Braking Resistor Protection

[Kani77]

Hi,

Do we need to protect the VFD braking resistor using fuses? If so how do I calculate the fuse rating? The braking resistor is 400W and 18Ohm, VFD output 600V, 3Phase, 60Hz. VFD rating is 45kW. I appreciate your feedback and help.

Thanks,
Kani

 

[waross]

Follow the manufacturer's instructions.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

What waross says. I have not seen protective devices inserted on breaking resistor circuits, either on VFD'd or on conventional DC Hoists. The protection goes on the branch circuit feeding the system.

Given that VFD breaking resistor circuits are often used in systems where loss of the breaking capability can result in loss of load control (a safety hazard) it would be highly unusual to find fuses in a breaking resistor circuit.

Should the breaking resistor circuit be operated above it's thermal capability, the resistor will be in distress long before there is a risk of overheating the wiring. As the energy input to breaking resistors is limited by the energy available for dissipation in the mechanical system, there is little risk of exceeding the resistor duty cycle if the resistor sizing instructions are followed.

If the resistor circuit faults to ground, the drives ground fault detection should trip. Check the specific drive manual to confirm.

 

[waross]

Given that VFD breaking resistor circuits are often used in systems where loss of the breaking capability can result in loss of load control (a safety hazard) it would be highly unusual to find fuses in a breaking resistor circuit.

I concur. All good advice and good information.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kani77]

Thank you very much for your reply. I considered the thermal OL contact of the resistor as a safety input to VFD.

 

[eeprom]

Braking resistors are usually protected by thermostats that trip the drive. This is pretty common

 

[jraef]

Some high-end VFDs include calculated braking resistor protection in programming, ie you enter the data on the resistor and the drive keeps track of what it is seeing to know when it is being over stressed.


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

 

[Compositepro]

Fuses and other protection for a braking resistor is like putting relief valves in the brake lines of a truck.