WOUND ROTOR ASYNCHRONOUS MACHINE OPERATING IN BOTH PHASE-SHIFTING TRANSFORMER & FREQUENCY CONVERTER MODE

4.1 Objective

This exercise presents the commissioning of a wound rotor asynchronous machine in two different scenarios that have applications in power grids: operated as a phase-shifting transformer (the asynchronous machine does not move) and as a frequency converter powered by an auxiliary motor (the test bench’s DC machine).
The goal of this exercise is to understand the operating principles of induction motors, considering it as a rotary transformer.
Therefore, when driving it at variable speed with the DC machine, we can observe the variations induced on the open rotor windings while the stator is energized at fixed voltage and frequency.

4.2 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 Oscilloscope1 tab, are disabled.  When enabled, triggers are active for oscilloscopes 1 and 2. See Table 4.

4.3 DC Machine Start Up And Speed Regulation Procedure

Follow these steps to start up the DC machine and vary its speed:

• Set the DC voltage source SC2 to maximum. This drives the DC machine field winding to start with a maximum excitation current I_f.
• Use the DC motor to slowly start the drive, increasing the DC voltage source SC1, which feeds the armature winding, to V_a= 460 V.
  Caution: at startup, speed and electromotive force voltage in the DC motor armature is null.
  The current is only limited by the small motor armature resistance R_a.
  The current I_a must be limited at startup by manually increasing V_a proportionate to motor speed, otherwise, fuse F_ia will be blown, and the drive will stop.
  If this happens, wait until speed is null and return to initial settings (see Section 4.2).
  Then, and only then, can we simulate replacing fuses by resetting the protection system using push-button
• To set drive speed, maintain voltage V_a provided by SC1 to V_an=460 V, and regulate speed by manipulating I_f, with SC2: as current I_f decreases, drive speed increases and vice versa.
  Speed can be precisely adjusted using the slider to the right of SC2.
  Speed can also be reduced by reducing the armature voltage using SC1 and leaving I_f

4.4 Asynchronous Machine Operating as a Phase Shifting-Transformer

4.4.1 Setup Diagram

Figure 4: Electrical diagram; Induction machine operating as a phase shifting transformer and as a rotating frequency converter

In this exercise, asynchronous machine rotor windings are in open-circuit and stator windings are fed by three phase autotransformer SA1.
We can see the no-load voltages induced in the rotor windings, for several rotor positions, when the machine is operating in phase-shifting transformer mode (the machine is not rotating).
The DC motor is used to move the rotor from one fixed position to the other.

4.4.2 Exercise

1. Ensure that the setup is in the initial configuration described in Section 4.2 by resetting voltage sources SC1, SC2 and SC3 to 0 V and all switches to their initial states.
2. Enable switch K1 to connect the asynchronous machine stator to the AC grid using autotransformer SA1.
3. Enable switch K6 to disconnect the two machines and block the asynchronous machine rotor. The asynchronous machine is stopped.
4. Enable switch K4 to open the rotor of the machine
5. In the AC Grid tab, slowly increase autotransformer SA1 voltage to the asynchronous machine’s nominal voltage U_sn (see the asynchronous machine’s data plate in Table 1).
6. Observe the asynchronous machine’s line-line voltages at the stator (U_s) and at the rotor (U_r_G1) on oscilloscope 1 in the Oscilloscope 1 tab.
   Adjust axis Y using the potentiometer on the left and check that switch K10 is enabled to synchronize signals to the stator voltage.
   This will ensure that waveforms are properly displayed on the oscilloscope.
   What is the voltage frequency at the asynchronous machine rotor?
   Do the voltages have the same phase?
   Measure both rotor and stator RMS line-line voltages.
7. Now, slowly move the asynchronous machine rotor using the DC motor and stop it at a different position than in the previous step: the machine shaft has been mechanically disconnected using K6.
   Begin by starting the DC motor and run at a very low speed (N=2 rpm) using the procedure from Section 4.3, set the voltage SC2 to 460 V and voltage SC1 to a very low value. Use the arrows on the right to fine tune speed settings.
8. Disable switch K6 to mechanically connect the two machines and drive the induction motor rotor at very low speed with the DC machine.
   The rotor can be stopped at a new position using switch K6.
9. Observe voltages at the rotor and the stator for 3 fixed and three varied positions using the method from the previous step.
   Take a screenshot of the oscilloscope screen to include in the lab report for each position.
   Did the asynchronous machine’s rotor voltage frequency fluctuate at any of the 3 stop positions?
   Measure the RMS line-line voltage values at the rotor and the stator for each of the three positions.
   Did voltage phase-shifting change from one position to the next?
   Include your results in the lab report for this exercise.

4.5 Using the Asynchronous Machine as a Frequency Converter

In this exercise, the asynchronous machine’s rotor windings are in open-circuit and stator windings are fed by the three-phase autotransformer SA1. The autotransformer SA1 voltage is kept at 460 V.
We can see the no-load voltages that are induced in the rotor windings when they are driven at variable speeds by the DC motor.
The wound rotor asynchronous machine operates as a running frequency converter.
Use voltage sources SC1 and SC2 to control the drive speed, as described in Section 4.3.

1. Use Oscilloscope1 tab, to observe line-line stator (U_s) and rotor (U_r_G2) voltages.
   Use the potentiometer to the left of the oscilloscope to adjust the Y-axis to better observe the waveforms.
   Make sure that switch K11 is enabled to synchronize the oscilloscope with the rotor voltage signal.
2. Disable switch K6 to mechanically connect the two machines, thereby using the DC machine to control the variable speed for the asynchronous machine’s rotor.
   Increase the DC machine’s speed (see Section 4.3) until the drive speed reaches 1800 rpm.
   Take a screenshot of the oscilloscope screen to include in the lab report.
   Measure the voltage frequency values at the asynchronous machine rotor and the RMS voltages at the rotor and the stator.
   Include the results in the lab report for this exercise.
3. Repeat step 2 with speed at 1200 rpm and at 900 rpm.
4. Use SC1 to stop the drive and reset it to 0 V.
   Enable switch K7 to reverse the asynchronous machine’s stator phase sequence.
   Repeat steps 2 and 3.

4.6 Lab Report

1. Present the results from steps 6 and 9 of Section 4.4.2 in tables (with units in SI).
2. Answer questions from steps 6 and 9 of Section 4.4.2
3. Show how this type of phase shifting transformer can be used in an AC network to regulate active power exchanged between two areas that are connected via an inductive line that shares identical frequencies and voltage RMS.
   Find the expression of active power exchanged between two mono-phased voltage sources connected by an inductive reactance X with the same frequency and RMS voltage and phase-shifted by a θ angle.
   Show how the active power flow between the two voltage sources can be controlled using θ.
4. Present the results from steps 2, 3 and 4 in Section 4.5 in tables (with units in SI).
   Explain the rotor voltage frequency measurement.
5. Determine the expression of the electrical frequency f_r observed in the asynchronous machine’s rotor as a function of (i) its mechanical speed N in rpm, (ii) of the stator electrical frequency f_s (60 Hz) and (iii) the number of pole pairs p deducted from the rating plate (see electric machines nameplate ratings).
6. The RMS value (called V_r0) for the line-neutral voltage at the rotor terminal when it is stopped, and the stator is energized as in step 6 of Section 4.4.2.
   Determine the expression of the RMS value for the line-neutral voltage at rotor terminals V_r as a function of (i) its mechanical speed N in rpm, (ii) the stator electrical frequency f_s, (iii) V_r0 and (iv) the number of pair poles of the machine.
7. This type of frequency converter can be used as a rotating transformer to exchange active power between two AC grids operating with slightly different frequencies.
   This is done by operating the asynchronous machine’s rotor as a rotating transformer with a motor whose speed is controlled at N₁₂ (rpm).
   Calculate N₁₂ as a function of f₁ and f₂ as well as the number of asynchronous machine p pole pairs used as rotating frequency converters.
   Demonstrate how autotransformers are also needed upstream and downstream to adjust voltages.
   Since 2003 such a device has been used at Hydro-Québec Langlois substation to achieve asynchronous interconnection with Ontario and New York power grids.