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 above a preset value, the movement of the strip releases a catch which opens the trip contacts. In recent years more modern electronic relays are used which simulate the thermal overload. Many of these relays also incorporate a memory, i.e. simulates the temperature rise / cooling curve of the winding. 6. How does the magnetic overload relay operate?
A solenoid connected in series with the supply contains a plunger whose movement is damped by a dashpot. When the safe current is exceeded, the solenoid pulls the plunger up - disconnecting the supply. The damping provided by the dashpot prevents unwarranted tripping on short-time overloads. 7. How many overload relays are required in the control gear? On three-phase supplies where the neutral point of the system is connected to earth, as is usually the case, three overload relays (one in each line) are necessary for complete protection. For 2-phase 3-wire and 4-wire supplies, two overload relays are required, one in each phase line, none being connected in any neutral or earth conductor. With single-phase motors one overload relay in any conductor except an earthed conductor or neutral. 8. What happens when one of the three lines supplying a three-phase induction motor becomes open-circuited? The motor, if already running, will continue to run as a single-phase motor on the remaining single-phase supply. The condition is called single-phasing. If the motor is loaded to more than about 30 per cent of full load, the currents in the motor windings tend to become excessive and overheating occurs. With one line broken, the motor will not start up and, due to the heavy standstill current, burn-out is likely unless the motor is quickly disconnected. 9. What currents flow in a single-phasing delta-connected motor? Assuming that supply line L1 is open circuited as shown, typical line and phase currents, given as percentages of normal full-load three-phase current, at various loads will be:- [pic]
Fig. 22-Open-circuited supply line of...