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Dynamic Braking Resistors

Dynamic Braking (DB)

The DC bus voltage level of an AC drive increases while the

motor is re-generating (i.e. ramping to a stop). Dynamic

braking resistors provide a means of rapidly stopping a

rotating motor and load while maintaining an acceptable

bus voltage level. The kinetic energy stored in the spinning

mass is converted into electrical energy and quickly

dissipated as heat through a resistor. Control Techniques

offers resistor kits available for both AC and DC drives.

Dynamic Braking for DC Drives

DC drives provide an exponential stopping prole when

a dynamic brake resistor is applied across the motor’s

armature circuit (when the motor acts as a DC generator).

This type of braking can occur only when the drive is

congured for coast stop and power has been removed

from the motor (stop commanded). To apply a dynamic

brake resistor to a DC drive, a motor contactor with a

DB pole is required. This motor contactor is standard on

Quantum MP drives up through 225hp (400A).

Control Techniques Mentor MP and Quantum MP DC drives

are available in regenerative models as an energy efcient

alternative to dynamic braking. See the Mentor MP/

Quantum MP DC drive brochure for details.

Dynamic Braking for AC Drives

AC drives provide a constant torque stopping prole

when a dynamic brake resistor is applied across the DC

bus circuit. Dynamic braking can be employed under a

stop command or anytime a decrease in motor speed

is commanded, provided the AC drive is enabled and

programmed for ramp stop (fast ramp mode).

Control Techniques offers two types of dynamic braking

kits for AC Drives. The E-stop duty kits are rated for light

start/stop or deceleration duty cycles.

The cyclic duty kits are intended for heavy duty

applications that need the capability to dissipate

regenerated energy on a more continuous or repetitive

basis such as downhill conveyors, hoists, feeders and

dynamometers.

E-Stop Duty

E-stop duty DB resistors are designed for absorbing energy

generated by infrequent motor stops or deceleration.

These kits are designed to meet or exceed NEMA standard

7-15-1970, which states “DB resistors will not exceed

their rated temperature rise when the drive is braked

from maximum speed to standstill three times in rapid

succession with a load inertia equal to or less than the

motor inertia.” They are designed to provide 150% braking

torque (peak at max speed for DC drives, constant for AC

drives) for 1800rpm base speed motors.

Cyclic Duty

The standard packages listed below are NEMA 1 rated

and include built-in junction box, terminal strip, normally

closed thermal contact (klixon switch) and resistors pre-

wired with high-temperature Teon wire.

These heavy-duty kits have been designed to provide

dynamic braking for cyclic and continuous braking

applications. There are three levels available: 10%, 25% and

50%. These levels refer to the continuous allowable braking

level (i.e. 25% refers to 25% of rated motor braking torque)

or the maximum allowable duty cycle rates with maximum

specied “on-time” limitations (refer to illustration below).

Galvanized NEMA1 with

normally closed thermostat

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