THREE PHASE SYNCHRONOUS MOTOR CONSTRUCTION DETAILS AND OPERATION
Three phase synchronous motor is normally comes in higher kilowatt and voltage rating. These motors are used for driving heavy loads such as Exhaustors fan, compressors etc. These motor are designed to run continuously for months together. In large industries these motors serve two fold purpose of driving mechanical load as well as power factor improvement. These motors can run at leading power factor with increase in excitation current. The main feature of synchronous motor is its constant speed operation. The motor runs at constant synchronous speed given by supply frequency and number of poles in stator winding.
CONSTRUCTION
The motor is mainly consist of a stator, rotor, bearing and cooling system for higher kilowatt motors.
Stator:- The stator is made of silicon steel lamination of desired diameter and stacked for suitable stator length. The motor kilowatt rating is depends on diameter of core at air gap and core length. Radial ventilation ducts are provided at regular distance of 70 to 80 mm for forced air cooling of motor. Stator is wound with a three phase winding with suitable number of poles depending on motor speed. High voltage winding is used for motors above 500 kw rating. voltage is usually 6.6 kv or 11 kv depending on supply system standards. The stator is fixed in motor body. Body is made of steel with box type fabricated construction.
Rotor:- there are two types of rotors used in synchronous motors.
1 Smooth cylindrical rotor- It is used in machine with 3000 RPM in India for 50 hz. Smooth cylindrical rotor has slots on the periphery which holds distributed winding to produce sinusoidal magnetic field.
2 Salient pole rotor- It is used in motors with 1500 RPM and lower RPMs.
Salient pole rotor- The number of poles in a rotor depends on the number of poles in stator winding. For example if a motor has 4 poles in stator winding then rotor will also have 4 poles and must be wound for 4 pole dc winding. In Salient pole rotor a spider is mounted on shaft. Rotor poles are fixed on spider with dovetails or T slots. The winding leads are connected to two slip rings mounted on shaft. A low voltage 24 volt to 50 volt high amperes DC supply is given to rotor for synchronizing the motor. This rotor current is normally 300 to 500 amp for 1 MW motor. In addition to this DC excitation winding there is a squirrel cage damper winding on the rotor poles face. This damper winding provide the starting torque to the motor which makes motor self starting.
Pedestal bearing :-There are two pedestal bearings to support motor shaft. Bearings are made of babbitt metal. A oil cooling system is used for cooling of bearings.
Heat exchanger :-A heat exchanger is used in forced air cooling circuit to transfer heat from cooling air to cooling water.
Exciter:- Exciter is a low voltage high current DC generator coupled to the main motor shaft. The DC supply generated in exciter is used for excitation supply of main motor.
Operation:- Synchronous motors are started at no load so first of all damper gate of air line is completely closed. Three phase power is given to motor by closing main ocb (oil circuit breaker). As the three phase supply is switched on a rotating magnetic field is established in stator core which induces torque in damper winding and rotor starts rotating. This is just like a simple induction motor starting. Gradually motor speed increases and reaches near synchronous speed, DC contactor is automatically picks up and DC excitation supply is switched on to rotor circuit through carbon brushes and slip rings. This excitation supply magnetize the rotor poles which pulls up the rotor poles to stator poles and then stator and rotor pole gets magnetically locked known as motor is synchronized. Then rotor excitation current is regulated as per load and power factor requirement. Now damper can be opened as per requirement.
Running parameter:- stator current, rotor current, winding temperature, bearing temperature, oil flow, motor vibration etc are monitored and recorded on hourly basis.
No-load-vector-diagram-of-transformer
Heat exchanger :-A heat exchanger is used in forced air cooling circuit to transfer heat from cooling air to cooling water.
Exciter:- Exciter is a low voltage high current DC generator coupled to the main motor shaft. The DC supply generated in exciter is used for excitation supply of main motor.
Operation:- Synchronous motors are started at no load so first of all damper gate of air line is completely closed. Three phase power is given to motor by closing main ocb (oil circuit breaker). As the three phase supply is switched on a rotating magnetic field is established in stator core which induces torque in damper winding and rotor starts rotating. This is just like a simple induction motor starting. Gradually motor speed increases and reaches near synchronous speed, DC contactor is automatically picks up and DC excitation supply is switched on to rotor circuit through carbon brushes and slip rings. This excitation supply magnetize the rotor poles which pulls up the rotor poles to stator poles and then stator and rotor pole gets magnetically locked known as motor is synchronized. Then rotor excitation current is regulated as per load and power factor requirement. Now damper can be opened as per requirement.
Running parameter:- stator current, rotor current, winding temperature, bearing temperature, oil flow, motor vibration etc are monitored and recorded on hourly basis.
No-load-vector-diagram-of-transformer
No comments:
Post a Comment