The frequency converter is protected against short circuits by means of current measurement in each of the three motor phases or in the DC link. A short circuit

between two output phases will cause an overcurrent in the inverter. The inverter will be turned off individually when the short circuit current exceeds the permitted

value (Alarm 16 Trip Lock).

To protect the frequency converter against a short circuit at the load sharing and brake outputs please see the design guidelines.

Switching on the Output

Switching on the output between the motor and the frequency converter is fully permitted. You cannot damage the frequency converter in any way by switching

on the output. However, fault messages may appear.

Motor-generated Over-voltage

The voltage in the intermediate circuit is increased when the motor acts as a generator. This occurs in following cases:

The load drives the motor (at constant output frequency from the frequency converter), ie. the load generates energy.

During deceleration if the moment of inertia is high, the friction is low and the decel time is too short for the energy to be dissipated as a loss in the

frequency converter, the motor and the installation.

Incorrect slip compensation setting may cause higher DC link voltage.

The control unit may attempt to correct the ramp if possible (par. B-17 Over-voltage Control.

The inverter turns off to protect the transistors and the intermediate circuit capacitors when a certain voltage level is reached.

See par. B-10 Brake Function and par. B-17 Over-voltage Control to select the method used for controlling the intermediate circuit voltage level.

Mains Drop-out

During a mains drop-out, the frequency converter keeps running until the intermediate circuit voltage drops below the minimum stop level, which is typically 15%

below the frequency converter's lowest rated supply voltage. The mains voltage before the drop-out and the motor load determines how long it takes for the

inverter to coast.

Static Overload in Advanced Vector Control mode

When the frequency converter is overloaded (the torque limit in par. F-40 Torque Limiter (Driving)/par. F-41 Torque Limiter (Braking) is reached), the controls reduces

the output frequency to reduce the load.

If the overload is excessive, a current may occur that makes the frequency converter cut out after approx. 5-10 s.

Operation within the torque limit is limited in time (0-60 s) in par. SP-25 Trip Delay at Torque Limit.

AF-650 GP Design Guide

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