Mechanical Brake Control; Extreme Running Conditions - Danfoss VLT AQUA Drive FC 202 Design Manual

Introduction
which the resistor is active. Illustration 2.25 shows a typical
braking cycle.
The intermittent duty cycle for the resistor is calculated as
follows:
Duty Cycle = t /T
b
T = cycle time in seconds
t is the braking time in seconds (as part of the total cycle
b
time)
Load
Speed
ta tc tb to
T
Illustration 2.25 Typical Braking Cycle
Danfoss offers brake resistors with duty cycles of 10% and
40% suitable for use with the VLT
10% duty cycle resistor is applied, it can absorb braking
power up to 10% of the cycle time with the remaining
90% being used to dissipate heat from the resistor.
For resistor selection information, refer to the Brake Resistor
Design Guide.
NOTICE
If a short circuit in the brake transistor occurs, power
dissipation in the brake resistor is only prevented by
using a mains switch or contactor to disconnect the
mains for the frequency converter. (The frequency
converter can control the contactor).
2.14.2 Control with Brake Function
The brake is protected against short-circuiting of the brake
resistor, and the brake transistor is monitored to ensure
that short-circuiting of the transistor is detected. A relay/
digital output can be used to protect the brake resistor
against overloading by generating a fault in the frequency
converter.
In addition, the brake makes it possible to read out the
momentary power and the mean power for the latest 120
s. The brake can also monitor the power energizing and
make sure that it does not exceed the limit selected in
MG20Z202
Design Guide
ta tc tb to ta
Time
®
AQUA Drive FC 202. If a
Danfoss A/S © Rev. 06/2014 All rights reserved.
2-12 Brake Power Limit (kW). Use 2-13 Brake Power
Monitoring to select what function occurs when the power
transmitted to the brake resistor exceeds the limit set in
2-12 Brake Power Limit (kW).
CAUTION
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.
Over voltage control (OVC) can be selected as an alternative
brake function in 2-17 Over-voltage Control. This function is
active for all units and ensures that if the DC link voltage
increases, the output frequency also increases to limit the
voltage from the DC link, thereby avoiding a trip.
NOTICE
OVC cannot be activated when running a PM motor,
while 1-10 Motor Construction is set to [1] PM non-salient
SPM.

2.15 Mechanical Brake Control

2.15.1 Brake Resistor Cabling
EMC (twisted cables/shielding)
Twist the wires to reduce electrical noise between the
brake resistor and the frequency converter. For enhanced
EMC performance, use a metal screen.

2.16 Extreme Running Conditions

Short circuit (motor phase – phase)
The frequency converter is protected against short circuits
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 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, see the design guidelines.
Switching on the output
Switching on the output between the motor and the
frequency converter is fully permitted and cannot damage
the frequency converter, but it can cause fault messages to
appear.
Motor-generated overvoltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator.
Overvoltage occurs in the following cases:
1. The load drives the motor, generating energy.
2. 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
2 2
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