1782 IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-15, NO. 6, NOVEMBER 1979

POLYPHASE CONVERTER FOR INDUCTION MOTOR CONTROL USING THE STATOR CORE SATURATION

Department of Electronics K. Harada

Kyushu University Fukuoka, Japan

ABSTRACT

This paper presents a new polyphase converter using saturation of the stator core of the induction motor. In this case, the commutation is performed by the saturation of the stator core even if the magnetic characteristic of the stator core is not of a sharp saturation. In order to compensate the effect of the

of the main transistors are supplied by independent slow saturation of the stator core, the base currents

dc sources. By changing the voltage of the dc sources, the ratio of the output voltage to the oscillation frequency can be arbitrarily controlled. The oscilla- tion frequency is derived theoretically by idealizing each element of the converter. The phase rotation of the output voltages is determined by a simple R-C net- work with an auxiliary transistor.

INTRODUCTION

The polyphase converter with transistors and sat- urable cores has interesting characteristics for power applications [1]-[3]. For example, the number of com- ponents is small, the oscillation frequency can be easily changed over a wide range without phase distor- tion and there is inherently no cross firing.

When the polyphase converter is used for the in- duction motor drive, the whole system is considerably

control device made of a transistor inverter. If the simplified in comparison with the conventional PWM

saturation of the stator core of the motor is applied for the converter, the control device becomes smaller. Because it is possible to eliminate the saturable cores in the polyphase converrer.

In this paper, a new control system for an induction motor drive by a polyphase converter is presented. The commutation of the converter is performed by the stator core saturation of the motor.

The oscillation frequency is derived analytically from the basic three-phase circuit. A circuit without transformers is proposed so that the induction motor may be the only magnetic element. In order to compensate the effect of the slow saturation of the stator core, the base currents of the main transistors are supplied by independent dc sources.

M. Nagao Department of Electrical Engineering

Nagasaki University Nagasaki, Japan

BASIC CIRCUIT

The basic circuit of a three-phase converter using the stator core saturation of an induction motor is shown in Fig.1, in which the commutation is performed by the stator core saturation and three-phase self- oscillation is established by positive feedback through the linear transformers LT1,LTz and LT3. Fig.2 shows idealized waveforms of phase voltages eUN,eVN and em of the motor.

When the induction motor is driven by the balanced three-phase voltages as shown in Fig.2, the equivalent circuit for the fundamental wave component is as shown in Fig.3.

stator core characteristic is represented by the three straight lines with the knee current IMOS as shown in Fig.4, and that the commutation occurs at the time when the following relation is satisfied:

Here we assume that the slow saturation of the

Big=iC=io+IMos+i~ (1)

where B : common-emitter current amplification factor of the transistor Tr,

iB : base current of the transistor Tr, ic : collector current of the transistor Tr, io : driving circuit current, iM2: secondary current of the motor.

For simplicity, we consider the condition of no load

Then Eq. (1) becomes (iM2=O), and neglect the driving circuit current io.

BiB'IMOS * (2)

Under these assumptions, the motor current iM1 is obtained from the equivalent circuit shown in Fig.3 as follows :

2EDSin(wt-e) im(t)= (3)

JrM12+w2(1No+lM1)2

where 8=tan-1(l~o+1m) /rm, rm: primary winding resistance of the motor, 1110: exciting inductance of the motor, lm: primary leakage inductance of the motor.

Fig.1. Basic three-phase converter using stator core saturation of a motor.

Fig.2. Idealized waveforms of the phase voltages of a motor.

Manuscript received June 1, 1979. 0018-9464/79/1100-1782900.75 0 1979 IEEE

'M1 'M1 'M2 'M2

Fig.3. Equivalent circuit of an induction motor.

Fig.4. Magnetization curve.

I I I ' I I Experimental value

ff

I I I I I 0 50 $00 150 200 250

dc supply voltage ED(V)

Fig.5. Oscillation frequency characteristics.

A s the commutation occurs a t t he time t = T / 2 , we get from Eq.(2) and (3 ) the following

Hence, t he o sc i l l a t ion frequency f becomes as follows:

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where NM is the number of turns per phase of the s t a t o r winding of the motor and 0s i s the f lux at the knee current IMOS of the stator core per phase of the motor.

Fig.5 shows the experimental and the calculated r e s u l t s of t he o sc i l l a t ion frequency f vs. dc supply voltage ED, taking Eg as a parameter. The experimental results in Fig.5 are obtained in the c i rcui t wi thout the capacitor Cs which w i l l be shown i n Fig.6 of the next chapter. The s o l i d l i n e shows the r e su l t s of calcula- t ions from Eq.(5) with an IMOS=1.26A, which corresponds to an Eg=2.5V. The experimental results seem to be d i f f e r from the calculat ions. This reason comes from the e f fec t of the slow saturation of the s ta tor core . The knee current IMOS should be a function of the dc source voltage Eg. The dot ted l ines are the calculated r e s u l t s , i n which IMOS is assumed proportional to EB.

INDUCTION MOTOR DRIVE

(1) Consideration on the base c i rcui t

co l lec tor of t he t r ans i s to r and in the motor a t t h e shown i n Fig.1, an excessive current flows in the

commutation interval because of the s low saturat ion of the stator core. Therefore a large loss is generated and the range of the osci l la t ion f requency i s small. The e f f e c t of the slow saturation of the motor can be compensated for , i f the base c i rcu i t res i s tance RB a r e proportional to the supply voltage ED, tha t i s , the base currents ig are kept constant independent of ED. However, i t is not easy to make RB proportional to ED. To so lve t h i s d i f f i cu l ty , independent dc sources Eg are used for the d r iv ing c i rcu i t s of t he t r ans i s to r s T r as shown i n Fig.6. In this case, the oscil lation fre- quency becomes also a function of Eg as descrived in the preceding chapter.

Linear transformers LT1,LTz and LT3 as shown i n Fig.1 result in the increase of the s ize and weight of the whole device. These can be eliminated by introduc- ing complementary connections of t r a n s i s t o r s as shown in Fig.6. Zener diodes Zd as well as capacitors CB a r e effect ive to shorten the commutation time between tran- s i s t o r s . Zener diodes Zd' a re used to protect the bases of t r a n s i s t o r s Trg.

e U N and the l ine current im of the motor. The wave- form of eUN i s almost the same as tha t of saturable cores already reported [3 ] . The specif icat ion of the tes t devices used in the experiments is given in Table I

(2) Phase rotation A circuit to determine the phase rotation of the

output voltages i s enclosed by the dot ted l ine in Fig.6. This c i rcu i t cons is t s of a simple R-C network and the aux i l i a ry t r ans i s to r TrR. When commutation occurs from the lower transistor to the upper, the pulse current for synchronizing flows from the terminal U to the terminal B2 o r B3 through the R-C network and T ~ R . Accordingly, the converter unit C U I is the master o s c i l l a t o r and the others, CU2 and CU3, become slaves. The phase rotation is posi t ive (CU1-tCU2-tCU3) for the switch SWR connected to B2 and a connection to E3 corresponds. to a negative rotation (CU1-tcU33cU2).

(3) Torque charac te r i s t ics

motor driven by t h e c i r c u i t i l l u s t r a t e d i n Fig.6 a r e shown in Fig.8, taking ED and Eg as parameters. The r a t i o of the voltage to the frequency P/f ,can be ad jus ted a rb i t ra r i ly by d c source voltage Eg. It can be seen from Fig.8 that the torque characterist ics can be controlled to some extent by a s&ll' s ignal Eg. It may correspond to the field control of a>dc motor. On the other hand, the operation of changing the dc supply voltage ED corresponds to the armature'control of i t . The'maximum torque i s increased considerably by inser t - ing the capacitor Cs between the supply and the neutral point N' . In some case, especial1y"for low speed oper- a t ion , fo r example E D = ~ O V , the maximuq torque can be twice by connecting the capacitor CS of a proper value.

When the motor speed i s accelerated in the c i rcui t

Fig.7 shows an oscillogram of the phase voltage

Torque-speed charac te r i s t ics of the induction

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Fig.6. Full circuit of the three-phase converter using the stator core saturation of a motor.

Fig.7. Oscillogram of the phase voltage eUN and of the l ine current iM1 of a motor.

Table I Test devices used.

Power

15uF. 350V (Electrolvtic Capacitor) Capacitor CC

PC: 2OOW (Hirel, Japan) ED205(npn) Transistor T r VCEO: 4 0 0 V , IC: 20A, EB205(pnp)

Induction Motor IM ZOOW, 200V, 6OHz

0 ~~

5 0 0 1000 1500 2 0 0 0 Speed N (rpm)

Fig.8. Torque-speed charac te r i s t ics .

The eff ic iency of t h i s system i s inherent ly the same a s t h a t of a convent ional t ransis tor inverter i f we neglect the power required for the base driving circuit. In our experiments, the efficiency i s about 60%.

CONCLUSIONS

From preceding discussion, the features of the new polyphase converter for induction motor control using the s ta tor core saturat ion are as follows.

1) Polyphase o s c i l l a t i o n i s possible by using the s ta tor core sa tura t ion of the motor instead of satura- ble cores used in the conventional polyphase converter. The e f f e c t of slow saturation of the stator core can be compensated fo r by adjusting the base current. 2) The V/f r a t i o i s a rb i t ra r i ly cont ro l led by an auxil- iary control voltage while keeping the dc supply volt- age constant. The converter has a wide range of frequency. 3) The induction motor i s the only magnetic element i n this converter, because linear transformers for the feedback of the output voltages become unnecessary. The converter is simple in construct ion.

ACKNOWLEDGMENT

The authors express their thanks to Mr. Y. Yoshida of Yasukawa Elec t r ic MFG. CO. , LTD., Yukuhashi, Japan, for his discussions.

I REFERENCES

[l]. K. Harada, IEEE Trans. Magn., Vol. MAG-3,

[2]. R.L. Hodkinson and J. Mills, Power Electronics- pp.117-124 (1967)..

Power Semiconductors and their Applications, pp.41-44 (1977).

[3]. K . Harada and M. Nagao , Proceeding of INTELEC, pp.335-343 (1978).