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ReWind the Transformer

Some years (10+) back I partially unwound the power transformer for my power supply on the Hurco mill so that I could stay within the safe margins for the servo drivers I chose. Sadly the servo drivers did not hold up. I replaced them with same more than once before finally upgrading to something that could handle a lot more.

I've got one servo motor that is getting pretty hot. Its also got the most load on the machine. The Y-axis motor. Its a belt drive and it has to sling around the weight of the table and the saddle. It gets hot enough after a few hours to give a 1st degree burn. Not quite enough to make instant skin death, but its hot. You would suffer injury if you wrapped your hand around it and held on.

I have considered a couple options including converting it from a belt drive to a direct drive. A direct drive would have much less parasitic load. There may be an issue with the ballscrew itself, but I'm holding out. I have loosened the gibbs slightly, and I have made sure the ball nut and the gibbs are getting plenty of oil from the automatic oiler.

I am wondering if I increase the voltage by rewinding a few wraps back onto the transformer to bump the voltage back up to the original 90(+/-) VDC instead of the current 78VDC. If the voltage is higher the current should be lower at the same load requirements. Will that result (within reasonable limits) in a slightly cooler motor?

I'm also considering wrapping a heat sink around the body of the motor with a generous slathering of heat sink compound. I believe I can make a bendable heat sink that could be warped to fit the motor.

In all the years I have owned this machine i have always had issues with the Y axis. Its burned up servo drives, and a while back the motor cooked. I do mean cooked.

Actually I am considering doing all of the above. My only real issue with any of these things is that making the axis direct drive will result in that huge motor (and protective cover) sticking out in front of the machine in the way of daily operation.

Reply to
Bob La Londe
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This is a horizontal mill? The Y axis is vertical? Does it have a counterweight or air spring to coutner the weight of the table? That would be the first thing I'd consider.

I really doubt the belt makes any difference.

There may be an issue with the ballscrew itself, but I'm holding

No, it will make no difference. The drive gives the motor the current required to deliver the torque needed to stay in position. Higher voltage to the drive MIGHT actually make it worse.

If this is a DC brush motor, that will have little benefit. It is the rotor (armature) that is producing all the heat. Possibly forced air cooling to the motor will help. If it is a brushless motor, then heat sinks of forced air cooling will help a lot, as all the hear is generated in the stator.

And, if you can't hold your hand on it, then you are COOKING it now!

Jon

Reply to
Jon Elson

On 10/10/2019 11:34 AM, Jon Elson wrote:> On Thu, 10 Oct 2019 10:29:22

-0700, Bob La Londe wrote: >

Any? mechanical power transmission always has some associated loss in usable power. The timing belts on this machine are pretty darned tight. I matched the deflection on the Y up with the deflection on the Z. They are the same center to center distance, and the Z has no issues.

I considered there might be an issue with back EMF at higher voltage, but generically total power is a function of amps*volts. Its my understanding that DC motors typically have a sweet spot, and it was originally powered at 90VDC. My thought was in addition to requiring maybe less current for operation it might also be its "sweet spot."

Do you think the heat might have an affect on the field of the permanent magnets? Its not an open motor, so forced air through the motor is not an option. Of course air could be blown over the motor, but that would not directly cool the rotor.

I am aware.

Reply to
Bob La Londe

Greetings Bob, Your DC brushed motor consumes current based on the torque requirement while the voltage is related to speed. So higher voltage will help it go faster but will not lower the current required. If you could change the torque requirement that would help. Can you change the gearing so that the motor spins faster? Years ago I ran into this exact problem on a home made 4th axis. And the solution was to speed up the motor while lowering the torque. You are correct that total power is voltage times current. Unfortunately supplying more voltage to the motor will not cause it to draw less current. Eric

Reply to
etpm

On 10/10/2019 1:48 PM, snipped-for-privacy@whidbey.com wrote: > On Thu, 10 Oct 2019 13:11:46 -0700, Bob La Londe snipped-for-privacy@none.com > wrote: >

90(+/-) >>>> VDC instead of the current 78VDC. If the voltage is higher the current >>>> should be lower at the same load requirements. Will that result (within >>>> reasonable limits) in a slightly cooler motor? >>>>

I have been looking for a band-aid (rather than surgery) for this application. I think that might just be the band-aid I need. Right now its a 1:1 belt drive. I will have to think about that some. It would be nice to keep the motor inside the knee where it is now.

Reply to
Bob La Londe

My bandaid solution would be to clamp a heat sink with fins to the servo.

Paul

Reply to
Paul Drahn

Higher voltage means more power but less time. The sweet spot you mentioned is in there somewhere. Winding the transformer back to normal first will give you an answer of some sort.

Hul

Bob La L> Some years (10+) back I partially unwound the power transformer for my

Reply to
Hul Tytus

I hope it works. Where is the encoder for the Y axis? If it's on the ballscrew then you can probably get away with changing the pulley ratio. If it's on the motor, then you will need to tell the control what the new ratio is. Or move the encoder. Eric

Reply to
etpm

Yeah, that's no big deal. I can set the PPI pretty easily. The math is simple too once you have done it once or twice.

Reply to
Bob La Londe

I think changing the motor drive ratio is the best solution. However if one wants to get heat out of a brush dc motor i would get a non contact temperature sensor ($30 at harbor freight) and find the hot spots. The hot spots are the place to apply cooling. If the motor has plain bearing than most of the heat is being rejected though the shaft to the housing. If they are ball bearing then the heat is likely from convection of the air internal to the motor to the housing.

If you put a fan on the output shaft that blows over the motor housing you attack to heat sources. You cool the motor shaft and you cool the motor housing with force air which provides much more cooling than a static heat sink. If one need more cooling wrap copper tube around the motor and pass cold water though it.

Reply to
Dan

What you loose in a toothed belt drive is barely noise in the grand scheme of things. How about a bigger motor? Changing the voltage is unlikely to help as the POWER dissipation won't change appreciably by raising the volts. So the amps drop a bit - the volt/amps stay the same. I'd be tempted to either add a big finned heat sink or wrap a few turns of copper tubing around it and add a pump and radiator - - -

- run glycol like in an automotive cooling system

Reply to
Clare Snyder

Sounds like you are sending a boy to do a man's job. Either change the ratio or put a bigger motor on it.

Reply to
Clare Snyder

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