Why do we need Starting methods of induction motors?
In induction motors, the magnitude of the induced emf in the rotor circuit depends on the slip of the induction motor. The is induced emf decides the magnitude of the rotor current. But at the start, the speed of the motor is zero and slip is maximum. As rotor conductors are short-circuited, the largely induced emf circulates very high current through rotor at the start. Due to which stator of the induction motor draws a very high current of the order 5 to 8 times the full load current at start. Due to this kind of situation, the appliances which are the load and the motor can be burnt up. To avoid such cataclysmic happening, various starting methods are used with the help of starters. The starter is a device that is basically used to limit high starting current by supplying reduced voltage to the motor at the time of starting. Such reduced voltage is applied only for a short period of time and once the rotor gets accelerated, full normal rated voltage is applied to the motor.
Top 6 Starting Methods of Induction Motors:
The following are the starting methods of induction motors explained.
Stator-Resistance Starting Method:
In order to apply the reduced voltage to the stator of the induction motor, three resistance is added in the series with each phase of the stator winding. Initially, the resistances are kept maximum in the circuit. Due to this, the large voltage gets dropped across the resistances. Hence, the reduced voltage is applied to the stator which reduces the high starting current. When the motor starts running, the resistances are gradually cut-off from the stator circuit. When the resistances are entirely removed from the stator circuit then rated voltage gets applied to the stator and the motor runs smoothly. This method can be used for both star and delta connected stator. But there are large power losses due to the resistances and the starting torque of the motor also reduces due to the reduced voltage applied to the stator at the starting of the induction motor.
Auto-Transformer Starting Method:
A three-phase star-connected auto-transformer can be used to reduce the voltage applied to the stator. It consists of a suitable change over switch. When the switch is in the start position, the stator winding is supplied with reduced voltage. This can be controlled by tapping’s provided with autotransformers. When the motor runs at normal of 80% speed to the rated one, the change over switch is thrown into the run position. Due to this, the rated voltage is applied to the stator winding and the motor start rotating with the rated speed. The changing of switches is automatically done by the relays. The power loss is much less in this type of method. It can be used for both star and delta connected motors. But it is expensive because of the autotransformer.
Star-Delta Starting Method:
It is the cheapest of all the methods and it is a very commonly used method for the induction motors. It uses a triple pole double throw (TPDT) switch. The switch connects the stator winding in the star at the start and due to this, the voltage per phase is reduced by the factor of 1/(3)1/2 and hence, the starting current gets limited. When the switch is thrown on the other side, the winding is connected in delta fashion across the supply due to which it gets normal rated voltage. The operation of the switch can be made automatically using the relays which ensures that the motor will not start with the switch in the run position. It is limited to the use of only delta connected motors and the factor by which voltage changes are fixed and it cannot be changed.
Rotor Resistance Starting Method:
To limit the rotor current which consequently reduces the current drawn by the motor stator from the supply, the resistance can be inserted in the rotor circuit at the start. And the resistance which is added in the start in the rotor circuit is in the form of a 3-phase star connected rheostat. The external resistance is inserted in each phase of the rotor winding through the slip rings and brush assembly. Initially, there is a maximum resistance in the circuit. As the motor gathers the speed, the resistance is gradually cut-off. The operation can be of manual or automatic nature. We know that the starting torque of the motor is directly proportional to the rotor resistance. And due to this method, starting toque gets improved while the starting current is limited. The only limitation for this method is that it is only used for the slip-ring induction motors because, in wound rotor induction motors, the rotor is permanently short-circuited.
Direct on-Line Starting Method:
In the case of small capacity motors like 5hp, the starting current is not very high due to which there is no need for any other type of starting method. Such motors are connected directly to the supply without any reduction of the supply voltage. But the starter used for this method is for avoiding any abnormality i.e. overloading, low voltage, etc. to happen. It is like the isolation process. The normally open start contact is usually open and normally closed stop contact is closed. At the start, normally open start contact is pushed for fraction of second due to which coil gets energized and attracts the contactor due to which stator gets directly supplied. Under overload condition, current drawn by the motor increases due to which there is excessive heat produced, which increases temperature beyond the limit and due to this, thermal relays get opened due to high temperature protecting the motor from being overload.
Soft Starting Method:
In some applications, it is necessary to control starting torque and speed smoothly. In textile machines, such type of control is necessary to avoid breaking threads. The thyristor voltage controller is used in a soft starting method of the induction motor. In this method, two thyristors are connected in anti-parallel in each phase of the input supply. The thyristors are triacs and to control the voltage applied to the stator, the firing angle of the triac is controlled. The interval between the successive firings is 600 while the firing frequency is six times the input frequency. Each triac conducts for 1200 once fired. As long as the thyristor is fired, it conducts and correspondingly voltage is available to the stator. Hence, controlled voltage is available to the stator due to which torque is controlled and the starting current is limited.