Characteristic Features of Synchronous Motors.
1. They run at synchronous speed. This machine will run at a constant speed which can be changed by varying the supply frequency.
2. They are not self starting.-The rotor and stator fields have to be excited by a separate D.C excitation voltage.
3. They can be operated under a wide range of power factors (whether lagging or leading).This is why they are used for power-factor correction in power systems that have loads containing lagging power factors.
Principle of Operation – How it works
When a 2phase winding is fed with a 3phase voltage supply, a magnetic field is produced according to Faraday’s 1st law of Magnetism. Therefore a magnetic field of constant magnitude is produced where the magnitude depends on the rate of magnetic flux according to Faraday’s 2nd Law of Magnetism. The magnetic field produced rotates at a synchronous speed after the rotor has rotated half the period. The rotor rotates back in the opposite direction because the direction of the induced current is opposing the change in magnetic field. (Lenz Law).This is the reason why synchronous motors are not self starting. At the half period point, the stator and rotor poles interchange their positions and therefore the motor rotates in the reverse direction. Due to the continuous and rapid rotation of the stator and rotor poles, the rotor is subjected to a torque which is rapidly reversing and that’s why a D.C excitation is required to start it. As the load on the motor is increased, the rotor tends to fall back in phase but still continuous to run at synchronous speed.
Procedure for Starting a Synchronous Motor.
1. The main field winding is short-circuited.
2. Reduced voltage with the help of an auto transformer is applied across the stator terminals.
3. The motor starts running.hen it reaches a steady speed as it will be judged with the wrecking sound, a weak D.C is supplied by removing the short circuit in the main field winding.
4. If the excitation is sufficient, the machine is put into synchronization and it starts running at synchronous speed.
5. Apply full supply voltage at the stator terminals by cutting out the auto transformer.
6. If you want to vary the power factor therefore you vary the D.C excitation. The motor maybe operated at any desired power factor by changing the D.C excitation.
Application of 3 Phase Synchronous Motors
Power Factor Correction
Overexcited synchronous motors with a leading power factor are usually used for power factor for those power systems which have a large number of induction motors and other devices having a lagging power factor like welders and fluorescent lamps. They can be seen in large establishments like Universities, Schools, Hospitals and Industries.
Constant Speed Applications
Because of high efficiency and high speed synchronous motors are used where constant speed is required Example: Centrifugal Pumps, Belt Driven Reciprocating Compressors and Line Shafts.
Voltage Regulator
The voltage at the end of a long transmission line varies greatly when large inductive loads are present. If the inductive loads are disconnected, suddenly, the voltage tends to rise considerably above its normal value because of its line capacitance Xc. By installing a synchronous motor with field regulator; this change in voltage can be controlled. This is called voltage regulation.
Am sorry no image in this post, but in case you come across a synchronous motor take a picture and e-mail me I will publish as soon as possible.You can see it at
http://www.diytrade.com/china/4/products/1609079/Ac_synchronous_motor.html
and more info at
http://en.wikipedia.org/wiki/Synchronous_motor
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