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Thread: VFD parallel motors
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08-03-2008, 05:26 PM #18
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Re: VFD parallel motors
Parallel connection of motors
The frequency converter is able to control several motors
connected in parallel. If the motors are to have
different rpm values, use motors with different rated
rpm values. Motor rpm is changed simultaneously,
which means that the ratio between the rated rpm values
is maintained across the range. The total current
consumption of the motors is not to exceed the maximum
rated output current IINV for the frequency converter.
Problems may arise at the start and at low rpm values
if the motor sizes are widely different. This is because
the small motors' relatively high ohmic resistance in
the stator calls for a higher voltage at the start and at
low rpm values.
In systems with motors connected in parallel, the electronic
thermal relay (ETR) of the frequency converter
cannot be used as motor protection for the individual
motor. For this reason further motor protection must
be used, e.g. thermistors in each motor or an individual
thermal relay. (Circuit breakers are not suitable as protection).
NB!
Parameter 107 Automatic motor adaption,
AMT cannot be used when motors are
connected in parallel. Parameter 101 Torque
characteristic must be set to Special
motor characteristics [8] when motors are
connected in parallel.
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