Induction Motor Speed Control Methods
Speed control methods of three phase Induction Motor Detailed explanation voltage pdf resistances V/f disadvantages of speed control
Speed control methods of three phase Induction Motor Detailed explanation:
A three phase induction motor is a constant speed motor
as dc shunt motor. But the speed of d.c. shunt motor can be changed
smoothly just by using rheostats. This maintains the speed regulation
and efficiency of dc. shunt motor. But in case of 3 phase
induction motors very difficult to achieve smooth speed control. if
the speed control is achieved by using some methods , the performance of
the induction motor in term of its power factor, efficiency etc. gets badly affected.
The three phase induction motor has advantages as follows :
- Low cost
- Low maintenance
- Simple construction
- Reliability and high efficiency
speed equation of induction motor is,
 |
| figure: induction motor speed control |
From
above equation it observed that speed of the induction motor can be
changed either the synchronous speed Ns or by changing value of slip s.
Torque produced in three phase induction motor is ,
 |
| Fig: Torque equation of induction motor |
if
the value of R and E are changed then to keep the torque constant for
constant load condition, motor reacts by change in its slip.
Effectively its speed changes.
Thus speed of the induction motor can be controlled by methods:
Control From Stator Side:
it includes following methods
a) speed control of 3 phase induction motor using v/f I.e v/f control.
b) Supply voltage control.
b) Supply voltage control.
c) speed control of induction motor by pole changing method
d) Adding rheostats in stator circuit
Let discus in detail Speed control methods of three phase Induction Motor:
a ) speed control of 3 phase induction motor by variable frequency method i.e V/F control
The synchronous speed is given by,
 |
| fig:speed equation of induction motor |
Thus by controlling the supply frequency ( f )smoothly, the synchronous speed can be controlled smoothly
b) speed control by supply voltage:
Torque equation
For low slip region which is operating region of induction motor (sX)^2 <<R and hence neglected
Now
From above expression if supply voltage reduced below rated value, torque produced also decrease
But
due to reduction in voltage, current drawn by motor increases. Large
change in voltage for small change in speed required is the biggest
disadvantage of this method. Due to increased in current motor may get
overheated hence this method rarely used in practice.
c) By Pole changing:
This method called pole changing method of controlling speed of induction motor. In this method it is possible to have various speed in steps by changing stater pole. Continues smooth speed control not possible by this method
d) Adding rheostat circuit:
 |
| fig:Rotor resistance starter |
If
the rotor resistance is increased, the torque produced decreases. But
when the load on the motor is same, motor has to supply same torque
as load demands. So motor reacts by increasing its slip to compensate
decrease in T torque due to resistance R and maintains the load
torque R constant So due to additional rotor resistance R
motor slip increases ie the speed of the motor decreases Thus by increasing the rotor resistance R. speeds below normal value can be achieved Another advantage of this method is that the starting torque of the motor increases Fi proportional to rotor resistance But this method has following disadvantages 1. The large speed changes are not possible This is because for large speed change.required large in rotar which causes large rotor losses, which reduce the efficiency of motor .
2) The speed above the normal values can not be obtained
3) Large power looses occur hence sufficient cooling arrangements required
4) Due to large power losses, efficiency is low Thus the method is rarely used
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motor slip increases ie the speed of the motor decreases Thus by increasing the rotor resistance R. speeds below normal value can be achieved Another advantage of this method is that the starting torque of the motor increases Fi proportional to rotor resistance But this method has following disadvantages 1. The large speed changes are not possible This is because for large speed change.required large in rotar which causes large rotor losses, which reduce the efficiency of motor .
disadvantages of speed control of induction motor by adding external resistances:
1) this method can not be used for the squirrel cage induction motors2) The speed above the normal values can not be obtained
3) Large power looses occur hence sufficient cooling arrangements required
4) Due to large power losses, efficiency is low Thus the method is rarely used
tags: dol starter,motor starter,star delta starter,single phase motor,three phase motor,3 phase induction motor, single phase to three phase,three phase induction motor,4 phase motor,dol starter,motor starter,star delta starter,single phase motor,three phase motor
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