Bridging of motor protection during starting [t-head2]

For small motors the bridging of the motor protection during starting is more economical. Because of the additional parallel contactor, the overload relay does not carry the full current during starting. Only when the motor has reached full speed is the bridging contactor switched off and the full motor current is then carried by the overload relay. Provided it has been set correctly to the rated motor current, this will ensure full motor protection during operation. Starting must be monitored.

The motor is a limiting factor with regard to the tripping delay of the current transformer-operated relay and the bridging period. One must ensure that the motor is able to tolerate the high temperature generated by direct starting, for the prescribed starting time. Motor and starting procedure have to be selected carefully when dealing with machines having a very large rotating mass, which are practically the only ones subject to this problem when direct starting is used.

Depending on the operating conditions adequate protection of the motor winding may no longer be given by an overload relay. In that case it must be weighed up whether an electronic motor-protective relay ZEV or a thermistor overload relay EMT 6 in conjunction with an overload relay Z meets the requirements.

Star-delta starter ( )
1 operating direction Changeover time with overload relay in position
A: < 15 s	B: > 15 < 40 s	C: > 40 s
Setting of the overload relay
0.58 × Ie	1 × Ie	0.58 × Ie
Full motor protection in (star) position	Only partial motor protection in position	Motor not protected in (star) position

Heavy starting duty [t-head3]