Synchronous Motor basic how to start method of synchronize difference of induction motor and synchronous motor
Synchronous motor
An alternator which runs as a motor is called as synchronous motor. 3 phase AC supply is required for the AC winding and suitable DC voltage is required for the field winding excitation. The synchronous motors are not self starting.
Working principle
When the stator winding of a three-phase synchronous motor is connected to a three-phase supply, a rotating field is set up in the machine. If the rotor is then started in the direction of rotation of the rotating field, the north pole of the rotating field draws the south pole of the rotor with it, and the south pole of the rotating field draws the north pole of the rotor. The rotor continues to turn at a speed of rotation which can be calculated from the familiar formula, Ns = 120f/p. It turns synchronously with the
rotating field. The machine is now working as a motor.
Construction
In construction, synchronous motors are almost identical with the corresponding alternator, and consist essentially of two elements.
1 Stator (armature)
2 Rotor (field)
A synchronous motor may have either a revolving armature or a revolving field, although most synchronous motors are of the revolving field type. The stationary armature which is wound for the same number of poles as rotor is attached to the stator frame while the field magnets are attached to a frame which revolves with the shaft. The field coils are excited by direct currents, either from a small DC generator (usually mounted on the same shaft as the motor and called as an exciter), or from other DC source. Figure 1 shows a directly connected exciter
Methods of Starting a Synchronous Motor
Action of Damper Winding at Start
1. By Using a Pony Motor
The small induction motor coupled to the synchronous machine for starting purposes is called the Pony Motor.
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Process: A three-phase current is fed to the stator winding.
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Speed Control: The rotor is started by a pony motor (having the same number of poles as the synchronous motor).
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Transition: The pony motor brings the motor very close to the synchronous speed.
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Lock-in: DC is supplied to the field, the pony motor is switched “off,” and the motor pulls itself into synchronous speed.
2. By Using Damper Windings
The damper winding is a squirrel cage winding consisting of copper bars embedded in the pole shoe and short-circuited at both sides.
The Starting Mechanism:
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The stator sets up a rotating magnetic field that cuts the damper winding on the rotor.
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This induces a current, developing torque that runs the motor slightly below synchronous speed (acting as an induction motor).
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DC excitation is then switched on, creating definite poles on the rotor.
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The two sets of poles (stator and rotor) lock together, pulling the motor into full synchronous speed.
Operational Steps:
Note: While starting, the main field winding is initially short-circuited. Once a steady speed is reached, the short is removed, and DC excitation is applied to achieve synchronism.
3. By Synchronisation
In this method, the synchronous motor is initially operated as an alternator.
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Step 1: The machine is run as an alternator and synchronised with the main supply bus using standard synchronisation methods.
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Step 2: Once synchronised, the prime mover is disconnected.
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Step 3: The machine continues to run at synchronous speed by drawing power directly from the supply mains.
| Method | Primary Component | Key Characteristic |
| Pony Motor | External Induction Motor | Matches pole count; disconnected after start. |
| Damper Winding | Squirrel Cage Bars | Starts as induction motor; locks via DC excitation. |
| Synchronisation | Prime Mover / Alternator Mode | Synchronised to busbar before removing drive. |
| Aspects | Synchronous Motor | Induction Motor |
|---|---|---|
| Speed | Synchronous speed is constant independent of load condition. | Less than synchronous speed. Decreases with increasing load. |
| Power Factor | Operates at all power factors whether lagging or leading. | Operates at only lagging power factor. |
| Efficiency | Very good | Good |
| Cost | Costlier | Cheaper |
| Starting | Not self-starting | Self-starting |
| Speed Control | No speed control | Can be controlled in small range |
| Application | Used for mechanical load and for power factor improvement (synchronous condenser) | Used mainly for mechanical loads |
