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Braking Resistor ? size

intrepid

Cast Iron
Joined
Aug 14, 2008
Location
Western Maryland
What should I be looking for in a braking resistor for my lathe? It is powered by a 1.5hp 3phase Leeson motor. The motor is driven by a VFD given to me. It is working fine but I would like to add braking capability. I can not find a brand name on the VFD, only that it is made by the "CH ina Company". Although I am electrically challenged, I have done all the wiring myself. Are the braking resistors rated for hp, amps, or????

thanks,
intrepid
 
Is it a 240VAC input drive? - you really need to know a couple of things before picking a resistor.

1.) does it have provisions for a resistor? (terminal blocks specifically labled for attaching a resistor?)

2.) does it have a manual that states the minimum resistance allowed in Ohms?

3.) does it have a manual that states the resistor duty cycle or that lists the parameters that allow adjusting the duty cycle calculator time constants?
 
motion,

I have no manual for the VFD. I really wish I did for this would make for a much easier set-up. I will ask my buddy I got it from last year if there is any chance he would be able to locate the manual or maybe locate one at his shop where he got it. I will not hold my breath though. As for terminals for the resistor. I have two marked with the letters "BR". I was thinking/hoping these are for "Braking Resistor".
So, I take it that without further information from the manual or manufacture,
I will not be able to figure out what braking resistor I could use?
thanks
 
Intrepid,

I'm not sure how you can program the drive with no manual! Most drives allow DC injection braking which is more than adequate for most applications, especially a small lathe powered by a 1 1/2HP motor.

I have several drives running in my shop, one on a 7 1/2HP 17" lathe. I can stop the machine in short order without the need of any resistor, merely by programming a DC brake parameter.

You may find it hard to access or program the drive with no manual but remember that a add on brake resistor isn't necessary in all (most) shop situations.

Stuart
 
Where do you get braking resistors

In the event that a braking resistor is desired, where would you acquire one. Radio Shack? Are they just typical high wattage resistors or do you need something specific here. I'm not talking ohmage, I'm talking packaging/grounding, etc. How big physically are typical braking resistors. My VFD calls for a minimum braking resistor of 10.3 ohms with peak power of 44KW. Now this is a 20 HP VFD that I am using with a 5 HP motor so I will not need anything near that much peak power.

Chuck
 
After looking on ebay, I have an idea of what these units look like and the cost. I don't believe the values that I posted previously are needed. A 40KW braking resistor would cost a fortune and be massive.

My VFD is 480V 3 phase input (I am using it with 460V 1 phase input), 20Hp output although I am only driving a 5 HP 3 phase motor.

The manual shows 10.3 ohms for 20 hp, 17.7 ohms for 10 hp and 32 ohms for 5 hp. The average power specified for 5 hp is 7kw. This still seems too high. There was one 8500w braking resistor on ebay for $500. Almost all of the rest were much smaller - to the tune of a 100-400watts.

Chuck
 
Chuck,

I'm still learning about the braking resistors also. I was able to install my VFD with my friends help only because he had marked the location of the previous install. But without a manual I cannot figure out the braking. However, if I understand atomarc correctly, I should be able to program braking without the need for a braking resistor. This would be great and would save space and money! But first I will have to find a manual, which is out of my hands.

danny
 
My understanding of the braking is that the VFD can do some braking without the installation of an external dynamic braking resistor but for some applications where heavier braking is desired, the add-on brake is the only option. I will be working with mine this weekend. I do have the manual so hopefully I can figure out how to maximize the braking without adding an external braking unit.

My VFD is an ABB unit and lots of docs are available.

Chuck
 
Check out HVAC heating elements. Some have taps
that you could use for lower ohm values.
A LOT cheaper than ceramic Ohmite types.
Paralleled water heater elements will also work.
<als>
 
Braking Resistor

Check out using a stove element, an oven element or a water heater element. You do not need a continuous rating unit. Low cost from any good 'bone yard'
JRW
 
JRW, can you provide some more details on this? What kind of elements will work? Can you site some specific examples? Do you have any pictures you could share? I like the idea but still feel a bit out of the know.

Thanks,
Chuck
 
Element

Clomb
Go to your kitchen and look at the electric stove top and then open the oven and look at the element. Any of the large "box" stores will have replacement elements for stoves, ovens and water heaters. The specific size will depend on your VFD requirement.
JRW
 
Be careful they don't mean "attach brake module here".

You can probably find out by measuring the voltage across those two points. Best to use a Simpson or similar that actually draws current, but a DVM will work.

Make sure that you do NOT measure the bus voltage on them when VFD is not spinning the motor. You might get some leakage. be careful, there is probably 325V DC available there.

Then make sure that you DO get some voltage when you brake the lathe hard..... I.e. in circumstances that give an O/V trip.

If either condition is not satisfied, it's a non-starter, forget about it.

As for the resistor, figure the brake IGBT is good for maybe 1/3 to 1/2 what the main ones are good for. 3 HP is about 10A, 240V, so figure 3A or so, which suggests about 100 ohms minimum. Hopefully that is conservative.

You want a resistor that is 100 ohms, and can stand at least 4A pulse current (1600W pulse rating), and has a continuous power rating of probably 250W or so..... I happen to know those are available.

That will, with no further information, probably get you a functioning brake resistor.

You will need to mount it on a heatsink with some "thermal mass" which really means it's got fins and some weight to it, with a goodly block of material where the resistor mounts.

The brake pulses to keep the DC bus down, so you don't need a full-power rated resistor. You DO need a "pulse power rated" resistor.

I don't know what a stove element has for resistance. I expect less than 100 ohms. the only guy I know of who tried to use one also blew the brake IGBT in his drive, and was NOT happy.
 
I have used the Chromalox strip heaters from McMaster - in this day and age of documentation on line - you should be able to come up with a manual that gives you braking resistor specifications.

Here is a shot of the last "Shop project drive" with a braking resistor setup.

DSC02355.JPG


McMaster-Carr Strip Heaters

We have sized these for drives between 1HP all the way up to banks for 100HP - they work well and are easy to mix and match to get the resistance and power capacity you need.
 
With no further information, the 240V 500W heater unit looks like your boy...... given the assumptions above.

No guarantees.
 
So, I'm not an electrician or an electronic expert. I proved that this afternoon trying to hook up a home made braking resistor using a chromalox heater strip.
The good thing is I did not get shocked or hurt. However the VFD did not fair as well. Some smoke and burnt wire smell let me know again I should let the wiring to the experts! At least it was not a big financial since it was given to me. At least now I know I do like and want a VFD again and I can but what I want this time. A well known brand WITH a manual!

danny
 
Ouch, at least figure out what you did wrong. Did you test the leads that would go to the resistor for voltage? They should not have power to the leads until the brake IGBT opens. If you just hooked the resistor up to the DC buss, you just created a huge short on the DC buss. I am not sure if other drives would protect against this or not but possibly your VFD is repairable? I say that but recently learning that newer drives use mostly integrated modules, you might be hosed just because of a few diode failures.
 
Yah... ouch....

That was the reason for mentioning checking the output pins for correct brake operation..... and for not liking the heaters in general for resistors.... (The ones M-G linked are potentially OK, in the lower watt ratings....since they are rated for current and the installation can therefore be "designed")

Your experience seems to be similar to the other person who tried heaters.
 








 
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