Thoughts on braking resistor

[Hood]

I am thinking about putting a braking resistor on the VFD that I have on the lathes spindle.
The Yaskawa info I have found says this

So basically 40 Ohm and 2050 Watts.

I have a few dead 22Kw servo drives and they have internal resistors that could stop the spindle from 2000rpm in 1/2 second. The ratings from the servo drive manual says this for the resistor.

So basically it is saying 20 Ohms but it gives 2 values for the power, 400 continuous and 32KW peak.
I have a few sets of these so could link in series to give me the 40 Ohm Yaskawa say but continuous would only be 800 Watts but 62kw peak.

What do you think, bearing in mind a spindle will be stopping in a second or so max, would that resistor have enough of a power rating?

 

[daleyd]

How often would it be stopping?

At a complete guess I’d say it should be ok, the peak rating is a fair bit higher than the specified one - the continuous rating is almost irrelevant given that I would imagine it spends most of its time running or stationary?

Keep an eye on the temperature after a few stops to make sure it’s not going to fry itself

 

[Hood]

It wouldn't be stopping and starting every second or two but maybe in a 10 minute period it could stop/reverse 2 or 3 times.

Will probably just do as you suggest and see what the temps get like.

 

[Luke]

Get the resistance right and as long as the power rating is in the right ballpark, it'll be fine for an e-stop.

As long as it isn't glowing orange (or heading that way) after a few E-stops it'll be fine.

If you're concerned, you could double up to give increased power but still the correct resistance.

Make sure the resistance isn't too low. That'd be bad for the drive's brake chopper.

 

[Luke]

Do you know enough about the drive train to work out the total moment of inertia? From there, we could work out the energy required to stop it.

Modelling shafts and chucks as hollow and solid cylinders will give pretty accurate results - they're pretty simple lumps.

 

[Pete.]

I don't know how much the factory ones cost but you could cobble one from series-parallel connecting cooker rings.

Wouldn't have to worry about them glowing either

 

[Hood]

Get the resistance right and as long as the power rating is in the right ballpark, it'll be fine for an e-stop.

As long as it isn't glowing orange (or heading that way) after a few E-stops it'll be fine.

If you're concerned, you could double up to give increased power but still the correct resistance.

Make sure the resistance isn't too low. That'd be bad for the drive's brake chopper.

The resistor in the servo drive is a bank of 4 in parallel they are 80 Ohm so I assume they must be 100 Watts continuous each. 80/4 giving the 20 Ohm and 100 x 4 giving the 400W. I have 3 dead drives so 12 resistors of 80 Ohm and 100W but will have to see if I can work out how to configure them (series and parallel ) to give me approx 40 Ohm.

Do you know enough about the drive train to work out the total moment of inertia? From there, we could work out the energy required to stop it.

Modelling shafts and chucks as hollow and solid cylinders will give pretty accurate results - they're pretty simple lumps.

Not really as I am not sure exactly what the motor will be turning when it is stopping. This is because it drives through a gearbox with electromagnetic clutches which give different ratios. What happens though is when you stop the clutches disengage the motor from the spindle and the clutches brake the spindle pretty quick, the motor however will still be driving some shafts in the gearbox and I have no idea what the shafts and gears inside being driven will be.

 

[Hood]

I don't know how much the factory ones cost but you could cobble one from series-parallel connecting cooker rings.

Wouldn't have to worry about them glowing either

Funny you should say that as that is basically what is in the active shunts on the small Allen Bradley servo drives.

 

[Hopefuldave]

The resistor in the servo drive is a bank of 4 in parallel they are 80 Ohm so I assume they must be 100 Watts continuous each. 80/4 giving the 20 Ohm and 100 x 4 giving the 400W. I have 3 dead drives so 12 resistors of 80 Ohm and 100W but will have to see if I can work out how to configure them (series and parallel ) to give me approx 40 Ohm.

Either 2 sets in series, each 4 in parallel, or 4 sets in parallel of two in series should do it, and give you 800w continuous, plenty I'd think. I got a pair of nice ally cased ones from a Chinese Gent, weren't an arm and leg.

Dave H. (the other one)

 

[Hood]

That seems to be as much as I can work out, only possible to use 8 to get the 40 Ohm rather than using all 12 to get the 1200 Watts. I will have to look and see what some others I have are resistance/power wise, maybe I can make a mongrel resistance pack

 

[Hood]

Here are the ones out the drives.

But I also have at least 3 of these so that may help a bit as two of them with the others should give me 4 sets of parallel resistors in series and 40 Ohm with a power rating of 1500W if my quick calcs are right. better get a move on, car to load and Catamaran to weld

 

[Hood]

Calcs are wrong, will work it out later

 

[Seadog]

Six in parallel will give you 13Ω, two of those banks in series is 26Ω. Series those with your 22Ω and you end up with 48Ω.

 

[Hood]

8x 80 ohm in parallel is 10 ohm plus 4 in parallel is 20 ohm plus two of the round ones in parallel is 11, so 41 ohm and 1500w if I series them I think.

 

[Hopefuldave]

Problem will be that you want the same current in each resistor, e.g 8 in parallel each get 1/8th the current, 2 in parallel get half, 2 in series both get all of it... dissipated power is resistance x square of the current, so taking 1/2 instead of 1/4 QUADRUPLES the power and heat - POP! Magic Smoke loss!

Safest is two identical parallel banks in series with each other, working with what you've got that's 2 sets in series of 4 in parallel, each resistor is taking 1/8th of the power at the same (1/4) current.

Dave H. (the other one)

 

[Luke]

Yup, this ^.

You've got more to play with if the resistors you've got have different ratings, but you've got to consider the individual elements as well as the total bank.

 

[Seadog]

Safest is two identical parallel banks in series with each other

Which is why I configured them as I did. If the increase in resistance is acceptable then everything balances nicely.

 

[Hood]

Ok, just so I am clear, I need to keep the resistance of each parallel grouping equal or as close as possible to each other , is that correct?

So either the way Seadog has said or just using two banks of the 80 Ohm ones would be the best route, the former actually being slightly better as it gives an extra 150W to the total.