Classification of Three Phase Motors

Classification of Three Phase Motors

Three Phase induction motors are classified on the basis of working voltage and there construction.

Classification on the basis of operating voltage:- 

1. HT motors:- These motors works on high voltage 11 kv, 6.6 kv etc. normally motors above 500 kw is comes with high voltage winding. H T motors are used in very big industries for heavy loads like Exhaustors fans, Induced draft fans, very big compressors etc. These motor are  of two types first one is synchronous motor. Synchronous motor stator consist of three phase high voltage winding. Rotor is generally sailient pole type with 4 pole ( same as stator winding ) dc winding normally 24 to 50 volt dc with 200 to 300 ampere ( 1MW motor). In addition to this dc winding rotor has also a damper winding made of solid copper conductor on the rotor pole face. This damper winding provide the starting torque to rotor whi!e starting and reduce huntung of motor during running. The power factor of synchronous motor can be adjusted with rotor current ( excitation current) so these motors are also used for power factor correction purpose. The second is simple induction motor.

2. LV motors:- these motors are working at 415 vac.

Classification on the basis of rotor winding:-

1. Squirrel cage induction motor:- These motors are having 3 phase winding in stator and a squirrel cage winding in rotor. The squirrel cage winding consist of  solid conductors of copper or alluminium in rotor slots with short circuiting end ring on both sides. These  motors are used with direct on line starter or star delta starter. The starting torque of motor is 350 percent of full load torque. These motors are used in floor mills, belt conveyors, ventilation  fans, compressors  in industry. These motors runs at approximatly constant speed near synchronous speed. Some double speed motors are also comes. The speed control of squirrel cage induction motor is done with the help variable voltage variable frequency drives.        2. Slip ring motor or wound rotor motor:- In this type of motor stator has 3 phase winding. Rotor has also three phase winding connected in star connection. Three leads of rotor winding is connected to slipring mounted on shaft. External connection to slipring is given with  help of  carbon brushes. In slipring motor 3 phase supply is given to stator and a three phase star connected external resistance is inserted in rotor circuit. At starting full resistance is kept in rotor circuit as speed increses gradually all resistance is cutoff that is external resistance is short circuited. The benifit of slipring motor is that  its maximum torque can be obtain from initial speed to full load speed. Slipring motors are used in electrcal overhead cranes where high starting  torque is required.