Friday, August 15, 2014

Three Phase Synchronous Motor

Cutaway view of  an eight pole, 2000 horsepower synchronous motor
Figure 1: Cutaway view of  an eight pole, 2000 horsepower synchronous motor.
The synchronous motor is one type of three phase AC motor which operates at a constant speed from no-load to full-load.  It is similar in construction to a three phase AC generator but it has a revolving field which must be separately excited from a direct current source.  By changing the DC field excitation current, the power factor of this type of motor can be varied over a side range of lagging and leading values.  This motor is used in many industry applications because of its fixed speed from no-load to full-load, its high efficiency and to initial cost. It is also used to improve the power factor of three phase AC industrial circuits.

The essential parts of a three phase synchronous motor are:
1.) Laminated core with three phase winding;
2.) Revolving field with slip rings and amortisseur winding;
3.) Brushes and brush holders; and
4.) Two end shields to house the bearing that support the rotating shaft.

Synchronous motor rotor with amortisseur winding
Figure 2: Synchronous motor rotor with amortisseur winding.
The stator core and windings of a synchronous motor are similar to those of a three phase squirrel cage induction motor or a wound-rotor induction motor. The leads for the stator winding marked T1, T2 and T3 terminate in a thermal box usually mounted on the side of the motor frame.

The rotor of the synchronous motor usually has salient field poles connected to give alternate polarity. The number of rotor fields must equal the number of stator field poles. The field circuit leads are brought out to two slip rings mounted on the rotor shaft. A squirrel cage or amortisseur winding is provided for starting purpose because the synchronous motor is not self-starting without this auxiliary winding.

Figure 2 illustrates a rotor with salient poles (easily recognized poles) complete with amortisseur winding. The amortisseur winding consists of copper bars embedded in the laminated metal structure of each pole face. The copper bars of this special squirrel-cage winding are bragged to end rings on each side of the rotor.

Carbon brushes mounted on brush holders make contact with the two slip rings. The terminals of the field circuit are brought out from the brush holders to a second terminal box mounted on the motor frame. The two leads for the field circuit are marked F1 and F2.

Operating principles:
When rated three phase voltage is applied to the stator windings, a rotating magnetic field is set-up which travels at synchronous speed. The synchronous speed of the magnetic field is determined by the same factor which affect the synchronous speed of induction motor. These factors are the frequency of the three phase supply and the number of stator poles.

Synchronous speed in R.P.M. (Revolution Per Minute) = 120 x frequency / Number of Poles or S = 120 F/P

The magnetic field, set up by the stator windings travelling at synchronous speed, cuts across the amortisseur or squirrel cage winding of the rotor and induces voltages and currents in the bars of this winding. The resultant magnetic field of the squirrel cage winding embedded in the rotor field poles react with the stator field in such a manner as to cause rotation of the rotor. The rotor will increase its speed to a point slightly below the synchronous speed of the stator field. In other words, there is a light slip of the rotor back of the magnetic field set up by the stator windings. The rotor of the typical synchronous motor accelerates to about 95 to 97 percent of synchronous speed when started as an induction motor with the amortisseur windings.

The field circuit is now excited from an outside source of direct current and magnetic poles of fixed polarity are set up in the rotor field cores. The fixed magnetic poles of the rotor are attracted to unlike poles of the rotating magnetic field set up by the stator windings. The motor is in step with the rotating magnetic field.

Figure 3 illustrates how the rotor field poles look with unlike poles of the stator field. The rotor speed then becomes the same as the speed of the stator field which is synchronous speed.

Principle of operation of a synchronous motor
Figure 3: Principle of operation of a synchronous motor.
DC Field Excitation:
In most synchronous motor installations, direct current for the field is obtained from a DC exciter circuit which may supply fixed excitation to several AC machines. Some synchronous motors have a DC generator coupled directly to the motor shaft. In other installations, electronic rectifiers supply the DC excitation current.
External connections for a synchronous motor
Figure 4: External connections for a synchronous motor.
Figure 4 shows the DC connections to the motor. A field rheostat controls the current in the separately excited field current. When the field switch is open, the field discharge resistor (FDR) is connected directly across the field winding.

Starting the Synchronous Motor:
A synchronous motor is never started with the DC field circuit energized for the rapid rotating field produces an alternating torque on the stationary rotor poles. The most common method of starting a synchronous motor is to connect its stator winding to the AC line, and to connect its field (rotor) winding to a field discharge resistor.

At the instant the motor is started as an induction motor, the rotating field of the stator turning at synchronous speed cuts the turns of the DC field coils many times per second and induces a high emf in the field windings as high as 1500 volts. Hence, the field circuits should be well insulated and enclosed to prevent personnel from coming in contact with them. By connecting the field discharge resistor across the field winding, the energy in the field circuit is expanded in the resistor and the voltage at the field terminal is limited to a lower value, yet still high enough to be a dangerous shock hazard.

After the motor has accelerated to about 95% of synchronous speed, the field circuit is energized from the DC source and the field discharge resistor is disconnected. The rotor then pulls into synchronism with the revolving armature (stator) flux and the motor will then operate at constant speed. Certain high-inertia, hard starting loads require special automatic field application equipment to apply field at the best position of the rotor slip-cycle to cause successful transition from induction motor operation to synchronous operation.

When the motor is shut down, the field circuits is de-energized by opening the field discharge switch. As teh field flux collapses, an induced voltage is produced in the field winding which can be of sufficient value to damage the insulation of these windings. However, with the field resistor connected across the field circuit, the energy stored in the magnetic field is expanded in the resistor. As a result, the induced voltage in the field circuit is kept at a low value.

Industrial Applications:
Synchronous motors are used to improve the power factor of the system. Decreasing the field current below normal by inserting all the resistances of the rheostat in the field circuits results in a poor lagging power factor. If the motor is normally excited, it will have a unity power factor. If the motor is overexcited, the power factor is decreased because of leading power factor.

Synchronous motors are used for constant speed applications in series of 20 horsepower and larger.

They are used to drive larger air gas compressors which must be operated at a fixed speed in order to maintain a constant output at maximum efficiency. They are also used to drive DC generator, where a source of direct current is required. Other applications include driving fans, blowers and large pumps in water pumping stations.

The three phase synchronous motor is used in some industrial applications to drive a mechanical load and also to correct power factor. Figure 5 illustrates a typical industrial feeder with a lagging power factor condition caused by two induction motors. The synchronous motor connected to this same feeder is operated with an over excited field. The leading reactive kilovars supplied by the synchronous motor compensate for the lagging kilovars caused by the induction motors or other load on the same three phase distribution system. If the DC field of the synchronous motor is overexcited sufficiently to supply leading kolovars equal to the lagging kilovars of the distribution system, then the power factor of the distribution system will be corrected to unity.
Synchronous motor used to correct power factor
Figure 5: Synchronous motor used to correct power factor.
When the synchronous motor is used only to correct power factor and does not drive any mechanical load, it serves the same purpose as a bank of capacitors. When used only to correct power factor, the synchronous motor is called a synchronous capacitor or a synchronous condenser.

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