Till now we know about an AC Motor with only one rotor, but this is a special motor which has 2 rotors. The 2 rotors are primary and secondary. Here the two rotors are coaxial but one inside the other that is the primary rotor will be inside the secondary.
Construction:
Construction:
The stator of this motor is similar to the stator of general 3 phase AC motor. This motor contains 2 different types of rotors; primary rotor is a salient pole rotor and the secondary is hallow cylindrical rotor. These rotors have sliprings. 
Working:
This motor can be operated in 3 different ways . They are
- Primary as synchronous motor and blocked secondary
- Both primary and secondary as synchronous motors
- Primary as synchronous motor and Secondary synchronous condenser
PRIMARY AS SYNCHRONOUS MOTOR WITH BLOKED SECONDARY:
As mentioned in this mode the secondary rotor is blocked and DC supply is given to the primary rotor. The 10 rotor also contains damper windings. When 3-phase supply is given to the stator windings a rotating magnetic field is generated. By the faraday’s law of electro magnetism emf is induced in the damper windings. As the damper windings are short circuited current flows through it and the rotor starts rotating with a speed near to synchronous speed. Because of magnetic locking between the rotor and stator magnetic field 10 rotor will rotate with synchronous speed.
As the 20 rotor is blocked and the windings of this rotor are in rotating magnetic filed emf is induced in these windings also. By connecting a load to the sliprings electrical energy can be taken out. There will small phase difference between the stator emf and the 20 rotor but the frequency of induced emf will be same.
Hence the magnetic locking will be strong and effect of hunting can be suppressed more when compared with normal synchronous motor, even there is sudden change in loads.
Advantages:- It draws less amount of current from the supply.
- Hunting effect can be reduced.
- Less variation in speed for sudden change in loads
Both primary and secondary as synchronous motor:
Here both 10 and 20 rotor windings are given dc supply. As mentioned above the 10 rotor will rotate but secondary rotor will not rotate so it must be stated as the induction motor. After reaching near to synchronous speed DC supply is given to the rotor. Then magnetic locking takes place and the rotor rotates with synchronous speed.
Primary as synchronous motor and Secondary synchronous condenser:
Here also dc supply is given to both the rotors but primary rotor is loaded and the secondary is not loaded. As the synchronous motor running under no-load condition act as synchronous condenser we can use this machine as both synchronous motor as well as synchronous condenser at a time itself.
ADVANTAGES OF DUAL ROTOR SYNCHRONOUS MACHINE:- It utilises less power.
- Hunting is suppressed well when compared with normal synchronous machine.
- It can be used as synchronous condenser as well as motor.
- By blocking secondary rotor we can take the electric supply and can be given to loads.
- Cheaper when compared with two separate synchronous motors.
- Same as normal synchronous motor this machine can also be used at various power factors
- By blocking secondary rotor and by short circuiting the windings of primary rotor we can use it as Induction machine.
- It occupies more space than normal synchronous machine
By
K. Vinod Krishna
10H71A0259
Categories:
TECHNICAL ARTICLES(session-2)
