Brake resistor cable size

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philly

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When sizing the cable to be connected between the drive chopper and the brake resistor is it simply a matter of taking the drives Max DC bus level and dividing it by the brake resistor value to determine the current through the resistor. This current can then be used to determine the wire size. So for example if the max DC bus level was 800V and the resistor value was 8ohms, then 10A would flow through the resistor and a #14, cable would be sufficient?

Is high temp wire typically recommended for this connection?

When setting up the brake resistor on the drive do you need to set the duty cycle of the chopper on the drive? Is this duty cycle set as a function of the duty cycle of the resistor? How is this value determined?
 
StephenSDH

StephenSDH

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Yes. The bus voltage divided by the ohm rating of the resistor will give you the maximum current draw.

This circuit is very noisy so it is best to use shielded cable if you have noise concerns.
 
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philly

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Is is recommended to use high temp wire when connecting to these brake resistors? Do the resistors ever get hot enough to cause heating issues with the cable? Does high temp wire typically have a higher ampacity per given size?

When determining what DC bus level to use in the calculation, which value should be chosen? The nominal bus voltage, the voltage the chopper turns on at, or the max voltage at which the drive will trip?
 
GeorgeB

GeorgeB

ElectroHydraulics engineer (retired)
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philly said:
When sizing the cable to be connected between the drive chopper and the brake resistor is it simply a matter of taking the drives Max DC bus level and dividing it by the brake resistor value to determine the current through the resistor. This current can then be used to determine the wire size. So for example if the max DC bus level was 800V and the resistor value was 8ohms, then 10A would flow through the resistor and a #14, cable would be sufficient?
Click to expand...
... 100A?
philly said:
Is high temp wire typically recommended for this connection?

When setting up the brake resistor on the drive do you need to set the duty cycle of the chopper on the drive? Is this duty cycle set as a function of the duty cycle of the resistor? How is this value determined?
Click to expand...
In sizing SERVO brake resistors and cabling, we typically look at peak and at RMS. It is not unusual to need the resistor for 1 or 2% of a 10 second cycle. In your example, E^2/R suggests 80kW peak, far above anything I've ever seen. I'm more used to requirements like 800W peak with a 100W resistor, the wattage of the resistor to handle peaks, not RMS in this case.
 
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philly

Senior Member
Cold Fusion said:
philly -
I'm still curious about the current value being 10A. How do you get 10A with 800V and 8 ohms?

cf
Click to expand...

I'm curious myself :grin: Thats a math error on my part. It should be 100A as stated.

I pulled 800V out of the air, so I'd guess I'd have to check the drive specs to see what the acutal bus voltage should be in the calc. That was why I was curious what bus voltgage I should use in my previous post.

I have hears other recommend that high temp wire should be used.
 
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philly

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The actual resistor value is 7.6 ohms.

I opened up the resistor today and found that from the resistor element to the terminal block on the resistor housing the manufacturer did not use high temp wiring, and the wiring appears to be small somewhere around #10 or so.

So seeing this I now wonder if the manufacturer did not use high temp wire is it necessary to use it in this case. Also if the manufacturer used small wire and only provided terminals for small wire, is it necessary to use a cable capable of carrying the full current as calculate above? Can we get away with a smaller wire due to the fact the current is intermittent and not continuous?

Even if I use a very low bus voltage of 650V DC, which is normal operating voltage I still come up with 85A.

Manufacturers wire was not THHN and appeard to have some type of silver conductor.
 
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philly

Senior Member
Were going to install a #4 High Temp wire because we have plenty of it laying around. Should this work for this scenario

I'm really curious how cable type and rating is factored into this brake resistor application.
 
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