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Appendix A-1

MN715

Whenever a motor is abruptly stopped or forced to slow down quicker than if allowed to
coast to a stop, the motor becomes a generator.  This energy appears on the DC Bus and
must be dissipated using dynamic braking hardware.  Dynamic braking (DB) hardware
can be a resistor or transistor load.  Table A-1 provides a matrix of DB turn ON and turn
OFF voltages.

Nominal Voltage

230VAC

460VAC

575VAC

Overvoltage Fault (Voltage exceeded)

400VDC

800VDC

992VDC

DB ON Voltage

381VDC

762VDC

952VDC

DB UTP *

388VDC

776VDC

970VDC

DB OFF Voltage

375VDC

750VDC

940VDC

*   DBUTP (DB Upper Tolerance Peak)

+

1.02 x

2

Ǹ

x V

L

*

L

Braking torque and time should not exceed the available drive braking torque and time
rating.  The drive braking torque is limited to the available peak current and peak current
time rating of the control. If the peak current or peak current time limit is exceeded during
braking, the control may trip on an over voltage or a regen power fault. Selecting an
oversized control or a line regenerative control should be considered in these cases.

1.

Calculate the watts to be dissipated using the following formulas for the
appropriate load type.

2.

Identify the control model number and determine which braking hardware is
required based on the model number suffix:  E, EO, ER, MO or MR.

3.

Select appropriate braking hardware from Baldor 501 Catalog or Tables A-2,
A-3 and A-4.

1.

Calculate braking duty cycle:

 

Duty Cycle

+

Lowering Time

Total Cycle Time

2.

Calculate braking watts to be dissipated in dynamic braking resistors: 

 

Watts

+

duty cycle

 

lbs

 

FPM

 

efficiency

44

where:

lbs = weight of load
FPM = Feet Per Minute
efficiency = mechanical efficiency 

i.e., 95% = 0.95