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Hoisting Motor


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I am dealing with design of a hoist motor( 3 phase induction motor ) control circuit.The supply of the braking rectifier is taken between one motor phase and motor neutral point .( star connected motor) . The hoist vendor tells me that in case of sudden loss of power when the hoist is lowering the load (regenerative mode) the regenerated voltage will provide supply to the braking rectifier which will cause delay in holding of brake(brake engages when supply is not available to brake coil). He advises me to put a contactor to cut off the brake power supply when main power fails. Is the regenerated energy so significant that I need to isolate the brake with a contactor to prevent regenerated feed .( the motor is of 9.5Kw ) . any advise is welcome .





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Hello sb1234


Welcome to the forum.


Yes, when the voltage is removed from the motor operating at full speed, there will be voltage generated by the rotating rotor field in the stator field.

This voltage will decay exponentially as the rotor field decays.

The duration of the voltage generated is a function of the time constant of the rotor circuit. High efficiency rotors have longer time constants. This could delay the brake for up to half a second under normal situations, but if there is static power factor correction permanently connected to the motor terminals, it could be considerably longer.


I would agree with the recommendation of the use of a "brake" contactor if such a delay is critical.


Best regards,


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