Sohail Ansari

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Speed control of an Induction Motor


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.

Capture

The curve suggests that the speed control and braking operation are available from
nearly zero speed to above synchronous speed.

fsd

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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Capacitor Start Induction Motor


Depending upon whether capacitor remains in the circuit permanently or is disconnected from the circuit using centrifugal switch, these motors are classified as,
           1. Capacitor start Induction run motor
           2. Capacitor start capacitor run motors
 1. Capacitor start Induction run motor:
 In case of capacitor start capacitor run motor, there is  centrifugal switch which is put to open after starting the motor that means capacitor remain temporarily in the circuit.
The connection of capacitor start motor is shown in the Fig.. The current Ilags the voltage by angle Φm while due to capacitor the current Ist leads the voltage by angle Φst. Hence there exists a large phase difference between the two currents which is almost 90, which is an ideal case. The phasor diagram is shown in the Fig.1(b).
                                         Fig 1.  Capacitor start Induction Run 
       The starting torque is proportional to ‘α ‘and hence such motors produce very high starting torque .
       When speed approaches to 75 to 80% of the synchronous speed, the starting winding gets disconnected due to operation of the centrifugal switch. The capacitor remains in the circuit only at start hence it is called capacitor start Induction run motors.
2. Capacitor start Capacitor run motor:
In case of capacitor start capacitor run motor, there is no centrifugal switch and capacitor remain permanently in the circuit. This improves the power factor.
       The schematic representation of such motor is shown in the Fig.
                              Fig. 2 Capacitor start capacitor run motor
 The phasor diagram remains same as shown in capacitor start induction run motor. The performance not only at start but in running condition also depends on the capacitor C hence its value is to be designed so as to compromise between best starting and best running condition. Hence the starting torque available in such type of motor is about 50 to 100% of full load torque.
       The direction of rotation, in both the types can be changed by interchanging the connection of main winding or auxiliary winding. The capacitor permanently in the circuit improves the power factor. These motors are more costly than split phase type motors.
       The capacitor value can be selected as per the requirement of starting torque, the starting torque can be as high as 350 to 400 % of full load torque. The torque-speed characteristics is as shown in the Fig.3.
                Fig.3  Torque speed characteristic of capacitor split phase motor
 Applications :
       These motors have high starting torque and hence are used for hard starting loads. These are used for compressors, conveyors, grinders, fans, blowers, refrigerators, air conditions etc. These are most commonly used motors. The capacitor start capacitor run motors are used in celling fans, blowers and air-circulations. These motors are available up to 6 kW.

why in most application prefer delta connection on induction motors? what happen if we use star connection?


Electrical EngineeringWe prefer star connection at the time of starting in order to reduce phase voltage and prevents the motor from getting damaged. Once the motor runs at some speed then we use delta connection so that all the line voltage goes to the phase and the motor runs at maximum speed.

It is so popular in rural water pump set motors in
India and Star Delta starters are manufactured by local artisans (these are so simple).

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