Farewell Day
Electrical Machines - Questions and Answers
Information courtesy of ALSTOM.
INDUCTION MOTOR CONTROL GEAR
1. What are the main functions of control gear? 1. To provide a means of starting and stopping the motor and, at the same time, of limiting the starting current if required. 2. To give adequate protection to the motor under all conditions. 3. To allow speed changing when required. 4. To provide means of braking the motor when required. 5. To reverse the direction of rotation when required.
Protection of the motor must be automatic, but the other operations may be arranged to be under the control of an operator, or may be partly or fully-automatic.
2. What devices are required to give adequate protection to the motor? 1. Under-voltage release to prevent automatic restarting after a stoppage due to a drop in voltage or failure of the supply, where unexpected restarting of the motor might cause injury to an operator. 2. Overload relays for protection against excessive current in the motor windings - e.g. in the event of overload or failure of the motor. 3. Earth fault. 4. Single phase protection.
3. What provision must be made for short-circuit conditions in motor circuits?
Since overload relays are not designed to operate and clear the circuit in the event of a short-circuit. Circuit-breaker or fuse protection of sufficient breaking capacity to deal with any possible short-circuit that may occur must be provided.
4. What are the usual forms of overload relay in motor-control gear?
In small contactor starters, generally thermal relays, either of the 'solder pot' or bimetal type. With large contactors or oil switches, magnetic relays of the solenoid type with dashpots. Either type of overload relay may be used within intermediate sizes.
5. How do thermal relays work?
The bimetallic thermal relay consists of a small bimetallic strip that is heated by an element connected in series with the supply. When the current rises
Information courtesy of ALSTOM.
INDUCTION MOTOR CONTROL GEAR
1. What are the main functions of control gear? 1. To provide a means of starting and stopping the motor and, at the same time, of limiting the starting current if required. 2. To give adequate protection to the motor under all conditions. 3. To allow speed changing when required. 4. To provide means of braking the motor when required. 5. To reverse the direction of rotation when required.
Protection of the motor must be automatic, but the other operations may be arranged to be under the control of an operator, or may be partly or fully-automatic.
2. What devices are required to give adequate protection to the motor? 1. Under-voltage release to prevent automatic restarting after a stoppage due to a drop in voltage or failure of the supply, where unexpected restarting of the motor might cause injury to an operator. 2. Overload relays for protection against excessive current in the motor windings - e.g. in the event of overload or failure of the motor. 3. Earth fault. 4. Single phase protection.
3. What provision must be made for short-circuit conditions in motor circuits?
Since overload relays are not designed to operate and clear the circuit in the event of a short-circuit. Circuit-breaker or fuse protection of sufficient breaking capacity to deal with any possible short-circuit that may occur must be provided.
4. What are the usual forms of overload relay in motor-control gear?
In small contactor starters, generally thermal relays, either of the 'solder pot' or bimetal type. With large contactors or oil switches, magnetic relays of the solenoid type with dashpots. Either type of overload relay may be used within intermediate sizes.
5. How do thermal relays work?
The bimetallic thermal relay consists of a small bimetallic strip that is heated by an element connected in series with the supply. When the current rises