3-80
Engineering Guide CDA3000
3 Selection of inverter module
Project planning notes for multi-motor operation
Subject Project planning notes
Current configura-
tion of inverter
module
The sum of the motor currents must be less than the rated output cur-
rent 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 (See Figure 3.36). The
motor choke limits the du/dt and thus the leakage currents, and pro-
tects 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 pro-
tected by the inverter module. For that reason, depending on specific
depending on the motor should be protected by means of external
motor circuit-breakers or thermistor protective relays; see 3.2.13.
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.
Table 3.37 Project planning notes for multi-motor operation
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