January 2012 6.16 The Control Loop 925
Using the voltage
protection module
If the power supply fails during weakened-field operation, and the
synchronous drive is running at a high speed at the same time, then the DC-
link voltage can rise rapidly (generator effect of the drive). If this voltage rises
to over 850 V, then inverters and possibly the motor itself can become
damaged. Reliable protection against this is offered by a voltage protection
module, which short-circuits the motor phases when the trigger threshold of
850 V is exceeded. (for example SM 110 or SM 130; see the "Inverter Systems
and Motors" Technical Manual).
Whether a voltage protection module is necessary for a drive depends on the
desired speed, the nominal speed N-N and the no-load voltage U0 of the drive.
See the motor table for these values.
The following formula calculates the motor speed at which a voltage greater
than 850 V would be induced by the generator effect. If the desired motor
speed is greater than the one calculated, then the voltage protection module
(SM 110 or SM 130) must be used!
Please refer to the "Inverter Systems and Motors" Technical Manual to see
which voltage protection module is needed for which motor types (depending
on the maximum phase current of the SM 110 or SM 130).
A braking resistor, such as PW xxx or UP 110, does not offer sufficient
protection.
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