Lenze EDB9300UES - Manual (Page 158)

Commissioning
BA8230 153
5.6.6 Multiple motor operation If two or more motors are always to be operated together on the 8230 frequency
/Group supply inverter with speed control, this group can be considered as a large motor with the
correct type data settings (the total for all the motors connected) for rated current,
rated voltage, rated speed and the power factor of the resultant machine in the
structure speed regulation without encoder. In our experience the power factor should
not be set to that of a comparably large motor. The trip torque/rated torque factor
should be reduced.
In speed control without encoder the inverter output frequency is regulated according
to the load and can assume values in excess of the type point frequency.
If the power of the motors differs significantly or when a large number of small motors
are used, the parameters can also be set for the frequency control structure as a pure
characteristic control, neglecting the speed and current control characteristics. Speed
instability may occur in this mode, especially with larger motors.
If motors of different power levels are connected and the parameters are set for the
resulting total, small motors will inevitably be underexcited because they receive a
smaller proportion of the compensation voltage due to the higher internal voltage
drop.
If several asynchronous motors of the same rating are connected in a mechanically
rigid coupling, speed regulation with encoder can be implemented, taking into account
the possible slip difference in asynchronous motors.
Depending on the mode of operation, the speed of the motors used and the load cycle
a limit on the effect of the magnetising controller can positively affect the current
control characteristics when operating at less than full load.
If the coupling is not rigid enough it is possible for the load to be distributed
incorrectly. This can only be corrected by a better coupling or a load compensation
control.
At least one common output choke is to be used on group supplies. Motor cables
must be limited in length so that the sum of the individual lengths lies within the
maximum motor cable length stated for the 8230 frequency inverter. The connection
instructions in section 3.5 apply accordingly.
The output choke is to be used on group supplies.
Suitable overcurrent protection facilities for pulse-shaped voltages are to be provided
for the individual motors on group drives. A higher adjustment current must be taken
into consideration for thermal overload protection with magnetic release. Overload
protection facilities with an indirect release via a auxiliary contact are recommended.
The current limit set with the parameters is decisive for an overload in controlled
operation, while the maximum permitted unit current can occur during frequency
characteristic control.
Switching the motors in the speed control structure is possible if it is guaranteed that
not more than 30 % of the motors used are switched off.
If the cable lengths stated are exceeded in large multiple motor systems one should
first check whether motor connection cables can be grouped for most of their length
and only branched off near the motors. As a second option the installation of output
chokes with higher inductance should be examined.
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