3 PHASE TRANSFORMERS CONNECTIONS

DC MOTORS

1TTOPICS TO DISCUSS Introduction to DC MOTORS. Principle. Construction. Working. Types. Speed control. Applications.

2A DC motor or Direct Current Motor converts electrical energy into mechanical energy.

A direct current (DC) motor is a fairly simple electric motor that uses electricity and a magnetic fieldto produce torque, which turns the rotor and hence give mechanical work.

DC MOTORPRINCIPLE OF DC MOTORSIn any electric motor, operation is based on simple electromagnetism.

When acurrent-carrying conductor is placed in an external magnetic field, it will experience a force i.e. Lorentz force.

Due to this force torque is produced which rotates the rotor of motor and hence a motor runs.

DC motor consist of 2 windings 1. Field winding. 2. Armature winding.

Field winding is stationary and armature winding can rotate freely.Field windings when given DC supply, magnetic field is produced.Armature winding when connected to DC source, shaft connected to it rotates due to Lorentz force.

CONSTRUCTION

WORKING OF DC MOTORS

TYPES OF DC MOTORSDC motor are of 3 types they are..DC SHUNT MOTOR

DC SERIES MOTOR

DC COMPOUND MOTOR1. DC SHUNT MOTOR

ArmatureThe parallel combination of two windings is connected across a common dc power supply.The resistance of shunt field winding (Rsh) is always higher than that is armature winding.This is because the number of turns for the field winding is more than that of armature winding.The cross-sectional area of the wire used for field winding is smaller than that of the wire used for armature winding.

2. DC SERIES MOTOR

The field winding is connected in series with the armature.The current passing through the series winding is same as the armature current .Therefore the series field winding has fewer turns of thick wire than the shunt field winding.Also therefore the field winding will posses a low resistance then the armature winding.3. DC COMPOUND MOTORLONG SHUNT COMPOUND MOTOR

In this the series winding is connected in series with the armature winding and the shunt winding is connected in parallel with the armature connection.

SHORT SHUNT COMPUND MOTOR

In short shunt compound motor the series winding is connected in series to the parallel combination of armature and the shunt winding This is done to get good starting torque and constant speed characteristics.CUMULATIVE COMPOUND DC MOTORSDIFFERENTIAL COMPOUND DC MOTORSIf the two field windings i.e. series and shunt are wounded in such a way that the fluxes produced by them add or assist each other If the two field winding i.e. series and shunt are wounded in such a way that the fluxes produced by them always try to oppose and try to cancel each other.METHODS OF SPEED CONTROL 1. FLUX CONTROL Speed of dc motor is inversely prop. to field fluxSo speed can be controlled above base speed It is constant power speed control.Speed below base speed is not possible in this method.

METHODS OF SPEED CONTROL 2. ARMATURE VOLTAGE CONTROL-

Speed of dc machine is directly prop. to armature voltage.So speed below base speed can be controlled.It is a constant torque method.Speed reversal is possible.

APPLICATIONS OF DC MOTORSMOTORS..APPLICATIONSD.C. SHUNT MOTOR

LATHES , FANS, PUMPS DISC AND BAND SAW DRIVE REQUIRING MODERATE TORQUES.

D.C. SERIES MOTOR

ELECTRIC TRACTION, HIGH SPEED TOOLS

D.C. COMPOUND MOTOR

ROLLING MILLS AND OTHER LOADS REQUIRING LARGE MOMENTARY TORQUES.

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