An induction motor is an
electric motor which works on Faraday's law of electromagnetic induction.
Induction motors are AC fed motors. Specifically, a three-phase induction motor fed by three phase AC supply. Three phase induction motors are the most widely
used motor in industries to drive heavy loads (large power applications).
An induction motor is a constant
speed motor, means a few percent speed drop from no load to full load like in
the case of a DC Shunt Motor. Speed of a DC Shunt Motor can be varied easily
with the help of various speed control methods DC Motor with good efficiency,
but in case of induction motor changing speed results decrease in efficiency
and poor power factor. Then a question arose in our mind, why are we interested
in changing the speed of a three-phase induction motor even if it results in
decreasing performance of the induction motor.
Why do we need to control the speed of a three-phase induction motor?
Controlling the speed of a three-phase induction motor is essential for
various industrial and commercial applications. Induction motors are most
commonly used to drive loads in industries. Various industrial applications
required different speeds to drive a load. So, these applications require an
induction motor that operates on a wide range of speeds. To fulfil the speed
requirements of various applications in industry Speed control of an induction
motor is required. Controlling the speed of a three-phase induction motor
provides flexibility, efficiency, and improved performance in a wide range of
industrial and commercial applications.
Methods of speed control of three phase induction motor
Before studying the methods of speed control of a three-phase induction motor we should know the basics of a three-phase induction motor.
As we know, a three phase Induction motor is a three phase AC fed motor which works on Faraday's law of electromagnetic induction. When a 3-phase supply is given to stator winding it produces a flux called rotating magnetic field which rotates at synchronous speed Ns. This rotating magnetic field induces a current in rotor winding. Simultaneously a secondary rotating magnetic field is produced called rotor RMF rotating magnetic field. Due to the interaction of these magnetic field, rotor experience a torque and rotate at speed denoted with N.
N = Ns (1-s) ………………………… (1)
Ns = 120 F / P ……………………….. (2)
Where,
- Ns = Synchronous speed at which stator RMF rotates.
- F = Frequency of given supply
- P = number of poles
- N = rotor’s speed
- s = slip in induction motor
- s = 1 at starting or under standstill condition.
Torque Equation in three phase induction motor
T = K sV^{2} /
R_{r} …………………………………. (3)
_{ }
_{}
Where,
- T = Torque in induction motor
- K = Proportionality Constant
- s = slip in induction motor
- V = Voltage applied to induction motor
- R_{r} = Rotor Resistance
It is seen from the equation (1) that there are two basic ways to control
the speed of 3-phase induction motor. First is by changing the synchronous
speed and the second method is depend on the slip in induction motor.
From equation (2) we can conclude that there are two ways of changing
synchronous speed that are by changing the supply frequency and second is by
changing the number of stator poles.
From equation (3) it is conclude that slip control method is depends on
supply voltage, and rotor
resistance.
Apart from above derived method there are three more method for speed
control of three phase induction motor. That are slip power recovery method,
cascading / Tandem connection method and rotor resistance method.
So, there are six different method of speed control of 3 phase induction
motor
- Voltage Control
- Frequency Control
- Pole Changing Method
- Stator Resistance Control
- Rotor Resistance Control
- Slip Power Recovery
- Cascading Tandem Connection
An induction motor has good speed regulation means a few percent speed
drop from no load to full load. They are also known as constant speed
motors. Then a question arose in our mind that why do we need to
control the speed of induction motors even though they have good speed
regulation.
Voltage Control / Stator Voltage Control
Voltage control method depend on slip control in which a variable voltage
at constant frequency is supplied to the motor stator. A variable supply
voltage tends to motor operate at different slip to deliver a constant
load torque this relationship follows equation (3). It is obvious that the
supply voltage cannot be raised beyond the rated value, it can only be
reduced. If the supply voltage is reduced then the motor will operate at
higher slip for delivering constant torque (as per eq (3) ) resulting the
speed of the motor reduced ( as per equation (1) ).
The voltage control method gives a small range of speed control due to
efficiency considerations. Because if slip increases then induced current
in the rotor is also increased. This increased current causes the
overheating of the motor as a result efficiency decreases.
This method requires a variac or any other device to supply reduced
voltage.
Stator Resistance Control
This method is similar to voltage control method in which a rheostat is
added in stator circuit. By adding the rheostat supply voltage is reduced
simultaneously slip increases to deliver constant load torque as a result
motor speed get decreased. (discussed in details in above section).
Frequency Control
This method is a synchronous speed control method In which by altering
the supply frequency, the synchronous speed of the induction motor can be
changed as per equation (2) simultaneously the speed of an induction motor
can be changed as per equation (1).
In an induction motor induced EMF in Rotor E_{r} is given
by
E_{r} = 4.44 É¸. F. T
And E_{r} is proportional to V
So, V = 4.44 É¸. F. T
…………………………… (4)
É¸ Î± V/F
…………………………… (5)
Where,
- F = frequency of supply voltage
- V = Applied Voltage
- É¸ = Flux
- T = Number of Turns.
Equation (5) indicates that if we vary the frequency, flux density is
also changed. It has to be taken care that flux density in an induction
motor should be constant because if flux density increases then the motor
will get saturated and draw a large amount of magnetizing current and if
flux density decreases the machine fails to operate.
So, with the variation in frequency supply voltage is also varied to make
flux density constant. This method is also called variable voltage
variable frequency control of induction motor or V/F control method of
induction motor.
If frequency increases then voltage should also be increased to keep v/f
constant. If voltage increases beyond the rated value, then insulation
failure may occur and the motor will get damaged. So, frequency cannot be
increased to high value.
If frequency decreases then voltage should also be decreased in order to
keep v/f constant. If voltage is decreased the machine fails to operate at
lower voltage. So, frequency cannot decrease to very low values.
So, v/f control is used over the specific range. This method provides the
highest speed control range among all the speed control methods.
In modern practices variable voltage and variable frequency supply
from a constant voltage and frequency supply is achieved by Power
Electronics converter - inverter arrangement shown in the given figure.
This
converter
- inverter arrangement employs
SCR
or any other controlled switch for getting variable voltage variable
frequency supply. This whole arrangement refers to variable frequency drive (VFD).
Pole Changing Method
Pole changing method of speed control is a synchronous speed control
method in which we can alter the synchronous speed by changing the number
of the stator poles This relationship is shown in equation (2). This
method requires two or more stator windings wound for different numbers of
poles and are isolated to each other. These two stator windings generate
RMF rotating at different synchronous speeds. By simply connecting one
winding to the supply and leaving the other open, desired synchronous
speed simultaneously motor speed can be obtained.
This method is applicable only for
squirrel cage induction motors
because in a squirrel cage induction motor rotor automatically creates the
same number of poles as the stator.
This method is not so popular and rarely preferable because it requires
another winding which increases the cost as well as size of the
motor.
Rotor Resistance Control
As the name implies, in this method rotor resistance is altered for
getting the desired speed of the induction motor. It is obvious that rotor
resistance cannot be decreased but it can only be increased by adding
external resistance. This type of speed control is only possible for slip ring induction motors
in which rotor winding is essentially star connected.
In this method additional resistance is added to the rotor in each phase through slip rings. By adding the additional resistance in the rotor, we will force the motor to operate in high slip region to deliver the constant load torque. In high slip region motor will be operated in low speed. In this method speed of the 3-phase induction motor cannot be increased to the rated value. (refer torque slip characteristics to get more details about effect of adding rotor resistance)
In this method additional power losses occur in rotor additional resistance. So, for efficiency consideration this method does not offer a wide range of speed control of three phase induction motors. However, this method will help in increasing the starting of the three-phase induction motor by increasing the starting torque.
Slip Power Recovery
This method will help to overcome the power losses that occur during the
speed control of a three-phase induction motor by rotor resistance control
method. In this method additional voltage is injected into the rotor
through the slip ring at the same frequency of the rotor. By injecting the
additional voltage into the rotor, we will force the motor to
operate at a low slip region. If slip of the three-phase induction
motor then the speed of the motor increases to deliver a constant load
torque.
Cascading / Tandem Connection
This
method of speed control of a three-phase induction motor requires two motors
which are mechanically coupled as well as electrically connected. Mechanically
coupled means these two motors are connected with a common shaft and the second
motor is electrically connected with the slip ring of the first motor as shown
in figure. In this method the first motor is essentially a slip ring induction
motor or the second motor can be any type, either slip ring or squirrel cage
induction motor. The first motor is called main motor which is used to give supply
at slip frequency to the second motor which is called auxiliary motor whose
speed control is required.
Let
Ns_{1}
= Synchronous speed of first motor
F
= Supply frequency of first Motor
F_{1}
= Frequency of induced EMF in first Motor’s
rotor
P_{1}
= Number of pole of first motor
N_{1}
= Speed of the first motor
s_{1}
= Slip of the first motor
Ns_{2 }
= Synchronous speed of second motor
F_{2}
= Supply frequency of second Motor
P_{2}
= number of pole of second motor
N_{2}
= Speed of the second motor
S_{2}
= Slip of the second motor
As both motors are mechanically coupled so, N1 = N2
By
solving the above equation we get,
In
above equation
- + sign indicates cumulative cascading
- -ve sign indicates differential cascading
In
this method a single drive operate at 4 different speeds
- When only main induction motor drives the load the speed of the drive is
Ns1
= 120 F
/P1
N
= (1- S1 ) Ns1
- When only auxiliary motor drives the load then the speed of the drive is
Ns2
= 120 F /P2
N
= (1- S2 ) Ns2
- When both motors are connected in cumulative cascading manner
N
= 120F / (P1+ P2)
- When both motors are connected in differential cascading manner
N = 120F / (P1 - P2)
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