SYNCHRONOUS MOTOR

7.1 Objective

This exercise presents the implementation of the synchronous machine operating as a motor.
This synchronous motor is fed by an AC grid and drives the DC machine running in a generator mode with a resistive load.
We will study (i) the motor start up and the synchronization process, (ii) the adjustments of its power factor and, (iii) its operation as a reactive synchronous compensator.
We will also investigate the stability limits of the synchronous motor.

7.2 Initialization of the Setup

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

1. Continuous voltage supplies SC1, SC2, SC3 set to 0 V.
2. Continuous voltage supply SC4, in the "Measurement Resistors" tab, set to 0 V.
3. Alternative power SA1, in the "AC Grid" tab, set to 0 V.
4. Switches K₁, K₂, K₃, K₄ opened: synchronous generator armature has no load, see Table 4.
5. Switch K₆ disabled: the synchronous machine rotor is driven by the DC motor, see Table 4.
6. Switch K₅ disabled: voltage source SC3 powers the synchronous machine field winding, see Table 4.
7. Trigger switches K₁₀, K₁₁, K₁₂, K₁₃, K₁₄, in the tab "Oscilloscope", disabled, see Table 4.

7.3 Starting and Synchronizing the Synchronous Motor to the Grid

In this exercise, the synchronous machine is operating in motor mode and is driving the DC machine, which is operating in generator mode and supplying a resistive load.

We start the machine in asynchronous mode and synchronize it to a constant voltage and frequency AC grid.
Then, we open switch K5 during startup so that the synchronous machine field is not fed by SC3.
The field is connected in parallel with the starting resistance which limits high voltages that could damage its isolation during asynchronous motor operation.
In addition to the field winding, the synchronous motor rotor is equipped with a damping bar cage that serves the same purpose as a conventional asynchronous machine's rotor squirrel cage.
Thus, the synchronous motor is transformed into an asynchronous motor during startup.

After closing switch K₁, we will use a conventional variable voltage starting process for the asynchronous motor with autotransformer SA1.
We increase the rotor speed as close as possible to the synchronous speed (1800 rpm).
Then we close switch K5, to synchronize the motor to the AC grid having previously set the value of SC3 to allow DC current Jf to circulate in the field.
The machine therefore synchronizes on the AC grid which has constant frequency and voltage and operates as a synchronous motor.
Since the rotor turns at synchronous speed, there is no voltage induced in the damping bar cage.

Once synchronized to the grid, the motor operates at constant speed equal to synchronous speed (1800 rpm), regardless of the resistant torque imposed on the shaft by the DC generator.
This operation is maintained as long as the motor does not reach its stability limit.
Otherwise, the motor disconnects, and protections must come into play to protect the windings.

After synchronizing the motor to the grid, the RMS voltage on the armature terminals is imposed by SA1.
The user now has two inputs to control the active power P and reactive power Q exchanges between the synchronous motor and the AC grid: (i) the resistant torque imposed on the shaft by the DC generator that can be controlled by I_f using SC2, and (ii) the synchronous machine's field current J_f, adjustable using SC3.
In this exercise, the features of these two control inputs are investigated, as well as the synchronous motor's operating limits.

7.3.1 Setup Diagram

Figure 12: Diagram of the synchronous machine operating in motor mode

7.3.2 Exercise

1. Ensure that all voltage sources are at 0.
2. Connect the synchronous machine field to the starting resistance by closing switch K₅ "Starting Resistor" and set the field voltage SC3 in such a way to obtain the nominal field current (see Table 1).
3. Close switch K₁ to connect the synchronous machine's armature to auto transformer SA1.
   Gradually increase voltage to the armature terminals until it reaches its nominal value U=U_sn=460 V using SA1 in the "AC Grid" tab.
4. Enable switch K₁₄ in the "Oscilloscopes5" tab located in the lower part of the panel to observe motor synchronization to the grid that will be done in the next step.
   Before synchronization, observe speed and torque oscillations.
5. Once the voltage at the armature terminals reaches its nominal value U_sn and the speed has stabilized to a value close to the synchronous speed, open switch K₅ and observe the damped oscillations of the speed and torque during synchronization of the motor to the grid.
   At that time, the current J_f imposed by SC3 is circulating in the synchronous machine's field.
   Once synchronization is complete, disable switch K₁₄.
6. The synchronous motor is now connected to the grid and its speed is set to 1800 rpm by means of the AC grid's frequency (60 Hz).
   The resistant torque imposed on the motor shaft by the DC generator is practically non-existent, since SC1 and SC2 are set to 0.
   We can say that the synchronous motor operates at no load, but in strict sense, there is a low active power consumption due to the mechanical and electrical losses of the drive train.

Procedure in case of non-synchronization, motor disconnection or triggering of protection systems:

• • If during the exercise, the drive speed is not exactly 1800 rpm after synchronization, the internal angle θ is not constant and the armature RMS current value is significant, the motor is disconnected, even though the protection system was not triggered.
    In this case, the synchronous machine is still operating in asynchronous mode and presents torque oscillations and circulating currents in the stator, the field and in the damper cage.
    These currents could produce too high temperatures if they are prolonged.
    In this case, reopen the switch K₅ and repeat the synchronization procedure.
  • If a fuse is blown at any time during the exercise, the drive stops, the synchronization procedure has failed, or the stability limit was exceeded, it is necessary to wait until the speed slows to 0 and then reset all parameters to their original settings (Section 7.2).
    Then, and only then we can simulate the fuse replacement by resetting the protection system using K₈.
    The entire synchronization process must be restarted by repeating the previous step.

7.4 No-load V-curve, Synchronous Compensator Operating Mode

1. Once startup and synchronization of the machine to the grid are successfully completed, the motor should turn at 1800 rpm with practically zero torque imposed on its shaft by the DC generator.
   This operation mode is called no load, because the motor absorbs a low value of P due to the rotation losses of both machines and the Joule losses of the stator of the synchronous machine.
   Varying the synchronous motor's field current J_f using SC3 without changing the settings for SC1 and SC2, P remains virtually constant while Q can vary greatly and even reverse.
   The synchronous motor is now operating as a synchronous compensator and absorbs very low P and can either produce or absorb a variable Q, which is controlled by its field current value J_f.
   From the AC grid's perspective, the synchronous motor is behaving as a three-phase capacitor or inductor controlled by J_f.
2. Starting from the operating point obtained after synchronization, vary the synchronous machine's field current J_f using SC3, while maintaining the motor's armature RMS voltage at its nominal value U_sn using SA1 (if the voltage is not kept constant during this exercise, the results will be unusable).
   Start increasing J_f until the current in the stator of the synchronous machine is equal to its nominal value I_sn.
   Take note of the values for Jf, I_s, U_s, θ, P and Q
3. Gradually decrease Jf following the previous procedure and observe the values of Is, θ, P and Q.
   The value of Q gradually decreases until it reaches Q=0 at a current Jf=Jf1.
   The synchronous motor's power factor is unitary and Is reaches a minimum value of Is = Ismin1.
   Take note of values Jf1, Ismin1, Us, θ, P and Q for the operating point at unitary power factor.
4. Increase J_f starting from J_f1 and take note of J_f, I_s, U_s, θ, P and Q for 5 operating points between I_smin1 and I_sn (take care to maintain U_s= U_sn for each point using SA1).
5. Return to the unitary power factor point (J_f=J_f1, Q=0) and gradually decrease J_f from this value.
   Note how reactive power Q reverses and increases its absolute value, as does the current I_s.
   Take note of the values J_f, I_s, U_s, θ, P and Q for 5 operating points between I_smin1 and I_sn (take care to maintain U_s= U_sn for each point using SA1).
   If the machine disconnects before reaching point I_s=I_sn or if a fuse blows, follow the procedure described in Section 7.3.
   Repeat step 5 until I_s is closest to value I_sn without causing the loss of synchronism of the motor.
6. After all measurements are taken, increase J_f again until it reaches the value measured at step 2 (J_f >J_f1,I_s close to I_sn).
   Do not stop the setup.

7.5 Constant Power V-curve, Operating Limits

1. Starting from the previous step, set the current I_f using SC2 to obtain a power Va*Ia produced by the DC generator equal to the absolute value of 2000 W (voltage at armature terminals Va should be around 200 V and armature current I_a should be in the order of 10 A) (SC1 must remain at 0 V).
   Do not modify the SC2 setting to ensure that the output power on the synchronous motor's shaft remains constant.
   Repeat steps 2 through 5 of the exercise described in Section 7.4. In this case, the synchronous motor has a load and is supplying mechanical power to the DC generator.
2. Increase J_f using SC3 until the armature current I_s is as close as possible to its nominal value I_sn. (maintain U_s= U_sn using SA1, or the results will be unusable).
   Take note of the values for J_f, I_s, U_s, θ, P and Q for this point.
3. Gradually decrease J_f and observe values Is, θ, P and Q.
   The value of Q gradually decreases until reach Q=0 at a current J_f =Jf₂.
   The synchronous motor's power factor is unitary and Is reaches a minimum value of I_s = I_smin2.
   Take note of Jf2, I_smin2, U_s, θ, P and Q for this unitary power factor operating point.
4. Increase J_f again, starting from J_f2 and take note of the values J_f, I_s, U_s, θ, P and Q for 5 operating points between I_smin2 and a value as close as possible to I_sn (take care to maintain U_s=U_sn for each point using SA1).
5. Return to the unitary power factor point (J_f =J_f2, Q=0) and gradually decrease J_f from this value.
   Take note of the values J_f, I_s, U_s, θ, P and Q for several operating points (take care to maintain U_s=U_sn for each point using SA1).
   Note how reactive power Q reverses and increases its absolute value, as does the current I_s.
   The absolute value of the internal angle also increases, and we get closer to the synchronous motor's stability limit.
   Continue gradually decreasing J_f until it reaches the stability limit at which the motor loss synchronism: the speed is no longer maintained at 1800 rpm and internal angle θ oscillates.
   Take note of this value.
   If the machine disconnects or the protection system is triggered, open switch K₁, Set SC2 back to 0, and repeat the synchronization procedure described in Section 7.3.2, and then the steps 1 through 5 of this exercise to return to the unitary power factor operating point (J_f=J_f2, Q=0).
   Start decreasing J_f again until reaching the stability limit.

7.6 Observation of the Rotor's Dynamic Behavior

This exercise shows the variations for internal angle θ and the dynamic behavior of the synchronous motor's rotor during a torque transient on the shaft.

1. Synchronize the motor to the AC grid using the procedure described in Section 7.3.2
2. Select the "Oscilloscopes4" tab and enable switch K₁₃.
   The scopes in this tab show the evolution in time of the motor's electromagnetic torque and internal angle θ.
3. Adjust voltage source SC3 to set the field current value J_f to 10 A.
4. Create a sudden variation of the synchronous motor's torque by suddenly changing the DC machine field current I_f using voltage source SC2.
5. Observe the transient torque and the damped oscillations in the internal angle θ on the scopes.
   Make sure to disable and re-enable switch K₁₃ if you wish to observe another transient phenomenon.
6. Repeat the exercise using different values for I_f and J_f.
   For example, for high I_f and low J_f values, the initial value of θ before applying the torque variation is close to the stability limit.
   Try with several torque values (changing I_f) and observe the variations in the frequency of the damped oscillations of θ, as well as the synchronous motor stability limits.
   If the motor disconnects or the protection system is triggered, repeat the procedure in Sections 7.4 and 7.5.
7. Return the setup to its initial conditions, as described in Section 7.2.

7.7 Lab Report

1. Present the results from Sections 7.4 and 7.5 in tables (SI units).
2. Based on these results, plot the two characteristics I_s vs J_f with and without the load on the same graph.
   Show the specific operating point values: (i) I_s=I_sn to the high J_f values (synchronous motor operating in over-excitation), (ii) unitary power factor operating mode (Q=0), and (iii) operating point limits for low J_f values (synchronous motor operating in under-excitation).
   These characteristics are called "V curves" or "Mordey curves" for the synchronous motor with constant power and voltage.
   For a given P, the "over-excitation zone" corresponds to the zone where J_f is higher than the unitary power factor operating point.
   The "under-excitation zone" corresponds to the zone where J_f is lower than the unitary power factor operating point.
3. Specify, on the graph, the zone in which the synchronous motor is supplying reactive power to the AC grid and the zone in which the synchronous motor is absorbing reactive power from the AC grid.
4. Explain, qualitatively, the reactive power exchanged between the synchronous machine and the AC grid when the machine is operating on a constant active power and field current J_f is varying.
5. Why must we use the autotransformer SA1 to maintain constant voltage on the motor armature equal to its nominal value U_sn in the experiments of Sections 7.4 and 7.5?
6. Plot the characteristic Q vs J_f for the synchronous motor operating as synchronous compensator using the results of exercise 7.4.
   In that exercise, P measured should be constant and very low, since the torque on the shaft is also low.
   Explain why the active power still varies a little with the changes of J_f.
7. List the static stability limits of θ obtained in the synchronous machine's motor operation (step 5 in Section 7.5).
   Compare it to the theoretical stability limit calculated for the synchronous machine connected to the AC grid.
8. Explain, qualitatively, the damped oscillations shown on in Section 7.6.
   Would these oscillations be damped if the rotor was not equipped with a damping bar cage?
   What would be the consequences for the synchronous motor's dynamic stability limit?
   Why would the frequency of these oscillations depend on the synchronous machine's initial load state (steady state resistant torque before applying variations to it)?
9. Using the synchronous machine's steady state model, whose parameters E(Jf), X_d, X_q and R_s were determined using the low power tests of Exercise 1, it is possible to predict and theoretically calculate characteristics I_s vs J_f and Q vs J_f from the exercises in Sections 7.4 and 7.5.
   Using the theoretical machine model, its parameters and circuit methods learned in the class (Kirchhoff and Ohm's laws, phasor diagram, etc.) determine the expressions of characteristics I_s vs J_f and Q vs J_f for an active power P and constant voltage U_s.
   Use these expressions to calculate theoretical characteristics for I_s vs J_f and Q vs J_f corresponding to the exercises in Section 7.4 and 7.5.
   To compare these curves, superimpose the theoretical calculated and measured characteristics on a single graph.
10. The synchronous motor, like any electric motor, can supply mechanical power to a load (pump, mineral crushers, refiners, etc.), but it can also be used to generate or absorb reactive power on the power grid where it is connected.
    It can also operate at unity power factor if we adjust J_f according to the P absorbed.
    What is the advantage of using a synchronous machine in a factory that also contains several induction motors that absorb reactive power?
    Discuss.