Design of Multi-Parallel Drive Techniquefor System with Numbers of Permanent Magnet
Synchronous Motors
Tsuyoshi Nagano

Jun-ichi Itoh

Nagaoka University of Technology
Niigata, Japanngn244@stn.nagaokaut.ac.jp

Nagaoka University of Technology
Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Abstract— This paper discusses a multi-parallel drive system,
which uses two types of inverters; A main inverter with V/fcontrol and numbers of auxiliary inverter with vector control
and damping control to control multiple numbers of Permanent
Magnet Synchronous Motors (PMSMs). In addition, in PMSMs,
the auxiliary windings are placed in the slots together with theconventional windings. The auxiliary windings are used in theauxiliary inverter with damping control to suppress the torquevibration when the motor speed is converged. From thesimulation and experimental results, the proposed systemachieves a stable operation with two parallel connected PMSMs.
Furthermore, the experimental results demonstrate theeffectiveness of the proposed system even if the output power of
the auxiliary inverter is less than 10% of the main inverter.
Moreover, the relationship among the response of the damping
control, the overshoot of the motor speed and the power
capacity of the auxiliary inverter are investigated in simulationand experiments. From the results, it can be confirmed that if
the application requires a slow response of the motor speed, thesmaller power capacity of the auxiliary inverter can be designed
to be 10% compared with the main inverter capacity.

I.

INTRODUCTION

Recently, driving multiple motors at a time shows goodinterests in community. One of the techniques to drive
multiple motors in parallel is to employ one large power
capacity inverter. One of the advantages of parallel motor
drive by one large power capacity inverter is to be able toreduce the numbers of the inverter and system cost. Multiple
numbers of parallel connected induction motor can be driven
by one large power capacity inverter. However, in the case of
PMSMs, since each pole position of the motor is different toeach other, the motor current cannot be controlled to match therotational coordinates by only one inverter. Therefore, it isdifficult to drive the parallel connected PMSMs by using only
one inverter. Due to the difference of each pole position, phasedifferences between the rotational coordinates of the inverter
and each of motor will occur eventually. In that case, thetorque vibration occurs due to the resonance between a
synchronous reactance and the inertia moment of the motor.

Therefore, the torque vibration needs to be suppressed in orderto drive the PMSMs in parallel.
On the other hand, in order to drive two PMSMs, twoparallel drive systems with a five-leg inverter [1-2] and a nineswitches inverter [3] have been proposed and studied [4-7].
However, the numbers of parallel units are limited by the
number of legs in the inverter. Especially, it is not practical todrive several ten units of PMSMs at the same time.
In this paper, in order to drive PMSMs in parallel
efficiently, multi-parallel drive system of PMSMs based on
V/f control is proposed [8]. The V/f control is suitable formulti-parallel drive for PMSM because the controller does not
require the information of the rotor position in the PMSM. In
addition, each of the PMSM is added with auxiliary windings[9]. The auxiliary windings, which are connected to a small
power capacity inverter, suppress the torque vibration. The
power capacity of the auxiliary inverter is much smaller thanthat of the main inverter because the power capacity of theauxiliary windings is required as much as the vibration forsuppression. Note that the application of this proposed system
is aiming for the constant speed drive system such as belt
conveyers, fans, process line control and so on. Variable speed
drive systems such as the electric train and electric vehicles
are not considered in this paper because the motor speed ineach motor is different in these applications.
This paper is organized as follows; first, the configurationof the proposed parallel drive system is introduced. Next, thecontrol method of the main inverter with V/f control and theauxiliary inverter with vector control is described. Next, thesimulation and experimental results demonstrate that the
proposed system can drive multiple of parallel connected
PMSMs efficiently by suppressing the torque vibration.
Finally, the relationships among the response of the damping
control and the overshoot of the motor speed and the power
capacity of the auxiliary inverter are clarified.

II.

PROPOSED SYSTEM AND CONTROL

Fig. 1 shows the configuration of the proposed system. In
PMSMs, the auxiliary windings which are used for thedamping control (with the auxiliary inverter) are placed in theslots together with the main windings as shown in Fig. 2. The
proposed system uses two different power rating inverters.
The first one is the large power capacity inverter for the main
windings to drive the motor. The second one is the small
power capacity inverter for the auxiliary windings to suppress
the torque vibration. In term of the effectiveness of the
proposed system, it is very important that the power capacityof the auxiliary inverter is enough small. In other words, thedamping power for the PMSM is much smaller than that of
the main drive power.
Fig. 3 shows the control block diagram of the proposed
Fig. 1.Configuration of the proposed system.The proposed system uses two
system. In the proposed system, the main inverter is applied different power rating inverters, the large power capacity inverter for thewith the V/f control and the auxiliary inverter is applied with main windings to drive the motor and the small power capacity inverter forthe vector control and the damping control. Each of theauxiliary inverter can control the current in auxiliary
windings in order to suppress the torque and speed vibration.
Figs. 4 (a) and (b) show the control block diagrams of
damping control which is implemented to the vector control
in the auxiliary inverter control. The torque vibration resultsfrom the fluctuation of the pole position. The fluctuation isthen intended to be suppressed by the damping control in theauxiliary inverter. The structure of Fig. 4 (a) is generally used
to suppress the fluctuation of the pole position. After thefluctuation is only extracted by a high pass filter, thisfluctuation is multiplied by a damping gain Kd where theproduct is the q-axis current command. However, there areproblems that it is influenced by the noise which is caused
due to differentiate the pole position error and that thefrequency bandwidth of the controller is limited. On the other
hand, taking account of the velocity, the error between therotational speed command and the detection value ismultiplied by the damping gain as shown in Fig 4 (b). The
structure of controller is similar to the velocity controller.
Therefore, there are not these problems essentially.

Fig. 2. Main windings and auxiliary windings. The number of the auxiliary
winding is less than the numbers of the main windings.
sTHPF
1 + sTHPF

Kd

Kd

Figs. 4.Damping control block diagram for the torque current command.
Type I is generally used to suppress the fluctuation of the pole position.
However, Type II is better than in term of the noise and the frequency
bandwidth.

1sFig. 3.Control block diagram of the proposed system. The V/f control is applied to the main inverter and the auxiliary inverters uses the vector
control for the damping control.

Figs. 5. Simulation and experimental model of the PMSM in addition auxiliary
windings for damping control. In order to neglect the magnetic coupling in thesimulation and the experiment, the proposed system is validated using (b)III.

SIMULATION RESULTS

A. Single motor drive with auxiliary windings
Figs. 5 (a) and (b) show the simulation and experimental
models that are used to verify the operation of the proposedsystem on the simulation. Due to the reason that the mutual
magnetic interference occurs between the main and theauxiliary windings, as a result the control for the auxiliary
inverter becomes complicated. Therefore, the proposed system
is validated using a model where two PMSMs are connected
in series via single shaft. Then, the rear end of the main
PMSM is connected to the load machine. It means that themagnetic coupling was neglected in the simulation and theexperiment. Table 1 shows the details of the simulation
condition.
Figs. 6 (a) and (b) show the simulation results when the
proposed control is applied (a) without and (b) with damping
control. In Fig. 6 (a), the vibration occurs in the torque and therotational speed during the acceleration. After the acceleration,
the vibration is maintained. The vibration of the load angle isalso maintained. On the other hand, in Fig. 6 (b), thevibrations in the torque and the rotational speed and the load
angle are suppressed by the auxiliary inverter by occurring theauxiliary torque to neutralize the vibration of the load angle.
Besides, it can be confirmed that the rotational speed is wellfollowing to the rotational speed command. Moreover, evenwhen the load fluctuation occurs after the acceleration, theeffectiveness of auxiliary torque can be realized, as thevibration in the main torque can be suppressed also. Looking
at the output power of each inverter, the output power of themain inverter is nearly 1.0 p.u., however, the largest power
produced from the auxiliary inverter is only 0.5 p.u.. From theresults, the power capacity of the auxiliary inverter is shownsmaller than that of the main inverter. Moreover, the auxiliary
inverter only operates when the torque vibration occurs. Notethat the power capacity of the auxiliary inverter is dominatedby the response time of the torque vibration suppression.

Figs. 6. Simulation results for a single motor drive without and with damping
control. (a) The vibration of torque and rotational speed occur during theacceleration. (b) It can be confirmed that the rotational speed is wellfollowing to the rotational speed command owing to the damping control.
The vibrations in the torque and the rotational speed are suppressed by theauxiliary inverter. Besides, it can be confirmed that the rotational speed iswell following to the rotational speed command.

Tm1
TA1

PM1

J

PM2

M

INV

INV

Auxiliary

Main

TA2

PM1

J

PM2

M

Tm2
INV
AuxiliaryFig. 7. Simulation model for parallel connected dual motor drive. In order toneglect the magnetic coupling in the simulation and the experiment, twoparallel drive of the proposed system is validated using Fig. 7 as shown in
Fig. 5(b).

B. Parallel motor drive with auxiliary windings
Fig. 7 shows the simulation models that are used to verify
the operation of two sets of parallel connected PMSM. As
mentioned above, in order to neglect the magnetic coupling inthe simulation, the model where two PMSMs are connected inseries via single shaft is used in the simulation.
Fig. 8 shows the simulation results when two sets ofparallel connected PMSMs are driven by the proposed systemwith the damping control as shown in Fig. 7. Smoothacceleration progresses are confirmed in the two set of parallel
connected PMSMs. Besides, when the rated motor speed, load
step applies to PMSM1 at 0.38s, and later also applies to
PMSM2 at 0.42s, the operation of the two auxiliary inverters
can be observed from the output power. The maximum output
power of the auxiliary inverter is approximately 0.25 p.u. of
the rated power of the main inverter. Although the power
capacity of main inverter increases with the increase ofparallel units, the power capacity of auxiliary inverter does notchange to the numbers of parallel units.
IV.

Fig. 8. Simulation results for parallel motor drive with damping control.
Smooth acceleration progresses are confirmed in the two parallel connected
PMSMs. After the rated motor speed, load step applies to each PMSM,
stable operation of each PMSM can be confirmed.

EXPERIMENTAL RESULTS

In previous section, the simulation results demonstrate theeffectiveness of damping control and stable parallel driving
operations. In this section, in order to confirm theeffectiveness of the damping control, the experiments are
conducted with a motor –generator set.

Figs. 9. Simulation model and motor-generator set. If the load machine isremoved from (a), then it is equal to the motor-generator set as shown (b).

Figs. 9 show the relationship between the simulation
model and the motor-generator set in this paper. Since themodel is constructed from two PMSMs that are connected inseries via single shaft as shown in Fig. 4 (b), if the load
machine is removed as shown in Fig. 9 (a), then it is equal tothe motor- generator set as shown in Fig. 9 (b).
Fig. 10 shows the experimental configuration to confirm
the effectiveness of the damping control in the proposedsystem. As mentioned above, the effectiveness of the proposedsystem can be verified with the motor-generator set as shown
Fig. 10. Therefore, in order to confirm the effectiveness of theFig. 10. Experimental configuration and condition

proposed damping control, the experiment is performed withthe motor-generator set as shown in Fig. 10.

Fig. 11. Acceleration test without/with damping control in motor-generator
set. (a) After the acceleration, the 400 r/min - speed vibration is maintained.
(b) The speed vibration is reduced from 400 r/min to nearly 0 r/min incompared with (a)Fig. 12. Deceleration test without/with damping control in motor-generator
set. (a) After the deceleration, the 500 r/min - speed vibration is maintained.
(b) The speed vibration is reduced from 500 r/min to nearly 0 r/min incompared with (a)Figs. 11 show the experimental results that illustrate themotor speed vibration when the proposed system is applied (a)without the damping control and (b) with the damping control
in acceleration test. In this experiment, it is difficult tomeasure the torque response directly. Therefore, the speed
vibration is evaluated instead of the torque response. In Fig.
11 (a), the proposed system is implemented without damping
control. The speed vibration can be noticed during theacceleration. After the acceleration, a 400 r/min of speed
vibration and the 10 Ap-p of current vibration in q-axis of themain inverter are maintained. On the other hand, Fig. 11 (b)demonstrates the experimental results, where the proposedsystem is implemented with the damping control. The
effectiveness of the auxiliary inverter can be noticed from theresult that the speed vibration is reduced from 400 r/min tonearly 0 r/min in compared with Fig. 11 (a). The 10 Ap-p ofcurrent vibration in the q-axis of the main inverter as shown
Fig. 11 (a) is suppressed as well.

of current vibration in q-axis of the main inverter are
maintained as shown in Fig. 12 (a). By contrast, it isconfirmed that, as shown Fig.12 (b), the speed vibration is
reduced from 500 r/8min to nearly 0 r/min and the current
vibration is reduced from 15 Ap-p to 0 Ap-p in compared with
Fig. 12 (a) in the same way as acceleration test.

Figs. 12 show the experimental results that illustrate themotor speed vibration when the proposed system is applied (a)without the damping control and (b) with the damping control
in deceleration test. In the same way as Fig. 11(a), after the
deceleration, a 500 r/min of speed vibration and the 15 Ap-p

Nevertheless, it is confirmed that the q-axis current of theauxiliary inverter flows only during acceleration anddeceleration. Moreover, the maximum q-axis current of theauxiliary inverter is 20% of the q-axis current of the main
inverter. Therefore, it is confirmed that the auxiliary inverter
can suppress the speed vibration via auxiliary windings with asmall q-axis current of the auxiliary inverter even if in theacceleration and deceleration test.
V.

RELATIONSHIP BETWEEN THE POWER CAPACITY AND
THE SUPPRESSION RESPONSE

The fundamental operation and the parallel drive of the
proposed system are discussed in the previous section. In
order to confirm the relationship among the output power of
the auxiliary inverter and the suppression effect of the speed
vibration, various tests on different instantaneous output

power are examined in simulation and experimented similarly
to Fig. 6 (b).
Fig. 13 shows the relationship among the output power of
the auxiliary inverter and the suppression response and theovershoot value of the speed under the same condition. Notethat 1p.u. of the output power of the auxiliary inverter is
1500W. The suppression response is evaluated in terms of thenatural angular frequency of the damping control. The power
capacity of the auxiliary inverter is evaluated with the output
power. And lastly, the suppression effect is evaluated with theovershoot of the motor speed. From the results in Fig. 13,
when the output power of the auxiliary inverter is larger, theresponse of the damping control becomes faster. Besides, it isconfirmed that decreasing the output power of the auxiliary
inverter results the overshoot of the speed becomes larger. In
other words, when the demand of the response in the speed
control is not so fast, the power capacity of the auxiliary
inverter can be reduced. Even if the auxiliary output power isonly 10% of the main inverter, the overshoot is possible tosuppress less than 10% (180 r/min) as shown Fig. 13.
Therefore, it is confirmed that the proposed system is useful to
fan and conveyor applications.
CONCLUSION
This paper proposes a control technique to dive multiple
numbers of parallel connected PMSMs. The features of the
proposed system are following;z

The proposed system is composed by two different
inverters, which are the main inverter for parallel
drive and the small power capacity auxiliary inverter
for damping control.

z

In addition to the conventional windings (mainwindings) for the motor drive, each of the PMSM is
added with auxiliary windings to suppress the torquevibration.

z

The power capacity of the auxiliary inverter is small
because the auxiliary inverter is operated when thetorque vibration needs to be suppressed.

The simulation and experimental results demonstrated that,the proposed system can drive parallel connected PMSMs
efficiently by suppressing the torque vibration by applying
damping control to each of the auxiliary inverter. Moreover,the experimental results demonstrated that the proposedsystem can reduce the speed vibration from 400r/min tonearly 0 r/min in spite of small q-axis current.
In addition, from the simulation and experimental results,
the relationship among the response of the damping control,
the overshoot of the motor speed and the auxiliary inverter
capacity are investigated. The results show that, if theapplication requires slow response, the power capacity of theauxiliary inverter can be designed to be 10% of the main
inverter. Therefore, it is confirmed that the proposed system isuseful in fan and conveyor applications.
In the future work, the internal structure of the auxiliary
windings in PMSM of the proposed system will be considered.

Fig. 13. Influence of the output power on the suppression effect and the
suppression response. When the output power of the auxiliary inverter islarger, the response of the damping control becomes faster. The output powerof the auxiliary inverter decrease, the overshoot of the speed becomes larger.
In other words, when the demand of the response of the speed control is not so
fast, the power capacity of the auxiliary inverter can be reduced due to thesmall output .power of it.

Secondly, driving several units of motor-generator in parallelwith the proposed system drives will be considered.
ACKNOWLEDGMENT
A part of this study was supported by Industrial
Technology Grant Program in 2009 from New Energy andIndustrial Technology Development Organization (NEDO) of
Japan.
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