Construction And Working:
1. the size and design of the motor
2. the kind of application de
3. the location of the motor in the distribution system and
4. the capacity of the power system and the rules governing such installations as established by power supply companies.
Squirrel cage motors of capacity up to 1.5 kW, double cage motors, and squirrel cage motors of large capacity having a large rotor resistance are started by this method.
This is the most economical method of starting of induction motors. As the name suggests, this method involves direct switching of polyphase squirrel cage induction motor to the supply mains, as illustrated in fig.
The starting period usually lasts a few seconds because starting torque for the induction motor is about twice the full-load torque.
The push-button type direct-on-line starter, which is very common in use, is shown in fig.
The start button is a momentary contact switch that is held normally open by a spring. The stop button is held normally closed by a spring.
When the start button is pressed the operating coil (or main contactor ) gets energized through the overload relay contacts
This closes the three main contacts M that connects the motor to the supply. At the same of auxiliary or maintaining contacts MC are closed. When the maintaining contacts MC close, a new circuit is established through the stop button, maintaining contacts MC and operating coil.
Since the operating coil of the circuit is now maintained by the auxiliary contacts MC, the start button may be released.
When the stop button is pressed, the operating coil gets de-energized, thereby opening all main contacts and auxiliary contacts.
If the supply fails or line voltage drops below a certain value, the main contacts and the maintaining contacts are both opened. Upon return of the supply, the contactor cannot close until the start button is again closed. Because a contactor that is controlled by a three-wire control circuit maintains the interruption of the circuit even after the supply is restored, it is said to provide under-voltage protection for the motor. This protection is employed when it is desired to prevent the unexpected starting of a motor.
Overload protection is employed for protecting the motor and control apparatus from excessive heating due to overloads on motor.
Thermal-overload relays are commonly used for motor overload protection. Thermal overload relays are of two types. Both types are operated from the heat generated in a thermal element through which the motor current flows. In one type, the heat bends a bimetallic strip, and in the other, the heat melts a film of solder.
Both act to open the motor control circuit and, therefore, disconnect the motor from the source of supply.
A bimetallic strip is made of two different metals whose surface has been welded together. One of the metals expands rapidly as compared to the other when heated.
When heat is applied the strip is caused to deflect or curl up due to unequal expansion of the two metals. When the motor current attains a predetermined value, the heat generated deflects the strip far enough to trip a latch that opens the motor control circuit.
When the strip cools down sufficiently, the relay may be reset and the motor re-started. In the solder-film type of overload relay, the heat generated at a given overload current melts the film,
which releases a latch arrangement and opens the motor control circuit. When the solder has cooled enough to hold the latch, the relay may be reset.
In either type of thermal relay, the time required to operate is determined by the magnitude of
current flowing in the heater. Thus, the relay operates slowly for light overloads but disconnects the motor in a short time for dangerously heavy overloads.
Relays in which the time of operation is inversely proportional to the amount of current flowing are known as inverse time relays.