Lust CDA34.006 - Project Planning Notes for Multi-Motor Operation

A-10
CDA3000 Operation Manual
A.5 Project planning
notes for multi-
motor operation
Subject Project planning notes
Current
configuration of
inverter module
The sum total of the motor currents must be less than the rated output
current of the inverter module
Σ of motor currents, (I
M1
+ I
M2
+ I
Mn
) < I
inverter
Motor control
method
Multi-motor operation is only permitted with the VFC motor control
method.
Motor choke A motor output choke must always be used. The motor choke limits the
du/dt and thus the leakage currents, and protects again switching
voltage overload resulting from switching of the motor inductance.
Motor cable length The total length of the overall motor cable is produced by adding the
individual lengths per motor.
Motor protection In multi-motor operation the parallel-connected motors cannot be
protected by the inverter module. As a result, depending on application
the motor protection should be provided by means of external motor
circuit-breakers or thermistor protective relays.
All motors have
the same power
output
In this application the torque characteristics of all motors remain
roughly equal.
The motors have
different power
outputs
If the motor outputs are very different, problems may occur on startup
and at low speeds. This is because of the high stator resistance of small
motors and the resultant high voltage drop on the stator coil.
In practice: With a power ratio of around 1:4 between the motors, the
starting torque of the smallest motor is still approx. 70%
of the nominal torque. If the torque of approx. 70% is not
sufficient, a larger motor must be used.
If all the motors are started together, the small motor will
start up later, because the slip frequency is higher.
Speed
proportionality
Differing motor output speeds can only be attained by using motors
with differing nominal speeds, e.g. 1440 rpm and 2880 rpm. The speed
ratio of approx. 1:2 is maintained during the speed change. The
accuracy depends on the slip and thus on the load.
Connecting
individual motors
When connecting motors, ensure that the connection current is not
higher than the inverter peak current. It is advantageous if the inverter
load is >40%.
This 40% base load backs up the output voltage of the inverter module
at the moment of connection of the motor.
During connection the motor must not be run in the field
weakening range, since the connected motor would
otherwise have to run at reduced runup torque.