The Synchronous Motor

Objective:

• To examine the construction of the 3Ф synchronous motor.
• To obtain the starting characteristics of the 3Ф synchronous motor.

Discussion:

The synchronous motor gets its name from the term synchronous speed, which is the natural speed of the rotating magnetic field of the stator. As you have learned, this natural speed of rotation is controlled strictly by the number of pole pairs and the frequency of the applied power.

Like the induction motor, the synchronous motor makes use of the rotating magnetic field. Unlike the induction motor, however, the torque developed does not depend on the induction currents in the rotor. Briefly, the principle of operation of the synchronous motor is as follows. A multiphase source of AC is applied to the stator windings and a rotating magnetic field is produced. A direct current is applied to the rotor windings and a fixed magnetic field is produced. The motor is so constructed that these two magnetic fields react upon each other causing the rotor to rotate at the same speed as the rotating magnetic field. If a load is applied to the rotor shaft, the rotor will momentarily fall behind the rotating field but will continue to rotate at the same synchronous speed.

The falling behind is analogous to the being tied to the rotating field with a rubber band. Heavier loads will cause stretching of the band so the rotor position lags the stator field but the rotor continues at the same speed. If the load is made too large, the rotor will pull out of synchronism with the rotating field and, as a result, will no longer rotate at the same speed. The motor is then said to be overloaded.

The synchronous motor is not a self-starting motor. The rotor is heavy and, from a dead stop, it is not possible to bring the rotor into magnetic lock with the rotating magnetic field. For this reason, all synchronous motors have some kind of starting device. A simple starter is another motor which brings the rotor up to approximately 90% of its synchronous speed. The starting motor is then disconnected and the rotor locks in step with the rotating field. The more commonly used starting method is to have the rotor include a squirrel cage induction winding. The induction winding brings the rotor almost to synchronous speed as an induction motor. The squirrel-cage is also useful even after the motor has attained synchronous speed, because it tends to dampen rotor oscillations caused by sudden changes in loading. Your Three-Phase Synchronous Motor/Generator contains a squirrel-cage-type rotor.

Equipment Required:

• Three-Phase Synchronous Motor/Generator
• Electro Dynamometer
• AC Ammeter
• AC Voltmeter
• Wires

Procedure:

CAUTION!!!

High voltages are Present In the Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement!!!

1. Examine the construction of the Three-Phase Synchronous Motor/Generator, paying particular attention to the motor, slip rings, rheostat, connection terminals and the wiring.

2. Viewing the motor from the rear of the module:

a. Identify the two slip rings and brushes.

b. Can the brushes be moved?

c. Note that the two rotor windings are brought out to the two slip rings via a slot in the rotor shaft.

d. Identify the DC damper windings on the rotor.(Although there are only two windings, they are connected so that their magnetomotive forces act in opposition, thus, creating four poles.

e. Identify the four salient poles just beneath the damper windings.

f. Identify the stator winding and note that it is identical to that of the three-phase squirrel cage and wound rotor motors.

3. Viewing the front face of the module:

a. The three separate stator windings are connected to terminals 1 and 4, 2 and 5, 3 and 6.

b. What is the rated voltage of the stator windings? 220 V

c. What is the rated current of the stator windings? 0.5 A

d. The rotor winding is connected through the 380 Ω rheostat (and a toggle switch S) to terminals 7 and 8.

e. What is the rated voltage of the rotor winding?

f. What is the rated speed and mechanical output power of the motor? Speed=1450 rpm and Power = 175W.

Starting Characteristics:

Figure – 1

4. Using your Three-Phase Synchronous Motor/Generator, Power Supply and AC Ammeter, connect the circuit shown in Figure – 1. Note that the three stator windings are wye-connected to the fixed 380 V 3 Ф output of the power supply, terminals 1, 2 and 3.

5.  a. Turn on the power supply. Note that the motor starts smoothly and continues to run as an ordinary induction motor.

b. Note the direction of rotation

Rotation = CCW, I₁ = 0.5 Aac

c. Turn off the power supply and interchange any two of the leads from the power supply.

d. turn on the power supply and note the direction of rotation.

Rotation = CW, I₁ = 0.5 Aac

e. Turn off the power supply.

6.  a. Connect the circuit shown in Figure – 2. Note that the synchronous motor is wired in its normal starting configuration (as a three-phase squirrel-cage induction motor).

b. Set the dynamometer control knob at its full CW position (to provide a maximum starting load for the synchronous motor).

c. Close the switch S.

Figure – 2

7.   a. Turn on the power supply and quickly measure E₁, E₂, I₁ and the developed starting torque. Turn off the power supply.

           E₁ = 398 Vac, E₂ = 410 Vac, I₁ = 1.5 Aac.

           Starting torque = 3.0 N-m.

       b. Calculate the full-load torque corresponding to 175W at 1500 rpm

           Full-load Torque = Pout ×60/2πN = 175×60 / 2π×1500 = 1.11N-m

       c. Calculate the ratio of starting torque to full-load torque.

           Ratio = 1.11 × 100/ 3.0 = 37%

       d. Why a large AC voltage E2 was induced in the rotor windings?

Because of a higher starting torque a large AC voltage is induced as both are proportional.