Introduction
Synchronous motor is electrically identical with an alternator or an ac generator.
The synchronous machine will then operate as a motor and delivers the mechanical energy to the load.
The speed of rotation of this motor is called as synchronous speed Ns which is directly proportional to supply frequency because
Ns = 120 f / P
Construction of Three Phase Synchronous Motor
Construction of three phase synchronous motor same as that of an alternator.
It has two windings stator (armature) rotor (field) windings.
The armature (stator) winding is placed in the slots which are distributed over the entire yoke.
The stator winding is connected to the 3 phase ac supply.
The field winding is placed in the rotor slots, and it is connected to the dc supply
The DC supply is also called as excitation, and it is obtained from a dc shunt generator mounted on the shaf.
The rotor can be a salient-projected type or cylindrical type. Normally the salient pole rotor is preferred.
Rotating Magnetic Field (RMF):
The stator winding (star or delta connected) is connected to a 3 phase ac supply, a rotating magnetic field (RMF) is produced in the air gap between the stator and rotor.
The RMF rotates at a speed called synchronous speed Ns which is given by,
Ns = 120 f / P
where
f = Supply frequency and
P = Number of stator poles.
Types of Synchronous Motor
- Permanent magnet synchronous motors.
- Wound field synchronous motor.
- Reluctance type synchronous motor.
Permanent magnet synchronous motors.
These motors do not have the field (rotor) winding Instead they have a permanent magnet rotor. No external dc excitation is needed.
This will eliminate the need for the sliprings and brushes. This reduces the winding losses and necessity of maintenance.
PMSM cannot produce very high torque and hence they are preferred in low power applications only.
Such motors are often referred to as "brushless dc experiences motors or electrically commutated motors. Because of the elimination of field winding they lose their ability to control the power factor.
Wound field synchronous motor
In synchronous motors the stator is known as of armature, and it has the three phase windings which are connected to the three phase supply.
The rotor is known as "field" and in the wound field motors, the field winding is connected to a dc source through brushes and sliprings.
This dc voltage is called as excitation. The field wound synchronous motor can be operated over a wide range of lagging and leading power factors by changing the field excitation. Thus these motors can be used in the applications that need the power factor correction.
Reluctance type synchronous motor
These motors also do not have the field (rotor) winding, but they have salient poles on the rotor.
The flux is the air-gap between the stator and the rotor is produced only due to the current drawn by the stator (armature) winding of the motor.
The torque producing capacity of these motors therefore low and the motor has low power factor.
It is therefore used for applications requiring low torque and power.
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