26 Chapter 5 - Electrical Connections
User Manual
Variable voltage output is derived from the intermediate circuit voltage using PWM technology. Special sinusoidal modulation
in conjunction with the motors own inductance produces an extremely good sinusoidal curve for the output current I
2
. The
voltage/frequency ratio is programmable and can be adapted to suit the motor being controlled.
More than one motor can be connected in parallel to the drive output. Motors may run at different speeds even though they
have the same number of terminal pairs, because motor slip can vary with the load applied and motor characteristics may
vary too. Motors can also be switched in and out individually, though great care must be taken when doing so.
Switching a motor in or out causes voltage peaks by interrupting an inductive current flow. These voltage peaks do not
normally disturb the drive output provided the motor is a low power model and other motors remain connected to the inverter
after it is switched out.
If the motor being switched out is the last motor connected to the drive, make sure that the motors
magnetising current has dropped to zero before switching it out. the best way of doing this is to lock the
inverter bridge and disconnect the motor only after a fixed delay calculated to suit the characteristics of
the motor, in practice from about 0.5 seconds up to a number of seconds.
Motors can likewise be switched in to an already functioning inverter one at a time. If you wish to do so, bear in mind that the
instant the motor is connected its inrush current far higher than its nominal current. The drive must therefore be carefully
selected so that inrush currents do not exceed the drives nominal current. You must also consider the overload that the drive
is able to cope with if the duty cycle during which the new motor is connected coincides with the limited period for which
overload is permitted.
More than one inverter cannot work directly in parallel.
