Brake Functions; Mechanical Holding Brake; Dynamic Braking; Selection Of Brake Resistor - Danfoss VLT FC 360 Design Manual

Product Overview
Leakage current [mA]
Illustration 2.22 Influence of Cut-off Frequency of the RCD on
what is Responded to/Measured
See also RCD Application Note, MN90GX02.

2.9 Brake Functions

2.9.1 Mechanical Holding Brake

A mechanical holding brake mounted directly on the
motor shaft normally performs static braking.
NOTE
When the holding brake is included in a safety chain:
A frequency converter cannot provide a safe control of a
mechanical brake. A redundancy circuitry for the brake
control must be included in the total installation.

2.9.2 Dynamic Braking

Dynamic Brake established by:
•
Resistor brake: A brake IGBT keep the overvoltage
under a certain threshold by directing the brake
energy from the motor to the connected brake
resistor (2-10 Brake Function = [1] Resistor brake).
•
AC brake: The brake energy is distributed in the
motor by changing the loss conditions in the
motor. The AC brake function cannot be used in
applications with high cycling frequency since
this will overheat the motor (2-10 Brake Function
= [2] AC brake).
•
DC brake: An over-modulated DC current added
to the AC current works as an eddy current brake
(2-10 Brake Function≠0 s ).
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VLT AutomationDrive FC 360 Design Guide

2.9.3 Selection of Brake Resistor

To handle higher demands by generatoric braking, a brake
resistor is necessary. Using a brake resistor ensures that the
energy is absorbed in the brake resistor and not in the
100 Hz
frequency converter. For more information, see the Brake
2 kHz Resistor Design Guide.
100 kHz
If the amount of kinetic energy transferred to the resistor
in each braking period is not known, the average power
can be calculated on the basis of the cycle time and
braking time also called intermittent duty cycle. The
resistor intermittent duty cycle is an indication of the duty
cycle at which the resistor is active. Illustration 2.23 shows
a typical braking cycle.
The intermittent duty cycle for the resistor is calculated as
follows:
Duty cycle = t
T = cycle time in seconds
t is the braking time in seconds (of the cycle time)
b
Illustration 2.23 Typical Braking Cycle
380-480 V
Cycle time (s)
Braking duty cycle at 100% torque
Braking duty cycle at over torque
(150/160%)
Table 2.14 Braking at High overload Torque Level
Danfoss offers brake resistors with duty cycle of 10% and
40%. If a 10% duty cycle is applied, the brake resistors are
able to absorb brake power for 10% of the cycle time. The
remaining 90% of the cycle time will be used on
dissipating excess heat.
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MG06B202 - VLT is a registered Danfoss trademark
/T
b
Load
Speed
ta tc tb to ta
T
2
tc tb to ta
Time
HK37-H75K
120
Continuous
40%
37
2