MOTORS CONTACTORS AND CONDITIONS

CONTACTORS: 

Contactor is a mechanical switching device capable of making carrying and breaking electric current under normal circuit conditions including operation overload conditions.

Contactors have a main circuit and control circuit. The contactors are designed according to the method of energizing the control circuit, namely Elector Magnetic, Pneumatic, and Electro-Pneumatic.

AC-3 Duty – Squirrel – cage induction motor: Starting, Switching off.

OVERLOAD RELAY SETTING:  

Overload protection given for almost all motors. The overload relay setting will be of 110% in general.

STARTING CURRENT: 

 Starting current of a induction motor generally will be 6 times the rated current.

FAULT EXPERIENCED: Individual’s experience.

SINGLE PHASING: 

One of the supply lines gets disconnected due to rupturing of fuse or open circuit in one of the three supply connection. In such cases the motor continues to run on a single phase supply of 440V. If the Rotor is locked to its rated full load, it will draw excessive currents on single phasing. The windings get overheated and damage is caused.

LOCKED ROTOR CURRENT: Due to mechanical overload in motor locked, rotor can sometimes happen and during this time the motor will draw the maximum current equal to the starting current. This is called the Locked rotor current.

STARTING TIME: The time taken for a motor form starting to attain its full speed is called the starting time. It will be generally 4 to 5 seconds for small and medium range motors and 8 to 10 seconds for large motors.

UNDER VOLTAGE CONDITIONS: Under voltage conditions in a motor greatly affect its performance. During this condition, the motor speed reduce it will draw more current and this will cause rise in temperature of windings. For protecting the motor from this condition, under voltage release is provided.

OVERLOAD RELAY TESTING:  

The characteristics of the relay should be selected and set to suit the protection requirement of particular motor, as the thermal time constant for the motor can vary wildly (15 minutes to 1 hour) curve A indicates characteristic of motor heating to reach maximum permissible temperature in 15 minutes for moderate overload (1.3 times full load currents). The relay will trip according to characteristics ‘B’ for overload of 200% the relay will in less than 4 minutes, motor can with stand 200% overload for 4 minutes.

PUSH BUTTON STATIONS: 

These are installed in the units near each motor for starting and stopping. The control supply for the stations is 110DC feeding from the respective switch gears or 230V A.C.

GREASING INTERVAL: 

The bearings of motor shall be greased according to the recommended interval and quantity, by the manufacturer, example Greasing interval – 1500 running hours. Quantity is 20grms.

MEGGER VALUE: 

The insulation resistance value of the winding of the motor shall be checked periodically during Preventive maintenance.

        For 3.3 KV motor, the IR value shall be checked with 1 KV megger.

        For 11 KV motor, the IR value shall be checked with 5 KV megger.

WINDING RESISTANCE:  

The winding resistance of the motor shall be checked during preventive maintenance with the bride. The winding resistance of the three windings should be equal.