Klockner Moeller Electric Parts from KlocknerMoeller.net
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Representing
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All about Motors
Motor protection |
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| Motor protection in special
applications |
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Heavy starting duty
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:
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Current transformer-operated relay ZW7
The ZW7 consists of three special saturable core current transformers,
supplying an overload relay Z00. It is used principally for medium and
large motors.
Up to two times rated current Ie, 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 × Ie) , 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.
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Adjusting the current transformer-operated relay ZW7
for lower motor ratings
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.
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