10, hydraulic reversing valve; 11, 25-pressure sensor; 12.1–12.4, safety ball valve; 13.1 and 13.2, high-pressure accumulator; 14, temperature sensor; 15, liquid level gauge; 16, cooler; 17, high-pressure oil filter; 18, oil pump motor unit (M2 is the drive motor for the oil refill pump); 19, flowmeter; 20, oil suction filter; 23.1 and 23.2, low-pressure accumulator; 26, shut-off valves.As a principle of energy recovery and utilization, the auxiliary system starts the hydraulic motor M2, filling the low-pressure accumulators with oil. The pressure sensor 25 measures the low-pressure accumulator’s filling pressure and changes in pressure. Once the low-pressure accumulator filling pressure reaches the set value, the oil pump motor unit 18 stops and the filling is complete.The electromagnetic clutch 2 is combined and the main drive motor 1 drives the rolls clockwise when the variable displacement pump/motor 6 rotates clockwise. Once the speed has been reached, the electromagnetic clutch 2 is disengaged, allowing the rolls to spin under inertia, and the variable pump/motor 6 brakes the rolls with the electromagnetic clutch 5 combined.Therefore, solenoid ball valves 8.3 and 8.4 operate in the lower position when the roller decelerates or brakes counterclockwise. The hydraulic pump/motor 6 continues to rotate due to the large inertia of the rolls. There are two ports on the hydraulic pump/motor 6: one on the high-pressure side and one on the low-pressure side. Currently, the hydraulic pump/motor 6 is in pump mode, with the low-pressure port on the A port. Through the upper position of the reversing valve 10, the B port of the hydraulic motor draws oil from the low-pressure accumulator 23.1 and 23.2. The A port of the hydraulic motor produces high-pressure oil, which enters the high-pressure accumulator 13.1 and 13.2 through the solenoid ball valve 8.3 at the lower position so that the roller braking process energy can be recovered, as shown in Figure5. RED arrows are in the direction of high-pressure oil, and BLUE arrows are in the demand of low-pressure oil.Figure 5
Deceleration or braking of the roll.The main drive motor becomes the primary power source when the roll rotation is started by electromagnetic clutches 2 and 5. The high-pressure accumulator is used as an auxiliary power source.When the roll accelerates counterclockwise or starts, the solenoid ball valve 8.4 gets the signal and opens, while the solenoid ball valve 8.3 closes. High-pressure oil is connected to port B and low-pressure oil is connected to port A of the hydraulic motor. The hydraulic reversing valve ten is in the on position after setting it to the lower position. With the upper position of the solenoid ball valve 8.4, the high-pressure accumulators 13.1 and 13.2 drive the rolls of the hydraulic motor. In contrast, the low-pressure oil from the A port is stored in the low-pressure accumulators 23.1 and 23.2 through the lower position of the reversing valve 10. Figure6 illustrates how the system reuses energy in this way. RED arrows are in the direction of high-pressure oil, and BLUE arrows are in the demand of low-pressure oil.Figure 6
Acceleration or starting of the roll.


## 4. Simulation Parameter Calculation of Energy Recovery System of Reversible Rolling Mills

### 4.1. The Rotational Inertia of Work Roll and a Maximum Speed of Roll

We take a model 650 four-roll reversible rolling mill as an example. Its work roll is the drive roll, and its main parameters are shown in Table4. The work roll material is ductile iron, and its density is 7.3 g/cm3. This study ignores the masses of the shaft and head due to their small sizes.Table 4
Main parameters of the 650 four-roller reversible rolling mill.
Types of reversible rolling millsRated power of main motor (kW)Rated speed of main motor (r/min)Max rolling force (kN)Max rolling torque (N·m)Max rolling speed (m/s)Size of the work roll (mm)The 650 four-roller reversible rolling mill6006005000350008φ220 × 650Equation (1) shows the moment of inertia of one work roll of the four-roller reversible rolling mill:(1)Jr=12mr2=12ρπh⋅r4,where m is the mass of the roll, r is the roll diameter, h is the roll length, and ρ is the density of the corresponding material of the roll. Different materials’ rolls or hollow rolls can be calculated according to the actual situation. If the quality of the roll neck and the shaft head cannot be ignored, then this part of the mass should also be added.This is usually calculated using equation (2) when it comes to electric drive:(2)Jm=GD24g,where Jm is the moment of inertia corresponding to the no-load torque of the motor, GD2 is the flywheel moment, which is obtained from the product catalog, and g is the gravitational acceleration taken as 10 m/s2.In the following equation, the roll line speed is calculated as(3)vmax=πDnrmax60,where the maximum rolling speed is vmax, the diameter of the roll is D, and the maximum speed of the roll is nrmax.We get the selected motor flywheel moment GD2 for 156 kg·m2 and bring in the data from (1) to (3) to get Jr for 1.091 (kg·m2), Jm for 3.9 (kg m2), and nrmax for 695 (r/min).The maximum speed of the ladder speed diagram can be selected within the maximum speed allowed by the motor. For determining the biting and throwing speed, the goal is to obtain a shorter rolling rhythm time, ensure smooth biting of the rolled piece, facilitate the operation, and adjust to the reasonable speed range of the main motor. We get the shortest rolling rhythm when the current pass’s throwing speed equals the next pass’s biting speed. There is no requirement for the shortest rolling rhythm in this article. When the roll speed is set tonp, the kinetic energy of the roll can be recovered. The roll can be rotated from static to ny when the kinetic energy is recovered. The rolled piece is assumed to bite in at 3.7 m/s during the idling acceleration period and throw out at 4.375 m/s during the idling deceleration period. Take a bite from the mill and throw out the mill speed based on the actual operating conditions of the selected rolling mill. Bringing in the data, ny is 321.4 r/min, np is 380 r/min, ωy is 33.64 rad/s, and ωp is 39.77 rad/s.


### 4.2. Speed and Time Required for Each Stage of One Pass for the Roll

We assume that the accelerationaj of the main motor in the acceleration period is 160 r/s2 and the deceleration bz in the deceleration period is 240 r/s2. According to the actual rolling time of the selected mill and rolls, the acceleration and deceleration are calculated. Table 5 shows the duration of each of the abovementioned six stages.Table 5
Time of each stage of one pass of the reversible rolling mills [2].
StageOperating modeTimeCalculation formulaValue (s)0 (0-t0)Idling constant speed periodt0Set idle constant speed periodt0 = 2 s21 (t0-t1)Idling acceleration periodtyty=ny−n0/aj1.696252 (t1-t2)Accelerated rolling periodtjtj=nw−ny/aj2.3353 (t2-t3)Constant speed rolling periodtwtw=60L1/πDnw−1/nwny+nw/2tj+nw+np/2tz1It is not calculated here; it is assumed that the constant speed rolling period tw is 2 s and L1 is the length of rolled piece after rolling24 (t3-t4)Slow rolling periodtz1tz1=nw−np/bz1.31255 (t4-t5)Idling deceleration periodtz2tz2=np/bz1.583


### 4.3. Torque Calculation of the Reversible Rolling Mills and Parameter Matching of Recovery System

As shown in the following equation, the main motor drive torque of the mill is calculated as(4)MD=MZi+Mf+Mkon±Mdon,where MD is the torque of the main motor, rolling torque is measured by MZ, and Mf is the additional friction torque occurring on the roller bearing and transmission mechanism during rolling. The idling torque, Mkon, is the friction torque during idling. Whenever the roller moves at nonuniform speeds, Mdon is required to overcome inertial forces. The first three moments of the transmission rolls for the static moment Mj, is calculated by the following equation:(5)Mj=MZi+Mf+Mkon.In the abovementioned formula, torque is measured when the roller rotates at a constant speed. These three items are indispensable for any rolling mill. Generally, rolling torque is the largest. The rolling torque can be determined in two ways: one by measuring the force of metal on the roller and the other by measuring energy consumption. Equation (6) is used in this article to calculate the rolling moment based on the metal force on the roller:(6)Mz=Pa,where P is the vertical pressure applied to the roller by the metal, i.e., the rolling force, and a is the length of the force arm. In order to eliminate the influence of geometric factors on the force arm, the force arm coefficient is often determined by Ψ, and the force arm value is determined by following equation. Cold-rolled strip, rolling force arm coefficient Ψ number for 0.33∼0.42, here, take Ψ = 0.42.(7)ψ=φ1αj=alj,where φ1 is the closing pressure action angle, the contact angle is αj, and lj is the contact arc length, which is calculated by(8)lj=R⋅∆h,where R is the roll radius and ∆h is the thickness difference before and after rolling.It is the rolling torque on a single roll that is calculated here. With the help of the following equation, we calculate the displacement of the recovery pump/motor based on the maximum rolling torque of a specific model 650 four-roller reversible rolling mill:(9)T=Vg⋅∆p⋅ηmh2π,where T is the torque. The hydraulic pump/motor’s maximum displacement is Vg, Δp stands for differential pressure, which is 31.5 MPa, and ηmh stands for mechanical efficiency, which is taken as 1. The displacement of the recovery pump/motor is calculated as 3488.9 ml/r according to equation (10).However, it can be seen from the abovementioned calculation that the rolling torque is ample and the recovered energy is limited in the onload rolling stage. If this part of the energy is recovered, it must match the recovery pump/motor with a large displacement, and the cost is high. Moreover, the rolling torque required during onload startup is large, and the torque provided by the recovered braking energy is small, which is not enough to overcome the rolling torque and drive the roll movement. Therefore, this article only gives the configuration of the recovery scheme.It is possible to use multiple recovery pumps/motors in parallel to reduce the input torque of the recovery pump/motor. Motors with high speeds can be used in conjunction with pumps and motors to avoid the problems associated with low-speed motors, such as energy efficiency or failure to recover braking energy. However, the abovementioned calculation indicates that the rolling torque is ample and the recovered energy is limited during the onload rolling stage. For this part of the energy to be recovered, the pump or motor must have a large displacement, and the cost is high. In addition, during onload startup, the rolling torque required is large, and the torque provided by the recovered braking energy is small, so the roll movement cannot be driven. The article only describes the configuration of this recovery scheme.In the no-load period, the static moment is not a rolling moment, and the dynamic moment is calculated by(10)Mdon=GD2375⋅dωdt,where G is the gravity of the rotating part, the rotating part’s inertia diameter is D, and ω stands for the angular velocity of the rotating part, calculated by equation (10).The formula for estimating no-load torque is complex, and it is usually estimated by using the following formula:(11)Mkon=0.03∼0.06Tm,Pm=Tmnm9549,where Tm represents the motor’s rated torque, Pm represents its rated power, and nm represents its rated speed. In this case, Mkon is 286.47 N·m, idling acceleration torque MD1 is 295.777 N·m, and idling deceleration torque MD5 is 272.509 N·m; MD1 matches the hydraulic pump/motor maximum displacement Vg, which is 58.97 ml/r. This article chooses a two-way variable pump/motor with a maximum displacement of 80 ml/r Figure 7 shows the working speed diagram for the reversible rolling mill roll.Figure 7
Working speed diagram of the reversible rolling mill roll.


## 4.1. The Rotational Inertia of Work Roll and a Maximum Speed of Roll

We take a model 650 four-roll reversible rolling mill as an example. Its work roll is the drive roll, and its main parameters are shown in Table4. The work roll material is ductile iron, and its density is 7.3 g/cm3. This study ignores the masses of the shaft and head due to their small sizes.Table 4
Main parameters of the 650 four-roller reversible rolling mill.
Types of reversible rolling millsRated power of main motor (kW)Rated speed of main motor (r/min)Max rolling force (kN)Max rolling torque (N·m)Max rolling speed (m/s)Size of the work roll (mm)The 650 four-roller reversible rolling mill6006005000350008φ220 × 650Equation (1) shows the moment of inertia of one work roll of the four-roller reversible rolling mill:(1)Jr=12mr2=12ρπh⋅r4,where m is the mass of the roll, r is the roll diameter, h is the roll length, and ρ is the density of the corresponding material of the roll. Different materials’ rolls or hollow rolls can be calculated according to the actual situation. If the quality of the roll neck and the shaft head cannot be ignored, then this part of the mass should also be added.This is usually calculated using equation (2) when it comes to electric drive:(2)Jm=GD24g,where Jm is the moment of inertia corresponding to the no-load torque of the motor, GD2 is the flywheel moment, which is obtained from the product catalog, and g is the gravitational acceleration taken as 10 m/s2.In the following equation, the roll line speed is calculated as(3)vmax=πDnrmax60,where the maximum rolling speed is vmax, the diameter of the roll is D, and the maximum speed of the roll is nrmax.We get the selected motor flywheel moment GD2 for 156 kg·m2 and bring in the data from (1) to (3) to get Jr for 1.091 (kg·m2), Jm for 3.9 (kg m2), and nrmax for 695 (r/min).The maximum speed of the ladder speed diagram can be selected within the maximum speed allowed by the motor. For determining the biting and throwing speed, the goal is to obtain a shorter rolling rhythm time, ensure smooth biting of the rolled piece, facilitate the operation, and adjust to the reasonable speed range of the main motor. We get the shortest rolling rhythm when the current pass’s throwing speed equals the next pass’s biting speed. There is no requirement for the shortest rolling rhythm in this article. When the roll speed is set tonp, the kinetic energy of the roll can be recovered. The roll can be rotated from static to ny when the kinetic energy is recovered. The rolled piece is assumed to bite in at 3.7 m/s during the idling acceleration period and throw out at 4.375 m/s during the idling deceleration period. Take a bite from the mill and throw out the mill speed based on the actual operating conditions of the selected rolling mill. Bringing in the data, ny is 321.4 r/min, np is 380 r/min, ωy is 33.64 rad/s, and ωp is 39.77 rad/s.


## 4.2. Speed and Time Required for Each Stage of One Pass for the Roll

We assume that the accelerationaj of the main motor in the acceleration period is 160 r/s2 and the deceleration bz in the deceleration period is 240 r/s2. According to the actual rolling time of the selected mill and rolls, the acceleration and deceleration are calculated. Table 5 shows the duration of each of the abovementioned six stages.Table 5
Time of each stage of one pass of the reversible rolling mills [2].
StageOperating modeTimeCalculation formulaValue (s)0 (0-t0)Idling constant speed periodt0Set idle constant speed periodt0 = 2 s21 (t0-t1)Idling acceleration periodtyty=ny−n0/aj1.696252 (t1-t2)Accelerated rolling periodtjtj=nw−ny/aj2.3353 (t2-t3)Constant speed rolling periodtwtw=60L1/πDnw−1/nwny+nw/2tj+nw+np/2tz1It is not calculated here; it is assumed that the constant speed rolling period tw is 2 s and L1 is the length of rolled piece after rolling24 (t3-t4)Slow rolling periodtz1tz1=nw−np/bz1.31255 (t4-t5)Idling deceleration periodtz2tz2=np/bz1.583


## 4.3. Torque Calculation of the Reversible Rolling Mills and Parameter Matching of Recovery System

As shown in the following equation, the main motor drive torque of the mill is calculated as(4)MD=MZi+Mf+Mkon±Mdon,where MD is the torque of the main motor, rolling torque is measured by MZ, and Mf is the additional friction torque occurring on the roller bearing and transmission mechanism during rolling. The idling torque, Mkon, is the friction torque during idling. Whenever the roller moves at nonuniform speeds, Mdon is required to overcome inertial forces. The first three moments of the transmission rolls for the static moment Mj, is calculated by the following equation:(5)Mj=MZi+Mf+Mkon.In the abovementioned formula, torque is measured when the roller rotates at a constant speed. These three items are indispensable for any rolling mill. Generally, rolling torque is the largest. The rolling torque can be determined in two ways: one by measuring the force of metal on the roller and the other by measuring energy consumption. Equation (6) is used in this article to calculate the rolling moment based on the metal force on the roller:(6)Mz=Pa,where P is the vertical pressure applied to the roller by the metal, i.e., the rolling force, and a is the length of the force arm. In order to eliminate the influence of geometric factors on the force arm, the force arm coefficient is often determined by Ψ, and the force arm value is determined by following equation. Cold-rolled strip, rolling force arm coefficient Ψ number for 0.33∼0.42, here, take Ψ = 0.42.(7)ψ=φ1αj=alj,where φ1 is the closing pressure action angle, the contact angle is αj, and lj is the contact arc length, which is calculated by(8)lj=R⋅∆h,where R is the roll radius and ∆h is the thickness difference before and after rolling.It is the rolling torque on a single roll that is calculated here. With the help of the following equation, we calculate the displacement of the recovery pump/motor based on the maximum rolling torque of a specific model 650 four-roller reversible rolling mill:(9)T=Vg⋅∆p⋅ηmh2π,where T is the torque. The hydraulic pump/motor’s maximum displacement is Vg, Δp stands for differential pressure, which is 31.5 MPa, and ηmh stands for mechanical efficiency, which is taken as 1. The displacement of the recovery pump/motor is calculated as 3488.9 ml/r according to equation (10).However, it can be seen from the abovementioned calculation that the rolling torque is ample and the recovered energy is limited in the onload rolling stage. If this part of the energy is recovered, it must match the recovery pump/motor with a large displacement, and the cost is high. Moreover, the rolling torque required during onload startup is large, and the torque provided by the recovered braking energy is small, which is not enough to overcome the rolling torque and drive the roll movement. Therefore, this article only gives the configuration of the recovery scheme.It is possible to use multiple recovery pumps/motors in parallel to reduce the input torque of the recovery pump/motor. Motors with high speeds can be used in conjunction with pumps and motors to avoid the problems associated with low-speed motors, such as energy efficiency or failure to recover braking energy. However, the abovementioned calculation indicates that the rolling torque is ample and the recovered energy is limited during the onload rolling stage. For this part of the energy to be recovered, the pump or motor must have a large displacement, and the cost is high. In addition, during onload startup, the rolling torque required is large, and the torque provided by the recovered braking energy is small, so the roll movement cannot be driven. The article only describes the configuration of this recovery scheme.In the no-load period, the static moment is not a rolling moment, and the dynamic moment is calculated by(10)Mdon=GD2375⋅dωdt,where G is the gravity of the rotating part, the rotating part’s inertia diameter is D, and ω stands for the angular velocity of the rotating part, calculated by equation (10).The formula for estimating no-load torque is complex, and it is usually estimated by using the following formula:(11)Mkon=0.03∼0.06Tm,Pm=Tmnm9549,where Tm represents the motor’s rated torque, Pm represents its rated power, and nm represents its rated speed. In this case, Mkon is 286.47 N·m, idling acceleration torque MD1 is 295.777 N·m, and idling deceleration torque MD5 is 272.509 N·m; MD1 matches the hydraulic pump/motor maximum displacement Vg, which is 58.97 ml/r. This article chooses a two-way variable pump/motor with a maximum displacement of 80 ml/r Figure 7 shows the working speed diagram for the reversible rolling mill roll.Figure 7
Working speed diagram of the reversible rolling mill roll.


## 5. Simulation Analysis and Testing

### 5.1. Modeling of System Simulations

Based on the working principle of the energy recovery system of the reversible rolling mill and the parameters of the components calculated in the previous article, a model of the energy recovery system of the reversible rolling mill is established by AMEsim. The model consists of a primary motor drive module, a speed control module, an energy recovery module, and an oil replenishment system module. For each stage of the reversible rolling mill, the speed of the main drive motor is fixed for a given thickness of a particular rolled part, so PI speed control is used. The torque input for rolling different mill parts is given by a signal, which controls whether the energy recovery system is connected to the original system by the clutch on/off. Figure8 illustrates the energy recovery model for reversible rolling mills, and Table 6 shows the system’s basic parameters.Figure 8
Energy recovery model based on the reversible rolling mill.Table 6
Basic parameters of the energy recovery model of reversible rolling mill.
NameParametersValueThree-phase permanent magnet synchronous motorRated power (kW)600Rated speed (rpm)600Moment of inertia GD2 (kg·m2)174RollMoment of inertia (kg·m2)1.091Axial plunger variable motorDisplacement (mL/r)80Volumetric efficiency0.97Mechanical efficiency0.90High-pressure accumulatorNominal volume (L)40Nominal pressure (MPa)31.5Preinflation pressure (MPa)18Low-pressure accumulatorNominal volume (L)40Nominal pressure (MPa)10Preinflation pressure (MPa)2


### 5.2. Simulation Results and Analysis

The following two central studies were carried out through this energy recovery system.(1)
For example, the energy recovery device is started in the first pass when the roll is thrown out to recover the energy of one working roll of the reversible rolling mill. In other words, during the rolling deceleration period at speednp, the electromagnetic clutch between the recovery pump/motor and the roll is engaged, driving the roll from 0 to ny using the recovered energy. In this period, the electromagnetic clutch between the main drive motor and the roll is disconnected, the motor is idle with low-speed n0, and the main drive motor starts driving the roll rotation when the roll speed reaches ny, and the energy recovery device stops working. Figure 9 shows the roll speed and high-pressure accumulator 13.1 pressure and volume changes. Recovery pump/motor braking, starting torque, and power changes are shown in Figure 10.(2)
In the case of a reversible rolling mill, for example, the energy recovery device is activated during the first bite and it throws of the rolled part. Furthermore, it absorbs the motor’s surplus power in addition to recovering the kinetic energy of the braking rolls. To recover the energy from the two parts independently, two high-pressure accumulators are used. As shown in Figure11, the clutch on/off signal is displayed. The ideal speed profile of the roll, the actual speed profile, and the speed profile of the recovery pump/motor are shown in Figures 12 and 13, which also illustrates the change in fluid pressure and volume during the bite-in and throw-out periods in the high-pressure accumulator. The change in torque and power of the energy recovery system is shown in Figure 14. The simulation results are shown in Tables 7–12, and due to space limitations, only some of the essential results are listed in this article.(3)
The energy recovery system brakes the roll at 380 r/min, with a braking time of 1.366 s, when the high-pressure accumulator 13.1 is inflated to 18.126 MPa. Gap time for each pass is 3.634 s, which can be adjusted according to the actual situation. 5 s into the energy utilization phase, the recovery pump/motor drives the roll, with a maximum speed of 321.4 r/min. As can be seen in Figure10, the pump/motor provides the required braking and starting torque for the rolls during the energy recovery and utilization phase, with a maximum value of 142.423 Nm and 142.344 Nm, respectively. In the energy recovery phase, the pump/motor has the maximum power at 0.089 s with a value of 5.354 kW. In the energy utilization phase, the pump/motor has a maximum power of 4.525 kW, and this power is obtained at 6.267 s.(4)
Figure11 shows the on/off signals for clutches 2 and 5, which indicate the different phases of energy recovery. Rolls are driven by the primary drive system between 0 and 3.696 s and 4.696 and 9.344 s. A transition occurs between 3.696 and 4.696 seconds, when the energy recovery system intervenes, which marks the beginning of the bite into the rolled part. During 9.344 seconds, the energy recovery system works; when the rolls are rolled and the energy recovery system brakes them, the braking time is slightly reduced compared to the original system (see Figure 12), which shortens the rolling cycle PI control of the motor speed which is used by the original mill drive system, as shown in Figure 12, and the actual speed is lower than the ideal speed and the pump motor speed fluctuates a little at 4.696 seconds. This is because the simulation sampling interval affects the antireverse valve’s operating time in the energy recovery system. As the hydraulic system is already disconnected from the original mill drive system at this point, this fluctuation does not affect the regular operation of the system. As can be seen from Figure 13, the energy recovery system starts to fill rapidly with liquid from 3.696 to 3.905 s. The fluid pressure in the high-pressure accumulator 13.1 increases rapidly from 0 to 18.0 MPa and then stabilizes. The maximum charge pressure was obtained at 4.719 s with a value of 18.188 MPa. The volume change in accumulator 13.1 was 40–39.706 L. At 9.344–9.548 s, the liquid pressure in high-pressure accumulator 13.2 increased rapidly from 0 to 18.0 MPa and then stabilized. The maximum charge pressure was obtained at 10.846 s with a value of 18.328 MPa. The volume of accumulator 13.2 varied from 40 to 39.792 L. As seen in Figure 14, the change in pump/motor torque for energy recovery stage 1 was a rapid increase from 0 to 141.328 Nm, followed by a maximum value of 143.140 Nm in 0.781 s. The pump/motor torque for energy recovery stage 2 rapidly increased from 0 to 141.328 Nm, followed by a maximum value of 143.140 Nm in 0.781 s. In energy recovery phase 2, the pump/motor torque reaches its maximum at 10.802 s with a value of 142.469 Nm. Energy recovery phase 1 reaches its maximum power at 4.686 s with a value of 7.002 kW, and energy recovery phase 2 reaches its maximum power at 9.548 s with a value of 5.284 kW.Figure 9
High-pressure accumulator change in pressure and volume and roll speed change.Figure 10
Variations in starting torque, pump/motor braking, and power.Figure 11
Signal for clutch on/off.Figure 12
Speeds of the recovery pump/motor, the ideal speed, and the actual speed for the roll.Figure 13
Pressure and volume changes in high-pressure accumulators during bite-in and dump-out.Figure 14
Variations in torque and power in energy recovery systems.


### 5.3. Calculation of Energy Recovery and Utilization Efficiency

The kinetic energy of the roller at the rotational speedny is calculated by equation (12), and the kinetic energy of the roller at the rotational speed np is calculated by equation (13). The value of Ey is calculated to be 2.824 kJ and the value of Ep is 3.947 kJ:(12)Ey=12Jωy,2(13)Ep=12Jωp2.Table 7
High-pressure accumulator change in pressure and volume and roll speed change.
Time (s)Roll speed (r/min)Pressure of high-pressure accumulator (MPa)Volume of high-pressure accumulator (L)0380040187.69718.11839.8151.366018.12639.8022018.12639.8023018.12639.8024018.12639.8025018.12639.802Table 8
Variations in starting torque, pump/motor braking, and power.
Time (s)Torque for pump/motor (Nm)Power of pump/motor (kW)0.0010.440970.0170.089141.500495.3541.24142.423430.2745.016142.34378−0.03806.267141.10065−4.525Table 9
Signal for clutch on/off.
Time (s)Signal of clutch 2Signal of clutch 50OnOff3.69625OnOn4.69625OnOff9.34375OffOnTable 10
Speeds of the recovery pump/motor, the ideal speed, and the actual speed for the roll.
Time (s)Expected speed of rolls (r/min)Actual speed of rolls (r/min)Speed of pump/motor (r/min)3.69625317.803317.461904.69625448.956442.845435.4629.34375371.214371.164370.973Table 11
Pressure and volume changes in high-pressure accumulators during bite-in and dump-out.
Time (s)Gas volume of high-pressure accumulator 13.1 (L)Pressure of high-pressure accumulator 13.1 (bar)Gas volume of high-pressure accumulator 13.2 (L)Pressure of high-pressure accumulator 13.2 (bar)00404003.905180.002404004.719181.88039.7064009.548181.88039.70640180.00110.846181.88039.70639.792181.328Table 12
Variations in torque and power in energy recovery systems.
Time (s)Input torque for pump/motor (Nm)Input power of pump/motor (kW)Speed of pump/motor (r/min)3.6960003.905141.3285.066342.2704.686143.1407.002467.1109.350.00380371.2149.548141.3575.284356.9649.57141.4335.197350.89410.802142.4690.18512.427The formula for Boyle’s law is as follows:(14)p0V0n=p1V1n=p2V2n=paVan=C,where p0 is the gas precharge pressure of the accumulator, the charging volume of the accumulator is V0, p1 is the lowest working pressure of the accumulator, V1 is the volume of the accumulator at the lowest working pressure, p2 is the highest working pressure of the accumulator, the volume of V2 corresponds to the accumulator’s highest working pressure, pa is the pressure under the free state of the accumulator, and the volume of the accumulator under free state pressure is Va.The energy equation of the accumulator is calculated by(15)Eacc=∫V2V1pdV=p1V1−p2V21−n=p2V11−np1p2−p1p21n,(16)Eer=EaccEp×100%,(17)Eeu=EyEacc×100%.Bringingp0: 18 MPa, V0: 40 L, Pa: 18.126 MPa, and Va: 39.802 L into equation (15), the energy recovery efficiency calculated by equations (16) and (17) represents the calculation of the energy utilization rate. The value of Eacc is calculated to be 3.594 kJ, the value of Eer is 91.056%, and the value of Eeu is 78.575%. The overall energy efficiency rate is around 71.547%.Based on the abovementioned calculation, the reversible rolling mill has a recoverable energy of 3.947 kJ, an actual energy recovery of 3.594 kJ, and an energy recovery efficiency of 91.056% during the no-load deceleration period. In the reversible rolling mill’s no-load acceleration period, the useable energy is 3.594 kJ, the utilized energy is 2.824 kJ, and the energy utilization rate is 78.575%. The coefficient of viscous friction, Coulomb friction moment, and static friction moment are all related to energy loss.Bringingp0: 18 MPa, V0: 40 L, Pa1: 18.188 MPa, Va1: 39.706 L, Pa2: 18.133 MPa, and Va2: 39.792 L into equation (16), the value of Eacc1 is calculated to be 5.358 kJ and the value of Eacc2 is 3.781 kJ.A single roll absorbs 5.358 kJ of extra motor power and recovers 3.781 kJ of roll braking kinetic energy so that the two rolls can recover 18.278 kJ of energy in one rolling pass. An energy recovery of 255.892 kJ can be achieved by rolling 14 passes. The part of the energy can be used for roll no-load start, at the sudden change in load as an auxiliary power source to provide instantaneous high power, for the mill and other auxiliary equipment to provide power, such as winders.


### 5.4. Experimental Verification

To verify the efficiency of the reversible rolling mills braking energy recovery, the flywheel is used as the load to simulate the roll braking. The following equivalent test scheme is designed based on the principle that the energy before roll braking equals the energy before flywheel braking. First, the electromagnetic clutch YC1 is engaged, and the motor drives the flywheel to rotate to the set speed. Next, the electromagnetic clutch YC1 is disconnected and the electromagnetic clutch YC2 is combined to brake the flywheel via the hydraulic pump/motor. When the hydraulic motor is in oil pump mode, oil is sucked from the low-pressure accumulator and supplied to the high-pressure accumulator, converting the rotational kinetic energy of the flywheel into hydraulic energy to be stored in the high-pressure accumulator until the flywheel stops spinning. The brake energy recovery test rig is shown in Figure15. The flywheel runs through the motor at a speed of 380 r/min. Therefore, the braking efficiency is calculated to be 56.92%.Figure 15
Brake energy recovery test bench. 1, a group of accumulators; 2, units for oil pump motors; 3, integrated valve blocks; 4, flowmeter; 5, energy recovery pump/motor; 6, rotational speed and torque sensors; 7, clutch 1; 8, flywheel; 9, clutch 2; 10, motor; 11, operation panel; 12, industrial controllers.


## 5.1. Modeling of System Simulations

Based on the working principle of the energy recovery system of the reversible rolling mill and the parameters of the components calculated in the previous article, a model of the energy recovery system of the reversible rolling mill is established by AMEsim. The model consists of a primary motor drive module, a speed control module, an energy recovery module, and an oil replenishment system module. For each stage of the reversible rolling mill, the speed of the main drive motor is fixed for a given thickness of a particular rolled part, so PI speed control is used. The torque input for rolling different mill parts is given by a signal, which controls whether the energy recovery system is connected to the original system by the clutch on/off. Figure8 illustrates the energy recovery model for reversible rolling mills, and Table 6 shows the system’s basic parameters.Figure 8
Energy recovery model based on the reversible rolling mill.Table 6
Basic parameters of the energy recovery model of reversible rolling mill.
NameParametersValueThree-phase permanent magnet synchronous motorRated power (kW)600Rated speed (rpm)600Moment of inertia GD2 (kg·m2)174RollMoment of inertia (kg·m2)1.091Axial plunger variable motorDisplacement (mL/r)80Volumetric efficiency0.97Mechanical efficiency0.90High-pressure accumulatorNominal volume (L)40Nominal pressure (MPa)31.5Preinflation pressure (MPa)18Low-pressure accumulatorNominal volume (L)40Nominal pressure (MPa)10Preinflation pressure (MPa)2


## 5.2. Simulation Results and Analysis

The following two central studies were carried out through this energy recovery system.(1)
For example, the energy recovery device is started in the first pass when the roll is thrown out to recover the energy of one working roll of the reversible rolling mill. In other words, during the rolling deceleration period at speednp, the electromagnetic clutch between the recovery pump/motor and the roll is engaged, driving the roll from 0 to ny using the recovered energy. In this period, the electromagnetic clutch between the main drive motor and the roll is disconnected, the motor is idle with low-speed n0, and the main drive motor starts driving the roll rotation when the roll speed reaches ny, and the energy recovery device stops working. Figure 9 shows the roll speed and high-pressure accumulator 13.1 pressure and volume changes. Recovery pump/motor braking, starting torque, and power changes are shown in Figure 10.(2)
In the case of a reversible rolling mill, for example, the energy recovery device is activated during the first bite and it throws of the rolled part. Furthermore, it absorbs the motor’s surplus power in addition to recovering the kinetic energy of the braking rolls. To recover the energy from the two parts independently, two high-pressure accumulators are used. As shown in Figure11, the clutch on/off signal is displayed. The ideal speed profile of the roll, the actual speed profile, and the speed profile of the recovery pump/motor are shown in Figures 12 and 13, which also illustrates the change in fluid pressure and volume during the bite-in and throw-out periods in the high-pressure accumulator. The change in torque and power of the energy recovery system is shown in Figure 14. The simulation results are shown in Tables 7–12, and due to space limitations, only some of the essential results are listed in this article.(3)
The energy recovery system brakes the roll at 380 r/min, with a braking time of 1.366 s, when the high-pressure accumulator 13.1 is inflated to 18.126 MPa. Gap time for each pass is 3.634 s, which can be adjusted according to the actual situation. 5 s into the energy utilization phase, the recovery pump/motor drives the roll, with a maximum speed of 321.4 r/min. As can be seen in Figure10, the pump/motor provides the required braking and starting torque for the rolls during the energy recovery and utilization phase, with a maximum value of 142.423 Nm and 142.344 Nm, respectively. In the energy recovery phase, the pump/motor has the maximum power at 0.089 s with a value of 5.354 kW. In the energy utilization phase, the pump/motor has a maximum power of 4.525 kW, and this power is obtained at 6.267 s.(4)
Figure11 shows the on/off signals for clutches 2 and 5, which indicate the different phases of energy recovery. Rolls are driven by the primary drive system between 0 and 3.696 s and 4.696 and 9.344 s. A transition occurs between 3.696 and 4.696 seconds, when the energy recovery system intervenes, which marks the beginning of the bite into the rolled part. During 9.344 seconds, the energy recovery system works; when the rolls are rolled and the energy recovery system brakes them, the braking time is slightly reduced compared to the original system (see Figure 12), which shortens the rolling cycle PI control of the motor speed which is used by the original mill drive system, as shown in Figure 12, and the actual speed is lower than the ideal speed and the pump motor speed fluctuates a little at 4.696 seconds. This is because the simulation sampling interval affects the antireverse valve’s operating time in the energy recovery system. As the hydraulic system is already disconnected from the original mill drive system at this point, this fluctuation does not affect the regular operation of the system. As can be seen from Figure 13, the energy recovery system starts to fill rapidly with liquid from 3.696 to 3.905 s. The fluid pressure in the high-pressure accumulator 13.1 increases rapidly from 0 to 18.0 MPa and then stabilizes. The maximum charge pressure was obtained at 4.719 s with a value of 18.188 MPa. The volume change in accumulator 13.1 was 40–39.706 L. At 9.344–9.548 s, the liquid pressure in high-pressure accumulator 13.2 increased rapidly from 0 to 18.0 MPa and then stabilized. The maximum charge pressure was obtained at 10.846 s with a value of 18.328 MPa. The volume of accumulator 13.2 varied from 40 to 39.792 L. As seen in Figure 14, the change in pump/motor torque for energy recovery stage 1 was a rapid increase from 0 to 141.328 Nm, followed by a maximum value of 143.140 Nm in 0.781 s. The pump/motor torque for energy recovery stage 2 rapidly increased from 0 to 141.328 Nm, followed by a maximum value of 143.140 Nm in 0.781 s. In energy recovery phase 2, the pump/motor torque reaches its maximum at 10.802 s with a value of 142.469 Nm. Energy recovery phase 1 reaches its maximum power at 4.686 s with a value of 7.002 kW, and energy recovery phase 2 reaches its maximum power at 9.548 s with a value of 5.284 kW.Figure 9
High-pressure accumulator change in pressure and volume and roll speed change.Figure 10
Variations in starting torque, pump/motor braking, and power.Figure 11
Signal for clutch on/off.Figure 12
Speeds of the recovery pump/motor, the ideal speed, and the actual speed for the roll.Figure 13
Pressure and volume changes in high-pressure accumulators during bite-in and dump-out.Figure 14
Variations in torque and power in energy recovery systems.


## 5.3. Calculation of Energy Recovery and Utilization Efficiency

The kinetic energy of the roller at the rotational speedny is calculated by equation (12), and the kinetic energy of the roller at the rotational speed np is calculated by equation (13). The value of Ey is calculated to be 2.824 kJ and the value of Ep is 3.947 kJ:(12)Ey=12Jωy,2(13)Ep=12Jωp2.Table 7
High-pressure accumulator change in pressure and volume and roll speed change.
Time (s)Roll speed (r/min)Pressure of high-pressure accumulator (MPa)Volume of high-pressure accumulator (L)0380040187.69718.11839.8151.366018.12639.8022018.12639.8023018.12639.8024018.12639.8025018.12639.802Table 8
Variations in starting torque, pump/motor braking, and power.
Time (s)Torque for pump/motor (Nm)Power of pump/motor (kW)0.0010.440970.0170.089141.500495.3541.24142.423430.2745.016142.34378−0.03806.267141.10065−4.525Table 9
Signal for clutch on/off.
Time (s)Signal of clutch 2Signal of clutch 50OnOff3.69625OnOn4.69625OnOff9.34375OffOnTable 10
Speeds of the recovery pump/motor, the ideal speed, and the actual speed for the roll.
Time (s)Expected speed of rolls (r/min)Actual speed of rolls (r/min)Speed of pump/motor (r/min)3.69625317.803317.461904.69625448.956442.845435.4629.34375371.214371.164370.973Table 11
Pressure and volume changes in high-pressure accumulators during bite-in and dump-out.
Time (s)Gas volume of high-pressure accumulator 13.1 (L)Pressure of high-pressure accumulator 13.1 (bar)Gas volume of high-pressure accumulator 13.2 (L)Pressure of high-pressure accumulator 13.2 (bar)00404003.905180.002404004.719181.88039.7064009.548181.88039.70640180.00110.846181.88039.70639.792181.328Table 12
Variations in torque and power in energy recovery systems.
Time (s)Input torque for pump/motor (Nm)Input power of pump/motor (kW)Speed of pump/motor (r/min)3.6960003.905141.3285.066342.2704.686143.1407.002467.1109.350.00380371.2149.548141.3575.284356.9649.57141.4335.197350.89410.802142.4690.18512.427The formula for Boyle’s law is as follows:(14)p0V0n=p1V1n=p2V2n=paVan=C,where p0 is the gas precharge pressure of the accumulator, the charging volume of the accumulator is V0, p1 is the lowest working pressure of the accumulator, V1 is the volume of the accumulator at the lowest working pressure, p2 is the highest working pressure of the accumulator, the volume of V2 corresponds to the accumulator’s highest working pressure, pa is the pressure under the free state of the accumulator, and the volume of the accumulator under free state pressure is Va.The energy equation of the accumulator is calculated by(15)Eacc=∫V2V1pdV=p1V1−p2V21−n=p2V11−np1p2−p1p21n,(16)Eer=EaccEp×100%,(17)Eeu=EyEacc×100%.Bringingp0: 18 MPa, V0: 40 L, Pa: 18.126 MPa, and Va: 39.802 L into equation (15), the energy recovery efficiency calculated by equations (16) and (17) represents the calculation of the energy utilization rate. The value of Eacc is calculated to be 3.594 kJ, the value of Eer is 91.056%, and the value of Eeu is 78.575%. The overall energy efficiency rate is around 71.547%.Based on the abovementioned calculation, the reversible rolling mill has a recoverable energy of 3.947 kJ, an actual energy recovery of 3.594 kJ, and an energy recovery efficiency of 91.056% during the no-load deceleration period. In the reversible rolling mill’s no-load acceleration period, the useable energy is 3.594 kJ, the utilized energy is 2.824 kJ, and the energy utilization rate is 78.575%. The coefficient of viscous friction, Coulomb friction moment, and static friction moment are all related to energy loss.Bringingp0: 18 MPa, V0: 40 L, Pa1: 18.188 MPa, Va1: 39.706 L, Pa2: 18.133 MPa, and Va2: 39.792 L into equation (16), the value of Eacc1 is calculated to be 5.358 kJ and the value of Eacc2 is 3.781 kJ.A single roll absorbs 5.358 kJ of extra motor power and recovers 3.781 kJ of roll braking kinetic energy so that the two rolls can recover 18.278 kJ of energy in one rolling pass. An energy recovery of 255.892 kJ can be achieved by rolling 14 passes. The part of the energy can be used for roll no-load start, at the sudden change in load as an auxiliary power source to provide instantaneous high power, for the mill and other auxiliary equipment to provide power, such as winders.


## 5.4. Experimental Verification

To verify the efficiency of the reversible rolling mills braking energy recovery, the flywheel is used as the load to simulate the roll braking. The following equivalent test scheme is designed based on the principle that the energy before roll braking equals the energy before flywheel braking. First, the electromagnetic clutch YC1 is engaged, and the motor drives the flywheel to rotate to the set speed. Next, the electromagnetic clutch YC1 is disconnected and the electromagnetic clutch YC2 is combined to brake the flywheel via the hydraulic pump/motor. When the hydraulic motor is in oil pump mode, oil is sucked from the low-pressure accumulator and supplied to the high-pressure accumulator, converting the rotational kinetic energy of the flywheel into hydraulic energy to be stored in the high-pressure accumulator until the flywheel stops spinning. The brake energy recovery test rig is shown in Figure15. The flywheel runs through the motor at a speed of 380 r/min. Therefore, the braking efficiency is calculated to be 56.92%.Figure 15
Brake energy recovery test bench. 1, a group of accumulators; 2, units for oil pump motors; 3, integrated valve blocks; 4, flowmeter; 5, energy recovery pump/motor; 6, rotational speed and torque sensors; 7, clutch 1; 8, flywheel; 9, clutch 2; 10, motor; 11, operation panel; 12, industrial controllers.


## 6. Conclusion

This article proposes a new type of noncoaxial parallel electrohydraulic hybrid drive system configuration and applies it to the reversible rolling mill. It is proposed to collect the braking energy of the previous pass of the reversible rolling mill roll and the surplus power of the original primary drive motor. Then, we start or accelerate the process for the next roll pass. As an auxiliary power source, this portion of energy can be used for no-load roll start, in load mutation to provide instantaneous high power and to power other auxiliary equipment in the rolling mill. During the no-load deceleration period of the reversible rolling mill, the energy recovery efficiency is 91.056%, while the energy utilization rate is 78.575% during the no-load acceleration period. The overall energy efficiency rate is around 71.547%. Experimental results indicate that the overall energy efficiency rate of the reversible rolling mill is 56.92%. The correctness of the simulation results and the effectiveness of the proposed scheme are well verified.Reversible rolling mills with energy recovery systems and innovative energy innovation drive schemes can reduce energy consumption, which lays the foundation for energy recovery systems in reversible steel rolling machinery. There are still some limitations to this article; however, as roll braking produces a small amount of kinetic energy, the energy recovery device contributes little to the original drive system after recovering this energy, so designing the electrohydraulic hybrid drive system for a reversible rolling mill presents a new challenge. In the future, our efforts will focus on developing an energy management strategy for reversible rolling mills.



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*Source: 3316392-2023-02-09.xml*