Download Induction motor control

UNIT-3
1.Stator voltage control
The speed can be controlled by varying the stator
voltage.
 Here the supply frequency is constant.
Equivalent circuit of induction motor
R1& X1 - the stator side resistance
& reactance.
V1 - stator voltage (or) supply
voltage.
f1 - the supply frequency.
I1, I2 - the stator and rotor side
current respectively.
R2 & X2 - the rotor resistance &
reactance respectively.
3I 22 R2
Torque T =
S S
-------------------------- (3.1)
Therefore the equation (3.3) becomes
Results 
 The starting torque α
Square of stator voltage, rotor resistance.
1/α The square of the leakage reactance.
 A small change in stator voltage results in a relatively large change in
torque.
 the slip at maximum
torque remains unchanged
Figure 3.2. Speed Vs Torque of Induction motor under stator voltage
 It is not a function of voltage.
 This method is not used for wide range of speed control and constant torque load.
Consider
The induction motor has no stator copper loss, friction loss, windage and core loss.
Electrical power converted into mechanical power (developed power in the rotor).
Pm = Total air gap power transferred across the air gap for a three phase induction
motor (Pag) – copper loss in the rotor (Pcur)
From equation (3.6)
 For low speed, slip will be very high. Therefore Efficiency
will be poor.
 It is an excellent method for reducing starting current and
increasing the efficiency during light load conditions.
 The losses are reduced, mainly core losses, which are
proportional to the square of the voltage.
This method is only suitable for speed control below the rated
speed, since the terminal voltage cannot exceeds rated value to
prevent the damage of the winding’s insulations.
We have seen that the torque is directly proportional to the
transformation ratio K.
1.1 AC Voltage Controllers fed drive
by connecting a reverse parallel pair of thyristors or Triac between
A.C supply and load, the voltage applied to the load can be
controlled.
without change in frequency.
applications:• speed control of poly phase induction motors,
• domestic and industrial heating, light controls,
• on load transformer tap changing static reactive power
compensators etc.
Classification
• depends on the supply.
 single phase controllers.
Three phase controllers.
Figure 3.3. Three phase Y connected AC voltage controller circuit
In the delta connection, the phase
current is
 1 


 3
times of line current,
 Thyrsitor current rating is reduced
by a factor of 3 Under normal
operation
Figure 3.4.Three phase delta connected AC voltage controller circuit
Each thyristors is the above circuits are fired in the sequence of their
numbers with a phase difference of 60 degree like three phase full
converter.
Let
Rin – Resistance in the induction motor.
Xin – Inductive reactance in the induction motor.
1  Xin 

Therefore, the phase angle   tan 
Rin


For the firing angle  , the motor terminal voltage remains constant and
nearly equal to the supply voltage. Both motor voltage and current are
sinusoidal. For higher values of firing angle  , the current flows
discontinuously and the motor voltage decreases with an increase in .The
zero motor voltage and current are reached at =150 degree and 180
degree
As per the emf equation.
Various Schemes of induction motor speed
control