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 () Non-reversing 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 Full motor protection in (star) position
1 × Ie Only partial motor protection in position
0.58 × Ie Motor not protected in (star) position
Multi-speed switch
2 speeds 2 separate windings
One tapped winding
3 speeds 1 × tapped winding + 1 winding
Attention must be paid to short-circuit protection of the overload relays.
Separate supply leads should be provided if required.
Heavy starting duty
Current transformer-operated relay ZW7
For medium and large motors
Bridging of motor protection during starting For small motors; no protection during starting
Bridging during starting using bridging relay Automatic cut out
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