INTRODUCTION :
As you know all refrigerating machines require some external source to run the compressor, fans pumps Electricity is an excellent source to transmit the electrical energy into mechanical energy So the electric motor is used as a Prime Mover for the compressor. Motors are generally grouped into two categories s.e. Alternating and Direct Current motors Because alternating current (A.C) is presently available in all parts of the country, AC motors are available in single phase 220 volts and three phase motors are 440 volts 50 c/s The motors with rating less than 1 HP are single phase or fractional HP motors and they are used for refrigerator,water coolers,deep freezer,room air conditioners/split a.c Single phase motors ther employed as Prime mover.
OBJECTIVES :
After studying this unit you will be able to
● explain the principle of electric motors
● explain the types of single phase motors
● explain the hermatic motors.
DEFINITION OF ELECTRIC MOTOR :
Electric motor is a machine which converts electrical energy into mechanical energy.
Principle of electric motor: Motors Electric motors works on principle that any conductor carrying current placed under magnetic field tends to move (rotate).
AC. single phase Induction run motor: Stationary part of ac motor is called starter and moving part is called rotor Starter is made of no. of stampings which are slitted to receive the winding Starter consists or contains Running and Starting windings which are connected to the source of power supply In single phase motor there is one main winding and other is starting winding The other part of motor is called Rotor Induction motors are generally equipped with squirrel cage rotor because it has simplest construction. Each bar is placed in slott The ends of rotor bars are welded with two short circuited metal plug. Starting winding is only ment for improving the starting torque As the speed of rotor reaches 75 % of its rated speed the starting winding is taken out of circuit and thus continue to operate on single winding Running Winding
TYPE OF SINGLE PHASE INDUCTION RUN MOTOR :
Single phase induction run motor in its simplest form is not self starting some means must be provided to bring the motor to its rated speed. The type of motor is called by name of starting method used On the basis of starting method, the types of a.c single phase induction run motors are as under:
(a) Split Phase Induction Run Motor (RSIR)
(b) Capacitor Start Induction Run Motor (CSIR)
(C) Capacitor Start Capacitor Run Motor (CSR)
(d) Permanent Capacitor Motor (PSC)
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Split phase induction run motor :
It has two windings in startor, main winding or running winding and starting winding spaced at 90 electrical angle apart The starting procedure is same like those of two phase motor When the spe reaches 75 % of its rated speed a centrifugal switch ( Relay ) actuated by weight counter and rod in rotor, opens the starting winding from circuit and continues to run on main winding or Running winding motor is used for small refrigeri tion machines like domestic refrigerators where some means are provided for unloading or equalizing the pressure of system. It is low starting torque motor.
Working : The starting winding is wound with small wire so that the winding has high resistance and low inductance whereas the running winding has wound with large wire so that winding has low resistance and high inductance When the power is supplied to the both windings, because of high winding, the current flows in starting winding Snce the current Dowing in both windings are 90 degree, single phase is split to give the effect of two phase and rotating field is set up in startor, which produces starting torque and causes the motor to rotate. When the motor speed reaches approximatly 75 % of it rated speed , a centrifugal switch / relay opens the starting winding with in few seconds and thus the motor continues to operate on running. Capacitor start induction run motor : The capacitor start motor is identical to the split phase motor in both conduction and operation except that a capacitor is installed in the seriese of starting winding which causes the current in winding to lead, the voltage Whereas current in running winding lags the voltage.
The starting torque of capacitor start motor is very high As the motor reaches 75 % of its rated speed , starting winding and starting capacitor are taken out of the circuit by centrifugal switch relay, The motor continues to operate on running winding.This type of motors are available in size ranging from 125 watts 6 kWs. Capacitor start capacitor run motor (CSR) : Capacitor start capacitor run motor is similer to that capacitor start motor with the exception that a second capacitor which is known as running capacitor. Running capacitor is directly installed in senes of starting winding, but in parallel with starting winding and centrifugal witch The operation of CSR motor differrs from that of other motors. In CSR motor the starting winding remains in the cireuit at all time Starting and running capacitors both are in series of starting winding A soon as the motor reaches the 75 % of its rated speed the starting switch open the circuit of starting capacitor from starting winding and motor continues to operate on both windings in circuit Running capacitor also remains with seres of starting winding Running capacitor cotrect the power factor and improve the running torque and thus run the motor in smooth action. This motor has high starting torque and excellent e ficiency , these motors are available in size of 375 w (1 % HP) 1.5 Kws (2HP).