STARTING OF 3-PHASE INDUCTION MOTORS
There are two important factors to be considered in starting of induction motors:
The starting current should be kept low to avoid overheating of motor and excessive voltage drops in the supply network. The starting torque must be about 50 to 100% more than the expected load torque to ensure that the motor runs up in a reasonably short time.
Hence the starting currents should be reduced. The most usual methods of starting 3-phase induction motors are:
(i) Direct-on -line starting
(ii) Star-delta starting
(iii) Autotransformer starting.
Rotor Resistance Starting
Direct-On-Line Starting
Star-Delta starting
Auto-Transformer Starting
Starting Torque: The torque developed the motor at the instant of starting is called starting torque.
where,
Ns = Synchronous speed in RPS
E2 = Rotor emf per phase at standstill
R2 = Rotor resistance per phase
X2 = Rotor reactance per phase at standstill
Full Load Torque and Maximum Torque
Sf = Slip corresponding to full load torque
Note. In general,
Starting Torque and Maximum Torque
Rotor Torque and Breakdown Torque: The rotor torque at any slip s can be expressed in .terms of the maximum torque
where, Tb = Maximum (or breakdown) torque
sb = Breakdown or pull out slip
No Load Test:
Power input = P0
No load current = I0 (average of 3 ammeter reading)
Voltage = V0 (line to line voltage)
Im = I0 sin φ0
Ic = I0 cos φ0
Ro = Vo/Io & Xo = Vo/Im
and
Rotation loss
Note: This test gives rotational losses and X0
Blocked Rotor Test: The shaft of the motor is clamped so that it cannot move and rotor winding is short-circuited.
VBR = Stator voltage (line to line) required to circulate IBR when rotor is blocked.
IBR = Stator current (average of three ammeter reading)
PBR = Total copper loss on full load at standstill
Blocked rotor impedance
Blocked rotor resistance
Blocked rotor reactance
Note:
Speed Control of Induction Motors: The rotor speed of an induction motor is given by
Nr = (1 – s)Ns and Ns = 120f/P
also
Speed Control by Frequency Changing: The synchronous speed of an induction motor is given by
The synchronous speed and therefore, the speed of the motor can be controlled by varying the supply frequency. The emf induced it the stator of the induction motor is given by
Speed Control by Pole Changing: The number of stator poles can be changed by (a) multiple stator windings, (b) method of consequent poles and (c) Pulse-Amplitude Modulation (PAM).
where p is the number of pole pairs, this would correspond to changing the synchronous speed.With the slip now corresponding to the new synchronous speed, the operating speed is changed.
Speed Control by Slip Changing: There are three ways of controlling slip. (i) Voltage control, (ii) Rotor-resistance control, (iii) Secondary foreign voltage control, and (iv) Speed control by cascade arrangement.
Voltage control
From the torque equation of the induction machine, we can see that the torque depends on the square of the applied voltage. The variation of speed torque curves with respect to the applied voltage is shown in figure below. These curves show that the slip at maximum torque remains same, while the value of stall torque comes down with decrease in applied voltage. The speed range for stable operation remains the same.
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Rotor Resistance Control
From the expression for the torque of the induction machine, torque is dependent on the rotor resistance. The maximum value is independent of the rotor resistance. The slip at maximum torque is dependent on the rotor resistance. Therefore, we may expect that if the rotor resistance is changed, the maximum torque point shifts to higher slip values, while retaining a constant torque. Figure below shows a family of torque-speed characteristic obtained by changing the rotor resistance.
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