DETERMINING THE ASYNCHRONOUS MACHINE’S STEADY STATE MODEL PARAMETERS
INDUCTION MOTOR WITH NO-LOAD

Section Content

5.1 Objective

This exercise is presented in two parts:

Part 1 deals with implementing standard reduced power tests that allow users to define the asynchronous machine’s steady-state model parameters.
Nominal power load tests for high-power asynchronous machines are difficult to prior to on-site commissioning because they require high power test facilities.
However, it is preferable to estimate load tests using asynchronous machine models developed in a lab through low power testing.
This exercise includes 3 identification tests: (i) no-load, (ii) locked rotor and (iii) winding resistance measurement.
The results will allow users to calculate the parameters of the steady-state single phase equivalent circuit of the asynchronous machine.

Part 2 will help users master the variable voltage startup of the induction motor with no load and energized by a constant frequency and voltage AC grid.

5.2 Initialization of the Setup

When the simulator is started up, the initial settings on screen are as follows:

DC voltage supplies SC1, SC2 set to 0 V.
AC voltage supply SA1, in the “AC Grid” tab, set to 0 V.
Switches K1, K2, K3 are open: asynchronous machine stator windings are in open-circuit. See Table 4.
Switch K6 is disabled: the asynchronous machine rotor is driven by the DC motor. See Table 4.
Switches K4 and K5 are disabled: asynchronous machine rotor windings are short-circuited. See Table 4.
Trigger switches K10 and K11, in the oscilloscope tabs, are enabled. Triggers are active for oscilloscope groups oscilloscopes1 and oscilloscopes2. See Table 4.

5.3 Part 1: Steady State Model Identification Test

In part 1, three standard low power tests are performed. The data obtained through these tests will help determine the asynchronous machine's steady state model parameters.

5.3.1 Setup Diagram, Part 1

Figure 5: Electric diagram setup for exercise 2, part 1

During the no-load, open-circuit test of the rotor (Figure 5, left), the asynchronous machine’s stator windings are powered by autotransformer SA1 and its rotor windings are in open-circuit.
In the short-circuited rotor test (Figure 5, center) the stator windings of the asynchronous machine are energized by SA1 and the windings of the rotor are short-circuited.
Finally, to measure asynchronous machine stator winding resistances, two out of three windings are supplied by DC voltage SC3 (Figure 5, right), in series with limiting resistance and rotor windings are in open-circuit.
In all tests, switch K6 remains enabled, which mechanically blocks the rotor (see Table 5).

5.3.2 Open Circuit Rotor No-load Test

Close switch K1 to connect the asynchronous machine stator to the grid using autotransformer SA1.
Enable switch K4 to set rotor windings to open-circuit.
Enable switch K6 to mechanically block the rotor and keep it idle.
Increase autotransformer SA1 voltage until it reaches nominal asynchronous machine line-to-line voltage U_sn (see Table 2).
Take note of (i) RMS line-line voltage U_o, (ii) RMS line current I_o and (iii) absorbed active power P_ofor this operating point.
Determine the stator-rotor transformation ratio a=U_s/U_r,using oscilloscope 1 in the oscilloscope1 tab.
Users can modify the offset for the y axis to get a more accurate reading of the rotor’s line-line voltage peak value and Calculate its RMS voltage U_r.

5.3.3 Locked Rotor No-load Test

Set autotransformer SA1 voltage to zero and disable switch K4 to short-circuit asynchronous machine rotor windings.
Ensure that switch K6 is enabled and that the rotor is locked.
Slowly increase voltage source SA1 until line current RMS value I_s at the asynchronous machine stator reaches its nominal value I_sn (see Table 2).
Take note of (i) RMS line-line voltage U_scc, (ii) RMS line current I_scc and (iii) absorbed active power P_ccfor this operating point.
When the exercise is completed, reset autotransformer SA1 voltage to 0 V.

5.3.4 Stator Winding Resistance Measurement

Open switch K1 to disconnect the asynchronous machine’s stator from autotransformer SA1.
Close switch K3 to energize stator windings with DC source SC3 (see Figure 5).
Ensure that switch K6 is enabled and that the rotor is locked.
Enable switch K4 to set rotor windings to open-circuit.
Click on the Measurement Resistors tab on the right of the panel (see Figure 3).
Two phases of the asynchronous machine’s stator are energized by variable DC voltage source SC3 (see Figure 5).
Because the asynchronous machine’s stator winding resistances R_s are low, two limiting resistances were inserted in series with SC3 to limit current I_dcirculating in the windings.
A voltmeter measures voltage U_dc at the two terminals into which flows I_dc.
Slowly increase SC3’s output voltage from zero, ensuring that DC current l_dc does not exceed the asynchronous machine stator’s nominal I_sn.
Measure U_dc and I_dc.
Extrapolate the R_s resistance for one stator winding:

5.4 Part 2: operating in motor mode with no driven mechanical load

5.4.1 Setup Diagram, Part 2

Figure 6: Electrical diagram setup for exercise 2, part 2

In this exercise, the asynchronous machine stator windings are supplied by autotransformer SA1 and its rotor windings are short-circuited. DC sources SC1 and SC2 are set to 0 V.
This way, the DC machine produces no electrical power and produces only a very low resistant torque equal to the mechanical losses of the two machines on the induction motor’s shaft. This is called no-load operation.

5.4.2 No Load Motor Operation Exercise

Restore the setup to the initial conditions described in Section 5.2 by resetting voltage sources SC3, SC1, SC2 and SA1 to 0 V and setting all switches to their initial states.
Select the AC Grid tab, then close switch K1 to connect the asynchronous machine’s stator windings to the grid using autotransformer SA1.
Using autotransformer SA1, slowly increase voltage U_s to start the asynchronous machine at low voltage.
Apply nominal RMS line-line U_sn to stator windings.
Make sure to keep I_s below I_sn at startup by manually increasing U_s in proportion to the motor speed increase, otherwise asynchronous machine stator fuses F_i1, F_i2 and F_i3 may be blown, and the drive will stop.
If this happens, wait until speed is null and return to initial settings (see Section 5.2).
Then, and only then can we simulate replacing fuses by resetting the protection system using push-button K8.
Once the steady state is reached and speed is stable, take note of (i) active power P absorbed by the induction motor, (ii) the RMS line-line voltage at stator U_s, (iii) the RMS line current at stator I_s, (iv) torque T_u and (v) rotation speed N.
Bring the motor to a stop by setting voltage to 0 V using SA1. Open switch K1.

5.5 Lab Report

5.5.1 Lab Report, part 1

Present the results from the low power exercise in tables (with units in SI).
Confirm that they are low power tests by comparing the active power measured on the tests to the nominal power listed on the asynchronous machine’s rating plate.
Using the results obtained, determine the elements of the single-phase equivalent circuit of the asynchronous machine referred to the stator (see Figure 7).

Figure 7: Asynchronous machine single phase standard equivalent circuit (at left)
(single phase circuit returned to the stator at right)

Use the following expressions and approximations:
In the locked rotor test: we measured (i) current I_scc, (ii) line-line voltage U_scc, (iii) absorbed active power P_cc, and can derive the power factor cosφ_cc.
We can neglect the iron losses and the no-load current because of the low voltage. We can calculate:

Assuming that:

In the no-load test, we measured (i) RMS line current I₀, (ii) RMS line-line voltage applied to the stator U₀, (iii) absorbed active power P₀. We can derive the power factor cosφ₀:

with

These equations make it possible to determine the parameters for the circuit (at the right in Figure 7) using the results obtained in the exercise.
Using the transformer ratio “a” measured on the no load test (step 6 in Section 5.3.2), determine the single-phase equivalent circuit parameters at the left in Figure 7.

5.5.2 Lab Report, part 2

Present the results from the low power exercise in Section 5.4.2 in tables (with units in SI).
Determine the asynchronous machine’s synchronous speed N_s using its nameplate rating data (see Table 2) and the slip value s for the operating point obtained in step 4 of Section 5.4.2.
Compare this value with the machine’s nominal slip value l s_ncalculated using its nameplate ratingdata (see Table 2).
Is it normal that it is lower than s_n? Discuss.
Compare the current value obtained in step 4 of Section 5.4.2 with the nominal current value (see Table 2). How do you explain this relatively high value, given that the motor had virtually no load?
Calculate the induction motor’s power factor for the operating point in step 4 of Section 5.4.2.
Compare this value with the induction motor’s nominal power factor FP_n provided in its nameplate rating data (see Table 2). Explain the differences.

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