DC Motors

Introduction

An electric motor is a machine which converts electrical energy into mechanical energy.The fundamental principle of the dc motors are identical with dc generators.A machine that operates as a motor will also operate as a generator.Motor and generator only differs in the mode of construction.

Working Principle of DC Motor

If the current carrying conductor is placed in a magnetic field,mechanical force is experienced on the conductor.The direction of rotation is given by the Fleming’s left hand rule  and the conductors moves in the direction of force.The magnitude of the mechanical force experienced on the conductor is given by:

F=BIcLc Newtons

where B=field strength in teslas(Wb/m^2)

Ic=current flowing through the conductor in amperes and

lc=length of conductor in meters.

When the motor is connected to the dc supply mains,a direct current passes through the brushes and commutator to the armature windings. when it passes through the commutator it is converted into ac.The group of conductors under the fields poles carry the current in the opposite direction.

 

pic source:Circuit globe(Flemings Left hand rule)

pic source:Circuit globe(working mechanism of DC motor)

When the field and armature circuits are connected across dc supply mains of 4 pole dc motor,current in armature conductors be outwards under  the N-poles(shown by dots) and inwards under s-poles (shown by crosses).The direction of force on each conductor can be determined by Fleming’s left hand rule.Each conductor experiences a force which tends to rotate the motor armature in clockwise direction.

commutator in dc motor:

The commutator is a form of rotating switch placed between the armature and the supply circuit and so arranged that it will reverse the connections to supply circuit at the instant of each reversal of current in the armature coils of the motor.In case of dc motor the current through the coils of the armature windings is necessary to be  reversed as a particular coil leaves one pole ,crosses the neutral line and comes under the influence of next pole which is of opposite polarity.

Back emf in dc motor:

when the motor continues to rotate due to motor action the armature conductors cuts the magnetic flux and emf is induced in them.The direction of induced emf is known as back emf as it oppose the applied voltage.Back emf acts like a governor i.e it makes the motor self-regulating so that it draws as much current as just required.

Back emf in a dc motor:

 

Types of DC MOTORS

The dc motor is classified as:

  1. Permanent magnet
  2. separately excited
  3. series wound
  4. shunt wound
  5. compound wound
  • cumulative compound wound
  • Differential compound wound

 

1.Permanent Magnet DC Motor

A permanent magnet dc motor is a dc motor whose poles are made of permanent magnet.The material used for permanent magnets are ceramics and rare earth magnetic materials having high residual flux and high corecivity. Field flux required in air gap of permanent magnets motor is developed by a set of permanent magnets fixed to stator. The rotor of such machine consists of armature core,armature windings and commutator. stationary carbon brushes are kept pressed on the commutator.

fig:2 pole permanent magnetized dc motor

Most of permanent magnet motor operate on 6V 12V or 24V dc supply obtained from batteries or rectifiers.

fig:equivalent circuit of a permanent magnet Dc motor

Advantages:

  • No need of  excitation  current as they doesn’t  have any field winding.
  • No loss of energy in the field .
  • smaller in size and cheaper in cost.

Limitations

  • The excessive current in the armature winding may demagnetize the permanent magnet.
  • The flux density produced in the air gap by permanent magnets is limited.
  • New permanent magnetic material like somarium cobalt and neodymium iron boron greatly reduced the risk of permanent magnetism.

Applications

used in automobiles as starter motors and for windshield wipers and washers.

used in heaters and air-conditioners for blowers.

used in raising and lowering windows.

used in slot cars and electric tooth brushes

used in personal computer disc drives.

 

2. separately Excited Dc motors:

Armature and filed coils are feed from different supply sources.

In separately excited dc motor,

power drawn from supply mains

p=VI where v is supply voltage

mechanical power developed,

Pm=power input to armature -power lost in armature

 

Applications

used for paper machinens,diesel electric propulsion of ships,in steel rolling mills etc.

 

3.series wound DC motors

Field coils consisting of few turns of thick wires are connected in series with the armature.The cross section area of the wire used for field coils has to be fairly large to carry the armature current.

In dc series motor,

power drawn from supply mains=VI

mechanical power developed,

Pm=power input-losses in armature and field

4.shunt wound Dc motor:

Fields coils are connecetd in parallel with the armature.The field windings consisits of large number of turns of comparatively fine wires to provide large resistance.The field current is less than the armature current.

The current supplied to the motor is divided into two paths,one through the field winding and second through the armature

 

wheree Ia is the armature current and Ish is the shunt field current

 

where V is the supply voltage and Rsh is the shunt field resistance.

 

mechanical power developed,

Pm=power input -losses in armature and shunt field

 

 

5.compound wound Dc motors:

This type of dc motor has both shunt and series field coils.The shunt fields has more ampere-turns.Basically there are two type of compound wound dc motors.

cumulative compound wound motor:

The field windings are connected in such a way that the direction of flow of current is same in both of the field windings.The flux due to to series field windings strengths the fields due to shunt field windings.

Application:used in conveyors,shears,crushers,bending rolls,punch presses,hoists,elevators,heavy planers,ice making machines,air compressors,rolling mills,printing presses etc.

Differential compound wound motor:

The field windings are connected in such a way that the direction of flow of current is opposite to each other in the two field windings. The flux due to to series field windings weakens the fields due to shunt field windings.

Application:For experimental and research work.

 

operating characteristics of DC motors

The performance characteristics is determined from the operating characteristics.The important characteristics of dc motors are:

1.Torque-Armature current characteristics:

This characteristics curve gives the relation between torque developed in the armature T and armature current Ia.This is also known as electrical characteristics.

2.speed-Armature current characteristics:

This characteristics gives relation between speed N and armature current Ia.This is also known as speed characteristics.

3.speed-Torque characteristics:

This characteristics gives relation between speed N and torque developed in armature T.This is also known as mechanical characteristics.

The important relations of motor characteristics are:

 

Be the first to comment

Leave a Reply

Your email address will not be published.


*