USING FIXED FREQUENCY SPEED VARIATION TO REGULATE THE INDUCTION MOTOR SPEED

6.1 Objective

In this exercise, we will test one of the drive speed variation procedures that use an induction motor. We will study how the induction motor’s speed-torque characteristics change when we modify the RMS voltage of the machine stator while maintaining a fixed frequency.

6.2 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.

6.3 Drive Speed Variation - Induction Motor Speed-torque Characteristics

Think of an industrial drive as a motor driving a mechanical load, such as a pump or a compressor.
The steady state operating speed for the drive is obtained when the torque from the electric motor is equal to the resistant torque from the mechanical load drive.

In terms of torque-speed, the balance point corresponds to the intersection of steady state speed-torque resistance characteristics T_r(Ω) of the mechanical load, and the steady state speed-torque T_u(Ω) of the electric motor for a fixed power supply (such as a constant frequency and voltage supply for an induction motor, as shown in Figure 8).

Modifying the drive speed is as simple as changing the intersection of the two torque-speed characteristics to obtain a new speed (changing the load).

One method for varying drive speed involves leaving the electric motor voltage setting unchanged, which maintains fixed speed-torque characteristics.
Add a resistant torque to the shaft, with an auxiliary braking device, to displace the mechanical load's resistant torque speed and decrease the speed.
This method is not energy efficient: speed variations consume more energy from the motor, since it must supply more energy to compensate for dissipated energy caused by using the braking device to regulate speed.

The second method for varying drive speed involves leaving the mechanical load's resistant torque speed at a fixed value and modifying the electric motor's voltage setting to displace its motor torque speed.
This method is energy efficient because the motor adapts its power consumption to the power used by the mechanical load drive for the new speed balance.
Since it is relatively easy to modify electrical power parameters for a variety of electric motors, this is the most common speed variation method for industrial uses.

In this exercise, we will study how an induction motor's torque speed characteristics react when we modify the RMS voltage and leave the frequency unchanged.
Thus, we obtain the torque speed characteristics for several power supply voltage values.

To find the various points of this characteristic, we will use a variable mechanical load drive whose resistant torque can be easily modified.
The variable mechanical load is achieved using the DC machine as a generator that feeds a fixed R_ch load resistance (see Figure 9).
We can adjust the mechanical load's current l_f, using SC2, to move the resistant torque-speed characteristic.
The resistant torque-speed T_r(Ω) characteristic can be expressed simply:

The DC generator's resistant torque-speed characteristic is a straight line whose slope k(I_f) varies with current I_f.
To determine the induction motor's speed-torque characteristic point by point for a fixed voltage and frequency, modify the slope k(I_f) for characteristic Tr(Ω) (see Figure 8)

Figure 8: Induction motor speed-torque characteristics supplied with constant voltage and frequency

6.4 Setup Diagram

Figure 9: Electrical diagram setup for exercise 3

In this exercise, the asynchronous machine's stator windings are powered with variable voltage U_s and fixed frequency f_s (f_s= 60 Hz) from autotransformer SA1 connected via switch K1.
Rotor windings are short circuited by switch K4. The asynchronous machine is operating as a motor and driving the 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).

6.5 Exercise

1. Restore the setup to its initial conditions, as described in Section 6.2; by resetting sources SC3, SC1, SC2 and SA1 and setting all switches to their initial states.
2. Select the “AC Grid” tab, then close switch K1 to connect the asynchronous machine’s stator windings to the grid using autotransformer SA1.
3. Using autotransformer SA1, slowly increase voltage U_sn to start the asynchronous machine at low voltage.
   Apply nominal RMS line-line voltage U_sn to the 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 6.2).
   Then, and only then can we simulate replacing fuses by resetting the protection system using push-button K8.
4. 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 current I_s at stator, (iv) torque T_u and (v) rotation speed N.
5. 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) (see Table 1).
   Take note of 10 points from this characteristic by increasing the DC generator's current l_f.
   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, note (i) the active power P absorbed by the induction motor, (ii) the RMS line-line voltage U_s at the stator, (iii) the RMS current I_s at the stator, torque T_u, and (iv) rotation speed N.
   Do not exceed the asynchronous machine’s nominal current I_sn at the stator, or the nominal value I_an of line current I_a in the DC generator’s armature, 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 6.2).
   Then, and only then can we simulate replacing fuses by resetting the protection system using push-button K8.
   Restart the exercise.
6. Restore SC2 to 0 V, set the RMS line-line stator voltage to U_s=345 V and repeat steps 3 to 5.
7. Repeat step 6 after setting the RMS line-line stator voltage to U_s=230 V.
8. Bring the motor to a stop by setting voltage to 0 V using SA1. Open K1.

6.6 Lab Report

1. Present the results from steps 1 to 7 in Section 6.5 in tables (with units in SI).
2. Using the results of the exercise, plot the three characteristics torque-speed T_u-N for the induction motor with the 3 voltage values U_s= U_sn, U_s=345V, U_s=230V.
3. Observe how the characteristics torque-speed T_u-N for the induction motor change with stator voltage U_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, (torque-speed characteristic Tr-Ω = constant regardless of Ω)?
   Would this be a safe solution? Discuss.
   Could it be used to drive a cooling fan propeller that has a characteristic type Tr-Ω = m Ω²  (resistant torque proportional to the square of the speed)? Discuss.
4. Calculate the induction motor cosφ power factor for each operating point of exercise 6.5. Plot all three cosφ-N characteristics with the three voltages U_s=U_sn, U_s=345 V, U_s=230 V. Explain the results.
5. Plot the three current-speed characteristics I_s-N with the three voltages U_s=U_sn, U_s=345 V, U_s=230 V. Explain the results.
6. We could also theoretically predict the induction motor’s speed-torque characteristic without the load tests run in Section 6.5 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.
   Calculate the theoretical electromagnetic torque characteristics T_em-speed N(rpm) for the induction motor in Section 6.5, 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.