Dynamic Modeling of the Universal Motor

444 Acemp - Electromotion 2011, 8 - 10 September 2011 İstanbul - Turkey

Dynamic Modeling of the Universal Motor used in Washer

A. POLAT L.T. ERGENE A. FIRAT Department of Electrical Engineering Department of Electrical Engineering Arçelik A.Ş. Istanbul Technical University Istanbul Technical University 59500, Tekirdağ, Turkey 34469, Istanbul, Turkey 34469, Istanbul, Turkey [email protected] [email protected] [email protected]

Abstract ─ In this paper, firstly, the structural features and operating conditions of a universal motor are presented. Alternating current (AC) and direct current (DC) operating conditions of the motor and the difference between them are explained. Universal motors give similar speed and torque characteristics on both AC and DC operations when the effective current is kept equal. When AC and DC operations are compared, the differences arise because of, transformer voltage, armature reaction, reactance voltage, saturation effects and commutation. The motor is modeled dynamically by using MATLAB/Simulink and these differences are analyzed and compared with the experimental.

Index Terms ─ Universal motors, equivalent circuit, reactance voltage, saturation effects.

I. INTRODUCTION

The universal motor is a series DC motor which is also designed to operate on AC. Universal motors have some advantageous: high-speed rotation, low cost, and high starting torque. Therefore, universal motors are widely used in home appliance such as sewing machines, washers, mixers, vacuum cleaners and small industrial power applications [1]. A conventional DC series motor has poor characteristics when operated on AC because of the high reactance of both the armature and field windings. This reactance limits AC current to a much lower value than DC current for the same line voltage. Using solid steel in the stator will also cause induced eddy currents and eventually extra heating and losses on AC operation. To overcome these disadvantages in the universal motors, some modifications such as using laminated steel on stator frame reducing the reactance are necessary. Universal motor is preferred over induction motor in some applications where high speed is required. Structurally, a universal motor has conventional DC machine’s rotor that has direct current winding and collector. The rotor and the excitation windings connected in series [1]. Even though the motor can be operated on both AC and DC, there are some speed and torque characteristic differences between these operations due to magnetic saturation and reactance voltage. The paper presents the dynamic models of the universal motor used in

the washers and shows the current, torque and speed differences between two different operating conditions. The experimental results for both operating conditions are also presented in the paper to verify the dynamic models.

II. METHODOLOGY

When the effective current value is equal in AC and DC operations, the current, torque and speed characteristics will be similar, but not the same. The main reason for this situation is the magnetic saturation. Magnetic flux will not increase linearly in the saturated case. The other reason is the current. The current will be limited by reactance as well as resistance unlike DC operation [2]. Universal motor has been considered as a series DC motor at the beginning and its equivalent circuit is obtained. Then, the model will be evaluated according to DC operation without the magnetic saturation and reactance. Later, voltage equations will be obtained for the AC operation and its model will be developed. The model will be modified according to AC operation with magnetization curve and reactance [3]. So, the effects of the saturation and reactance voltage on the speed and torque characteristics in different current, speed and load torque values will be investigated.

III. MATHEMATICAL MODEL

Equivalent circuit of the universal motor for DC operation is shown in Fig. 1. The current and voltage equations for this operating condition are similar to the series DC machine and given as follows [4]: If = Ia (1) Vf = rf . Ia + Lff . (dIa / dt) (2) E = Laf . ω . Ia (3) Va = ra . Ia + Laa . (dIa / dt) + E (4) Va = ra . Ia + Laa . (dIa / dt) + (Laf . ω . Ia) (5) Vt = Va + Vf (6) Vt = Ia .(rf+ra+Laf .ω)+(dIa/dt)(Lff+Laa) (7)

Where E is back EMF, If is excitation current, Ia is armature current,Va is armature voltage, Vf is excitation voltage and Vt is supply voltage (sum of the Va and Vf), ra is

978-1-4673-5003-7/11$26.00©2011 IEEE

445Acemp - Electromotion 2011, 8 - 10 September 2011 İstanbul - Turkey

Dynamic Modeling of the Universal Motor used in Washer

A. POLAT L.T. ERGENE A. FIRAT Department of Electrical Engineering Department of Electrical Engineering Arçelik A.Ş. Istanbul Technical University Istanbul Technical University 59500, Tekirdağ, Turkey 34469, Istanbul, Turkey 34469, Istanbul, Turkey [email protected] [email protected] [email protected]

Abstract ─ In this paper, firstly, the structural features and operating conditions of a universal motor are presented. Alternating current (AC) and direct current (DC) operating conditions of the motor and the difference between them are explained. Universal motors give similar speed and torque characteristics on both AC and DC operations when the effective current is kept equal. When AC and DC operations are compared, the differences arise because of, transformer voltage, armature reaction, reactance voltage, saturation effects and commutation. The motor is modeled dynamically by using MATLAB/Simulink and these differences are analyzed and compared with the experimental.

Index Terms ─ Universal motors, equivalent circuit, reactance voltage, saturation effects.

I. INTRODUCTION

The universal motor is a series DC motor which is also designed to operate on AC. Universal motors have some advantageous: high-speed rotation, low cost, and high starting torque. Therefore, universal motors are widely used in home appliance such as sewing machines, washers, mixers, vacuum cleaners and small industrial power applications [1]. A conventional DC series motor has poor characteristics when operated on AC because of the high reactance of both the armature and field windings. This reactance limits AC current to a much lower value than DC current for the same line voltage. Using solid steel in the stator will also cause induced eddy currents and eventually extra heating and losses on AC operation. To overcome these disadvantages in the universal motors, some modifications such as using laminated steel on stator frame reducing the reactance are necessary. Universal motor is preferred over induction motor in some applications where high speed is required. Structurally, a universal motor has conventional DC machine’s rotor that has direct current winding and collector. The rotor and the excitation windings connected in series [1]. Even though the motor can be operated on both AC and DC, there are some speed and torque characteristic differences between these operations due to magnetic saturation and reactance voltage. The paper presents the dynamic models of the universal motor used in

the washers and shows the current, torque and speed differences between two different operating conditions. The experimental results for both operating conditions are also presented in the paper to verify the dynamic models.

II. METHODOLOGY

When the effective current value is equal in AC and DC operations, the current, torque and speed characteristics will be similar, but not the same. The main reason for this situation is the magnetic saturation. Magnetic flux will not increase linearly in the saturated case. The other reason is the current. The current will be limited by reactance as well as resistance unlike DC operation [2]. Universal motor has been considered as a series DC motor at the beginning and its equivalent circuit is obtained. Then, the model will be evaluated according to DC operation without the magnetic saturation and reactance. Later, voltage equations will be obtained for the AC operation and its model will be developed. The model will be modified according to AC operation with magnetization curve and reactance [3]. So, the effects of the saturation and reactance voltage on the speed and torque characteristics in different current, speed and load torque values will be investigated.

III. MATHEMATICAL MODEL

Equivalent circuit of the universal motor for DC operation is shown in Fig. 1. The current and voltage equations for this operating condition are similar to the series DC machine and given as follows [4]: If = Ia (1) Vf = rf . Ia + Lff . (dIa / dt) (2) E = Laf . ω . Ia (3) Va = ra . Ia + Laa . (dIa / dt) + E (4) Va = ra . Ia + Laa . (dIa / dt) + (Laf . ω . Ia) (5) Vt = Va + Vf (6) Vt = Ia .(rf+ra+Laf .ω)+(dIa/dt)(Lff+Laa) (7)

Where E is back EMF, If is excitation current, Ia is armature current,Va is armature voltage, Vf is excitation voltage and Vt is supply voltage (sum of the Va and Vf), ra is

the resistance of stator and rf is the resistance of rotor, Laa is the self-inductance of stator, Lff is the self-inductance of rotor and Laf = Lfa are the mutual-inductance.

Fig. 1. DC Equivalent circuit of the universal motor

The mechanical equations of the motor are as follows [4],

Te = Laf . Ia . If (8) Te = J . dω/dt + B.ω + Tl (9)

dω/dt = (-B/J) .ω + (Te – TL) / J (10)

ω = ∫ ( (-B/J) .ω + (Te – TL) / J )dt (11) where B is the coefficient of friction, J is the coefficient of inertia, TL is load torque and ω is the angular velocity. In order to determine the back EMF value, the V-I characteristic is added to the model [5]. The B-H characteristic for lamination of the universal motor that used in model is shown in Fig. 2.

Fig. 2. The magnetization curve

The effect saturation effect can be seen on the voltage-current characteristic (Fig. 3). During the washing test of the motor, speed is kept constant at 860 min-1 and the current values are measured for different voltages. The V-I characteristic which presents saturation effect on model is also obtained.

Fig. 3. The V-I characteristic caused by B-H curve

The rated values of the test motor are 220V,50 Hz, 585W, 3,1A, and 10750 min-1. The circuit elements and the mechanical coefficients which are used in the dynamic equations are given in Table I.

TABLE I MOTOR PARAMETERS

Resistance of rotor 1,6 Ω

Resistance of stator 2,04 Ω

Rotor’s self-inductance 87 mH

Stator’s self-inductance 73 mH

Coefficient of inertia 3,2.10-4 kg.m2 Coefficient of friction 1,686. 10-5 Nms Initial angular speed 500 rad/s

IV. MODELING OF UNIVERSAL MOTOR AND RESULTS

In this paper, mathematical models of the motor for both cases are formed and simulated by using MATLAB/Simulink to obtain current, torque and speed characteristics. Fig. 4 shows the motor model for the DC operation. In that case, the current will be limited only by the resistance and there is no saturation effect.

Fig. 4. The DC model of the universal motor

The AC model of the motor will be achieved by adding the reactance besides resistance and the saturation effect to the DC model [5]. The AC model of the motor is given in Fig. 5.

446 Acemp - Electromotion 2011, 8 - 10 September 2011 İstanbul - Turkey

Fig. 5. The AC model of the universal motor

Speed, current and torque characteristics are obtained for AC and DC models at no-load operation.

Fig. 6. Characteristics at No-load operation with DC supply

Fig. 7. Characteristics at no-load operation with AC supply

Speed, torque and current characteristics indicate a rising trend in starting or in another words transient state as expected, and stabilize to nominal operation values in steady state as seen in Fig. 6 and Fig. 7 for DC and AC operations respectively.

TABLE II COMPARISON OF AC AND DC OPERATION (MODEL)

DC AC ML[Nm] n [rpm] n [rpm]

0 25580 27940 0,1 17510 18700 0,2 13710 13970 0,3 11750 11300 0,4 10580 9498 0,5 9802 8098 0,6 9218 6893 0,7 8763 5847 0,8 8397 4871

The torque and speed values for AC and DC operations show that the speed values at AC operational conditions are higher than DC operational conditions at lower load torque values. However, speed rapidly drops due to reactance at AC case as a result of the load torque increase [6].

Fig. 8. Speed-Moment characteristics for AC and DC operations (model)

The Fig. 8 shows that clearly, the universal motor has series characteristic that the torque is decreasing when the speed is increasing.

V. TESTING OF UNIVERSAL MOTOR AND EXPERIMENTAL RESULTS

The MAGTROL© dynamometer with Hysteresis braking system which provides frictionless torque loading independent of shaft speed is used to obtain experimental results of the universal motor. The current, torque, speed, power and efficiency of the motor are measured for different load torques for AC and DC operations. These tests were performed for both the spinning and washing conditions. The test system is given in Fig. 9.

447Acemp - Electromotion 2011, 8 - 10 September 2011 İstanbul - Turkey

Fig. 9. Test System

In the case of spinning, the speed decreases as the load torque increases.

Fig. 10. Speed-torque characteristics for AC and DC operations (test)

According to test results; as shown in the Fig. 10, the universal motor has higher speed values when it’s supplied by AC source than the DC ones under lower load torque and higher speed operation conditions. Besides, the difference between the speed-torque characteristics is not as clear as the characteristics at higher speed. However, the speed in DC is higher than the speed in AC when the universal motor is supplied by DC with the higher load torques and lower speeds. The speed-torque characteristics of the universal motor for AC and DC sources were compared with each other in terms of experimental results and simulation. In addition, the test results for AC operational condition and simulation results for AC case, similarly, DC results for both test and simulation will be compared with each other. Therefore, the differences between results of test and model will be seen and the reason of these differences will be explained.

Fig. 11. Comparison of DC test and simulation model

The speed-torque characteristics for DC case were shown for both experimental and simulation results in Fig.11. The characteristics are almost same except for higher speeds and lower load torques. The commutation is not included in the model. This affects the results especially at high speeds [7]. Losses caused by the rectifier have also contributed to this difference.

Fig. 12. Comparison of AC test and simulation model.

In the AC case, the simulation and experimental results are different almost 15% for higher load torque because of the design priorities. Almost all universal motors which are produced for washing machines are operated at AC. However, the AC case has more non-linear operating condition than DC one because of the saturation effects and the reactance voltage.

Fig. 13. Comparison of AC test and AC model (rated load torque)

448 Acemp - Electromotion 2011, 8 - 10 September 2011 İstanbul - Turkey

In the rated condition, the results are almost same for both test and simulation as seen in Fig. 13.

VI. CONCLUSION

In the paper, the dynamic models of an universal motor which is used in washers are built for AC and DC operating conditions and the speed-torque characteristics are obtained for various load torques. The comparison between the dynamic models is explained and the results are verified for both operating conditions with the experimental data. As a result, the characteristics which are obtained with testing and simulation are parallel, and the values of both test and simulation are compatible. The results show that, the effective value of the flux is less in AC operation than the one in DC because of the saturation. So decrease in flux increases the speed. The reactance voltage also decreases the speed. The saturation is more influential in the case of the lower current and higher speed. Otherwise reactance voltage is more influential. Therefore the speed of the universal motor in AC operation is less than the one in DC case at the rated condition.

REFERENCES

[1] Papa, G., Korousic, B., Benedicic, B., Kmecl, T., ‘‘Universal Motor Efficiency Improvement Using Evolutionary Optimization’’, IEEE Transactions on Industrial Electronics, Volume: 20, No: 3, pp. 602-611, 2003.

[2] Zhou, P., Brauer, J.R., Stanton, S., Cendes, Z., ‘‘Dynamic Modeling of Universal Motor’’, International Conference IEMD’99, pp.419-421, 1999.

[3] Tuncay, R.N., Yılmaz, M., Onculoglu, C., Kanca, G., ‘‘The Design Methodology to Develop New Generation Universal Motors for Vacuum Cleaners’’, IEEE, pp.926-930, 2001.

[4] Krause, P.C., Wasynczuk, O., Sudhoff, S.D., ‘‘Analysis of Electric Machinery and Drive Systems’’, Wiley IEEE Press, 2002.

[5] Ong, C.M., ‘‘ Dynamic Simulation of Electrical Machinery Using MATLAB/ Simulink’’, Prentice-Hall Inc., 1998.

[6] Polat, A., Ergene, L.T., ‘‘ Modeling of Universal Motors’’, Undergraduate Thesis, Istanbul Technical University, 2010.

[7] Kurihara, K., Yamamoto, K., Kubota, T., ‘‘Commutation Analysis of Universal Motor Taking into Account Brush-to-Bar Voltage Drop’’, ICEMS 2009.

Comparison of Current Balancing Configurations for Primary Parallel Isolated Boost Converter

Gokhan Sen1, S. M. Dehghan2, Ole C. Thomsen1, Michael A. E. Andersen1, Lars Møller3

1Ørsteds Plads 349, DK-2800 2Enginnering Faculty 3H2 Logic A/S Kgs. Lyngby, Denmark Qom University of Technology Herning DK-7400 [email protected] Qom, Iran Denmark

Abstract— Different current balancing configurations have been investigated for Primary Parallel Isolated Boost Converter (PPIBC). It has been shown that parallel branch current balancing is possible with several configurations of coupled/uncoupled inductors. Analytical expressions for branch currents have been derived for different cases of gate signal mismatch causing current imbalance. It has been observed that turn-on and turn-off delays in parallel power stages of the PPIBC have different effects in the branch currents deviating from ideal. It has also been observed that in some configurations inductance differences due to core tolerances play an important role in current imbalance. Analytical and simulation results have shown that another side effect of the gate signal delay and inductor value difference is additional voltage stress over the switches during the mismatch times. Advantages of each configuration in terms of effective current balancing, efficiency and manufacturing simplicity have been highlighted. Simulations with ideal components for each case have been carried out to confirm the analytical derivations. Experimental results have also been included to show the performances of different configurations where component non-idealities like transformer leakage inductances also become effective.

I. INTRODUCTION

Primary Parallel Isolated Boost Converter (PPIBC) is a high efficient topology for high input current, step up applications [1]. The effectiveness of the topology is coming from the unique parallel power stage structure in the input side where the voltage is low and the current is high. The primary side switches of each parallel power stage operate synchronously with the corresponding switches in the other parallel power stages. However due to propagation delays and rise-fall time differences of the ICs as well as component tolerances in the gate drive circuitry like gate resistances result in switching delays. These delays not only cause the branch current values to deviate but also switch voltage over-stress conditions to occur which may increase the switching losses.

Although various configurations have been claimed in the patent [2], only one of them, current balancing transformer (CBT), has been implemented so far for proof of concept. Recently integrated magnetic solutions have been proposed for this topology [3] and [4]. Deviations from ideal current waveforms in parallel branches have been reported in [5]. Similar effects can be observed if two separate inductors (TSI) or partially coupled inductors (PCI) are used. In this paper these configurations have been analytically investigated. Expressions have been derived for branch current deviations which then

compared to ideal circuit simulation as well as experimental results.

II. PRIMARY PARALLEL ISOLATED BOOST CONVERTER

Fig.1 shows PPIBC topology suitable for handling high input currents for fuel cell applications. The current is forced to be equal in both primary windings by the series secondary connection of the two transformers. Primary switches share the same control signals with the same phase switching sequence which allows a simple control. Output rectification unit as well as input and output filters are common to both primary stages.

The input inductor in Fig. 1 serves as an energy storage element for both primary power stages. As long as switches S1, S2, S3, S4 and S5, S6, S7, S8 work in the same pattern (Fig. 2a), the inductor current will be shared equally by the two full bridges. In case of a mismatch in switching, the CBT (effectively an inverse coupled inductor) in series with the input inductor shows high impedance in the differential path which limits the rate of change of the differential current (Fig. 2b).

III. CURRENT BALANCING CONFIGURATIONS

In order to simplify the analysis, the converter in Fig. 1 has been reduced to the circuit in Fig. 3a. This circuit will be used through out the paper except the coupled/uncoupled inductor combinations will replace the single inductor. A. Single inductor:

Figs. 3b-e show possible configurations of the switches both in normal and extra operation modes. Here a switch being on is equal to all four switches being on (inductor charging state). Similarly a switch being off in Fig. 3a corresponds to two diagonal switches being on in Fig. 1 (discharging state).

T1

Vin

S1

S4

S3

S2

Vo

1:n

C RL

vL2+ −

vT1

iL1

iS

+

+

+

−

−

−

D3

D2

iD1

D1

D4

io

iTP1 iTS

ic

T2

S5

S8

S7

S61:n

vT2

iL2

iS

+

−

iTP2

iin

vL1+ −L

T3A B

C

D

Fig. 1. PPIBC with coupled inductors for current balancing