INTRODUCTION OF DC MOTOR KRISHNA KARIYA nd YEAR ELECTRONICS AND TELECOMMUNICATION BIRLA VISVAKARMA MAHAVIDHYALAYA

DC Motor Applications

DC Motor The construction of a DC motor is virtually the same as that of a DC generator and many machines may be operated as either, such as a starter-generator.

Construction of DC motor  armature  the yoke (or casing)  the field coils

Construction of DC motor  an armature mounted in bearings  a stationary magnetic field  commutator and brushes

Direction of conductor tend to move

Armature The armature is typically a soft iron drum mounted on the motor shaft, with the armature conductors set axially into the surface of the drum. Also mounted on the armature shaft are the commutator segments, to which the armature conductors are connected. The armature shaft is mounted in ball bearings at each end, the bearings being held in the ends of the motor casing.

Field windings The field windings are attached to the inside of the yoke and form two poles fitting closely around the armature with a running clearance of about 2.5mm.

Section through a DC Motor

DC MOTOR PRINCIPLE OF OPERATION

Back EMF The loop of an armature is moving through the stationary field as the armature rotates and this inevitably induces an EMF in the armature. This EMF produces a current flow that opposes the applied current from the battery and therefore reduces the total armature current flow. The induced voltage is known as back EMF.

net EMF  The difference between the applied EMF and the back EMF is known as the net EMF, and it is this that determines the torque produced in the armature shaft.  In order to ensure that the net EMF is sufficient the resistance of the armature windings is kept as low as possible.

Armature current The initial current flow through the armature, before it begins to rotate, is determined by the applied voltage and the armature resistance. If the resistance is low the current flow will be very high. As the motor gains speed the back EMF increases and reduces the current flow through the armature.

Why there is a surge current when DC motor start? To avoid excess starting current, some DC motors have a resistance built in to the armature windings, which automatically cuts out as motor speed increases.

TYPES OF DC MOTORS  series wound  shunt wound  compound wound

Series Wound Motor

Series Wound Motor Continued ---  The field coils are connected in series with the armature.  At starting, when the current flow is very high. Consequently a characteristic of the series wound motor is high starting torque. This is useful in circumstances where the motor will be required to start against a high load and where the running load is also high.

Series Wound Motor Continued --- Examples of instances where series wound motors are used are engine starter motors, flap operating motors and landing gear operating motors.

NOTE  Series wound motors should never be allowed to operate without a mechanical load applied.  This is because they are liable to over- speed, possibly to destruction.

Shunt Wound Motors

Shunt Wound Motors Continued--  The field coils are connected in parallel with the armature windings.  The resistance of the field coils is deliberately set to limit the field current to that required for normal operation of the motor, and is much higher than the armature resistance.

Shunt Wound Motors Continued---  On start up the current flow through the armature is high, because of its low resistance.  a characteristic of the shunt wound DC motor is low starting torque.  As the armature speed increases, increasing back EMF will cause the armature current to decrease.

Shunt Wound Motors Continued---  Shunt wound motors are used when starting torque is low and increases with motor speed.  They are particularly useful where constant speed under varying load conditions is a requirement.  Typical applications in aircraft are fuel pumps and fans.

Compound Wound Motors

Compound Wound Motors Continued--- It has two sets of field windings, one connected in series with the armature and the other in parallel. The low resistance series windings are shown in heavy lining, the higher resistance shunt windings in lighter lining.

Compound Wound Motors Continued--- The compound wound motor combines the characteristics of the series wound and the shunt wound motor. It is capable of high starting torque, but will not over-speed under light mechanical loading and will maintain a reasonably constant speed under varying conditions of load.

Compound Wound Motors Continued---  The compound wound motor is suited to applications where loads may vary from zero to maximum and where starting loads may be high.  In aircraft they are often used to drive hydraulic pumps and used as a starter/ generator.