Product Features; Automated Operational Features; Short Circuit Protection; Overvoltage Protection - Danfoss VLT AutomationDrive FC 301 Series Design Manual

Product Features

4 Product Features

4.1 Automated Operational Features

These features are active as soon as the adjustable
frequency drive is operating. They require no programming
or set-up. Understanding that these features are present
can optimize a system design and possibly avoid
introducing redundant components or functionality.
The adjustable frequency drive has a range of built-in
protection functions to protect itself and the motor it is
running.

4.1.1 Short Circuit Protection

Motor (phase-phase)
The adjustable frequency drive is protected against short
circuits on the motor-side by current measurement in each
of the three motor phases or in the DC link. A short circuit
between two output phases causes an overcurrent in the
inverter. The inverter is turned off when the short circuit
current exceeds the permitted value (Alarm 16 Trip Lock).
Line power side
An adjustable frequency drive that works correctly limits
the current it can draw from the supply. Still, it is
recommended to use fuses and/or circuit breakers on the
supply side as protection in case of component break-
down inside the adjustable frequency drive (first fault). See
chapter 9.3 AC line input connections for more information.
NOTICE!
This is mandatory to ensure compliance with IEC 60364
for CE or NEC 2009 for UL.
Brake resistor
The adjustable frequency drive is protected from a short
circuit in the brake resistor.
Load sharing
To protect the DC bus against short circuits and the
adjustable frequency drives from overload, install DC fuses
in series with the load sharing terminals of all connected
units. See chapter 9.6.3 Load Sharing for more information.
MG33BF22
Design Guide

4.1.2 Overvoltage Protection

Motor-generated overvoltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in the following
cases:
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NOTICE!
To avoid that the motor overspeeds (e.g., due to
excessive windmilling effects), equip the adjustable
frequency drive with a brake resistor.
The overvoltage can handled either via using a brake
function (2-10 Brake Function) and/or using overvoltage
control (2-17 Over-voltage Control).
Brake functions
Connect a brake resistor for dissipation of surplus brake
energy. Connecting a brake resistor allows a higher DC link
voltage during braking.
AC brake is an alternative to improve breaking without
using a brake resistor. This function controls an overmag-
netization of the motor when running generatoric. This
function can improve the OVC. Increasing the electrical
losses in the motor allows the OVC function to increase
the breaking torque without exceeding the overvoltage
limit.
Rev. 2014-04-04 All rights reserved.
The load drives the motor (at constant output
frequency from the adjustable frequency drive),
i.e., 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 adjustable frequency
drive, 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 adjustable frequency drive 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.
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