Research Report

University of Wisconsin-MadisonCollege of Engineering

Wisconsin Power Electronics Research Center2559D Engineering Hall1415 Engineering DriveMadison WI 53706-1691

© Confidential

2012-42

Field Weakening of a Surface Mounted PermanentMagnet Motor by Winding Switching

S. Hemmati, T. A. Lipo

Department of Electrical andComputer Engineering

K. N. Toosi University of ThechnologyTehran, Iran

Department of Electrical andComputer Engineering

University of Wisconsin - MadisonMadison, WI, US A

2012 International Symposium on Power Electronics, Electrical Drives, Automation and Motion

Field Weakening of a Surface Mounted Permanent Magnet Motor by Winding Switching

S. Hemmati Department of Electrical and

Computer Engineering

K. N. Toosi University of Thechnology

Tehran, Iran

Email: hemmati@ee.kntu.ac.ir

Ahstract-A new method for flux weakening in surface per­manent magnet (SPM) machines is proposed in this paper. This method is called winding switching technique and can reliably double the field weakening speed range of PM motors. This is significant because extending the constant power speed ratio (CPSR) of SPM machines is usually challenging task due to the presence of low-permeability surface magnets and the resulting low machine inductance. In the new method of field weakening the usual three phase motor windings are separated into two portions and each portion is connected to an inverter. The paper discusses the stator winding arrangement which allows the normal and high speed operation of the machine along with the constant output power and reports simulation results taken from Finite Element Analysis.

Index Terms-PM Motors, Field Weakening, High Speed Operation, Electric Vehicle.

I. IN TRODUCTION

Permanent magnet motor drives are the topology of choice

for modern traction applications such as hybrid and electric

cars [1], [2]. The Toyota Prius is a typical example of a vehicle

using a permanent magnet motor drive in which the permanent

magnet motor is powered by a combination from a pulse width

modulated voltage source inverter. Such applications require

that the motor operate over a very wide speed range which

corresponds to a wide frequency range of the inverter. When

a permanent magnet motor is operated at a variable speed,

the internal voltage (emf) induced in the motor stator winding

increases linearly in proportion to the speed (or frequency).

Eventually, as speed increases beyond a certain point, this

induced voltage becomes greater than the voltage available

from the inverter power source. At this point the inverter is

no longer capable of feeding current into the motor since

the voltage of the load (i.e the motor) is greater than that

of the source (i.e. the inverter). Beyond this point the emf

must be counteracted by impressing a component of current

which opposes the fiux created by the permanent magnets.

This is called demagnetizing current. In other words, above

the base speed, flux weakening allows the machine to operate

in constant power. In this region, as shown in Figure 1, part

of the stator current Id (demagnetizing current) is used to

oppose the fiux from the Magnets and the Iq is used for torque

production. Below the base speed all of the stator current Iq is

978-1-4673-1301-8/12/$31.00 ©20 12 IEEE 736

T. A. Lipo Department of Electrical and

Computer Engineering

University of Wisconsin - Madison

Madison, WI, USA

Rated Torque

Email: lipo@engr. wisc.edu

Torque & Power

--_._----_._----_._---_.,

� 1\.\ P Rated

Power ··'·" .. T .... " ... _ ..... :

Iq & Id Rated ........................ ...

Current , .

•.•..

Iq

: .

................... .

,

,

,

Speed �------��----------�----�

Base Speed Max Constant

Power Speed

Fig. I. Constant power and torque operating region.

used to produce constant torque and the Id is equal to zero in

this region. The ability of a demagnetizing component varies

with the design of the motor and a maximum speed beyond

the point at which field weakening begins can typically only

be extended to about twice or three times the onset of field

weakening.

Several authors have addressed flux weakening in PM ma­

chines, [3]- [5] and [6]. The traditional FW method, which

is accomplished by applying a large demagnetizing current in

the d-axis of the PMs [3], is a way of weakening the air-gap

magnetic field and enlarge the constant-power operating range;

however, FW obtained by this method increases the winding

copper losses and also risks irreversible demagnetization of

the PMs. In [4], the main design criterion for optimal fiux

weakening has been presented. The purpose of this criterion

is to make the magnet flux linkage equal to the d-axis stator

flux linkage (valid for both smooth air gap and salient-pole

machines). An improved method to extend the constant power

speed range of the brushless dc motor using dual-mode inverter

+

Fig. 2. Circuit diagram of the concept.

Fig. 3. Circuit diagram of Normal operation - Current directed into the dots.

control has been proposed in [7] and verified experimentally in

[8]. This method combines both the phase-advance technique

and back-to-back thyristors in each phase. The reason or

justification for using back-to-back thyristors, as described

by the authors, is that the thyristors block the conduction

of the anti-parallel diodes. This eliminates the regenerative

torque component produced by the diodes conduction, hence

increasing the net torque and power produced by the machine.

Most hybrid vehicles require a field weakening range of 5-6

to l. Thus permanent magnet machines with only an inherent

2 or 3 to 1 field weakening range using demagnetizing current

can be employed as a motor for a hybrid vehicle. Machine used

today such as the Prius realize their 5 to 1 field weakening

range by intentionally introducing extra leakage inductance

into the machine. The approach inherently makes the machine

larger and heavier than would be necessary for the purpose of

simply providing the necessary torque.

In this paper a new configuration of stator windings is

proposed. Our goal is to extend the constant power speed

in PM motors up to about 4 times the base speed without

using the demagnetizing current. Since most well designed,

and optimized machine fall into the category of 2-3 to 1 field

weakening ability this new strategy could be an important

control function which would allow well designed machine

with minimum size and weight to be used for this application.

II. ANALYSIS OF SPM WI T H NEW WINDING

CONFIGURATION

Figure 2 shows a circuit diagram of the concept. The usual

three phase motor windings are separated into two portions,

portion 1 (left half portion on Figure 2) and 2 (right half

737

+

----+ --+

Fig. 4. Circuit diagram of High speed operation - Current reversed in half of the three stator winding.

portion on Figure 2). Both ends of the windings are brought

out of the machine, resulting in 3x4 or 12 leads. Three of

the leads from portion 1, normally forming the input of three

phase winding are connected to one inverter. Also the three

of the leads from portion 2 ( i.e the dot ends of portion 2)

are connected to a second inverter. The other 6 leads are now

connected together in each phase to form the mid points. These

mid points are now connected to three thyristors as shown

in Figure 2. As an alternative to thyristors, three contactors

could also be used. Figure 3 shows the resulting circuit when

the three thyristors are turned on during normal low and

moderate speed operation. In this case the three thyristors that

are turned on form a wye connection for portion 1 and also

for the other portion. In this case the currents on both the

right half and left half of the split three phase windings are

directed into the "dots" indicating that the flux produced is in

an additive polarity meaning that the flux produced in each

pole by each of the winding halves are additive. In this case

the voltage induced into each half of the winding from the

rotating magnets is also additive resulting in normal low and

moderate condition. In Figure 4 is shown the situation when

the three thyristors are turned off (or contactors are opened).

It should be noted that now the currents flowing into the right

half portion of the three phase windings (portion 2) are flowing

out of the dots rather than into the dots. This means that the

fluxes produced by the two windings are subtractive rather than

additive. While some net flux is produced, it is substantially

less than when the winding fluxes are additive. However, most

importantly, the voltage induced into the two portions of the

phase windings are also subtractive consequently reducing the

emf and alleviating the condition wherein the internal voltage

is beginning to exceed the terminal voltage provided by the

two inverters.

A brushless surface mounted permanent magnet ac motor is

considered as a simulation drive motor in this paper. This is an

eight-pole motor that operates up to about 6000 rpm. Figure 5

shows the cross-section and the basic dimensions of the motor.

It has to be mentioned that the basic dimensions of simulated

motor is as same as the Prius 2004 motor. The only difference

is that the non-IPM type as well as double layer winding for

the stator is considered here. Figure 6 shows the lap winding

configuration of phase A in which the q = 2 slots/pole/phase.

Basic dimensions [mm] Outer stator dia.: 270.0

Inner stator dia.: 162.0

Inner rotor dia.: 110.0

Outer rotor dia.: 160.6

Air-gap length: 0.7

Axial core length: 100

Magnet thickness: 6.54

Fig. 5. Basic dimension of the drive motor.

Fig. 6. Two layer lap winding of stator phase A.

The normalized air gap MMF produced by current flow in the

winding of one of the three phases (phase A) in the low speed

operation, is shown in Figure 7(a). Assuming for simplicity

that 112 amp flows in the coils of the winding, the amplitude

of fundamental component of the MMF can be calculated as

2 17r 1 2 1 5(;' 1 2 y'3 FI = - (-)sin(¢)d¢+- (-)sin(¢)d¢= -+-'if 0 2 'if ]' 2 'if 'if

Figure 7(b) shows the MMF that is produced when the

current is reversed in half of the three stator windings. In this

case the amplitude of fundamental component of the MMF

reduces to

2 Jif 1 1 F2 = - ( -)cos( ¢) d¢ = -'if � 2 'if -"6

The ratio of the MMF for the case of Figure 7(a) to the

MMF of the case of Figure 7(b) becomes

. F2 1 Ratw = -= 2 7r V3 = 0.268 FI 7r + 7r

Because of this change of current flow it can be shown that the

synchronous reactance as well as the induced EMF also drops

by the same amount, i.e. 0.268. Figures 8 shows an example

of this situation in terms of phasors. The per unit synchronous

reactance of the machine at rated speed is 0.86 and the emf

738

: ................................................................................................................................................................ .

MMF A

112 o n/6 5n/6

l ............................................................................. (.iif ............................................................................ ,

(b)

Fig. 7. Normalized MMF distribution of stator phase A winding assuming 112 amp flows in the coils at (a) normal and (b) high speed operation condition.

Vs= 1 Xs1s= l.72

I.� O.875 i7S I 1qs=0.5 E= 2 I I

d-axis (a)

q-axls -+-

X,1s= 046

.

21:S=0.7

___ +X1S I E=0.53 1s=l

I d-axis (b)

Fig. 8. Phasor diagram for operation at 2 pu speed: (a) before and (b) after the switching event.

at rated speed assumed as 1.0. The parameters of machine are

given in the Appendix. At two per unit speed the synchronous

reactance will rise to 0.86*2.0 = l.72 per unit while the emf

will rise to 2.0 per unit. The vector diagram of Figure 8 (a)

will exist in which the torque producing component of the

current has dropped to 0.5 pu and hence the torque drops to

0.5 per unit such that the power remains at 0.5*2.0 = l.0 per

unit. At this point the reactive component of current has risen

to -0.875 per unit, nearly the maximum permissible value of

-1.0 per unit. If the speed continue to rise just above 2 per unit

the reactive component will equal 1 and the torque producing

component to zero. At this point the power drops to zero and

the motor is unable to accelerate further.

Figure 8(b) shows what happens when the thyristors are

4oo,-----�----�_,--------�--�--------, - At 1500 rpm (MTPA) 350

300 -

250

........ At 3000 rpm and before switching event _._.- At 3000 rpm and after switching event

E : : : :Z : : : -; 200 : : : ::::s I I I

! 150 .... ............... ... , •••• J., ..... �'I,. ••• \.-••• � ..... \ ••••• �'" ••••• , ••••• � ..... �l, .... '" .............................. i, .... .. .

" .........• _ ,- • .- • r r . ....

.... ..". ... .. ... � ..... 'r ... :.� • , , , , , , , , , , , , , , , , , , 1 00 �:;.�.�:..---;:,:;�":;.�.�;,---.r�:.;.�,,.:;�.�:..:\.;.�:.;�.\.:;��.�:\:;.�;.:;.,.r�;

.�-':;�;..�:,.;.-,;;.�.,:,;�;..

50 -

OL-----�----�----�----�------�----� o 10 15 20 25 30 Time (ms)

Fig. 9. Torque at 1 per unit speed (1500 rpm) and 2 per unit speed (before and after switching event).

suddenly turned otf. Now the emf and the synchronous reac­

tance drops to 0.268*2.0 = 0.53 pu and 0.268*1.72 = 0.46 pu

respectively. The phasor diagram shows that the component of

current needed to provide demagnetizing mmf (d-axis current)

now drops to zero while the torque producing component

becomes 1.0. The output power is now E*I = 0.53* I = 0.53 per

unit. This condition now allows the speed to increase beyond

2.0 per unit.

III. SIMULATION RESULTS

The Ansoft-Maxwell package was used for running sim­

ulations. The machine was simulated in different operating

points starting from 1500 rpm up to end at 5600 rpm. Figure 9 shows the electromagnetic torque developed by the machine at

nominal speed 1500 rpm and also at twice nominal speed 3000

rpm befor and after switching event. At the nominal speed

machine is operating in maximum torque per amp (MTPA),

in which the input stator current vector is aligned with the

q-axis. Before switching event when the machine is operating

at 3000 rpm the torque is the half of that in nominal speed

and the angle between current and q-axis is now 60 degrees.

Figure 8(a) shows this situation in terms of phasor. When the

thyristors are suddenly turned off, the torque is decreased to

about 84 N.m. This was predictable, because it was already

shown that after switching event the flux is weakened by 0.268

in compare with the case of operating at nominal speed, so the

torque drops by that factor to 310*0.268 = 83.08 N.m. Table 1

shows the simulation result for all operating points. It can be

seen in table 1 that after switching event the input voltage is

120 volts. Now this voltage can be increased and this makes

the speed to increase up to 1500/0.268 = 5600 rpm. In the new

field weakening region the torque is constant and the output

power is increased as the speed is raised by increasing the

input voltage. In fact after switching the windings the field is

weakened so that the back-emf is decreased enough and this

situation allows the speed to goes up till 5600 rpm (3.73 per

unit).

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TABLE I FEM MODEL RESULTS

Speed [rpm] 1500 3000 3000 (FW) 4500 5600 Torque [N.m] 310 155 87 87 87

Vph (Inv I +Inv2) [V] 230 176 120 180 230 Iph[A] 95.45 95.45 95.45 95.45 95.45

Power Factor 0.77 I 0.86 0.86 0.86 Pout [KW] 48.69 48.69 27.33 40.99 51

Etliciency [%]

300 E � 200 � E" {:. 100

9l.6 92.2

°0L---�20�0�0---4�0�00------::-:'6000

50 .. _� __ ... , .. [ 40 ----------,--- -i ---�,���-� 30 �,/

a. '[20 ---------->---

:; 010 OL---�--------�

o 2000 4000 Speed [rpm] 6000

88.2 88.2 85.9

� 250 ,---------�------, � .'''' : ,� � 200 , ------�----/----'; '-, j,---:§. 150 ---------------1---'- .-------

'0 � ...... : E 100 g 50 ---------------------'---� 0 L-__ --:-::-:--__ :-:':-:: __ ____::_:' a. 0 2000 4000 6000

C ,.. g . ·u E UJ

100 80 ..----.... ---� ... ---.

60 40 20

OL---�--------� o 2000 4000 Speed [rpm] 6000

Fig. 10. Torque, Output Power, Input Voltage and Efficiency V s. Speed.

In Figure 10 the torque, voltage, output power and efficiency

vs speed for all simulated operating points from 1500 rpm up

to 5600 rpm are shown . In this figure it can be seen that in

the new field weakening region the torque produced by the

machine remains constant whereas the output power increases

as the speed goes up. The amount of power delivered at the

output at 5600 rpm now is as same as the power at nominal

speed. This means that the maximum constant power speed is

extended up to 3.73 times the nominal speed. It should be

noted that in this point there is no demagnetizing current.

In other words, this is the maximum speed without using

the demagnetizing current. It can be shown that using the

demagnetizing current allows the speed to increase beyond

the 4 per unit speed.

IV. CONCLUSION

The winding switching method has provided a convenient

way of extending the machines constant power operating

range. The analysis and simulation result has shown that

The maximum CPSR achieved using this technique without

using the demagnetizing current is 3.73: 1. This is significant

because the traditional field weakening method obtained by

demagnetizing current increases the winding copper losses

and also risks irreversible demagnetization of the PMs. Using

demagnetizing current allows the speed to increase beyond the

4 per unit speed.

ApPENDIX

Machine parameters and specifications is provided in table

2.

TABLE IT MACHINE SPECIFICATION AND PARAMETERS

Base Speed Rated Torque Phase voltage Line Current Stator slots

Number of poles Stator turn per coil

Stator phase resistance D and Q axis inductance

Magnet residual flux density

REFERENCES

1500 rpm 310 N.m

230 V 95.45 A 48 slots 8 Poles 8 Turns 0.13 S1

3.31 mH 1.1 tesla

[I] Y. Dai, L. Song, and S. Cui, Development of PMSM drives of hybrid electric car applications, IEEE Trans. Magn., vol. 43, no. 1, pp. 434437, Jan. 2007.

[2] X. Yanliang, X. Jiaqun, W. Wenbin, and T. Renyuan, Development of permanent magnet synchronous motor used in electric vehicle, in Proc. 5th Int. Can! Elect. Mach. Syst., 2001, vol. 2, pp. 884887.

[3] T. M. Jans, Flux-weakening regime operation of an interior permanent­magnet synchronous motor drive, IEEE Trans. Ind. Appl., vol. IA-23, no. 4, pp. 681689, Jul. 1987.

[4] R. F. Schiferl and T. A. Lipo, Power capability of salient pole permanent magnet synchronous motor in variable speed drive applications, IEEE Trans. Ind. Applicat., vol. 26, pp. 115123, Jan. Feb. 1990.

[5] T. Sebastian and G. R. SIemon, Operating limits of inverter-driven permanent magnet motor drives, IEEE Trans. Ind. Applicat., vol. 23, pp. 327333, Mar.Apr. 1987.

[6] B. Sneyers, D. W. Novotny, and T. A. Lipo, Field weakening in buried permanent magnet Ac motor drives, IEEE Trans. Ind. Applicat., vol. 21, pp. 398407, Mar.Apr. 1985.

[7] 1. S. Lawler, 1.M. Bailey, and 1.W. McKeever, Extended constant power speed range of the brushless DC motor through dual mode inverter control, , Oak Ridge National Lab., UT-Battelle, LLC, 2001.

[8] 1. M. Bailey et aI., Dual mode inverter control test verification, , Oak Ridge National Lab., UT-Battelle, LLC, ORNUTM-20001172, 2001.

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