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Wattage of a Braking resistor

Wattage of a Braking resistor

Wattage of a Braking resistor

Hi we have a 20 tonnes crane run by a 60kw electric motor and its VFD. The dynamic braking resistor connected is 3.5ohm but we need to know the wattage of it and its model qnd brand is not clear.

Is there a way to measure or calculate the wattage of the braking resistor?

RE: Wattage of a Braking resistor

If you want your hoist down speed to equal your up speed I would start by sizing the resistor to match the motor power. As you'll likely spend an equal amount of time hoisting vs braking, you may be able to reduce the resistor size for a 50% duty cycle.

RE: Wattage of a Braking resistor

That’s something you need to discuss with the VFD mfr. The drive has a “chopper transistor” that is connected to the brake resistor. That transistor has a maximum current rating and duty cycle. The current rating of the transistor dictates ohmic value of the resistor, but the duty cycle dictates the wattage rating of it. If you get too low of a watt rating on the resistor, it can burn up. Most VFDs do not provide protection for the braking resistor itself, some don’t even monitor the brake chopper transistor other than a heat sink temperature switch. Too high of a watt rating is not a problem though, other that physical size constraints and cost.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Wattage of a Braking resistor

To select a breaking resistor you need to know:
Maximum instantaneous energy (often this is limited by the drive to 200% motor design torque). This leads to the resistance value of the resistor, the selection process will be described in the drive manual.
Duty type (Crane Hoist is particularly severe, crane travel less so)
Duty Cycle (Duty Cycle must consider the worst case, which might be lowering a test weight the full hook travel distance, and then decelerating the load.)

The linked (in the next post) ABB technical manual 8 has an accurate, but overly complex explication of the requirements for selecting DB resistors.

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