DC MOTOR

INTRODUCTION

An electrical motor is a electric device which converts electrical energy into mechanical energy (one form of energy into another form).Dc motors are widely used in applications that require control of speed ,initially high torque(used in elevator , vacuum cleaner)engine starters etc . In general in every mechanical machine such as lathe, drill machine, grinding machine, we use series motor due to its capacity of supplying constant speed. They are made in sizes form 1/100 hp to thousands of hp. We know that Dc electric devices are basically of two types: Dc motors and Dc generators, both are similar in appearance and construction.

Construction of Dc motors:

Yoke is the outer most part of the Dc motor which protects the Dc motor and act as cover. The other main parts are as follow:

Pole core/pole shoe:-It contains the pole coil (provide support to the pole coil). It also provide low resistance path for current (which is carried by pole coil) by providing large area.

We know that according to faradays law of electromagnetism, when a conductor moves in magnetic field it produces electrical energy (emf is produced) or when magnetic field constantly change its direction in the presence of conductor, electrical energy is produced.

Armature is the most important part of Dc motor where the armature slots contain the armature conductors.  The magnetic field is provided by pole core /shoe and when armature conductors move in the magnetic field, electrical energy is produced. This produced (AC) electrical energy is converted into DC form with the help of commentator. The commentator is supplied with current by means of carbon brushes. Bearing are also used in the motor with brushes to reduce the friction.

Dc motors are generally of two types:-

1-Self exited Dc motors

2-Seperately exited Dc motors

In general we use self exited Dc motors because it doesn’t require separate source for initial supply of current.

Self exited Dc motors

1:- Dc shunt motors

2:-Dc series motors

3:-Dc compound motors

Compound motors are deferentially compounded long shunt/short shunt and commutatively compounded long /short shunt motors.

These motors differ in the construction of their field coils and in the connection between the field coils and armature.

BACK EMF:-

It is the electromotive force produced by the motor to oppose the source which produces the emf. When voltage is applied across the motor it generate (in the presence of magnetic field) force. The armature conductors move in the direction of induced force. When armature conductors move in magnetic field, emf is induced. According to the Lenz law the direction of induced emf always opposes the cause which produced the emf. Here the cause is the applied voltage source.

Small Dc motors of less than ½ hp size consume very little current and therefore can be started by placing full voltage across the motor terminals , but in the large Dc motor ,it full voltage is applied ,excessive current will flow which damages the motor . To start a large motor, it is necessary to connect a resistance unit in series with the armature so that the starting current is reduced to a safe value. This external resistance is called as starter. Now a day 3-point motor starter is generally used in a shunt motor.

As the motor starts, the resistance is completely cut off after the motor has reached the desired speed. At the high speed the counter /back emf produced in the armature prevents an excessive current flow.  Rotation of Dc motors can change by reversing the direction of the current flow in either the armature or field circuit.