Physical Laboratory Model of Typical Load Torque Characteristics for Teaching Electric Drives

8/7/2019 Physical Laboratory Model of Typical Load Torque Characteristics for Teaching Electric Drives

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Proceedings of the 2008 International Conference on Electrical Machines Paper ID 1214

978-1-4244-1736-0/08/$25.00 2008 IEEE 1

Physical Laboratory Model of Typical Load TorqueCharacteristics for Teaching Electric Drives

Goran Rovian, Tanja Vei, Damir arkoFaculty of Electrical Engineering and Computing

Department of Electric Machines, Drives and AutomationUnska 3, 10000 Zagreb, CroatiaTel: (+385 1)-6129-613, fax: (+385 1)-6129-705

e-mail: : [email protected], [email protected], [email protected]

Abstract-The realization of different load torquecharacteristics in the laboratory for the purpose of teachingelectric drives is presented. The torque characteristics with linearand quadratic dependence on speed are achieved by mechanicallycoupling the induction motor with a DC generator connected to aresistor. By controlling the excitation current of the DC generatordepending on the measured speed the desired torquecharacteristics can be achieved. This laboratory setup replaces theactual loads like centrifugal pumps, fans or brakes based onviscous friction.

I. INTRODUCTIONThe realization of various load torque characteristics in a

laboratory for teaching electric drives is often a problem.Usually an abundance of electric machines of various types canbe found in the laboratory, but common loads like fans orcentrifugal pumps with torque characteristics dependent onspeed are difficult to install and utilize. However, for thepurpose of teaching students the basic principles of electricdrives the torque characteristics can be simulated using a DC

generator connected to a resistor and coupled to an inductionmotor powered from a frequency converter. Since torqueproduced by the DC generator is dependent on the armatureand field current, and in turn the armature current is dependenton speed and the field current, it is possible simply bycontrolling the field current supplied from the regulated currentsource to achieve the desired torque characteristic dependenton speed. The drawback of this approach is limited dynamicssince field current cannot be controlled rapidly, but this can beovercome by setting the sufficiently long acceleration ordeceleration time of the drive.

II. REALIZATION OF LOAD TORQUE CHARACTERISTICSAn old elevator drive consisting of a 26 kW induction motor,

10 kW DC machine and 2.8 kW DC exciter, all coupled on thesame shaft, has been used. The induction motor is poweredfrom an ABB ACS 600 AC drive and the voltage from the DCexciter is used as a signal for speed measurement. The fieldcurrent of the DC machine is supplied from controlled currentsource SIMOREG E300/22. For data acquisition andprocessing Iotech Personal Daq/3000 AD converter with

Dasylab 7.0 software is used. The scheme of the laboratorysetup is shown in Fig. 1, while Fig. 2 shows the actuallaboratory setup.

(a) Elevator drive

(b) Data acqusition

Fig. 2 The actual laboratory setup

Fig. 1 Scheme of the laboratory setup


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Proceedings of the 2008 International Conference on Electrical Machines

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0 500 1000 15000

50

100

150

200

250

300

350

Speed (rpm)

Vrm

s-motor(V)

Fig. 4 Voltage change during start-up of AC motor drive, scalar mode

The RMS values of current and voltage during start-up areshown in Fig. 5 together with motor power factor. The signalsof AC motor voltage and current in steady state for quadratic

load characteristic are shown in Fig. 6.

0 5 10 150

100

200

300

400

Time (s)

RMSvalues

0 5 10 150

5

10

15

20

Vrms-motor (V)Irms-motor (A)

(a) voltage and current

0 5 10 15-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Time (s)

Cosphi

cos phi - motor

(b) power factor

Fig. 5 RMS values of motor voltage and current, motor power factor duringstart-up - quadratic load characteristic

15 15.01 15.02 15.03 15.04 15.05 15.06 15.07 15.08

-500

-400

-300

-200

-100

0

100

200

300

400

500

Time (s)

M

otorvoltage

Vmotor (V)

(a) voltage- quadratic load characteristic

15 15.01 15.02 15.03 15.04 15.05 15.06 15.07 15.08-30

-20

-10

0

10

20

30

40

Time (s)

Motorcurrent

Imotor (A)

(b) current quadratic load characteristic

Fig. 6 Motor voltage and current signals in steady state - quadratic loadcharacteristic

IV. CALCULATIONS BASED ON MEASURED VALUESTwo tests are carried out: with linear and with quadratic load

torque characteristic.To check if desired load torque characteristics are correctly

realized, simple calculation can be done, knowing equation fortorque equilibrium in dynamic behaviour of rotating machines

= +M L

dT T J

dt

(5)

where J is the polar moment of inertia, known from

previously conducted tests or from motor data.The second addend on the right-hand side of (5) is the

acceleration torque which represents the difference betweenmotor torque developed on the shaft and the load torquedeveloped by the DC generator.

The load torque is calculated form (2) using measured valuesof Ia, If and ct. The estimate of the motor torque is obtainedusing the analog output of the frequency converter. Using the


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value of load and motor torque it is possible to calculate theacceleration torque and polar moment of inertia.

All torque characteristics are approximated by polynomialsof degree 7 or higher resulting with smooth curves (Fig. 7)which are needed for further differentiation. In the signal ofmotor torque there is an unexpected hump for speed below200 rpm which is not present in load torque. This can beexplained by frequency converter error when estimating motor

torque for speed near zero which could be attributed to staticfriction. It can be noticed in Fig. 7 and in Fig. 8 that at steadystate the motor and load torque are the same, which is expectedsince acceleration torque drops down to zero once the steadystate speed is reached.

The acceleration torque is calculated according to (5) as adifference between the motor torque and the load torque. Thusthe same error is present for speed near zero as in the motortorque. This error will be neglected in further procedure andthe calculus is made for all speeds higher then 200 rpm untilreaching the steady state near 1455 rpm.

To determine the polar moment of inertia experimentally,

the acceleration torque is to be divided by a derivative of the

0 5 10 150

20

40

60

80

100

120

140

160

Time(s)

Torque(Nm)

MotorLoadAcceleratingMotor - polynomial fitLoad - polynomial fitAccelerating - polynomial fit

(a) linear

0 5 10 150

20

40

60

80

100

120

140

160

Time(s)

Torque(Nm)

MotorLoadAcceleratingMotor - polynomial fitLoad - polynomial fitAccelerating - polynomial fit

(b) quadratic

Fig. 7 Measured and calculated torques compared with polynomialapproximations

drives angular speed. Fig. 9 shows a time graph of rotationalspeed and angular acceleration. Only values for speed higherthen 200 rpm are considered. The result of calculation is shownin Fig. 10. During start-up there are variations of polar momentof inertia Jbetween 0.75 and 1.75 kgm2 for linear, and between0.51 and 1.94 kgm2 for quadratic load torque characteristic.Calculating the average ofJ from these two tests gives 1.19kgm2 and 1.13 kgm2 respectively.

Calculating the mean value ofJfrom both experiments givesthe average value of 1.16 kgm2, which is 27 % higher then 0.91kgm2 determinate from the slowdown test at no-load.

200 400 600 800 1000 1200 1400 16000

20

40

60

80

100

120

140

160

180

Speed (rpm)

Torque(Nm)

MotorLoadAccelerating

(a) linear

200 400 600 800 1000 1200 1400 16000

20

40

60

80

100

120

140

160

180

Speed (rpm)

Torque(Nm)

MotorLoadAccelerating

(b) quadratic

Fig. 8 Measured and calculated torques as function of speed


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0 2 4 6 8 10 12 14 16-500

0

500

1000

1500

2000

Time (s)

Speed(rpm

)anddw/dt(rpm)

0 2 4 6 8 10 12 14 16-5

0

5

10

15

20

Speed (rpm)dw/dt (rpm/s

(a) linear

0 2 4 6 8 10 12 14 160

500

1000

1500

Time (s)

Speed(rpm)anddw/dt(rpm

)

0 2 4 6 8 10 12 14 160

10

20

Speed (rpm)dw/dt (rpm/s

(b) quadratic

Fig. 9 Speed and angular speed derivation

V. CONCLUSIONThis paper shows how various load torque characteristics

can be generated by controlling the field current of a DCgenerator for the purpose of teaching students the fundamentalsof electric drives. This laboratory model is simple in itsimplementation and is suitable for educational purposes. Itsmain advantage over virtual laboratories based on simulationsis the opportunity for the students to work with real electricmachines, AC drives and equipment for measurement and dataacquisition.

It is confirmed that it is possible to simulate various load

torque characteristics using basic static behaviour of a DCmachine. The torque developed on its shaft is proportional tothe product of its armature current, field current and the back-emf constant.

This principle cannot be used to realize load torquecharacteristics where torque is present at speed near zero. Atzero speed there is no voltage induced in the DC machine todrive the armature current and hence the load torque cannot beproduced.

5 6 7 8 9 10 11 12 13 14 150

0.5

1

1.5

2

2.5

Time (s)

J

(kgm

2)

J (kg m2

(a) linear load torque characteristic

900 1000 1100 1200 1300 14000

0.5

1

1.5

2

2.5

Speed (rpm)

J(kgm

2)

J (kg m2)

(b) quadratic load torque characteristic

Fig. 10 Graph of polar moment of inertia during start-up

Two tests with different load torque characteristics (quadraticand linear) have been carried out showing similar behaviour ofcalculated polar moment of inertia. This indicates that a goodestimate of motor and load torque has been achieved. The onlynoticeable problem is at speed near zero where frequencyconverter fails to correctly estimate the motor torque.

REFERENCES

[1] Chen Yongjun, Huang Shenghua, Yang Xiongping, Li Junjie, Modelsand Developing of Load Torque Simulator with Permanent MagnetSynchronous Motor for Ship Electric Propulsion, Proceedings of UPEC'06, Vol 2, pp. 724-728, 6-8 Sept. 2006

[2] Standard Application Program 5.x for ACS 600 Frequency Converters,ABB Industry Oy, 1998.

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Physical Laboratory Model of Typical Load Torque Characteristics for Teaching Electric Drives