induction motor simulation

ECE-P 352 Experiment 4 Induction Motors Tests Using Simulink Modified (1/21/05)

S.A./C.N.

1

Induction Motor Tests Using Simulink

Objectives: The main objective of this simulation is to obtain induction motor equivalent circuit parameters by simulating four induction motor tests. This part of the lab consists of the following simulations:

1. DC test for stator resistance (DC_test) 2. No load test (no_load) 3. Variable frequency blocked rotor test (blocked_rotor) 4. Load test (load_test)

Simulations are designed to follow the actual hardware experiments as closely as possible. That will give you a chance to compare the simulation results to those of the actual experiment. For the tests, we use the Matlab Power System Blockset and Simulink which provide models of power systems such as induction motors, transformers, etc. A Simulink diagram for each test will be provided during the experiment. The induction motor used in these simulations has the following nameplate information and the equivalent circuit is given in Fig. 1.

Nameplate Information: Rated Power: 5 HP (5x746 W) Rated Voltage: 208 V Rated current: 15.7 A Rated Frequency: 60 Hz Rated Speed: 1735 rpm Number of poles: 4 Y-connected

R1 X1 X2

XmV R2(1-s)/s

R2

Figure 1: Per-phase equivalent circuit of an induction motor

Equivalent circuit parameters:

mH 2.31L and 551.0R mH, 926.1LL , 428.0R m2211 =Ω===Ω=


S.A./C.N.

2

DC Test for Stator Resistance: The DC test will provide data that enables you to compute the stator winding resistance R1 shown in Fig. 1. The Simulink simulation diagram is depicted in Fig. 2. Please open the Simulink diagram called DC_test and spend some time to understand the diagram. As shown in Fig. 2, a DC voltage source is applied to the phase A and B of the induction motor through a series RC branch while the phase C is grounded. Observe that the diagram has an induction motor block. Please double-click on the induction motor to see its parameters. These are the parameters that you will determine with induction motor tests and compare to hardware experiment results. For simulation, please follow steps below:

1. Double click on the DC voltage source to check if it is equal to 120 V. 2. Make sure the mechanical load Tm to the induction motor is exactly zero. 3. Run the simulation (Simulation/Start, final time is set up to 5 seconds) 4. Obtain the DC voltage VDC across the phases A and B of induction motor through the Voltage Display.

Read the voltage value on the voltage display after the simulation is over and record it. 5. Obtain the DC current IDC going through the DC source from the Current Display after the simulation is

over and record it. 6. Repeat the steps 3-5 for two different values of resistance connected to the DC voltage source. The

resistance values are 12 ohms and 4 ohms. In order to change resistance, double click on the Series resistance and change the value of the field named Resistance R (ohms).

7. For your report provide the values of the DC voltages and DC currents 8. Compute the stator winding resistance for each data set with the following formula:

DC

DC1 I2

VR =

8. Explain why such a formula for R1 works. 9. Take the average of the three values of R1. 10. Record R1 for comparison to your hardware results.


S.A./C.N.

3

Figure 2: Simulink diagram for the DC test

No-Load Test: The no-load test on the induction motor measures the rotational losses of the motor and it is able to evaluate its magnetizing current. In this test, a rated balance AC voltage (120 V rms per-phase) with rated frequency (60 Hz) is applied to the stator and rotor runs without any load. The Simulink diagram for the no-load test is given in Fig. 3. We use the same induction motor as in the DC test. Observe that a Y-connected 3-phase ideal voltage (phase A, phase B and phase C) source is connected to the stator windings. In order to find out the parameters of voltage source, please double click on any of the voltage sources. Do not change any parameters of the voltage sources. Notice that a zero mechanical load is applied to the rotor of the induction motor (input terminal Tm) to simulate to the no-load condition. Some measurement blocks have been added to the diagram to measure some of the electrical and mechanical quantities. Of those are: Real and reactive power for phase A (phase_A_power), the rms currents of the phases (abc_rms_currents), mechanical speed (mech_speed, Wm), electrical torque (elect_torque, Te) etc. Using the data, you will able to compute the rotational losses of the motor and the sum of the stator leakage reactance and magnetizing reactance ( m1 xx + ). For simulation, please follow the steps below:

1. Run simulation (Simulation/Start) 2. Double click on the powergui (blue box on the left) to obtain per-phase rms voltage of phase A, say VA 3. Obtain the phase A rms current, say IA from the phase A current display 4. Double click on the power_scope (orange color) to see the waveforms of phase A real and reactive input

powers. Note that purple color is the reactive input power while yellow color is the active input power. In order to get the numerical values, type phase_A_power in the MATLAB workspace. The first component of the last row is the phase A input real power, say PA and the second component is the phase A input reactive power, say QA

5. Double click on the scope (red color) called Wm to see the waveform of the mechanical speed in rad per second. In order to get the numerical value, type mech_speed in the MATLAB workspace and again the last row is the mechanical speed in rad per second, Wm.

6. For your lab report:

• Provide all the data in table format such as:

VA (V)

VB (V)

VC (V)

IA (A)

IB (A)

IC (A)

PA (W)

QA (Var)

Te (Nm)

Wm (rad/second

) • Compute the slip speed (s %) in no load condition. • Compute three-phase total-input real power: A3 P3P =φ

• Compute total stator copper losses: 12ASCL RI3P = (R1 is available from the DC test)

• Compute the rotational losses: SCL3micsW&Fcorerot PPPPPP −=++= φ

• Compute the sum of the stator leakage reactance and magnetizing reactance ( m1 xx + ) with two different formulas given below:

2A

Am1

A

Am1 I

Qxxor

IV

xx =+=+

• Explain the derivations of these equations in your report.


S.A./C.N.

4

• Show that you understand these equations by giving a short explanation of how they are derived. • Compare your results to the results of hardware experiment. Which formula gives a result close to the hardware experiment? Why do you believe that this is the case?

Figure 3: Simulink diagram for the no-load test

Variable Frequency Blocked Rotor Test The blocked rotor test (locked rotor test) is performed to determine the equivalent circuit parameters of an induction motor. This test corresponds to the short-circuit test on a transformer. In this test, the rotor is blocked so that it cannot move, a voltage less than the rated voltage is applied to the motor. The resulting current, voltage and power measurements enable us to compute the induction motor parameters. Fig. 4 shows the Simulink diagram for the blocked rotor test. We use the same induction motor parameters as the no-load test. However, in order to simulate blocked rotor condition we set the inertia of the rotor to infinite. Please double click on the induction motor block to see its parameters including the infinite (inf) inertia. Make sure that you understand all the blocks and their properties in the Simulink diagram before you run the simulation. In this test, the rotor is locked. A three-phase AC voltage is applied to the motor and adjusted to an appropriate value so that the current flow of each phase is equal to its rated value. Recall that the rated current is 15.7 A. You will run the simulation at various frequencies and obtain data on phase A current (IA), phase A rms voltage (VA) and phase A input real and reactive powers (PA, QA). For simulation, please follow the steps below:


S.A./C.N.

5

1. Double click on each AC voltage source and set the peak amplitude and frequency to 42 V and 70 Hz.

Do not change the phase. 2. Double click each of the following blocks: power (light blue box above the motor), phase A signal rms

(green box above the motor) and signal rms to the right of the motor and set their frequency attributes to 70 Hz.

3. Run simulation (Simulation/Start) 4. Double click on the powergui (blue box on the left) to obtain per-phase rms voltage of phase A, say VA

Make sure that you select 70 Hz in the powergui window to get the rms voltage of phase A 5. Obtain the phase A rms current, say IA from the phase A current display. Make sure that it is

approximately equal to the rated current, 15.7 A. If not, you should adjust the voltage source such that you will get IA close to the rated current.

6. Double click on the power_scope (orange color) to see the waveforms of phase A real and reactive input powers. Note that purple color is the reactive input power while yellow color is the active input power. In order to get the numerical values, type phase_A_power in the MATLAB workspace. The first component of the last row is the phase A input real power, say PA and the second component is the phase A input reactive power, say QA

7. Double click on the scope (red color) called wm to see the waveform of the mechanical speed in rad per second. It must be exactly zero since the rotor is blocked.

8. Record all the data at this frequency, f = 70 Hz 9. Repeat the steps from 1-8 for the following frequencies: 60 Hz, 45 Hz, 30 Hz, and 15 Hz. At each

frequency, you should adjust the peak amplitude of the each AC voltage source such that the resulting phase A current (check the phase A current display) should be around 15.7 A. For example peak amplitude should be around 37-38 V for 60 Hz. Make sure that whenever you change the frequency of the AC source, you must change the frequency attributes of the following blocks: power (light blue box above the motor), phase A signal rms (green box above the motor) and signal rms to the right of the motor.


• Provide all the data at each frequency in a table format such as:

Test Frequenc

y (Hz)_

VA peak (V)

VA rms(V)

IA (A)

PA (W)

QA (Var)

wm (rad/second

)

70 60 45 30 15

• At each frequency compute RLR and XLR using the following formulas:


S.A./C.N.

6

Hz 60f ;Xff

XXX

IQ

X and IP

R

;OR

sinZjcosZXjR Z,IV

PcosPF ,

IV

Z

ratedLRtest

rated21LR

2A

ALR2

A

ALR

LRLRLRLRLRAA

A

A

ALR

=′=+=

=′=

θ+θ=′+==θ==

• Take the average of XLR and RLR • Compute R2:

,RRR 1LR2 −= (Note that you already know R1 from the DC test) • Compute X1 and X2:

LR21 X5.0XX ==

• Compute Xm (Recall that m1 xx + is known from the no load test). • Compare your results with the hardware experiment and draw the per phase equivalent circuit of the induction motor indicating the parameter that you have computed.

Figure 4: Simulink diagram for variable frequency blocked rotor test Load Test:


S.A./C.N.

7

In this simulation, a rated voltage is applied to the stator through a Y-connected AC voltage source. Recall that the per phase rms voltage is 120 V. Therefore; we choose the peak amplitude as 170 V for each AC voltage source. Fig. 5 shows the Simulink diagram for the load test. Observe that the induction motor block has an input terminal labeled as Tm. Through this terminal you will be to put different mechanical load to the shaft of the motor. The mechanical load Tm is specified in terms for torque (N.m). You will run the simulation for various values of Tm and you will study how the mechanical speed, slip speed, output power, and motor efficiency change with the load. For simulation, please follow the steps below:

1. Check the peak amplitude and the frequency of the each voltage source. They should be 170 V and 60 Hz. 2. Set the mechanical load Tm to 5 N.m 3. Run simulation (Simulation/Start) 4. Double click on the powergui (blue box on the left) to obtain per-phase rms voltage of phase A, say VA 5. Obtain the phase A rms current, say IA from the phase A current display 6. Double click on the power_scope (orange color) to see the wave forms of phase A real and reactive input

powers. Note that purple color is the reactive input power while yellow color is the active input power. In order to get the numerical values, type phase_A_power in the MATLAB workspace. The first component of the last row is the phase A input real power, say PA and the second component is the phase A input reactive power, say QA

7. Double click on the scope (red color) called wm to see the waveform of the mechanical speed in rad per second. In order to get the numerical value, type mech_speed in the MATLAB workspace and again the last row is the mechanical speed in rad per second, wm.

8. Record all the data. 9. Repeat the steps 1-7 for the following mechanical loads Tm = 10, 15, 20, 25, 30 and 35 Nm


• Provide the data at each load level in table format such as:

Mechanical Load, Tm

(Nm)

VA rms (V)

IA (A)

PA (W)

QA (Var)

wm (rad/second

) 5

10 15 20 25 30 35

• Compute the output power at each load level: mmout TP ω=

• Compute the total input power at each load level: Ain P3P =

• Compute the efficiency of the motor at each load level: 100PP%

in

out=η

• Compute the slip at each load level:1800

n1800s m−=

• Construct the following plots: o nm vs. Pout o Tm vs. Pout o IA vs. Pout o Tm vs. s


S.A./C.N.

8

Figure 5: Simulink diagram for the load test

induction motor simulation

Documents