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 drive 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 Overvoltage

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, ie. the load generates energy.

During deceleration ("ramp-down") if the moment of inertia is high, the friction is low and the ramp-down time is too short for the energy to be

dissipated as a loss in the frequency converter, the motor and the installation.

In-correct slip compensation setting may cause higher DC link voltage.

The control unit may attempt to correct the ramp if possible (par. 2-17

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

See par. 2-10 and par. 2-17 to select the method used for controlling the intermediate circuit voltage level.

High Temperature

High ambient temperature may overheat the frequency converter.

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 VVC

plus

mode

When the frequency converter is overloaded (the torque limit in par. 4-16/4-17 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. 14-25.

2.15.1. Motor Thermal Protection

The motor temperature is calculated on the basis of motor current, output frequency, and time or thermistor. See par. 1-90 in the Programming Guide.

Over-voltage Control

MG.20.N3.02 - VLT

is a registered Danfoss trademark

2. Introduction to VLT AQUA Drive

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