CURRENT REGULATION IN AIR CONDITIONING SYSTEMS

An air conditioning system includes a compressor including a motor, a condenser, and an evaporator; a drive including an inverter providing a multiphase, AC output voltage to the motor; a current sensor configured to sense an output current generated by the inverter; a controller configured to communicate with the current sensor and the inverter, the controller configured to provide a voltage command to the inverter, the controller configured to execute operations including: monitor the output current generated by the inverter; compare the output current to a threshold; determine the occurrence of an over current condition when the output current exceeds the threshold; when an over current condition is detected, reducing the voltage command to the inverter.

BACKGROUND

Exemplary embodiments of this disclosure pertain to the art of air conditioning systems, and more specifically, to current regulation in air conditioning systems.

Air conditioning systems have transitioned from mechanically driven components to electrically driven components. Electrically driven air conditioning systems utilize power electronics to generate electrical power suitable for various components of the air conditioning systems. Power electronics drives have inrush currents much higher than their steady-state current due to the charging current of the input capacitance when they start to operate. This inrush current can damage components or make the drive not able to operate because of the over current protection.

SUMMARY

According to an embodiment, an air conditioning system includes a compressor including a motor, a condenser, and an evaporator; a drive including an inverter providing a multiphase, AC output voltage to the motor; a current sensor configured to sense an output current generated by the inverter; a controller configured to communicate with the current sensor and the inverter, the controller configured to provide a voltage command to the inverter, the controller configured to execute operations including: monitor the output current generated by the inverter; compare the output current to a threshold; determine the occurrence of an over current condition when the output current exceeds the threshold; when an over current condition is detected, reducing the voltage command to the inverter.

In addition to one or more of the features described herein, or as an alternative, further embodiments may include wherein if no over current condition is detected, the controller is configured to increase the voltage command to the inverter.

In addition to one or more of the features described herein, or as an alternative, further embodiments may include wherein the controller is configured to increase the voltage command to the inverter in a step wise manner.

In addition to one or more of the features described herein, or as an alternative, further embodiments may include wherein the controller is configured to increase the voltage command to the inverter until the voltage command to the inverter reaches an upper limit.

In addition to one or more of the features described herein, or as an alternative, further embodiments may include wherein reducing the voltage command to the inverter comprises reducing the voltage command to the inverter by a percentage.

In addition to one or more of the features described herein, or as an alternative, further embodiments may include wherein reducing the voltage command to the inverter comprises reducing the voltage command to the inverter by a fixed amount.

In addition to one or more of the features described herein, or as an alternative, further embodiments may include wherein the voltage command to the inverter corresponds to duty cycle of a pulse width modulation (PWM) signal provided to the inverter.

According to another embodiment, a method of controlling an air conditioning system including a compressor including a motor, a condenser, an evaporator and a drive including an inverter providing a multiphase, AC output voltage to the motor, includes: monitoring an output current generated by the inverter; comparing the output current to a threshold; determining the occurrence of an over current condition when the output current exceeds the threshold; and when an over current condition is detected, reducing a voltage command to the inverter.

In addition to one or more of the features described herein, or as an alternative, further embodiments of the method may include increasing the voltage command to the inverter when no over current condition is detected.

In addition to one or more of the features described herein, or as an alternative, further embodiments of the method may include wherein increasing the voltage command to the inverter occurs a step wise manner.

In addition to one or more of the features described herein, or as an alternative, further embodiments of the method may include wherein the increasing the voltage command to the inverter includes increasing the voltage command to the inverter until the voltage command to the inverter reaches an upper limit.

In addition to one or more of the features described herein, or as an alternative, further embodiments of the method may include wherein reducing the voltage command to the inverter comprises reducing the voltage command to the inverter by a percentage.

In addition to one or more of the features described herein, or as an alternative, further embodiments of the method may include wherein reducing the voltage command to the inverter comprises reducing the voltage command to the inverter by a fixed amount.

In addition to one or more of the features described herein, or as an alternative, further embodiments of the method may include wherein the voltage command to the inverter corresponds to a duty cycle of a pulse width modulation (PWM) signal provided to the inverter.

According to another embodiment, an air conditioning system includes a compressor including a motor, a condenser, and an evaporator; a drive including an inverter providing a multiphase, AC output voltage to the motor; a current sensor configured to sense an output current generated by the inverter; a controller configured to communicate with the current sensor and the inverter, the controller configured to provide a frequency command to the inverter, the controller configured to execute operations including: monitor the output current generated by the inverter; compare the output current to a threshold; determine the occurrence of an over current condition when the output current exceeds the threshold; when an over current condition is detected, reducing the frequency command to the inverter.

Additional technical features and benefits are realized through the techniques of the present disclosure. Embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed subject matter. For a better understanding, refer to the detailed description and to the drawings.

DETAILED DESCRIPTION

FIG.1is a block diagram of air conditioning system100in an example embodiment. The air conditioning system100may be configured to condition air in a building, such as a chiller, roof top unit, heat pump, etc. The air conditioning system100may be configured to condition air for refrigerated environments, such as a refrigerated container, a refrigerated trailer, refrigerator/freezer, etc.

The air conditioning system100includes a variable speed motor102that is coupled to a compressor104. The compressor104includes an impeller/rotor that rotates and compresses liquid refrigerant to a superheated refrigerant vapor for delivery to a condenser106. In the condenser106, the refrigerant vapor is liquefied at high pressure and rejects heat (e.g., to the outside air via a condenser fan in an air-cooled application). The liquid refrigerant exiting condenser106is delivered to an evaporator108through an expansion valve107. The refrigerant passes through the expansion valve107where a pressure drop causes the high-pressure liquid refrigerant to achieve a lower pressure combination of liquid and vapor. As fluid passes the evaporator108, the low-pressure liquid refrigerant evaporates, absorbing heat from the fluid, thereby cooling the fluid and evaporating the refrigerant. The low-pressure refrigerant is again delivered to compressor104where it is compressed to a high-pressure, high temperature gas, and delivered to condenser106to start the refrigeration cycle again. It is to be appreciated that while a specific air conditioning system is shown inFIG.1, the present teachings may be applicable to any air conditioning system.

As shown inFIG.1, the compressor104is driven by a variable speed motor102coupled to a drive101including an alternating current (AC)—direct current (DC) converter200and a DC-AC inverter110. The drive101is powered from one or more of an AC power source177and a DC power source179. The AC power source177may be an engine-driven generator or an AC grid (also referred to as mains). AC input voltage from the AC power source177is supplied through the AC-DC converter200. The DC power source179may be a battery or other charge storage device. The DC power source179is connected to a DC bus201between the AC-DC converter200and the DC-AC inverter110. The motor102may be driven by one of the AC power source177, the DC power source179, or a combination of both. In addition to the motor102, the drive101may power other components of the air conditioning system, such as an evaporator fan, a condenser fan, a battery charger, etc.

The AC-DC converter200includes solid-state electronics to convert the AC input voltage from AC power source177to a DC voltage across a DC bus201. The converter200may be controlled by a controller340using controls signals. The DC-AC inverter110includes solid-state electronics to produce multiphase, AC output voltage. In an embodiment, DC-AC inverter110converts the DC voltage from the converter200into a multiphase, AC output voltage, at a desired frequency and/or magnitude in order to drive the multiphase motor102. The DC-AC inverter110may also convert DC voltage from DC power source179into a multiphase, AC output voltage, at a desired frequency and/or magnitude in order to drive the multiphase motor102. The DC-AC inverter110may be controlled by a controller340using controls signals (e.g., PWM control).

The controller340may be implemented using a general-purpose microprocessor executing a computer program stored on a storage medium to perform the operations described herein. Alternatively, the controller340may be implemented in hardware (e.g., ASIC, FPGA) or in a combination of hardware/software. The controller340may also be part of an air conditioning control system.

The controller340operates the inverter110to control the current supplied to the motor102(or other loads of the air conditioning system). One or more sensors402are used to monitor current supplied from the inverter110to the motor102(or other loads of the air conditioning system). The sensors402may be located on one phase or multiple phases of the output of the inverter110. The sensors402may communicate with the controller340by a wired and/or wireless connection.

FIG.2is a flowchart of a process for controlling current output by the inverter110. By controlling the current output by the inverter110, the controller340can avoid the occurrence of high inrush currents that may damage power electronics in the drive101or cause the drive101to shut down due to internal protection.

The process ofFIG.2begins at502, when the inverter110is powered on. At504, the controller340determines if an over current condition is detected. The controller340detects an over current condition by monitoring the output current of the inverter110using sensor(s)402. The controller340compares the output current of the inverter110to a current threshold. If the output current is less than a current threshold, then the process flow to506.

At506, the controller340determines if a voltage command, Vcmd, is less than an upper limit. The voltage command, Vcmd, is provided from the controller340to the inverter110to regulate the output voltage of the inverter110. The voltage command may be represented as a numerical range, for example, 0 to 1. The voltage command range of 0 to 1 may correspond to a duty cycle of 0% to 100% of pulse width modulation (PWM) signals used to control switches in the inverter110. A voltage command of 0 corresponds to a duty cycle of 0%, and a minimum output voltage by the inverter110. A voltage command of 1 corresponds to a duty cycle of 100%, and a maximum output voltage by the inverter110.

As noted above, at506, the controller340determines if a voltage command, Vcmd, is less than an upper limit, e.g., 1. If the voltage command, Vcmd, is less than an upper limit, flow proceeds to508where the controller340increases the voltage command, Vcmd, by a step value (for example, 0.1 or a 10% increase in duty cycle). In this manner, the controller340ramps up the output voltage of the inverter110step wise over time. At506, if the voltage command, Vcmd, has reached the upper limit, e.g., 1, then flow proceeds to510.

At510, the controller340confirms that the inverter110is still on. If the inverter is not on at510, then the process ends at512, and restarts when the inverter110is turned on again. If at510, the inverter510is on, flow returns to504, where the controller340determines if an over current condition is detected. If the output current of the inverter110is greater than the current threshold, an over current condition is detected at504, and flow proceeds to514where the controller340reduces the voltage command, Vcmd. The voltage command, Vcmd, may be reduced by a percentage of a current value, for example, 0.5 times the current voltage command, as show in514. In other embodiments, the voltage command, Vcmd, may be reduced by a fixed amount, for example, 0.3 or 30% duty cycle reduction.

FIG.3illustrates an example of the process ofFIG.2.FIG.3depicts the inverter voltage command, Vcmd, versus time and the inverter output current versus time. The inverter voltage command, Vcmd, increases from 0% to 75% over 6 seconds. At the 6 second time, an over current condition occurs as shown at1inFIG.3. In response to the over current condition, the controller340reduces the inverter voltage command, Vcmd, from 75% to 37.5%, as shown at2ofFIG.3. The controller340then increases the inverter voltage command, Vcmd, in a step wise manner as shown at3inFIG.3.

In another embodiment, the controller340controls a frequency of the output of the inverter110when an over current condition is detected. Control of the frequency of the output of the inverter110operates in a similar manner as the process shown inFIG.2. Instead of adjusting the voltage command, Vcmd, the controller340adjusts a frequency command, Fcmd, which controls the frequency of the output of the inverter110. Similar to the process inFIG.2, the frequency command, Fcmd, may be reduced at514(in response to an over current condition) and then increased in a step wise manner until an upper limit is reached. In other embodiments, both the voltage command, Vcmd, and the frequency command, Fcmd, are controlled in accordance with the process inFIG.2.

Embodiments of this disclosure provide an inrush current handling method to decrease the peak current of the inrush current. This allows the drive to run through the inrush current without triggering a shutdown or damage to devices in the drive.