USING ROTOR RESISTANCE TO REGULATE WOUND ROTOR INDUCTION MOTOR SPEED

7.1 Objective

In this exercise, we will work with a speed variation process for industrial drives that use a wound rotor asynchronous machine (not applicable for squirrel cage induction motors).
We will study how a wound rotor induction motor's speed-torque characteristic moves when we modify the rotor winding resistance while maintaining fixed RMS voltage and frequency. 

7.2 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.

7.3 Setup Diagram

Figure 10: Electrical diagram setup for exercise 4

In this exercise, the asynchronous machine's stator windings are powered with fixed voltage U_s and frequency f_s (f_s=60 Hz) from autotransformer SA1 connected via switch K1.
Rotor windings are connected to a series of external resistances that can be adjusted using switch K5.
The asynchronous machine is operating in motor mode, driving a DC generator that is discharging on its fixed load resistance R_ch.
Use SC2 to vary the current I_f. DC voltage source SC1 is set to 0 V for this exercise and does not appear in the diagram when the DC machine is operating in generator mode because it has unidirectional voltage and current (it has a serial diode not shown here).
This allows us to identify the resistant torque-speed characteristic for several rotor winding resistance values.

7.4 Exercise

1. Restore the setup to its initial conditions, as described in Section 7.2; by resetting sources SC3, SC1, SC2 and SA1 and setting all switches to their initial states.
2. Close switch K5 to connect external adjustable resistors in series with rotor winding.
3. Place the "resistor value" dial, In the asynchronous machine panel, in the 2.5 Ω position to add 2.5 Ω resistor in series with the rotor.
4. Select the “AC Grid” tab, then close switch K1 to connect the asynchronous machine’s stator windings to the grid using autotransformer SA1.
5. Using autotransformer SA1, slowly increase voltage U_s to start the asynchronous machine at low voltage.
   Apply nominal RMS line-line voltage U_sn to stator.
   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 winding 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 7.2).
   Then, and only then can we simulate replacing fuses by resetting the protection system using push-button K8.
6. Once the steady state is reached and speed is stable, take note of (i) active power P absorbed by the induction motor, (ii) RMS line-line voltage U_s at stator, (iii) RMS line current I_s at stator, (iv) torque T_u and (v) rotation speed N.
7. Vary the DC generator’s resistant speed-torque characteristic slope k(I_f) by adjusting the current I_f using SC2.
   This will define a new operating point for the induction motor's speed-torque for voltage U_s = U_sn at a fixed frequency (f_s=60 Hz) and 2.5 Ω resistor in series with the rotor.
   Take note of 10 points from this characteristic by increasing the DC generator's current l_f using source SC2.
   Take care to maintain a constant voltage U_s for each operating point using SA1 or the results will be unusable.
   For each steady state operating point, take note of (i) the active power P absorbed by the induction motor, (ii) RMS line-line voltage U_s at the stator, (iii) RMS line current I_s at the stator, (iv) torque T_u, and (v) rotation speed N.
   Do not exceed the asynchronous machine’s nominal current I_sn or the DC generator’s nominal armature I_an, otherwise, fuses will be blown, and the drive will stop.
   If this happens, wait until speed is null and return to initial settings (see Section 7.2).
   Then, and only then, can we simulate replacing fuses by resetting the protection system using push-button K8.
   Restart the exercise.
8. Restore SC2 to 0 V and stop the motor by bringing voltage U_s to 0 using SA1. 
9. Place the "resistor value" dial, in the asynchronous machine panel, in the 1.5 Ω position to add a resistor of 1.5 Ω in series with the rotor
   Repeat steps 4 to 8. 
10. Open switches K5 to put the asynchronous machine rotor windings in short-circuit (series resistance with the rotor is null).
    Repeat steps 4 to 8.

7.5 Lab Report

1. Present the results obtained from Section 7.4 in tables (with units in SI).
2. Using the results of the exercise, plot the three characteristics for the induction motor torque-speed T_u-N with the 3 resistors in series with the rotor 1.5 Ω, 2.5 Ω and 0 (in the case of the rotor short circuit from step 10 in Section 7.4)
3. Observe how the characteristics for the induction motor torque-speed T_u-N change with series resistance values in the rotor and with fixed stator voltage U_sn and frequency f_s.
   Can we then assume that the process of using variable voltage to vary speed can be used to drive an elevator with a constant resistant torque (resistant torque-speed characteristic Tr-Ω = constant regardless of Ω)?
   Would this be a safe solution?
   Discuss.
4. Calculate the induction motor cosφ power factor for each operating point in Section 7.4.
   Plot all three cosφ-N characteristics with the three resistors in series: 1.5 Ω, 2.5 Ω and 0.
   Explain the results.
5. Plot the three current-speed characteristics I_s-N with the three resistors in series: 1.5 Ω, 2.5 Ω and 0.
   Explain the results.
6. We could also theoretically predict the induction motor’s speed-torque without the load tests run in Section 7.4 by using a steady state asynchronous machine model.
   All it requires is using a theoretical expression of the induction motor torque T_em ^{(Note 1)} to implement the circuit parameters identified using the low power tests in exercise 2 and resistors R_ext in series with the rotor.
   Calculate the theoretical electromagnetic torque characteristics T_em-speed N(rpm) for the induction motor in Section 7.4, using this simplified expression:
   
   V_s is the stator RMS line-neutral, p is the number of pair poles for the induction motor and ω_s is the angular speed.
   s is the motor slip when it turns at speed N:
   
   N_s is the motor synchronous speed corresponding to f_s.
7. Superimpose the 3 theoretical motor speed-torque characteristics for the induction motor T_em-N and the three characteristics T_u-N from step 2 on the same graph.
   Compare theoretical and measured characteristics and discuss the differences.