Motor protection in special applications [t-head1]

Heavy starting duty [t-head3]

An adequate tripping delay is essential in order to allow a motor to start up smoothly. In the majority of cases, overload relays ZB, motor-protective circuit-breakers PKZ(M) or circuit-breakers NZM can be used. The tripping delays can be taken from the tripping characteristics in the Moeller Main Catalogue, Industrial Switchgear.

In the case of especially high-inertia motors, whose run-up time exceeds the tripping delay of the above devices, it would be completely wrong to adjust an overload relay which tripped out before the run-up time expired, to a current level higher than the rated motor current. This would, it is true, solve the starting problem, but the motor would no longer be adequately protected during normal operation. However, there are other solutions to the problem:

Current transformer-operated overload relays ZW7 [t-head3]

The ZW7 consists of three special saturable core current transformers, supplying an overload relay Z.... It is used principally for medium and large motors.

Up to two times rated current I_e, the transformation ratio I₁/I₂ of the saturable core current transformers is practically linear. Within this range it does not differ from the normal overload relay, i.e. it provides normal overload protection during normal operation. However, within the transformer characteristic range (I > 2 × I_e) , the secondary current no longer increases proportionally to the primary current.

This non-linear increase in the secondary current produces an extended tripping delay if overcurrents greater than twice rated current occur, and hence permits longer starting times.

Adjusting the current transformer-operated overload relay ZW7 for lower rated motor current [t-head3]

The setting ranges quoted in the Moeller Main Catalogue, Industrial Switchgear apply when the incoming cable is looped once through the transformer relay.

If the current transformer-operated overload relay ZW7 is required to provide protection to a motor of below 42 A rating (minimum value in the setting range of 42 A to 63 A), the necessary range adjustment is achieved by looping the incomer several times through the aperture in the relay. The change in the rated motor current quoted on the rating plate is inversely proportional to the number of loops.

Example: [t-head3]

With the ZW7-63 relay, which has a setting range from 42 A to 63 A, a motor rating of 21 A to 31.5 A can be accommodated by looping the leads twice through the relay.