Electronic Torque Meter for Low-Power Induction Motor Applications

Transcription

1 Electrical and Power Engineering Frontier Mar. 04, Vol. Iss., PP. -7 Electronic orque Meter for Low-Power Induction Motor Applications Kamen Yanev *, Cheddi Kiravu Department of Electrical Engineering, University of Botswana Private Bag 00, Botswana * yanevkm@yahoo.com cheddi@brob .co.bw Abstract-An important parameter for a drive system is its developed torque. In practice up to now, torque-speed converters or torque-sensors are usually used to determine the developed torque. hese devices are costly and in many cases difficult to fix to a motor shaft due to mechanical or constructive constraints. he research paper presented here proposes an alternative indirect method for motor torque assessment. he method focuses on low power induction motor applications and is based on the evaluation of the power factor considering its variation with motor load. he lowest power factor is at no-load operation, and the highest value occurs at rated motor load. An electronic circuit for implementing the proposed method is suggested. he experimental results based on known conventional torque measurement methods are used to benchmark the corresponding results obtained via the proposed torque measurement method. he results corroborate each other. Keywords- Induction Motor Characteristics, Power Factor, orque evaluation, Electronics; I. IRODUCIO he torque developed by three-phase induction motors is a very important factor for any drive system. orques cannot be measured directly. Usually speed-torque converters or torque sensors are used to determine the torque of such motors [,, ]. hese devices are fixed on the motor shaft and produce signals proportional to the speed. It is not always possible to attach them on the motor shaft due to mechanical or constructive constraints [4, 5]. In addition, these devices are quite expensive and there are limitations to their accuracy. he method of torque assessment proposed in this paper does not require any speed converters. he torque is evaluated from basic motor characteristics like the output power P out, current I, power factor cos φ and efficiency η. It is well known that at escalating motor loading, the speed n reduces, but the input and output power P in, P out, the current I, the power factor cos φ, and the efficiency η, all increase. At full load the a.c. motor parameters are at their nominal/rated values: P, I, cos φ, and η as indicated on the motor s manufacturer data plate. At no-load, the power factor and the current are much lower than the rated values. Accordingly, cos φ 0 < cos φ, and I 0 < I. If not specified, the parameters like I and cos φ could be easily measured or calculated to reveal the induction motor characteristics as shown in Fig.. cosφ, n, I, Pin, Pout, η , pu cosφ n/0000 I/0 Pin/000 Pout/000 η Fig. Basic induction motor characteristics - -

2 Electrical and Power Engineering Frontier Mar. 04, Vol. Iss., PP. -7 II. EXPERIMES Experiments were conducted on two cage motors whose rated parameters are shown in able. ABLE : CAGE MOOR PARAMEERS Parameter Motor Motor Power, W Phase Voltage, V 0 40 Current, A Speed, rpm PF cosφ PF cosφ orque, m he experimental results are shown in able and able. he PF- characteristics for motors and are shown in Fig. and Fig. respectively where the torque is expressed in per-unit (pu). ABLE : MOOR A V = 0V, F = 50 HZ, m /, pu n, rpm I, A P in, W P out, W cos φ ABLE : MOOR A V = 5V, F = 50 HZ, m /, pu n, rpm I, A P in, W P out, W cos φ PF , pu cos φ Line Appr Fig. Approximation of the PF- characteristic of motor - -

3 Electrical and Power Engineering Frontier Mar. 04, Vol. Iss., PP. -7 PF , pu PF Line Appr Fig. Approximation of the (PF-) characteristic of motor III. BASIS OF HE PROPOSED MEHOD he method is based on the evaluation of the segmented line approximation of the power factor-torque PF- characteristic. he power factor depends not only on the loading but also on the size of the motor. For motors with low power in the range of P < 500W, the following are true: he rated currents are smaller and the number of stator turns is bigger in order to produce the necessary flux. As a result the active coil resistances and copper losses are high, and efficiency is reduced. he air-gaps between rotors and stators cannot be reduced in proportion to the motor sizes. he air gaps are of the same magnitudes as those for large motors due to constructive and technological constraints. his causes high magnetizing and no-load currents. As a result the motors with low power in the range of 00 < P < 500 W are characterized by higher PF at no-load and lower PF at full load, also lower efficiencies and larger no-load currents are typically related to bigger motors in the rated parameter ranges [, 6], as shown in able 4. ABLE 4: YPICAL PARAMEER RAGES FOR LOW POWER MOORS Motor Parameter orque Description Parameter Motor Motor o-load Power Factor cos φ Rated Power Factor cos φ Efficiency η o-load Current Ratio I 0 / I he PF- characteristic is non-linear. he slope of the curve is high at low loading but decreases at higher loading. It could be accepted that the slope changes at a load torque = 0.5 considered as the turning loading point value. he power factor corresponding to this load is referred to as cos φ. Hence the PF- characteristic can be represented by two straight lines as shown in Fig. and Fig.. aking into account the area of low loadings for which 0 < <, the resulting equations are as follows: - o - o =, () o - o =. () - - -

4 Electrical and Power Engineering Frontier Mar. 04, Vol. Iss., PP. -7 aking into account the area of high loadings where < <, the resulting equations are: - - =, () - - ( - ) + ( - ) =. (4) ( - ) he rated full-load torque is pu. Equations () and (4) reveal that at any loading, the power factors cosφ, cos φ and cos φ could be used to provide an estimation of the torque developed by the motor. he power factors cos φ 0 and cos φ are assumed to be known from the manufacturer s data. he determination of the torque and the power factor cos φ at the turning point is critical for the accuracy of this method. From the studies of the experimental motor characteristics and from general experience, at half the rated loading = 0.5, the power factor increases up to the value of cos φ = 0.6. It is also essential to determine the angle φ between the phase voltage and current at any loading to enable the calculation of the corresponding torque. At = 0.5, equations () and (4) become: - o =, - o - =. (5) - ( - ) IV. PROPOSED ELECROIC MEHOD FOR IMPLEMEIG MOOR ORQUE MEASUREME A block diagram of the torque meter derived from implementation of equations () to (4) above is shown in Fig. 4. Voltage ransformer Rectifier Zero Comparator R-S rigger Current ransformer Rectifier Zero Comparator Filter Zero & Span Converter cos φ Meter (0.-0.6) Calculator orque Meter ( ) Zero & Span Converter cos φ Meter ( ) Calculator orque Meter (0.5-.0) Fig. 4 Block diagram of the torque meter he block diagram in Fig. 4 is implemented using the electronic circuit diagram shown in Fig. 5 that is used to simulate the measurement of induction motor torque at different power factors. A voltage and a current transformer take into account the supply voltage and current of the induction motor respectively. wo Zero Comparators produce pulses with phase difference corresponds to the phase lag φ between the voltage and the current. hese pulses control an R-S rigger, resulting in its square wave output voltage. he width of the square pulses is equal to the phase difference φ. hese pulses are filtered, resulting in a filter DC output voltage proportional to the angle φ

5 Electrical and Power Engineering Frontier Mar. 04, Vol. Iss., PP. -7 he filter output is applied simultaneously to two parallel channels in order to achieve better linearity in measuring the power factor and the torque. Each channel includes a Zero and Span Converter [7, 8], producing output signals directly proportional to the power factor cos φ. he first channel generates the readings of cos φ ranging from cos φ 0 to cos φ. hese readings are measured by the meter XMM while the second meter, XMM, measures cos φ within the range of cos φ to cos φ. Calculators and are incorporated into each channel [9], enabling the calculation of the torques and from the values cos φ, cos φ and cos φ using equations () and (4). he torque results at low loadings are displayed by XMM, the first torque meter while at the display on XMM4, the second torque meter, indicates the results at high loadings. V 7.07V 5.00V_rms 50Hz 0Deg V 7.07V 5.00V_rms 50Hz 6.87Deg 4 4 R D 50ohm 48 R D 50ohm 48 ZCA 74S R 680ohm ZCA 74S R4 680ohm C nf C nf S U SR_FF R5 68kohm R6 68kohm Q R ~Q R7 0kohm C uf R8 5kohm R9 0kohm U R0.5kohm U R 0kohm C4 uf R 50kohm R 5kohm R6 R7 R0 R R R kohm R4 7.97kohm 00.5kohm U4 R5 6kohm R8.kohm.kohm U5 R9.kohm 9kohm XMM U6 kohm R kohm V4 0.V R5 kohm kohm U7 Y X A V/V 0V V5 0.8V XMM V6 5V R8 5.8kohm R kohm R9 00kohm U8 R7 6kohm R0.kohm R.kohm U9 R.kohm R 9kohm XMM U0 R4 kohm R5 kohm V7 0.4V R7 kohm R6 kohm U Y X A V/V 0V V8 0.4V XMM4 Fig. 5 Electronic circuit diagram for the proposed induction motor torque meter V. PROCEDURE FOR HE PROPOSED IMPLEMEAIO he torque meter of Fig. 5 was used for torque measurements by simulating loads with different phase lags and thus power factors. he corresponding torques were read by the two properly calibrated instruments as indicated in able 5: ABLE 5: ORQUE AD POWER FACOR MEASUREMES FOR HE CASE OF MOOR AD MOOR Parameter Motor Motor 0.5 pu 0.5 pu cosφ cosφ cosφ

6 Electrical and Power Engineering Frontier Mar. 04, Vol. Iss., PP. -7 he torque meters could be calibrated and properly tuned with the aid of Calculator and Calculator for any other value of the loading turning point corresponding to torque. VI. RESULS FROM HE IMPLEMEAIO OF HE PROPOSED MEHOD One set of experimental results was obtained with the aid of a traditional torque meter. he second set of results was obtained by calculation with equation (5). Finally, the last set of results was obtained by means of a MULISIM-based electronic simulation, using the electronic circuit of Fig. 5. Comparisons of the experimental and the calculated results with the simulated results obtained from the proposed method are presented in able 6 and able 7. It is seen that the results corroborate each other closely. While the results of calculations and simulation are almost identical, the error between the simulation and experimental results at different loadings lies within the acceptable tolerance of less than 0% [8, 9, 0]. ABLE 6: COMPARISO OF RESULS FOR HE CASE OF MOOR cos φ, pu, pu, pu Error, % Experimental Calculated Simulated ABLE 7: COMPARISO OF RESULS FOR HE CASE OF MOOR cos φ, pu Experimental, pu Calculated, pu Simulated Error, % VII. COCLUSIOS he proposed method is applicable for automatic measurement of the torque developed by low power induction motors. he method is based on the evaluation of the power factor premised on the linearization of the motor s PF- characteristic that typically increases non-linearly with its loading. he method utilizes the nominal motor data indicated on the motor s nameplate. An electronic circuit of a torques-meter is proposed. It enables the measurement of the power factor, as well as the torque by evaluating the phase lag between the phase voltage and the motor current. he circuit of the torque meter is easy to be - 6 -

7 Electrical and Power Engineering Frontier Mar. 04, Vol. Iss., PP. -7 realized and calibrated. As a result, such torque-meters could be assembled and implemented in practice as well as be employed to advance the induction motors studies. his paper provides a theoretical and experimental analysis of the torque meter performance at different induction motor loads. he analysis results back up the results obtained from the actual realisation of the suggested practical electronic torque meter device. heoretical, experimental, and simulation results substantiate each other very closely. Whereas outcome result differences may be due to linearization of the PF- characteristic, the errors are however less than 0%, which is well within acceptable error tolerances. REFERECES [] John Hindmarsh, Electrical Machines and their Applications, Oxford, Pergamon Press, ISB , 99. [] Liang, Xiaodong; Ilochonwu, Obinna (Jan 0). Induction Motor Starting in Practical Industrial Applications IEEE ransactions on Industry Applications 47 (), pp. 7 80, 0. [] Keljik, Jeffrey (009). he hree-phase, Squirrel-Cage Induction Motor, Clifton Park, Y, USA, Delmar, Learning, pp. 5, ISB , 009. [4] Bozhilov G. Electrical Micro Machines, Manual for Design, echnika, Bulgaria, 006. [5] Jacob M., Industrial Control Electronics, Prentice-Hall International, Inc., London, 00, pp. -6. [6] Schilling D.L, Electronic Circuits Discrete and Integrated, McGraw-Hill Book Company, Singapore, 009. [7] Savant C.J., Roden M.S., Carpenter G.L., Electronic Design Circuits and Systems, Redwood, California, USA, he Benjamin/Cummings Publishing Company, Inc., ISB , 0. [8] Jamil Asghar, M.S., Speed control of wound rotor induction motors by AC regulator based optimum voltage control, he Fifth International IEEE Conference on Power Electronics and Drive Systems: , 00. [9] Syed. A. asar, Handbook of Electric Machines, ew York, McGraw-Hill Book Company, 987. [0] Jianfang Mao, Qunli Shang, Shanen Yu, Studies Review for orque Measurement on Electronic Actuator, Communications in Information Science and Management Engineering, Vol. Iss. 7, pp , July 0 AUHORS IFORMAIO Prof. Kamen Yanev started his academic career in 974, following a number of academic promotions and being involved in considerable research, service and teaching at different Commonwealth Universities around the world. Currently he works as Associate Professor in Control and Instrumentation Engineering at the Department of Electrical Engineering, University of Botswana. His major research is in the field of Control Engineering as well as in the subject of Electronics and Instrumentation. He has more than 9 publications in international journals and conference proceedings in the area of Control Engineering, Electronics and Instrumentation. Most of his latest publications and current research interests are in the field of Analysis of Control Systems with Variable Parameters and Robust Control Design. Prof. Yanev is a member of the Institution of Electrical and Electronic Engineering (IEEE), a member of the Academic Community of International Congress for Global Science and echnology (ICGS), a member of the Instrumentation, Systems and Automation Society (ISA), a member of the editorial board of the International Journal of Energy Systems, Computers and Control (IJESCC) at International Science Press, a reviewer for ACA Press (a Scientific and echnical Publishing Company) and a member of the Botswana Institute of Engineers (BIE). Cheddi Kiravu is holder of the Dipl.-Ing. ( Univ. ) in Electrical Power Engineering from the University of Erlangen- uremberg Germany since 98. He also holds a B.Sc. (Hons.) degree in Mathematics, Physics, and Didactics from the University of Dar es salaam. He is currently Senior Lecturer in the Electrical Engineering Department of the Faculty of Engineering and echnology and Acting Director of the Clean Energy Research Centre at the University of Botswana. His main research interest is computer modeling and applications in electrical power, electric machinery, and renewable energy systems