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Revision 8.6
SECTION 6 : OPERATION OF CONTROL PANEL
90
Rotor contactors are closed all the time, at start the motor starts with very small percentage
of rotor resistances, which causes sudden high currents.
Mechanical failure
6.8.8
H Loss of Torque (Hoist loss of torque) Applicable to Hoist application only
A loss of torque occurred during hoisting and the motor is actually turning in the lowering direction due
to the pull caused by the load.
Possible causes
Open rotor resistance circuit, one, two or all three phases.
Rotor brush gear faulty.
Motor windings starting to fail by partial shortening of the coils.
The load to be lifted is excessively high and the motor is not capable to develop the required
torque to lift it.
Single phasing in stator or rotor causing a loss of torque on the motor
Damaged stator or rotor cables
Hoist loss of torque detection is disabled once the motor has achieved a speed > 75% in 4
th
notch.
This is required due to the rotor voltage and frequency being so close to 0 volts and 0 Hz at this stage,
making it difficult to obtain any reliable frequency measurement.
6.8.9
Lower Overspeed (Hoists only), Quadrant IV operation only:
Lower overspeed detection is only active during Quadrant IV operations, i.e. lowering in counter torque
and during lowering plugging.
The trip level is at 130% speed lowering.
CAUTION
VERY IMPORTANT
In the event that the installation and operation
of the Hoist motion requires overspeed
protection,
a
separate
and
independent
overspeed device should be installed, which
can trip the main supply to the motor and the
respective brake.
Possible causes
Quadrant IV overspeed indicates a loss of torque on the motor, due to similar reasons as
detailed in paragraph 6.8.8
