الفهرس 
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Abstract
The D.C.motor is widely used in variable speed drives owing to the ease by which its speed can be varied.either by armature or field voltage control.However due to the presence of the commutator and the brushgear,it has the following disadvantages:
1-It requires periodic maintenance due to the presence of moving contacts subjected to wear.
2-The commutator increase the size and cost of the machine.
-The commutator limits the output power for a given frame size beacause of the limitations on current voltage and speed.
4-It can not be used in hazardous areas or working conditions which requires sparkless operation.A conventiala.cmachine either of the synchronous or introduction type does not require a commutator.As a result of this,it is considerably less expensive than the d.c.machine of the same rating,as well as being more robust mechanically,therefore less maintenance is required.The only disadvantage of these types of machines is that the speed is dependent on the frequency of the supply,which is normally fixed.The speed of the synchronous motor is constant independent of the load on the motor shaft as long as the supply frequency is constant.The speed of an induction motor may be controlled by the following convential methods:
1-Changing the number of poles,gives discrete speed variation.
2-Change of supply frequency
Changing the rotor circuit resistance(in slip-ring type only.
4-Cascade connection(Kramer and sherbuis,Slip-ring type)