Energy Losses in Generator And Their Reduction

Like dynamos, the motors are also of three types.
1.    Series wound motor. Here the field coils and the armature coils are in series and the same current passes through both. The number of turns of the field coil is small. At small loads, current in small, the field is weak and the speed is high. A series motor is never used for belt-driven machinery, because if the belt breaks, the speed will become very high. It is always used in a loaded state. Torque is proportional to the square of the current,
    A series motor exerts large torque at low speed and is therefore used of cranes, lifts, electric tractin and in sewing machine.

2.    Shunt wound motor. In the case, the field coils are connected in parallel to the armature coils.
The resistance of the field coils is high and field current is independent of the load. The motor is connected to a constant voltage and thus the field coils behave in a manner as separately excited cols. The variation of speed with load is very small and the shunt motor is known as a constant speed machine. The curve relating to torque and armature current is a straight line. The torque is proportional to the current, τ ∞ I. This type of motor is generally used in fans, blowers and other machines where constant speed is required.

3.    Compound wound motor. Here the field coils are connected both in parallel and in series with the armature. The characteristics are midway between stunt and series wound motor. Such motors are designed to give variation of speed of torque with armature current. They are used to drive machinery involving very heavy loads of having a flywheel that helps in fluctuations of varying load and also where large torque is required.
The characteristic of series, shunt and compound wound motors.



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