About the Experimental Results of an Electric Driving System Based on Asynchronous Motor

and PWM Converter

Petre-Marian Nicolae*, Dan-Gabriel St nescu* and Ioana-Gabriela Sîrbu*

* University of Craiova/Department of Fundamental Electrotechnics, Craiova, Romania, e-mail: pnicolae@elth.ucv.ro; dstanescu@elth.ucv.ro; osirbu@elth.ucv.ro

Abstract— The paper deals with special features related to the a.c. electric driving system of a trolley-bus. One presents the driving system consisting of an asynchronous motor and inverter and some aspects regarding the PWM inverter control. The tests over the manufactured asynchronous motor were performed at a frequency equal to 50 Hz (for the rated speed) and a frequency equal to 106.6 Hz corresponding to a synchronous speed equal to 3200 rpm.The inverter was separately tested, before its inclusion in the driving system and some of the tests results presented (heating tests for the main components, the behavior at the discontinuous variation of line voltage, a.o.). Some tests concerning the behavior of the entire system at forward and backward running regimes are also presented.

Keywords—Electric drives, PWM inverter, Asynchronous motor, Efficiency.

I. INTRODUCTION

The developing of public transportation systems in cities is a practice in many European countries. Its purpose is to reduce the transportation jams and intense pollution. A significant role is played by the public transportation systems that use electric energy in electro-mechanic drive systems. An example in this sense consists of the urban transportation systems by trolley-bus, where one can notice a trend in substituting the d.c. drive by a.c. drive. Traction converters for feeding asynchronous machines are for more than 20 years a dominant application for power semiconductors in the high-power range. The introduction of IGBT1exhibits advantages as: control through voltage, simpler driver technology, no snubber circuit, less complexity and increased efficiency for partial load, low switching losses and therefore higher possible switching frequency.

The driving alternative that uses choppers, with performances superior to those from the classic solution (d.c. motor and velocity control through resistances), due to the d.c. motors utilization, does not entirely eliminates the potential sources of fault (collector, brushes, contactors). One can provide the trolley-buses with intelligent electronic systems by substituting the classic d.c. driving systems through their a.c. counterparts (static power converters and asynchronous motors) in order to increase the reliability, the energetic efficiency and

1 This work was supported by ANCS Romania through the 4-th Program-Partnerships and under the Contract 71-145/2007

exploiting safeness and also to significantly reduce the maintenance and exploiting costs.

The application in traction exhibits requirements that exceed the standard industrial conditions.

II. ELECTRIC DRIVING SYSTEM CONTROL

The electric driving system control (Fig.1) is based on a structure described below.

The electronic control block is used to perform: the interface with the power electric circuits, the interface with the vehicle (running conditions), the prescription of electric quantities, the control and regulation, the determination of parameters on the vehicle, the acquisition of numerical signals and respectively the generation of signal for numerical control, diagnosis and protection.

The control equipment provides pulses to the inverter through driver circuits so as to provide continuous control for two quantities: statoric current and rotoric frequency. In Fig. 2 a block diagram of a IGBT driver is presented.

The function spectrum of these circuits mostly comprises:

- gate voltage generator; - input for VCEsat- monitoring, sometimes also input

for shunt or sense-emitter; - monitoring of too low supply voltage; - error memory and error feedback output ; - adjustable dead time generation of the TOP-driver.

Fig. 1. Driving system structure.

Line filter

Supply Network

Asynchronous Motor

Electronic control block

Block with diodes andthyristors

PWM Inverter

Auxiliary services

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978-1-4244-1742-1/08/$25.00 c© 2008 IEEE

These standard drivers do not provide a real potential isolation. For some variants, the control input may be configured for connection of opto-couplers or pulse transformers. Moreover, progress is being made in the development of fast optic couplers with power driver output which have already integrate supply under voltage- and VCEsat- or VDS (on)-monitoring. To achieve simple driver units, DC/DC-converter and few passive components merely have to be added [1].

With the growing variety of function and protection parameters in driver circuits, the assemblies necessary on the primary side also have to meet more sophisticated requirements, for example, input signal logic, short-pulse suppression, dead time generation, error memory and error evaluation and drive of the pulse transformers.

During turn-on, switching speed is limited by a ramping up of the control voltage, which allows a good compromise between soft recovery and low turn-on losses. During turn-off, a feedback-control of the collector voltage will provide a safeguard against a too high voltage [2]. A voltage divider in combination with analogue amplifier which is coupled to the gate-control voltage is used here. For short-circuit protection the collector voltage is compared to a reference voltage.

The system requires information on the angular velocity from the system so that the microcontroller should be able to perform a real time computation of the asynchronous motor control frequency as described by:

221 ffff N Δ+±= (1) where: f1- stator supply frequency; fN- frequency proportional to the vehicle velocity; f2- frequency of slip; f2: frequency correction; ± : +running; - braking. After the determination of the starting period, when

the motor’s terminals (Fig.3.) are submitted to the entire line voltage, one performs an automated flow weakening, so that the vehicle might reach full speed.

III. ELECTRIC DRIVING SYSTEM

In the brake regime, the control block controls the current through motor and the voltage in the intermediate circuit (as depicted by Fig.1). Depending on the voltages from the intermediate circuit and respectively from the supplying network, the braking can be either electrical-recovering when the braking energy is transferred in the supplying network, or electrical rheostatic when there aren’t any consumers on the line and the braking energy is dissipated along the braking resistances through the braking chopper.

The driving system’s design relies on the indirect vector control. The objective of the asynchronous motor‘s vector control is to allow it to be controlled just like a separately excited d.c. machine. This is achieved through transformations of variables to the coordinate reference frame that rotate along with the rotor flux.

An advantage of this type of control consists in the linearity of mechanical characteristics, respectively of the characteristic torque versus motor speed [3].

Further on there are given some features of the main elements from the driving systems. The inverter (Fig. 4) was designed considering the following rated data:

- supplying voltage: 750 V d.c. - maximum apparent power: 300 kVA

Fig. 3. Driving asynchronous motor.

Fig. 4. PWM Traction inverter.

Logic t

On

t

Off

t

+1

Umax

Over-voltage limitation

Over-current diagnosis

Collector

Emitter

Status feedback

Firing signal

Fig. 2. Block diagram of IGBT driver.

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- maximum active power: 200 kW - maximum phase current: 410 A - switching frequency: 2 kHz The designed motor should meet some special requests

owing to the particular features of urban electric drives: - rated current (RMS): 228 A - rated speed: 1479 rot/min - rated voltage: 500 V - power factor: 0.86 - efficiency: 91.5 - rated frequency: 50 Hz - maximum frequency: 106.5 Hz The following data were yielded during designing: - stator inner diameter: 520 mm - rotor inner diameter: 105 mm - air gap: 2 mm - stator phase resistance: 0.04054 - rotor phase resistance: 7.078 .10-5 - magnetization current: 40.11%

IV. EXPERIMENTAL RESULTS OF THE ELECTRIC DRIVING SYSTEM TESTING

A. Experimental Tests with Asynchronous Motor The motor designing was realized in co-operation with

S.C. Electroputere S.A. Craiova. To achieve an efficient design and manufacturing one performed numerical simulations (by means of a specialized MATHCAD software) [4]. This way the asynchronous motor characteristics were deduced. One could notice a good agreement between the designing parameters and those required for a motor used in the electric urban traction [5].

The tests over the manufactured asynchronous motor (from Fig.3) were done at S.C. Electroputere S.A. Craiova, on special stands that provide the possibility to make operational schema with energy recovering (for braking). The tests were performed at a frequency equal to 50 Hz (for the rated speed) and a frequency equal to 106.6 Hz corresponding to a synchronous speed equal to 3200 rpm. The supplying voltage was sinusoidal.

One considered the specifications of the standard EN 60034-1 (IEC 349-2). The tests results yielded are presented by Tables I and II.

B. Experimental Tests with PWM Inverter The realized converter was submitted to a series of tests

according to present standards and the results are presented below:

1) Heating tests for the main components of the inverter (according to EN 50207 par. 4.5.3.11)

The converter operated at rated parameters, the sub-ensemblies temperatures being lower than the maximum values presented in Table III.

2) Tests on the behavior at the discontinuous variation of line voltage (according to EN 50207 par. 4.5.3.21.)

The verification was performed at rated load. The inverter’s supplying voltage was varied in 2 steps:

- from 525 V d.c. to 1200 V d.c.; - from 1200 V d.c. to 525 V d.c. After tests performing one noticed that during the

discontinuous variation of the supplying voltage within the range 525 V d.c. ÷ 1200 V d.c., the converter operates normally, at prescribed parameters.

3) Test of the behavior to short lasting interruption of supplying voltage (according to EN 50207 par. 4.5.3.22.)

At the interruption of converter’s supplying voltage (750 V d.c.) , the converter entered in the state “STOP” and the IGBT-s control pulses were blocked.

When the supplying voltage reappeared the inverter starting cycle was replayed and the inverter supplied an output voltage with the frequency given by the motor’s angular velocity at the start time.

4) Tests on the behavior at start and restart moments (according to EN 50207 alin. 7.4.2.)

The tests were performed when the supplying voltage varies within the range 525 V d.c. - 1200 V d.c. The converter started directly when it got a supplying voltage within the range 525 V d.c. - 1200 V d.c. (according to EN 50207 par.7.4.2.), providing the specified voltages at the output.

TABLE I.PARAMETERS FROM TESTS AT FUNDAMENTAL FREQUENCY

Frequency - 50 Hz; Synchronous speed - 1500 rpm

Power (kW)

Line Voltage (V)

R1 (1500C)

( )

Current (A)

Speed (rpm)

Mn (N.m)

Slip (%) cos φ Efficiency

(%) Mmax (N.m)

155 500 0.0413 227 1479 1000 1.4 0.86 91.5 3800

TABLE II.PARAMETERS FROM TESTS AT A FREQUENCY OF 106.6 HZ

Frequency – 106.6 Hz; Synchronous speed - 3200 rpm

Power (kW)

Line Voltage (V)

R1 (1500C)

( )

Current (A)

Speed (rpm)

Mn (N.m)

Slip (%) cos φ Efficiency

(%) Mmax (N.m)

155 500 0.0413 200 3120 477 2.5 0.94 94 1097

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TABLE III. SUBENSEMBLIES TEMPERATURES OBTAINED AFTER THE HEATING

TESTS

No. Component Imposed temperature

Registered temperature

1 radiators <+70 OC 52 OC

2 filter’s capacities <+70 OC 50 OC

3 inductivities, transformers <+70 OC 92 OC

4 bind bars <+70 OC 50 OC

5) Tests on the behavior at short-circuit (according to EN 50207 par. 7.4.3.)

After these tests one could see that the equipment did not suffer any damages during external short-circuits, along the a.c. current outputs. The inverter was set in the fault state „SHORT-CIRCUIT”, the IGBT control pulses were blocked and the inverter starting was accomplished through the „RESET” procedure.

6) Tests on the overload operation abilities (according to EN 50207 par. 7.4.6.)

The converter can provide on the a.c. output a current of 300 kVA for 5 sec. with no damages and no critical temperature overcoming.

The converter provides at the output a maximum current limited by 480 A a.c.+3%.

The tests revealed that the equipment supplies the over-load applied on the a.c. output for 5 sec. and limits the output current to the prescribed value (480A).

C. Experimental tests of the entire driving system testing Afterward a series of tests were made for the entire

driving system according to the electric schema presented in Fig. 5. with asynchronous motor supplied through PWM inverter.

The motor speed can be estimated from the time length of a pulse received from the position sensor as Halls, using a high frequency timer to count between two consecutive transitions of a pulse from the position sensors.

The accuracy of this speed estimator depends on the maximum motor speed max, the number p of pole pairs of the asynchronous motor and the frequency of the count timer, fclk. The above are descried by:

Ω⋅

⋅⋅=max

22logpfAcc clk

bitsπ

. (1)

For smaller speeds important dead times between the speed control loop and the speed estimator will arise. A possible correction of this effect can be implemented. One supposes that the speed estimate obtained from the time length of the i-th pulse is 0i. This estimate, average speed over the previous pulse, can be quite far from the real speed real of the asynchronous motor, mainly during transients of motion.

Some correction terms will be used to obtain a better speed estimate corr, between two consecutive pulse, using the relation:

iqiicorr CC ++Ω=Ω 00 (2)

where Ci0 is a constant term modeling the static load torque and Ciq is a constant modeling the torque constant [6], [7]. The measured phase currents, ia and ib, are transformed into the stator reference frame components ids and iqs.Then, based on the rotor field position information (computed from the rotor position and the corrective term given by the slip calculator book), these components are transformed into the rotor frame direct and quadrature components id and iq.

Fig. 5: Scheme of the asynchronous motor operating in torque/current mode

d,q

a,b

d,q

a,b

INVERTER

M

PI Current controller

PI Current controller

Speed transducer

Slip Calculator

m

i*d

i*q

Reference Generator

v*a

v*b

v*c

v*q

v*d

ia

ib

id

iqSpeed Estimator

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The current controllers are PI discrete controllers. The inverse coordinates transformation is used for computation of the phase voltages references v*a, v*b and v*c applied to the inverter, starting from the values of voltage references computed in the d and q reference frame (v*q, v*d) [8]. Thus, the six full compare PWM outputs of the microcontroller are directly driven by the program, based on the reference voltages. For frequency over the rated value, the motor flux must be reduced in order to rotate the asynchronous motor at speeds greater than the rated value.

To calculate the parameters required for the implementation of flux weakening procedure one must know the magnetization current, im. Using im one can compute the parameters idmax and idmin used in the microcontroller specific language. The maximum magnetization current is calculated whereas the minimum value is imposed by user.

The following quantities are used for the magnetization current calculation: motor rated voltage U1, rated frequency f1, phase magnetization inductance Lm, phase stator resistance Rs and electric time-constant . The electromagnetic torque is calculated with the equation:

qdm

e iiLLpT ⋅⋅⋅⋅=

2

2

23 (3)

where p is the number of pole pairs, Lm is the magnetization inductance, L2 is the rotor equivalent self inductance, id is the stator reactive current and iq is the stator active current in the rotor frame.

Fig. 6 depicts the asynchronous motor estimated and measured resistance. The signal denoted by “1” corresponds to the estimated resistance and the signal denoted by “2” corresponds to the asynchronous motor measured resistance.

The resistance from rotor reported to stator was calculated using the following equation:

r

Err I

UksR2

12

⋅⋅= (4)

where sr is the rated slip, kE is a constant, I2r is the rotor current reported to the stator.

During the motor idle starting at relative high acceleration one can notice the resistance diminishing up

to the moment when the motor inertial couple is cancelled when this tends to reach the minimum value orresponding to the limited maxim current supplied by the inverter.

In an ideal case both graphics must fit (the mathematic model of the control block must be very close to reality).

Fig. 7 depicts the waveform for the " forward " sense. The signal denoted by “1” stands for the motor axis reference speed whereas that denoted by “2” represents the real speed.

Motor starting was accomplished under the conditions of its blocking along axis in the first moment, followed by its sudden release.

This thing emphasizes the answer of the driving system inverter-motor in this situation.

One can notice that when the brake used to block the motor axis is released the speed tends to reach the reference speed and that after an over-regulation and a damped oscillation both values become equal.

Both the speed maximum magnitude and the oscillation period can be controlled by means of the parameters of speed regulator modeled by the inverter’s control software.

Fig. 8 depicts the waveforms for the “backward” rotation sense considering that: the signal denoted by “1” stands for the motor’s axis reference speed and the one denoted by “2” stands for the real speed. The characteristics speed versus time are identically shaped with those from Fig. 7, but they have reversed senses.

0.0 0.5 1.0 1.5 2.0

Fig. 7. Waveforms for the motor’s axis reference speed (“1”) and real speed (“2”) for the rotation sense “forward”.

Rad

/s

0.0 0.5 1.0 1.5 2.0 2.5

Acquisition Time (s)

1

2

250.0 200.0 150.0 100.0 50.0

0.0 -50.0

-100.0 -150.0 -200.0 -250.0

Fig. 6: The motor estimated and measured resistance

Acquisition time (s)

12

Fig. 8. Waveforms for the motor’s axis reference speed (“1”) and real speed (“2”) for the rotation sense “backward”.

Rad

/s

0.0 0.5 1.0 1.5 2.0 2.5

Acquisition Time (s)

1

2

250.0 200.0 150.0 100.0 50.0

0.0 -50.0

-100.0 -150.0 -200.0 -250.0

2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008) 1185

V. CONCLUSIONS

The experimental determinations made on stand concerning the asynchronous motor testing revealed the existence of a power reserve that satisfy the requirements of the present application. Considering the numerical simulations results and all the factors that might influence the asynchronous motor behaviour, one can see that is possible to perform an optimised design, reducing the sizes with (8-10)% for the same characteristics of the motor [4].

The tests over the motor and inverter, made separately, were performed considering the European standards (EN 60034-1 and EN 50207). The experimental waveforms verified the validity and correctness of the driving control scheme.

The employed technology and tests were in agreement with the operational conditions of the electric driving system that is to be done.

The driving systems based on the asynchronous motor will eliminate most of the disadvantages presented by the d.c. driving systems due to the inverter control scheme based on the vectorial control.

A superior energetic efficiency will appear because: • the electric braking might be regenerative until stop

and the energy dissipation over the breaking resistances should no longer be present;

• the asynchronous motor has identical characteristics for both regimes: operating as motor and during braking;

• the asynchronous motor exhibits insignificant losses and no thermal problems; the recovered braking energy can used by auxiliary services when no other consumers are present on the line.

Considering the European trends – for the development of environmental - friendly urban transportation system in big cities, one can conclude that the developed technological solution can be improved

considering both the asynchronous motor and respectively the PWM inverter (using last - generation electronic components).

ACKNOWLEDGMENT

The authors thank to their partners from S.C. INDAELTRAC S.A. and S.C. ELECTROPUTERE S.A. Craiova, who helped them to develop and implement this technological solution.

REFERENCES

[1] R. Lorentz, “Advances in electric drive control”, Proceedings of IEEE International Electric Machines and Drives Conference,Seattle, Washington, USA, 1999, p.9-16

[2] N. Ravisekhar Raju., “Rippled dc link twelve-pulse converter with sinusoidal ac waveform”, Proceedings of IPEC 2005 - The International Power Electronics Conference, Toki Messe, Session Paper S63, pp.16-20, Niigata, Japan, April 4-8, 2005.

[3] D. Mori and T. Ishikawa., “Force and vibration analysis of a PWM inverter-fed induction motor”, Proceedings of IPEC 2005 - The International Power Electronics Conference, Toki Messe, Session Paper S12, pp.9-15, Niigata, Japan, April 4-8, 2005

[4] P.M. Nicolae, D.G., Stanescu and V.D.,Vitan, “Designing, realization and test of asynchronous motors used for trolley-buses electric drive”, Proceedings of Iasi Polytechnic Institue, Tomul LII(LVI), Fasc.5.A, Electrotechnics, Energetics, Electronics, Oct. 2006, pp. 50-55

[5] M. El-Hami and S. Abu-Sharkh, “A general design model for electric motors”, Proceedings of IEEE International Electric Machines and Drives Conference, Seattle, Washington, USA,1999, p.186-188

[6] M. Harakawa, T. Nagano and E. Hayasaka., “Improvement of vector control in consideration of voltage saturation”, Proceedings of IPEC 2005 - The International Power Electronics Conference,Toki Messe, Session Paper S65, pp.8-14, Niigata, Japan, April 4-8, 2005

[7] U. K. Madawala and C. A. Baguley., “Transient modeling and parameter estimation of field aligned starting”, IEEE Trans. On Energy Conversion, vol. 23, pp.15-24, March 2008

[8] B. Singh and V. Verma., “An indirect current control of hybrid power filter for varying loads”, IEEE Trans. On Power Delivery,vol. 21, pp.178-184, January 2006

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