The speed control of induction motor is more complicated than that of dc motor, especially
when comparable accuracy is desired. The main reason for this can be attributed to the
complexity of the mathematical model of the induction machine, as well as the non-linear
power converters supplying this motor. It is very important to control the speed of induction
motor for the application in industries and in engineering.
There are many types of speed control. Speed control techniques of induction motors can be broadly classified into two types scalar control and vector control. Scalar method only the magnitude of voltage or frequency of the induction motor.
Types of Speed Control:
Mathematically, the relation between the speed of an induction motor and the synchronous
speed(speed of rotating flux) can be stated as:
Nr = (1-s) Ns & Ns = 120f/p Where, Nr = rotor speed
Ns = synchronous speed.
s = slip
f = supply frequency
As speed is a function of frequency and no. of poles , speed can be varied by varying these parameters.
Different ways of controlling speed of induction motor are:
1. Changing no. of poles
2. Stator voltage control
3. Rotor resistance control
4. Slip power recovery scheme, and
5. Constant V/f control
AC motors have traditionally operated at fixed frequency and speed .when load changes speed also gets changed. With increasing load, speed gets decreased and with decrease of load, speed rises. but as that drop is small percentage of full load speed, so that speed is considered to be constant with changing load.
Out of all the above methods induction motor speed variation can be easily achieved for a short range by either stator voltage control or rotor resistance control. But it may leads to lower efficiency. Also in stator voltage control method, as voltage is varied to vary the speed and torque is proportional to square of applied stator voltage, so in this method to vary speed, torque also gets affected. Also in other methods like rotor resistance control, part of power get lost in the resistor. So, efficiency gets reduced. So, this is also not a suitable control.
The most efficient scheme for speed control of induction motor is by varying supply frequency.
V/f Control Overview:
Induction motor speed variation can be easily achieved for a short range by either stator voltage control or rotor resistance control. But at low speed it result in low efficiency. The most efficient scheme for speed control of induction motor is by varying supply frequency. This results in scheme with wide speed range but also improves the starting performance.
The v/f ratio is kept constant, when the machine is operating at speed below base speed,
so that flux remains constant. Maximum torque remains constant in this case. At frequency less than rated frequency, the torque capability decrease and this drop in torque has to be compensated by increasing the applied voltage.
The curve suggests that the speed control and braking operation are available from
nearly zero speed to above synchronous speed.
In Fig. it is noted that frequency is increasing keeping voltage constant after reaching
the rated speed.The variable frequency control provides good running and transient performance because of the following features:
(a) Speed control can be possible from zero to above base speed.
(b) During starting, braking and speed reversal, the operation can be done at the maximum torque.
(c) Copper losses gets decreased, efficiency and power factor are improved.
(d) No load to full load speed drop is small.
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