Spindle motor rewind

I have an HSD spindle which I plan to use on my VTX1 rebuild, it is rated at 5Kw 380v 2 pole 400hz. I am running it on a vfd at 240v at the moment.

I am looking into the possibility of getting it rewound as ~ 3Kw 240v 8 pole 400hz. I think this would run at more suitable speeds for a mill and have more torque at the lower speeds. My VFD will actually go to 1000hz so could still get~ 18000rpm.

Perhaps even a pole changing winding, though I am not sure how the VFD would cope with that.

Has anyone gone down this path - what are the snags etc - there are always some :rolleyes:

I did note from a search that John S had dabbled a little in rewinds. :eagerness:

Did you get a price for a rewind ? it might be best to sell the one you have and buy another with the correct parameters you need.

Haven't got as far as pricing yet, just exploring the possibilities at the moment but, even if I found a spindle with those specs, I think the price gap between selling and buying would be a lot more than a rewind. :concern:

It's a 24 slot stator so an 8 pole single layer winding is simple and well within DIY territory:smile:

Ok need to get better advise on this after the holiday but don't think a 2 pole can be changed into an 8 pole just by winding.
OK the stator has 24 slots but the rotor has to match.

Also at 8 pole you will get approx 5.700 revs at 400 hz and there will be a limit you can go to on hz before it all gets saturated or egg bound in technical terms.
One guy on another forum has just had a quote for a standard 3~ rewind at £350 I can supply him a new motor for £125

Ok need to get better advise on this after the holiday but don't think a 2 pole can be changed into an 8 pole just by winding.
OK the stator has 24 slots but the rotor has to match.

Is there an easy way to find out the slots/bars in the rotor, the laminations are ground smooth and appear continuous, it is 58 dia x 130 long, the stator is 108 o/d

Also at 8 pole you will get approx 5.700 revs at 400 hz and there will be a limit you can go to on hz before it all gets saturated or egg bound in technical terms.

Can't find the specs for my exact motor but HSD do give figures for their 4 pole motors to 28000 rpm (933hz). It is 5 Kw continuously rated so I was hoping that derating to ~3 Kw would give a bit of headroom. Because its a mill I am more interested in some low speed torque than high speed, I think ~6000 rpm max would be OK

Duplicate post, please ignore this one.

I don't know exactly which motor you have but can you change the connections from star to delta ? it won't change the number of poles obviously but it would give a lot more torque. I've worked with motors for a long time and have never heard of rewinding an ac motor being described as, "simple and well within DIY territory".

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Originally Posted by EddyCurrent

I don't know exactly which motor you have but can you change the connections from star to delta ? it won't change the number of poles obviously but it would give a lot more torque. I've worked with motors for a long time and have never heard of rewinding an ac motor being described as, "simple and well within DIY territory".

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My first thought was to look the at connections to see if I could reconnect as delta . The motor windings are well constructed with every thing tightly wrapped up varnished and baked, I don't know if it is star or delta at the moment and it would take a bit of risky digging to find out.

I have also worked with motors for a long time, I have in the past (~60 yrs ago) rewound the odd motor or two, things had not used to be so throwaway then, I also had the advantage of working at a motor manufacturing plant, so all the materials were readily available.

The actual laying in of the coils is a fairly easy mechanical task, particularly with 8 poles 24 slot single layer, and is well within my DIY territory.

The more technical aspect of rotor slots, flux densities,wire gauge and turns is a bit more challenging, as John S inferred, which is why I was looking for the experience of others who may be designers of motors. There are several programs available to help with this but was looking/hoping for a shortcut.

[QUOTE=EddyCurrent;52560]I don't know exactly which motor you have but can you change the connections from star to delta ? it won't change the number of poles obviously but it would give a lot more torque. .[/Q

Changing star to delta connections will change the operating voltage and ~ double the amps required, doesn't change the torque curve of the motor.

More torque will require even more amps, which I haven't got, or more poles. This is in my garage on a 240v supply with my existing 3Kw VFD which is why I am looking at changing pole count.

Update

I have eventually rewound my 2 pole spindle to 8 pole, dragged on a bit for a variety of reasons. Did a little research on rotor slot numbers, seems to be a bit of a black art with different motor manufactures doing their own thing, seems to be more to do with starting torque and efficiencies than any thing else.

Invested in a winding design service on the net, asked for 240v 400Hz delta connection, came up with 20 turns per slot.

Wound the stator at 20 turns two in hand 0.75mm wire, :dejection: ~20A in delta no load running - ~12A in star, this was about 8 times the original motor.

Some emails back and forth getting me nowhere, seemed obvious to me that it was lack of turns (flux).

Decided to think it through on the basis that I knew the turns and current of the original motor and that HSD had got the ampere turns right.

Fortunately I had tried the motor before varnishing it so I was able to strip it down again very easily.

Wound the stator at 50 turns two in hand 0.55 wire, :thumsup: ~2.5A in delta no load running similar to the original motor, runs OK up to 500Hz which is more than I want.

From when I was a young lad my Father repeatedly told me ' Beware the Expert' :rolleyes:

Not particularly neat but does the job OK


Attachment 12031Attachment 12032Attachment 12033Attachment 12034

Very interesting John, Just goes to show with knowledge and patience a lot can be achieved ..Clive

Brilliant, well done. I don,t suppose that you want to rewind my HF (1000hz, 55volt) to 400hz 230volt), for a (small) bag of gold of course. This is a serious enquiry as I too have been told that it is not possible. G.

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Originally Posted by GEOFFREY

I don,t suppose that you want to rewind my HF (1000hz, 55volt) to 400hz 230volt), for a (small) bag of gold of course. This is a serious inquiry as I too have been told that it is not possible. G.

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Sorry but modifying some one else's motor would be a bit scary :eek:, I persevered with mine mostly because I was told it's not possible and I could see no reason that it could not be done, just a matter of finding the correct info, I have spent a lifetime solving engineering problems, many of which were not possible.

There are motor design programs out there but they are very expensive, need to find a friendly face in one of the universities. I tried a couple of re-winders for info but they really don't want to know about anything non standard.

Modifying an existing motor should not be too difficult if you accept that the manufacturer got it right with respect to flux densities in the stator (ampere turns in old money).

This something I found somewhere on the net.

Rewinding for a change in voltage

If a 220 volt motor is to be rewound to operate on 440 volts, use twice as many turns on each coil and one-half the circular-mills area of wire. In other words, if 40 turns of NO. 17 wire were used on the original motor, 80 turns of NO. 20 should be used on the new motor

Some motors rated for 230 volts will not handle the load on 208 volts if loaded to the maximum. The turns must be reduced to the ratio of the voltage change. As an example, 230-volt motor has 40 turns: 230/208 = 1.1, 40 turns/1.1 = 36 turns. If there is enough room, the next larger wire size should be used. An easy way to determine whether there is enough room is to cut the required number of of lengths of wire of this size and fit then into the slot.

Changes for New frequency

There are two ways to convert these motors; one keeps the same horsepower for the new speed, and the other keeps the same torque for the new speed (more horsepower). For the same horsepower, use the following formula: old turns*Sqrt(old Hz / new Hz) = new turns. If you want the same torque then it is: old turns*old Hz/new Hz = new turns.

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Originally Posted by johnsattuk

There are motor design programs out there but they are very expensive

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Here's a free one:
https://www.emetor.com/
Although lately I've been using this one as it's on the university computers.

Anyway, Geoffrey, have you got a picture of the stator inside your motor? I'd be interested in rewinding it / working out how for you. Who told you that it wasn't possible and what reason did they give?

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Originally Posted by Jonathan

Here's a free one:
https://www.emetor.com/
Although lately I've been using this one as it's on the university computers.

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I did find and play with emetor and also Koil, and some demo versions of others, but really my only requirement was no of turns per slot and I did not find that these gave me that info.

I originally tried the forum in the hope that some kind soul would help with some positive info. :rolleyes:

It is highly unlikely that I will wind another motor, this was a just a whim that became a challenge.

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Originally Posted by Jonathan

Here's a free one:
https://www.emetor.com/
Although lately I've been using this one as it's on the university computers.

Anyway, Geoffrey, have you got a picture of the stator inside your motor? I'd be interested in rewinding it / working out how for you. Who told you that it wasn't possible and what reason did they give?

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Thanks for your reply and offer Jonathan. I do not have any pictures of the internals of my spindle as I have never thought about dismantling it. I have a couple of these spindles which I love to use because of the air release collet chuck and work around the fact that I am limited to 3.172mm shank tools. As I have a "spare" spindle I would be happy to take a chance that it does not work if you are prepared to try. The impossible comment came from various commercial rewinders and I had very little faith in that I believe that nothing is impossible!!! Perhaps a little difficult sometimes. The most logical comment came from one rewinder who said that it would be difficult on a small diameter spindle to use the heavier duty wire necessary to change. These spindles are lovely (ex pcb machine), but I only have one HF inverter and I would like to get one running on a standard 400hz inverter as I have started to strip down another machine which has a cutting area of 540x600mm. G.

I apologise if I seem to be hi-jacking this thread, but I did start a somewhat similar thread about my spindle and had no response. G.

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Originally Posted by GEOFFREY

The most logical comment came from one rewinder who said that it would be difficult on a small diameter spindle to use the heavier duty wire necessary to change.

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Not sure it was a very logical comment, I would expect the requirement would be for more turns of a smaller dia wire.

Impressive ! good job (Edit: The rewind I mean)

Oops!! That could be my misinterpretation. Perhaps he meant not enough room to get the turns in. Anyway, they said it would be impossible - I guess they were just not interested. G.

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Originally Posted by GEOFFREY

Perhaps he meant not enough room to get the turns in. Anyway, they said it would be impossible - I guess they were just not interested. G.

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I think you guess correctly, getting the turns in is not generally a problem, once you know the turns you have to decide on the wire dia that will fill the slots as much as possible, bigger is better but it depends on the practicalities of getting them in. It may be that the number of turns required reduces the dia of the wire enough to limit the current, and thus the power of the motor.

What is the current rating of the existing motors, may be cheaper to get a VFD with enough amps to drive them, you would be able to program it for the volts.

Be interesting to see Jonathan's winding solution.

Thanks for the reply, but it is not possible to find a suitable replacement VFD. The problem as I understand it is that the spindle (900w max) is a 2 pole 3ph motor with a speed range of 15-60krevs at 15-60volts. I do not need that speed, so would be happy with 24krevs at 230volts and use an off the shelf inverter. I have been advised that whilst I may be able to set the voltage down that low with a 400hz inverter, these inverters are PWM and would still be applying a higher voltage for a short period to achieve the lower working voltage, but that the spindle low voltage windings will not be insulated sufficiently to cope with that. I hope that makes some sense. I certainly had the impression that the "impossible" comment was because there would not be enough space for the revised windings, or indeed that they were not really interested. The spindle body diameter is 62mm, but none of the rewinders I spoke to have seen the spindle, let alone examine it. G.

Rewinding a motor will only increase its torque rating if you manage to fit more copper in the stator slots than the original. Changing the number of turns in series/parallel (etc) primarily changes the voltage rating - it is the *total* current through the slot that matter when calculating the torque output. The total current you can put through the slot depends on how much copper you can fit - too much current or too little copper = more heat.

If you just copied the existing winding, but increased the number of turns by the ratio of the new to old voltage rating, then it would be OK. We can go into a lot more detail if you want to, but it's hard to say how much there is to gain from this without actually seeing the motor inside.

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Originally Posted by GEOFFREY

Thanks for the reply, but it is not possible to find a suitable replacement VFD. The problem as I understand it is that the spindle (900w max) is a 2 pole 3ph motor with a speed range of 15-60krevs at 15-60volts.

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That is well within the rating of the VFD I have designed/made.

Can you share your email? I would like to do this modifications too. Just a couple of questions. 😅 My mail is [email protected]