An electric motor is a machine that converts electrical energy into mechanical energy. The fundamental principle of dc motors is identical to dc generators. A device that operates as a motor will also operate as a generator. The motor and generator only differ 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 Fleming’s left-hand rule and the conductors move in the direction of a force. The magnitude of the mechanical force experienced on the conductor is given by:
where B=field strength in teslas(Wb/m^2)
Ic=current flowing through the conductor in amperes and
lc=length of the 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 poles of the field carry the current in the opposite direction.
When the field and armature circuits are connected across the dc supply mains of the 4-pole dc motor, the current in armature conductors is outwards under the N-poles(shown by dots) and inwards under s-poles (shown by crosses). The direction of the force on each conductor can be determined by Fleming’s left-hand rule. Each conductor experiences a force that tends to rotate the motor armature in a clockwise direction.
Commutator in dc motor
The commutator is a form of a rorotatingswitch placed between the armature and the supply circuit and so arranged that it will reverse the connections to the supply circuit at the instant of each reversal of current in the armature coils of the motor. In the case of a 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 the next pole which is of the opposite polarity.
Back emf in dc motor
When the motor continues to rotate due to motor action the armature conductors cut the magnetic flux and emf is induced in them. The direction of induced emf is known as back emf as it opposes 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:
- Permanent magnet
- separately excited
- series wound
- compound wound
- cumulative compound wound
- Differential compound wound
Permanent Magnet DC Motor
A permanent magnet dc motor is a dc motor whose poles are made of a permanent magnet. The material used for permanent magnets are ceramics and rare earth magnetic materials having high residual flux and high coercivity. The field flux required in the air gap of the permanent magnet motor is developed by a set of permanent magnets fixed to the stator. The rotor of such a machine consists of an armature core, armature windings, and a commutator. stationary carbon brushes are kept pressed on the commutator.
Most of permanent magnet motors operate on 6V 12V or 24V dc supply obtained from batteries or rectifiers.
- No need for excitation current as they don’t have any field winding.
- No loss of energy in the field.
- smaller in size and cheaper in cost.
- 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 materials like samarium cobalt and neodymium iron boron greatly reduced the risk of permanent magnetism.
- 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 toothbrushes
- used in personal computer disc drives.
Separately Excited Dc motors
Armature and field coils are fed from different supply sources.
In separately excited dc motor,
power is drawn from supply mains
p=VI where v is the supply voltage
mechanical power developed,
Pm=power input to armature -power lost in the armature
- Used for paper machines, diesel-electric propulsion of ships, steel rolling mills, etc.
Series wound DC motors
Field coils consisting of a 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 the dc series motor,
power is drawn from supply mains=VI
mechanical power developed,
Pm=power input-losses in armature and field
Shunt-wound Dc motor
Fields coils are connected in parallel with the armature. The field windings consist of a 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 the second through the armature
where 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 iin armature and shunt field
Compound wound Dc motors
This type of dc motor has both shunt and series field coils. The shunt fields have more ampere-turns. There are two types of compound wound dc motors.
cumulative compound wound motor:
The field windings are connected in such a way that the direction of the flow of current is the same in both of the field windings.
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 the flow of current is opposite to each other in the two field windings. The flux due to series field windings weakens the fields due to shunt field windings.
- For experimental and research work.
Operating characteristics of DC motors
The performance characteristics are determined from the operating characteristics. The important characteristics of dc motors are:
Torque-Armature current characteristics
This characteristics curve gindicatesthe relation between torque developed in the armature T and aurrent Ia. This is also known as electrical characteristics.
Speed-Armature current characteristics
This characteristic gives the relation between speed N and armature current Ia. This is also known as speed characteristics.
This characteristic gives the relation between speed N and torque developed in the armature. This is also known as mechanical characteristics.
The important relations of motor characteristics are: