Brake Functions In Af-650 Gp; Mechanical Holding Brake; Dynamic Braking; Selection Of Brake Resistor - GE AF-650 GP Design Manual

3.8 Brake Functions in AF-650 GP

Braking function is applied for braking the load on the motor shaft, either as dynamic braking or static braking.

3.8.1 Mechanical Holding Brake

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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).
NB!
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
(par. B-10 = [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 (par. B-10 = [2]).
DC brake: An over-modulated DC current added to the AC current works as an eddy current brake (par. B-02 and B-03 ≠ off ).
•

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.
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 intermitted duty cycle. The resistor intermittent duty cycle is an indication of the duty cycle at which the resistor is active. The below
figure shows a typical braking cycle.
NB!
Motor suppliers often use S5 when stating the permissible load which is an expression of intermittent duty 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 (of the cycle time)
b
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AF-650 GP Design Guide