Efficiency improvement by Changeover of Phase Windings of Multiphase Permanent Magnet Synchronous Motor with Outer-Rotor type

Young-Gook Kim Chae-Bong Bae, Jang-Mok Kim

Dept. of Electrical Engineering, Pusan National University Busan, 609-735, Korea

rain@pusan.ac.kr

Hyun-Cheol Kim Agency for Defense Development

Chin hae, South Korea hckim@add.re.kr

Abstract – In this paper, a new control algorithm is proposed to improve the efficiency of multiphase permanent magnet synchronous motor with outer-rotor type (MPMSM). The improved efficiency is obtained by using the change of the phase number from 6-phase motor to 3-phase motor and reducing the switching loss by using the bi-direction switch between two H-bridge inverter of six phase to change the number of the phase. The simulation and experimental results are presented to demonstrate the feasibility and advantages of the proposed algorithm.

I. INTRODUCTION High phase order or Multi-phase (phase order more than

three) machine drives are gaining growing attention in recent years, due to multiphase motor drives posse many other advantages over the traditional three-phase motor drives such as reducing the amplitude and increasing the frequency of torque pulsation, reducing the rotor harmonic currents and the stator current per phase without increasing the voltage per phase, lowering the dc-link current harmonics, higher reliability and an improvement of the fault tolerance. By increasing the number of phases it is also possible to increase the torque per rms ampere for the same volume machine. The additional degrees of freedom in multi phase system also enable us to inject harmonic current of supply multi motors from a single inverter. The high phase order drive is likely to remain limited to specialized applications where high reliability is demanded such as electric/hybrid vehicles, aerospace applications, ship propulsion, and high power application where a combination of several solid state devices form one leg of the drive. Therefore, the requirement of n separate drive units in a multi-phase system is not oppressive for large drives since many of the necessary components are presented in the contemporary designs [1-3].

In this paper, the efficiency improvement of multi-phase permanent magnet synchronous motor(MPMSM) can be obtained by using the changing of the phase number according to the rotor speed and reducing the switching losses of the inverters at three phase operation or the low speed range. At the low speed, the number of the phase winding is

three, and at the high speed, the number is six because the application of this motor is the electric ship propulsion. The load of the electric ship is propositional to the cube of the rotor speed. In the low speed range(or 3-phase operation), the number of the switching devices is 12, but three switching elements are added to change the number of the phase from six phases to three phases. So the total switching elements of the independent three phases are 15. And the stator current of three phase is the same to the six phases windings, but the amplitude of three phase back-emf are twice than the six phases because two three phases are directly connected by cascade. Therefore total power loss can be reduced by changing the number of the phase and reducing the number of the switching elements especially in the low speed range. To improve the additional efficiency, the new switching method by using bi-direction switch is proposed. The experimental results verify the usefulness of the proposed control algorithm.

II. MPMSM WITH OUTER-ROTOR TYPE

A. Configuration of MPMSM MPMSM is composed of the separated six phase

windings as shown in Fig. 1. And each phase is connected to separate six H-bridge inverters [7]. In general, H-bridge inverter system can be used for the single phase of ac machine or the chopper of a DC machine. In this paper, H-brides are utilized for the inverter of MPMSM as shown in Fig.1. Total H-bridge is six because MPMSM is six phase machine.

Fig. 1. The configuration of MPMSM

978-1-4244-4783-1/10/$25.00 ©2010 IEEE 112

B. Mathematical modeling of MPMSM

Fig. 2 shows the equivalent circuit of the stator winding of MPMSM. The end of each phase is not connected to neutral point unlike the conventional three phase ac machine. The winding of MPMSM can be divided into two groups of three phase motors to easily implement the vector control of this motor. One group is aV , cV , and eV . The other is bV , dV , and fV . So the control system of MPMSM can be considered as a parallel operation of two- three phase motors.

Fig. 2. Equivalent winding circuit of MPMSM

Fig.3 shows six phase back-emf of MPMSM which has the sinusoidal waveform. And the difference angle of each phase is 60 degree.

Fig. 3. Waveform of back-emf

Voltage equation of MPMSM is given at (1).

xxxx eidxdBAiv ++= (1)

where x means the specific phase, i is current, e is back-emf of the electric parameters and A, B are matrix defined as

⎥⎥⎥⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢⎢⎢⎢

⎣

⎡

=

RR

RR

RR

A

000000000000000000000000000000

⎥⎥⎥⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢⎢⎢⎢

⎣

⎡

=

LL

LL

LL

B

000000000000000000000000000000

where L is the synchronous inductance, and R is the stator resistance. If θe is the rotor position, back-emf can be expressed as in (2).

meex )(ke ωθ= (2)

The torque equation can be derived from the input power as shown in (3).

m

ffeeddccbbaa

m

ee

ieieieieieiePTωω

+++++== (3)

where Te is produced torque, Pe is output power, respectively. And ωm is rotational angle speed of the rotor. Mechanical motion equation of the motor can be expressed as in (4). This is the same equation form of the conventional three phases motor.

m

e

m

Lm

m

mm J

TJT

JB

dtd +−−= ωω (4)

The block diagram of 6-phase of MPMSM is shown in Fig. 4.

Fig. 4. Block diagram of MPMSM

III. OPERATION CHARACTERISTIC

A. Analysis of power losses in inverter [6]

The losses of the inverter can be classified as conduction loss, off state loss, and switching loss. Since the leakage current of the off-state of the device is negligibly small, the power loss during the off-state can be neglected in real inverter system. So, in the power loss of the switching device, the dominant term is conduction loss and switching loss.

Conduction losses occur at the state of the switching device at the on-state and the current follows through the

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switching device. Therefore, power dissipation during conduction is computed by multiplying the on state saturation voltage by on-state current.

oncon vip ⋅= (5)

where, onv is the on-state saturation voltage and ci is the load current.

Simple expression of the average conduction loss of each device can be expressed as follows:

∫ ⋅=β

αontion_lossave.conduc dθ kP

2π1P (6)

where, α , β are defined as the start and the end of the conduction state interval of each device over one period, and k is on-state ratio of conducting device, respectively.

Switching loss is the power dissipation during turn-on and turn–off switching transitions. In high frequency, PWM switching losses can be substantial and must be considered to improve the efficiency of the inverter system.

Simple expression of the average switching loss for diode and controllable switch can be expressed follows:

∫ ⋅⋅=β

αccrecching_lossD_ave.swit dθ fiE

2π1P (12)

( )∫ ⋅⋅+=β

αccoffonching_lossS_ave.swit dθ fiEE

2π1P (13)

Where cf is the frequency of carrier wave,

recE [J/A], onE [J/A] and offE [J/A] are reverse-recovery energy coefficient, turn-on switching energy and turn-off switching energy coefficient, respectively.

The efficiency of the inverter system is nearly affected by load current, carrier wave frequency, the saturation voltage, and energy coefficients. If the same device is used, the saturation voltage and energy coefficients are also same. And the same of the carrier wave frequency will be used for the measurement of the efficiency. So, these factors can be neglected for the test.

B. Operation concept of MPSM with outer-rotor type

Fig. 5(a) shows 6-phase inverter system. This is composed of 6 H-bridge inverters and the total number of the switching elements is 24. If there is the variation of the phase number from 6 to 3-phase inverter systems, Fig. 5(b) shows the transformed 3-phase inverter systems from 6-phase. As shown from Fig. 5(b), the bi-directional switching elements are inserted between a pair of 6 phase H-bridge inverters so that load1 and load 2 are connected in series. This series connection makes 3-phase inverter circuit from 6-phase inverter circuit. Therefore, the total switching elements of three phase inverter is 15, and two diodes are added for the condition path of the bi directional switch.

3-phase system has less power losses than 6-phase system because the power losses of both the conduction and switching are increased in proportion to the number of switching devices.

(a) Configuration of 6-phase of MPMSM

(b) Configuration of 3-phase of MPMSM

Fig. 5. Configuration of changeover phase windings

Fig. 6 shows the concept of the variation of the phase number of the machine from 3 to 6-phases according to the variation of the ship speed and the required power of the specific speed. As known from this figure, in the low speed range, three phase operation is effective, but in the high speed range, six phase operation is more useful because this motor will be used for the eclectic propulsion of the ship. In the ship application, the load or the required power is propositional to the cubic of the ship speed. The maximum power and the terminal voltage of 3-phase system are obtained at the changeover point. After this changeover point, the changeover operation is needed.

Fig. 6. Concept of changing the number of the phase system

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Fig 7(a) shows the simple equivalent circuit of 6 phase windings system including the phase current, the stator resistance, the inductance and back-emf. The transformed equivalent circuit of 3-phase system can be expressed as shown in Fig 7. (b). And the load current of 3-phase system is the same to the six –phase inverter systems, but the amplitude of three phase back-emf is twice than six phase because two phases are directly connected by cascade.

(a) The equivalent circuit of 6-phase of MPMSM

(b) Transformed equivalent circuit of 3-phase of MPMSM

Fig. 7. The equivalent circuit of changeover of the phase windings system from six phases to the three phase.

And the phase voltage of each winding can be expressed

as follows:

XX

XXXphase edt

dILIRV ++=−6 (14)

phase6XX

XXXphase3 2V)edt

dILI2(RV −− ≈++= (15)

Where Ix=I, Rx=R, Lx=L, ex=e, and x means specific phase.

From (14) and (15), the phase voltage of 3-phase system is about twice than 6-phase system because two phases are directly connected by cascade.

The limitation of phase voltage is equal to dc-link voltage, DCV because each phase is connected to separate six H-

bridge inverters. Phase voltage of both 3-phase system and 6-phase system are limited to dc-link voltage DCV as shown in Fig. 7(a) and (b), respectively.

Fig. 8 shows the marginal voltage of the dc-link for the operation of 3-phase mode and 6-phase mode on the basis of the relationship of the speed and the required power at the specific speed.

SPEE

D

Fig. 8. The condition of operating mode in the changeover of

phase windings system.

From Fig. 9, in the low speed range, three phase operation is useful, but in the high speed range, six phase operation is more effective due to the margin of the phase voltage. Therefore, the changeover of the phase windings is very effective to improve the efficiency of the total inverter system.

VDC

SPEED

Phas

e Vo

ltage

3-phase system region

6-phase system region

Maximum power of 3-phase system

V3-Phase = VDC

Fig. 9. The phase voltage according to motor speed

C. New switching method for reducing switching loss

The connection switch is needed for the changeover of the phase number. The mechanical relay can be used for the connection path between two H-bridge inverters. But this relay has many problems of the low response and the mechanical contact parts of the copper.

Therefore, in this paper, bi-directional switch is proposed for this connection between two H-bridge inverters. This bi-directional switch is consisted of 4 diodes and one IGBT as shown in Fig. 10. For the additional improvement of the efficiency of the total inverter system, this switch provides not only the conduction path of the changeover but also is the main switching element in three phase inverter system.

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Fig. 10. A configuration of system for changeover of phase windings

1) Operation modes of 6-phase inverter system

Fig. 11 shows the operation modes of 6-phase inverters system of MPMSM. One is positive on and off modes as shown in Fig. 11 (a) and (b). The other is negative on and off modes as shown in Fig. 11 (c) and (d), respectively. In 6-phase mode, a bi-direction switch Sc is always off as shown in Fig. 11.

(a) Positive cycle S1, S4, S5, S8 is on

(b) Positive cycle S4, S8 is on, S1, S5 is off

(c) Negative cycle S2, S3, S6, S7 is on

(d) Negative cycle S3, S7 is on, S2 S6 is off

Fig. 11. Operation modes of 6-phase system

2) Operation modes of 3-phase inverter system

Fig. 12 shows the operation modes of 3-phase inverter system. In 3-phase mode, a bi-direction switch is used for not only main PWM switching element, but also switch for changeover of phase windings. The other switching elements, S1 and S8, provide not the switching elements but the conduction path as shown in Fig. 12(a) and (c).

Fig. 12(a) shows the proposed switching method by using bi-directional switch. In this paper, S1 and S8 provide only commutative path, the main switching element is Sc. By using the proposed switching method, total number of the switching elements is not total switches number of 3-phase inverter system, 15 but only total bi-direction switches of 3-phase inverter system, 3. Therefore, the switching loss of three inverters system can be reduced considerably because most of switching loss is happened only at Sc.

(a) Positive cycle Sc, S1, S8 is on

(b) Positive cycle Sc is off, S1, S8 is on

(c) Negative cycle Sc , S3, S6 is on

(d) Negative cycle Sc is off, S3, S6 is on

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(e) Detailed waveforms of the proposed PWM method

Fig. 12. Operation modes of 3-phase system

IV. SIMULATION RESULT MATLAB/Simulink is used for the verification of the

proposed algorithm. The limitation voltage of dc-link, DCV , is 100[v].

Fig. 13 shows the simulation waveforms of rotor speed, the produced torque, power, phase current, back-emf and phase voltage when the motor operates at 600[rpm]. As known from this waveform, the changeover of the phase windings happens at 330[rpm] when the phase voltage of 3-phase system is equal to dc-link voltage DCV . The maximum power of 3-phase inverter system is produced at this speed.

(a) Speed

(b) Torque

(c) Power

(d) Current

(e) Back-emf

(f) Phase voltage

Fig. 13. Simulation result of changeover of phase windings system

Fig. 14 (a) is the sinusoidal reference voltage for the operation of the three phases. Fig. 14 (b) shows the switching waveform of the bi-directional switch. Fig.14(c) shows the positive switching waveform of S1 and S8 to provide the conduction path for the operation of the three phases as

shown in Fig. 12(a) and (b). Fig. 14(d) shows the negative switching waveform of S3 and S6 to provide the conduction path for the operation of the three phases as shown in Fig. 12(c) and (d). And Fig. 14 (e) shows one of output voltages of three phase inverter systems. As know from these resultant waveforms, the three phase inverter system using the proposed switching method operates well

Fig. 14. The simulation result of proposed PWM method

V. EXPERIMENT RESULT The MPMSM is used for the experimentation as shown in

Fig. 15. The type of inverter is H-bridge. And the number of this inverter is six as shown in the front of Fig. 15.

Fig. 15. Configuration of the experimental setup

Fig. 16. shows the experimental waveform of a-phase current, voltage and rotor speed from 0 to 600[rpm]. The changeover of phase windings happens at 300[rpm]. This speed is about 50% of the rated speed. The phase voltage is nearly DCV at the changeover point as shown in Fig. 16 (b).

(a) The phase current in 3-phase system

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(b) The phase voltage in 3-phase system

(c) Seed

Fig. 16. Phase current and voltage according to the rotor speed.

The control block diagram is used for the measurement of the efficiency of MPMSM as shown Fig. 17.

Fig. 17. Determine the efficiency of MPMSM

Fig. 18 and 19 are measured the voltage and current at the dc-link and load of each system. These are used to decide the input and output power for the measurement of the efficiency of MPMSM.

(a) 3-phase system input

(b) 6-phase system input

Fig. 18. The dc-link voltage and current in MPMSM

(a) 3-phase system output

(b) 6-phase system output

Fig. 19. The voltage and current of a phase at the load

Fig. 20 shows the resulting efficiency curve of MPMSM according to the rotor speed. From this figure, the 3-phase system has high efficiency than 6-phase system in the low speed range. And the 6-phase system of MPMSM is used to produce the full torque in the high speed range. So the total efficiency of MPMSM can be improved by changing the number of phase in the low speed range.

Fig. 20. The resulting efficiency curve of MPMSM according to the rotor

speed

VI. CONCLUSION In this paper, a new control algorithm was proposed to

improve the efficiency of multi-phase inverter system. The improvement of the efficiency was obtained by using the change of the phase number from six to three phase windings. And bi-directional switching element was introduced to provide not only conduction path but also the main switching function of three phase inverter system. The total efficiency of three phase inverter is much high than six phase inverter system. And in the three phase inverter, the improvement of the efficiency could be obtained because the switching elements are not H-bridge switching element but bi-directional switching elements.

And the usefulness of the proposed algorithm verified through computer simulation and the experimental results.

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