Specifications

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function to carry out when the power transmitted to the

brake resistor exceeds the limit set in 2-12 Brake Power

Limit (kW).

NOTICE

Monitoring the brake power is not a safety function; a

thermal switch is required for that purpose. The brake

resistor circuit is not earth leakage protected.

Overvoltage control (OVC) (exclusive brake resistor) can be

selected as an alternative brake function in 2-17 Over-

voltage Control. This function is active for all units. The

function ensures that a trip can be avoided, if the DC-link

voltage increases. This is done by increasing the output

frequency to limit the voltage from the DC-link. It is a

useful function, e.g. if the ramp-down time is too short

since tripping of the frequency converter is avoided. In this

situation, the ramp-down time is extended.

NOTICE

OVC cannot be activated when running a PM motor

(when 1-10 Motor Construction is set to [1] PM non salient

SPM).

2.12.4 Brake Resistor Cabling

EMC (twisted cables/shielding)

Twist the wires to reduce the electrical noise from the

wires between the brake resistor and the frequency

converter.

For enhanced EMC performance, use a metal screen.

2.13

Extreme Running Conditions

Short Circuit (Motor Phase – Phase)

The frequency converter is protected against short circuits

by current measurement in each of the 3 motor phases or

in the DC-link. A short circuit between 2 output phases

causes an overcurrent in the inverter. The inverter is

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, see the design

guidelines.

Switching on the output

Switching on the output between the motor and the

frequency converter is permitted. Fault messages may

appear. Enable flying start to catch a spinning motor.

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 (at constant output

frequency from the frequency converter), 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.

•

Incorrect slip compensation setting may cause

higher DC-link voltage.

•

Back-EMF from PM motor operation. If coasted at

high RPM, the PM motor back-EMF may

potentially exceed the maximum voltage

tolerance of the frequency converter and cause

damage. To help prevent this, the value of

4-19 Max Output Frequency is automatically

limited based on an internal calculation based on

the value of 1-40 Back EMF at 1000 RPM,

1-25 Motor Nominal Speed and 1-39 Motor Poles.

If it is possible that the motor may overspeed

(e.g. due to excessive windmilling effects),

Danfoss recommends using a brake resistor.

WARNING

The frequency converter must be equipped with a brake

chopper.

The control unit may attempt to correct the ramp if

possible (2-17 Over-voltage Control).

The inverter turns off to protect the transistors and the

intermediate circuit capacitors when a certain voltage level

is reached.

See 2-10 Brake Function and 2-17 Over-voltage Control to

select the method used for controlling the intermediate

circuit voltage level.

NOTICE

OVC cannot be activated when running a PM motor

(when 1-10 Motor Construction is set to [1] PM non

salient SPM).

Introduction to VLT® HVAC D... Design Guide

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