Mechanical Holding Brake; Dynamic Braking; Selection Of Brake Resistor - Danfoss VLT AutomationDrive FC 300 Design Manual

Introduction

3.8.1 Mechanical Holding Brake

A mechanical holding brake mounted directly on the
motor shaft normally performs static braking. In some
applications the static holding torque is working as static
holding of the motor shaft (usually synchronous
permanent motors). A holding brake is either controlled by
a PLC or directly by a digital output from the frequency
converter (relay or solid state).
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.

3.8.2 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]).
•
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]).
•
DC brake: An over-modulated DC current added
to the AC current works as an eddy current brake
(2-02 DC Braking Time≠ 0 s ).

3.8.3 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
frequency converter. For more information see the Brake
Resistor Design Guide.
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 3.32 shows
a typical braking cycle.
NOTE
Motor suppliers often use S5 when stating the permissible
load which is an expression of intermittent duty cycle.
RSPSupply - 1-888-532-2706 - www.RSPSupply.com
http://www.RSPSupply.com/p-21462-Danfoss-131B8966-VLT-Automation-VT-Drive-VFD-FC301-230V-5-HP.aspx
®
VLT AutomationDrive FC 300 Design Guide, 0.25-75 kW
The intermittent duty cycle for the resistor is calculated as
follows:
Duty cycle = t
T = cycle time in s
t is the braking time in s (of the cycle time)
b
Illustration 3.32 Typical Braking Cycle
200-240 V
PK25-P11K
P15K-P37K
380-500 V
PK37-P75K
P90K-P160
P200-P800
525-600 V
PK75-P75K
525-690 V
P37K-P400
P500-P560
P630-P1M0
Table 3.17 Braking at High Overload Torque Level
1) 500 kW at 86% braking torque
560 kW at 76% braking torque
2) 500 kW at 130% braking torque
560 kW at 115% braking torque
Danfoss offers brake resistors with duty cycle of 5%, 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.
®
MG33BE02 - VLT is a registered Danfoss trademark
/T
b
Load
Speed
ta tc tb to ta
Braking duty
Cycle time (s) cycle at 100%
torque
120 Continuous
300
120 Continuous
600 Continuous
600
120 Continuous
600
600 40%
600
tc tb to ta
Time
Braking duty
cycle at over
torque
(150/160%)
40%
10% 10%
40%
10%
40% 10%
40%
40% 10%
1) 10% 2)
40% 10%
45
3 3