Adjustable-Speed Single-Phase Induction Motor Drive M B de R. Correa1j2, C.B. Jacobinal, A.M.N. Lima1 and E.R.C. da Silval . .

‘Dep. de Eng. Eletrica - CCT - UFPB - Campus II - Caixa Postal 10.105 58109-970 Campina Grande, PB - Brazil

Phone: +55.83.3101407/1136, Fax: +55.83.3101418/1015 E-mail: jacobina@dee.ufpb.br

2CEFET - AL, UNED - Palmeira dos Indios, AL - Brazil

Abstract: This paper investigates the perfor- mance of adjustable-speed drive systems based on start-capacitor (SC) and split-phase capacitor (SPC) single-phase motors. It is demonstrated that operating single-phase motors in two-phase mode contributes to increase the motor torque, re- duces the torque pulsation and provides smooth start-up currents. Simulation results for SC and SPC motor as well as experimental results for an adjustable-speed drive based on a SC motor are presented and demonstrate the performance, cor- rectness and feasibility of the proposed solution.

I. INTRODUCTION

The single-phase motors are widely used in fractional horsepower applications. The use of single-phase motors in adjustable-speed motor drive systems has been quite unusual over the years. However, in recent years several research groups started to investigate different single-phase adjustable-speed motor drive systems [l-5]. The main mo- tivation behind this interest is the improvement of the per- formance of the existing single-phase standard solution. The cost of this solution can be reduced because the con- tinuous decrease in the prices of the semiconductor power switches.

In general there are two different alternatives to imple- ment an adjustable-speed single-phase motor drive system: in the first one the single-phase motor is treated as single- phase motor (single-phase mode) [6-91 and in the second one the single-phase motor is considered as a two-phase motor (two-phase mode) [1,3].

In the single-phase mode it is required to use a split- phase capacitor (SPC) motor. However, it is important to remark that this capacitor is designed for the nominal speed and consequently the performance is not optimized for variable-speed operation. The use of a start capacitor (SC) motor it is not recommended for adjustable-speed motor drive systems due to the presence of the centrifugal switch that changes the motor characteristics during the operation.

In the two-phase mode, as mentioned before, the single- phase motor is treated as a two-phase motor. The SPC motor has a main and an auxiliary windings that are used

Fig. 1. Single-phase sdjustable-speed motor drive system.

during the entire motor operation. Then, this type of mo- tor can be chosen as the standard choice for designing single-phase adjustable-speed motor drive systems either in single-phase or in the two-phase modes. However, it must be noted that it is also possible to use an SC mo- tor for implementing single-phase adjustable-speed motor drive systems. Moreover, it is important to evaluate the use of SC motors in the two-phase mode because its use is more widespread and it is cheaper than a SPC motor of the same power.

The windings of the SPC and SC motors have different characteristics and this will influence its performance when operated in the two-phase mode. The objectives of this article is to compare these two types of motors with respect to its performances when operated in the two-phase mode and single-phase mode.

II. SYSTEM CONFIGURATION

The configuration of the single-phase adjustable-speed motor drive system for the two-phase operation used in this paper is illustrated in Fig. 1. It is composed by a rectifier and a two-leg converter and the single-phase motor (SC or SPC).

III. SINGLE-PHASE MOTOR MODEL

According to the general theory of electric machines a single-phase motor with two quadrature windings can be described by the following set of equations:

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0 = ‘0’ [ 1 ,” T f#)idq = [ “; lo ] i& + [ m;d m;T, ] $fq> c3)

s*

(i&f* = [ “G 1” ] $fq + [ m;d m;T, ] i:dq, c4) T

Te = P(msTq+#$d - %rd%#&), (5)

P (T, - T,) = J% + Fw,, (6)

where v,Sdp = [w,Sd w&IT, iidp = [iid i&IT, iFdp = k;d i&lTJ &dp = L&d &,I’ and @dp = [@d @dplT are dq voltages, currents and fluxes of the stator and ro- tor in the stator reference frame (superscript s); r&, r,* and r, denote the stator and rotor resistances; I&, lsp, I,, msrd and msTq denote the stator, the rotor self and mutual inductances; w,, T, and T, are the motor speed, the elec- tromagnetic torque and the load torque; P, J and F are the motor pole pairs, the moment of inertia and viscous friction coefficient, respectively.

If a SC motor operates in the single-phase mode, the voltage w,Sd depends on the capacitor voltage, i.e.,

?& = t& - w, (7)

with

J’

t w, = &(T)dT + Q(h), (8)

to

if the centrifugal switch is closed. In this case, the capacitor and the centrifugal switch are connected in series with the auxiliary winding. If the switch is opened iid = 0 and then the electromagnetic torque is calculated by

In the two-phase mode the SC motor operates as a two- phase motor, the capacitor and the centrifugal switch are removed, and then the two windings are supplied from the voltage source converter as shown in Fig. 1.

If a SPC motor operates in the single-phase mode, the voltage w,Sd is given by (7) and (8). For the SPC motor, this relationship is valid at all operating conditions since the ca- pacitor is permanently in series with the auxiliary winding. In the two-phase mode the SPC motor also operates as a two-phase motor, and then the capacitor is removed.

IV. STEADY STATE ANALYSIS

A. Start capacitor The operation of the SC motor in the two-phase mode

can provide significant improvement to its performance. The performance gain obtained by operating the motor in the two-phase mode occurs in two different ways. The first one is the elimination of the centrifugal switch that

results in a wide speed range and increases the steady-state torque. The second one is the reduction of the pulsating torque.

Figures 2(a) and 3(a) show the steady T, x (w, - w,) /ws characteristics curves of a SC motor operated in the two- phase and single-phase mode, respectively. These curves are parameterized by the frequency of stator voltage w, = 2nfs and the motor speed is represented by w, = 2nfr. The comparison of these curves clearly reveals the torque increase obtained in the two-phase mode. The jumps observed in the torque curves of the single-phase mode [Fig. 3(a)] occurs when the centrifugal switch disconnects the capacitor.

Figures 2(b) and 3(b) h s ow the torque pulsation versus (w, - w,) /ws characteristics curves of a SC motor operat- ing in the two-phase and single-phase mode, respectively. The comparison of these curves also demonstrates that the operation in the two-phase mode exhibits less torque pulsation than the operation in the single-phase mode if lw:dl = E Iw:qI, [lag].

B. Split-Phase Capacitor Similarly, the operation of the SPC motor in the two-

phase mode can also provide significant improvement to its performance. To evaluate the performance gain for the SPC motor, the same characteristics curves used to study the SC motor were calculated and plotted. Figures 4(a) and 5(a) show the steady T, x (w, - w,) /ws character- istics curves of a SPC motor operated in the two-phase and single-phase mode, respectively. Figures 4(b) and 5(b) show the torque pulsation versus (w, - w,) /ws character- istics curves of a SPC motor operating in the two-phase and single-phase mode, respectively. As it was observed for the case of the SC motor, the operation of a SPC mo- tor in the two-phase mode increases the motor torque and reduces the torque pulsation.

V. TRANSIENT ANALYSIS

A. Start Capacitor Figures 6 and 7 show the torque and motor currents

corresponding to the start-up of a SC motor operating in the single-phase and two-phase modes, respectively. In the two modes, the voltage supplied to the SC motor windings follows a constant V/f8 law and the load torque is consid- ered constant. In the two-phase mode, the motor torque and stator currents have a smoother envelope and reach smaller levels during the entire start-up transient.

B. Split-Phase Capacitor Figures 8 and 9 show the torque and motor currents

corresponding to the start-up of the a SPC motor operating in the single-phase and two-phase modes, respectively. In the two modes, the voltage supplied to the SPC motor windings follows a constant V/f8 law and the load torque

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0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 ( 0 ( 0 s s - Or)/Os - Or)/Os

(b) Torque pulsation. (b) Torque pulsation

4

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3

2.5

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Fig. 2. Torque characteristics of the SC motor operating in the two- phase mode, without capacitor and centrifugal switch.

is considered constant. Also, in the two-phase mode the torque and currents have a smoother envelope and reach small amplitude levels during the entire start-up transient.

VI. EXPERIMENTAL RESULTS

in our laboratory. The overall control scheme of the drive was implemented by software in a Pentium II - 266MH.z microcomputer platform equipped with dedicated plug-in interface boards. The switching frequency of the power converter used in the experimental tests was 1OlcH.z but it can be varied over a wide range.

The simulation results presented in the previous sections The experimental tests were conducted for a SC motor indicates that there is a significant improvement in the per- and the converter provided a constant V/f8 pulse width formance, measured in terms of the average motor torque, modulated voltage source to supply the stator windings. torque pulsation and start-up currents, when SC and SPC Figures 10 and 11 show the experimental results corre- motors are operated in the two-phase mode. sponding to the start-up transients of the SC motor op-

To check the actual extent of this performance improve- erating in the single-phase and two-phase modes, respec-

1, f, =6OHz ' ' ,

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' ' , f, =5OHz 1

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f, =2OHz \x

0.8 0.6 0.4 0.2

(b) Torque pulsation

Fig. 3. Torque characteristics of the SC motor operating in the single- phase mode, with capacitor and centrifugal switch.

ments, the configuration shown in Fig. 1 was implemented tively. It is worth noting that the speed evolution the two-

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(a) Steady state torque.

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0.8 0.6 0.4 0.2 ( 0 s - Or)/Os

(b) Torque pulsation

1

0

Fig. 4. Torque characteristics of the SPC motor operating in the two-phase mode, without capacitor.

phase mode [see Fig. 11(a)] is faster than in the single- phase mode [see Fig. 10(a)]. It is also important to re- mark that in the two-phase mode the motor current has a smoother envelope and reach small amplitude levels during the entire start-up transient as it can be seen by comparing Figs. 10(b) 11(b).

VII. CONCLUSION

This paper has demonstrated that the operation of SC and SPC motors in the two-phase mode provides a signif- icant improvement of the motor performance. This indi- cates the feasibility for employing these two types of motors in adjustable-speed drive systems.

Both types of single-phase motors (SC and SPC) can be

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0.8 0.6 0.4 0.2 ( 0 s - Or)/Os

(a) Steady state torque.

0.8 0.6 0.4 0.2 ( 0 s - Or)/Os

(b) Torque pulsation.

Fig. 5. Torque characteristics of the SPC motor operating in the single-phase mode.

successfully treated as a two-phase motor. The advantages with respect to the standard single-phase mode are related with the reduction of the start-up current and the elimina- tion of the torque pulsations. Besides, that it is possible to increase up to the double the value of the average motor torque, in the case of the SC motor. However, this is de- pendent on the maximum allowed voltage for the auxiliary winding.

The experimental results have demonstrated the perfor- mance, correctness and feasibility of the proposed method- ology for the case of the SC motor.

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Fig. 6. Evolution of the torque and currents of a SC motor during Fig. 8. Evolution of the torque and currents of a SPC motor during the start-up transient for V/fs constant (with the start capacitor) - the start-up transient for V/fs constant (with the start capacitor) - single-phase mode. single-phase mode.

0 0.2 0.4 0.6 0.8 1 time (s)

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0 0.2 0.4 0.6 0.8 1 time (s)

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Fig. 7. Evolution of the torque and currents of a SC motor during Fig. 9. Evolution of the torque and currents of a SPC motor during the start-up transient for V/fs constant - two-phase mode. the start-up transient for V/fs constant - two-phase mode.

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(b) (b)

Fig. 10. Evolution of the speed and currents of a SC motor during Fig. 11. Evolution of the speed and currents of a SC motor during the start-up transient for V/f3 constant (with the start capacitor] - the start-up transient for V/fs constant (without the start capacitor) single-phase mode. - two-phase mode.

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TABLE 1. Parameters of the start capacitor motor (l/8 CV) controlled capacitor. IEEE Transactions on Industry Apphcations, 27(1):38-43, Jan/Feb 1991.

TABLE 2. Parameters of the split-phase capacitor motor (l/8 CV)

r s* = 10.30 Lq = 0.2081H rsd = 13.20 lsd = 0.2274H r, = 9.7714 I, = 0.2133H m srp = 0.1821H m,,d = 0.189H c= 25 uF

APPENDIX - MACHINE PARAMETERS

Tables 1 and 2 show the parameters of the single-phase motors used in this paper.

REFERENCES

[l] D.G. Holmes and A. Kotsopoulos. Variable speed con- trol of single and two phase induction motors using a three phase voltage source inverter. In Conf. Rec. IAS, pages 613-620,1993.

[2] M.F. Rahman and L. Zhong. A currente-forced re- versible rectifier fed single-phase variable speed induc- tion motor drive. In Conf. Rec. PESC, pages 114-119, 1996.

[3] E.R. Benedict and T.A. Lipo. Improved pwm modu- lation for a permanent-split capacitor motor. In Conf. Rec. IAS, pages 2004-2010,200O.

[4] M. Chomat and T.A. Lipo. Adjustable speed drive with single-phase induction machine for hvac applications. In Conf. Rec. PESC, pages 1446-1451, 2001.

[5] M.B.R. Correa, C.B. Jacobina, A.M.N. Lima, and E.R.C. da Silva. Rotor-flux-oriented control of a single- phase induction motor drive. IEEE Transactions on Industrial Electronics, 47(4):832-841, August 2000.

[6] E.R. Collins Jr. and R.E. Ashley. Operating charac- teristics of single-phase capacitor motors driven from variable speed supplies. In Conf. Rec. IAS, pages 52- 57, Dearborn, Mi, 1991.

[7] E.R. Collins. Torque and slip behavior of single-phase induction motors driven from variable-frequency sup- plies. IEEE Transactions on Industry Applications, 28(3):710-715, May/June 1992.

[8] E. Muljadi, Y. Zhao, T.H. Liu, and T.A. Lipo. Ad- justable ac capacitor for a single-phase induction mo- tor. In Conf. Rec. IAS, pages 185-190, Dearborn, Mi, 1991.

[9] T.A. Lettenmaier, D.W. Novotny, and T.A. Lipo. Single-phase induction motor with an electronically

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