In this article we will discuss about the classification of electric motors.
Type # 1. D.C. Motor:
A motor which receives its supply of electrical energy from a direct current voltage is called D.C. motor.
D.C. motor mainly consists of:
i. Armature, and
ii. Field coil
i. Armature:
The rotating part of the motor is called Armature. It is an assembly consisting of:
(a) Armature core
(b) Armature winding
(c) Commutator
The armature has got electromagnetic coils, distributed in slots around its periphery. The current is conducted in armature winding which is achieved by connecting leads from the armature winding to the commutator and brushes. The armature is designed to withstand electrical and mechanical stresses and to hold the coils of the wire. The material used in armature winding is of high electrical resistance and good mechanical strength.
ii. Field Coil:
Field coil is mounted on field magnets. The field magnet consists of a circular yoke with inwardly projecting poles, the polarity of which is alternately north and south poles. The poles may be cast steel or wrought iron or laminated sheets. Suitable coils are wound round the circular yoke. The field coil remains stationary whereas the armature coil rotates uniformly.
D.C. motor is of three classes depending upon the modes of winding:
(a) Series wound motor
(b) Shunt wound motor
(c) Compound wound motor series wound motor.
(a) Series Wound Motor:
It has got its field winding, connected in series with the armature. This is mostly used for direct coupling and other works where the load is permanently coupled with the motor.
At light load, the speed becomes very high and so it becomes dangerous.
(b) Shunt Wound Motor:
It has got its field winding connected in parallel (shunt) with the armature. Its speed is nearly constant.
(c) Compound Wound Motor:
It is a combination of series and shunt wound motor. This type of motor is mainly used for cranes and other heavy duty works, where an overload may have to be carried at a heavy starting torque.
D.C. Motor Starter:
Very small motors are started from rest by connecting them directly to the domestic supply line.
Usually small motors do not have a starter because:
(i) Such motors have a relatively high armature resistance than large motors hence their starting current is not very high.
(ii) It has got low moment of inertia, hence it attains speed quickly.
The box type starter used for starting shunt and compound motors are of two kinds:
i. Three point starter and
ii. Four point starter
i. Three Point Starter:
There are three terminals of the starter box. One line is directly connected to one terminal. The other line is connected to a point which is further connected to the starting arm through the overload.
In order to start the motor, the main switch is first closed and then the starting arm is slowly moved to the right. As soon as the arm makes contact with first point, the field circuit is directly connected across the line and at the same time, full starting resistance is placed in series with the armature. As the arm is further moved, the starting resistance is gradually cut down.
Till the arm reaches the running position, the resistance is all cut. There is a soft iron piece S attached to the arm. In running position it is attracted and held by an electromagnet energised by the shunt current. It is known as Hold on coil. The function of the Hold on coil is to hold the arm in the full running position when the motor is in normal operation. But in case of power supply failure or break in the field circuit, it is de-energised, thereby releasing the arm which is pulled back by the spring to the opposition.
ii. Four Point Starter:
In this starter, the Hold on coil is taken out of the shunt field circuit and has been connected directly across the line through a protecting resistance.
When the arm touches the first point, the line current divides into three parts:
(a) One part passes through starting resistance
(b) The second part passes through shunt field and to field rheostat and
(c) The third part passes through the Hold on coil and current protects resistance R.
Due to change in the shunt, field circuit does not affect the current passing through the Hold on coil because two circuits are not connected to each other.
Type # 2. A.C. Motors:
A.C. motors are those motors which run on alternating-current.
A.C. motor can be classed as follows:
i. Induction motor
ii. Synchronous motor, and
iii. Series type motor.
i. Induction Motor:
Induction motor is the most popular type of motor, used in agricultural purposes. According to the source of power, the motors are commonly divided into two groups i.e. single phase motor and three phase motor.
The essential principle of an induction motor is that the current in the starter, produces a rotating flux which induces a current in the winding of the rotor thus producing necessary torque. Induction motors have a few special features of having constant speed and variable horse power. It is so called because the rotating member obtains energy through the process of magnetic induction.
The main parts of induction motor are:
(a) Rotor and
(b) Stator
Rotor is the rotating member of the motor.
There are two types of rotor:
(i) Squirrel cage rotor, and
(ii) Wound rotor
Squirrel cage rotor consists of a series of bars placed in slots around the periphery of the rotor. There are end rings provided for short circuiting the conductors.
The wound rotors are not short circuited but are connected to slip rings on one end of the shaft. The slip rings provide a means for connecting external resistance with the rotor circuit. The resistance changes the input current, torque and motor speed. Wound rotor is not usually used on single phase motor. It is used only on polyphase system.
In the induction motor, the stator winding sets up a rotating magnetic field. When the magnetic field cuts across the rotor conductor, torque is developed. At full load, the rotor speed is usually about 97% of that of rotating field.
Motor which uses squirrel cage type of rotor is called Squirrel cage motor and the motor which uses wound rotor is called Wound Rotor or Slip ring motor.
Squirrel Cage Motor:
In this type of motor, the conductors are not insulated, they are bare conductor, set into the slots of the rotor. The conductors are connected together solidly by a ring at each end of the motor.
These motors are so termed on account of the fact that the construction of the rotor circuit is similar to a revolving cage used for tame animals. The rotor consists of solid bars of copper, arranged round the periphery of the rotor and connected at each end by a ring of copper. Squirrel cage motors are popular due to their robustness, simplicity, lower cost and lower maintenance compared to a Slip ring motor.
Slip Ring Motor:
A motor with an insulated rotor winding is called Slip ring motor or Wound rotor motor, because the rotor has to be wound with insulated conductors similar to those used for the stator. In case of large motors, an arrangement is fitted to the rotor shaft, enabling the slip rings to be short circuited and the brushes lifted off the slip rings, thus reducing both electrical and frictional losses.
ii. Synchronous Motor:
The synchronous motor runs at an exact speed for all loads. If the load is increased to a predetermined value, the motor stops. It is usually used in industries. This type of motor usually requires a d.c. source for exciting the field poles.
iii. A.C. Series Motor:
A.C. series motor has brushes, a commutator and a d.c. type wound rotor. The rotor is usually connected in series with the stator winding. The torque is developed because of the interaction between rotor current and field flux. They do not depend upon the rotating magnetic field. These motors give high starting torque, variable speed and constant horse power output. As the load on the motor increases, the speed reduces to a great extent. At no load, the speed is very high.
According to the source of power, the motors are commonly divided into two groups:
A. Three phase motor
B. Single phase motor
A. Three Phase Motor:
This type of motor has three main parts:
(a) Rotor
(b) Stator, and
(c) End plates
The motor operates by the interaction of magnetic fields without a centrifugal switch. Here one winding is connected to the supply line. The stator consists of laminated iron core and a frame made of cast iron.
The coils in the slots of the core are connected to form separate windings called phases. The windings are arranged in such a way that a magnetic field is formed inside the stator. These conductors form a system of closed circuit, around which current can flow. In this case it is not necessary to use a commutator since a.c. poles change the polarity at equal intervals.
B. Single Phase A.C. Motor:
In general, the single phase motor is employed only for light work, where the single phase electric supply is available.
Single phase motors are classed as:
a. Split phase type
b. Capacitor type
c. Repulsion type, and
d. Shaded pole type
a. Split Phase Motor:
It works on a single phase power circuit.
The main parts are:
(i) Rotor
(ii) Stator
(iii) Centrifugal switch, and
(iv) End plates
The rotor unit has got three parts:
(i) Winding
(ii) Laminated core and
(iii) Shaft.
There are copper rings, placed on both ends of the core. The copper bars are fitted into the slots of iron core. The stator unit consists of laminated iron core, two windings (running winding and starting winding) and a frame made of cast iron.
Both windings are connected to the two terminals of the main power line at the time of starting the motor. With the flow of the current, a magnetic field is established inside the motor. This induces a current in the rotor winding, providing another magnetic field.
Due to interaction of these magnetic fields the rotor starts rotating. A centrifugal switch is provided which automatically disconnects the starting winding when the motor reaches a 75% of the full load speed. In this case, starting current is high as much as five to seven times full load current. The direction of the rotation is reversed by interchanging the loads of the starting winding.
b. Capacitor Motor:
Capacitor motors are a form of split phase motor with a condenser on the side or on the top. The starting current is three to four times of full load current and the starting ability is about twice that of a similar split phase motor. They are reversed by interchanging the leads of the starting winding. Usually the capacitor is mounted on the top of the motor. It develops high starting torque at low starting current. The capacitor is able to store electrical energy.
In order to produce a starting torque, a magnetic field is established by placing the starting winding 90 electrical degrees out of phase with the running winding. Current reaches a maximum value in the starting winding before the current becomes maximum in running winding. This induces the current in the stator as well as rotor winding, producing rotation in the system.
c. Repulsion Motor:
In repulsion motor, the stator winding is used with a wound rotor having a commutator which is short circuited. The brushes are set at an angle to the main field. Due to transformer action, the field and armature fluxes repel each other and the rotor produces a torque.
d. Shaded Pole Motor:
This motor consists of a squirrel cage rotor and two or more coil with an iron core to increase the magnetic effect. Part of one end of this core is surrounded by heavy copper loops known as shading ring. This ring delays the flow of magnetism through it.
With flow of alternating current to the coil, the magnetism is strong at one point and weak at other end. This produces a rotating field that causes the rotation of the rotor in the direction in which the shading ring points. A motor thus constructed is known as shaded pole motor.