Author Topic: Rewiring old 3 phase motor for low voltage  (Read 13921 times)

Offline loply

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Rewiring old 3 phase motor for low voltage
« on: October 22, 2012, 04:15:19 PM »
Hi folks,

I acquired this old beast a week ago - .

As you can see from the photo, it's an inverted horizontally oriented wall mounted Eagle surface grinder, a little known model. Either that or I held my camera sideways, I don't remember  :hammer:

Can't wait to get it operational, alas, it has a three phase motor on it. Motor weighs a ton and is mounted on a steel plate that's probably 3/4inch thick.

I have a VFD to run the motor, but the problem is it's undoubtedly wired for "high voltage" or in star configuration.

I gather from reading around that I should be able to rewire the inside of the motor into delta so that I can run it at 240v 3ph. I've done this on modern motors where you just move some jumpers, but I gather that on old ones you have to work a bit more.

I don't really know where to start here - do I need to start snipping some of the tie material that's holding all the wires together, to try to find the ending of each circuit? Should I just dive in with some careful snipping action? Am I even looking at the right end?!

Would appreciate any advice.

Cheers,
Rich




Offline BillTodd

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Re: Rewiring old 3 phase motor for low voltage
« Reply #1 on: October 22, 2012, 04:46:24 PM »
Get yourself a 400-500v VFD and run it from a single phase step-up transformer.

(see my website under Haighton single phase conversion)

http://wktodd.webspace.virginmedia.com/major/diagrams.html

You'll need to de-rate the VFD some to run it on single phase, so choose a 2-3hp type.

P.S. It is possible to modify some VFDs internally so that the input capacitors act as a voltage doubler:

http://www.homeworkshop.org.uk/index.php?option=com_agora&task=topic&id=928&Itemid=47
Bill

Rob.Wilson

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Re: Rewiring old 3 phase motor for low voltage
« Reply #2 on: October 22, 2012, 04:46:54 PM »
Nice bit of kit you have yourself there Rich  :headbang: :headbang:

Sorry i cant help with your motor problem  :palm: what about fitting a single phase motor ?


Rob

Offline loply

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Re: Rewiring old 3 phase motor for low voltage
« Reply #3 on: October 22, 2012, 05:02:07 PM »
I'd like to keep the original motor as I already have the VFD, and I have read that 3ph motors (even with a VFD) run smoother than their single phase counterparts, which becomes relevant when fine finishes are sought. Also, this motor seems real well built.

Trouble is I don't know exactly how to do this rewiring malarkey...

Offline BillTodd

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Re: Rewiring old 3 phase motor for low voltage
« Reply #4 on: October 22, 2012, 05:18:28 PM »
Quote
Trouble is I don't know exactly how to do this rewiring malarkey...

You'll need to find the star point of the existing windings ( i.e. the place where all the poles are commoned ). you need to be careful not to wreck the motor in the process, especially if the star-point is buried deep.

It is a two pole motor so there shouldn't be too many wires to figure. If you are lucky, then all you need to do is strip back the wiring and  trace each of the existing connections which should leave a joint that connects the three pole pairs together (i.e. three wires connecting together instead of two).

(A step-up transformer can save a whole lot of grief)

Bill
 

Bill

Offline loply

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Re: Rewiring old 3 phase motor for low voltage
« Reply #5 on: October 22, 2012, 05:57:30 PM »
Thanks for the help folks.

Armed with the entirely safe combination of an undue sense of confidence, a small pair of scissors and a little bit of knowledge, I set about chopping up a 50 year old motor.

After some searching I found what I presume is the star point, as it's the only bit that looks like a star.



And after removing a battery from a less worthy device to power my multimeter, I can confirm that each point of the star, when separated, connects only to one of the original screw terminals.



Now the only thing I don't know is how to reconnect them. I've read a few pages but they seem to conflict with each other. Can I just randomly label the pairs as U/V/W as I see fit, then connect them in Delta as per usual?

Cheers,
Rich

lordedmond

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Re: Rewiring old 3 phase motor for low voltage
« Reply #6 on: October 23, 2012, 03:04:37 AM »
yes you can  ,but the rotation would be random , if its wrong just swap two power leads and you will be fine


make sure you clean the enamel off completely ( scraping is best )


I use to use silberloy barraging rod ( no flux required and above all no cleaning of the enamel


good luck and dont forget to tie the wires in tight and put some varnish on them ( poly is ok for this its to stop the wire rubbing together)




long time ago ( during my apprenticeship ) I did stator and armature winding


Stuart

Offline BillTodd

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Re: Rewiring old 3 phase motor for low voltage
« Reply #7 on: October 23, 2012, 03:10:59 PM »
Well done  :thumbup: Glad it went well for you :)

Quote
Can I just randomly label the pairs as U/V/W as I see fit, then connect them in Delta as per usual?

The original wires should identify the phases so just wire and label the tails as U2,V2 & W2.

If you connect

U2 - V1
V2 - W1
W2 -U1

It should rotate as expected.

Bill
Bill

Offline loply

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Re: Rewiring old 3 phase motor for low voltage
« Reply #8 on: October 24, 2012, 06:03:44 AM »
Bill, Stuart - Thanks to both of you for the help.

I stripped the varnish from the 'star' leads and soldered on some new extensions, then hooked up this 3 point junction box bringing all 6 ends to the electrical access point and wiring it into Delta at the same time:



Slightly non-standard cable colours admittedly, but I got it sealed back up and popped a new grommet and armoured cable on for good measure:



One quick hoist with the engine crane later and I ground my first parts - the jaws of this old WW2 Paramo vice (which were too bad to get completely flat!) and this triangular chuck adapter plate for my RT. Motor ran perfectly and the finish was impressive for my first shot with an unidentified wheel!

Thanks folks,
Rich


Offline David Jupp

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Re: Rewiring old 3 phase motor for low voltage
« Reply #9 on: October 24, 2012, 01:53:53 PM »
Great - I have a similar original motor for my Smart & Brown lathe (in my case a 6 pole, 970 rpm) - every time I've looked so far I've chickened out from digging to find the star point - I have a modern 4 pole motor in place temporarily.

I really must have another go at finding the star point - no obvious joints that 3 wires head into....  it would be really good to get the speeds back to original plus have VFD.

Offline NickG

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Re: Rewiring old 3 phase motor for low voltage
« Reply #10 on: October 27, 2012, 06:31:53 AM »
Useful thread this, I thought you needed the motors that were 220/440v to run from an inverter, does this mean you should potentially be able to re-wire any 3 phase motor in delta?

Cheers

Nick
Location: County Durham (North East England)

Offline BillTodd

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Re: Rewiring old 3 phase motor for low voltage
« Reply #11 on: October 27, 2012, 08:46:41 AM »
Useful thread this, I thought you needed the motors that were 220/440v to run from an inverter, does this mean you should potentially be able to re-wire any 3 phase motor in delta?

Cheers

Nick

Potentially yes, but the problem with rewiring, is that it is not always possible to find the star point without risking the destruction of the motor wiring;

I had a go at finding the star point on my HLV-H speed control motor, a small 1/8 hp device. The windings were so tightly bound and bonded with epoxy it would have simply  broken the copper wire if I had tried to pull it apart.  In the end I choose to drive it in star mode at 415vac from the main motor's VFD 

In the case of my Hardinge 2 speed (dual windings) main motor, the rewiring would have been so complicated by extra wire, that it just isn't worth it; Stepping up the 240 mains to 415 with a transformer and using a high voltage (=cheaper on ebay) VFD was easier and meant I didn't need to touch the, awkward to get at , motor at all (nor did I need to disturb the motor's 0.0003" balance!) .

Stepping up the mains voltage and using a high voltage VFD is easy :)

Bill
« Last Edit: October 27, 2012, 12:25:35 PM by BillTodd »
Bill

Offline awemawson

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Re: Rewiring old 3 phase motor for low voltage
« Reply #12 on: October 27, 2012, 09:01:36 AM »
There are another couple of issues with running 'non-inverter designed' motors on an inverter. The fast rise time of the chopped square waves can lead to insulation break down in the wires due to the high frequency component of the waveform, and also the cooling fan will really only be intended to shift enough air at standard rpm. In practice I've never had problems with the voltage break down, but the cooling at lower revs is definately to be watched.
Andrew Mawson
East Sussex

Offline loply

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Re: Rewiring old 3 phase motor for low voltage
« Reply #13 on: October 27, 2012, 10:27:15 AM »
Oddly, this particular motor doesn't have a cooling fan of any kind, which I found odd.

It does have several kilos of metal enclosure though, perhaps they expect it to simply conduct away.

Offline NickG

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Re: Rewiring old 3 phase motor for low voltage
« Reply #14 on: October 27, 2012, 11:01:38 AM »
Thanks guys, I thought the cheap type inverters wouldn't do the high voltage and you needed a proper phase converter to use them ... it has been a while since I looked, was looking into it when I bought my harrison lathe, but because it wasn't a dual voltage motor I thought I could either pay the high cost of a phase converter or swap for a single phase motor - so I put a single phase on. Still got the big 3 phase, looks a good quality motor!
Location: County Durham (North East England)

Offline David Jupp

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Re: Rewiring old 3 phase motor for low voltage
« Reply #15 on: October 27, 2012, 12:25:44 PM »
Oddly, this particular motor doesn't have a cooling fan of any kind, which I found odd.

It does have several kilos of metal enclosure though, perhaps they expect it to simply conduct away.

It probably has some small 'fins' on one end of rotor ?- which will act as a very crude centrifugal fan.  I have a similar one and the fins really don't amount to much.  As you say, all that iron will help!

Offline Noitoen

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Re: Rewiring old 3 phase motor for low voltage
« Reply #16 on: November 01, 2012, 10:29:24 AM »
If you have to pay to get a motor rewound, you can ask them to calculate the winding to a even lower voltage than the original. A 1 HP motor 3~220V/50Hz, rewound for 3~110V/50Hz, will do, on a 2Hp/220V inverter, configured for a 110V/50Hz motor, 2Hp at 220V/100Hz, without any derating. It's usually wise to remove the fan from the motor's shaft and assemble a forced cooling fan. Motors have a linear V/Hz curve and although you can "over-speed" a standard 220V/50Hz motor on a regular 220V inverter, as soon as you overshoot the original motor's design frequency, if you don't increase the voltage in proportion, you will lose torque.

Offline hopefuldave

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Re: Rewiring old 3 phase motor for low voltage
« Reply #17 on: November 01, 2012, 01:58:56 PM »
Quote from: BillTodd


I had a go at finding the star point on my HLV-H speed control motor, a small 1/8 hp device. The windings were so tightly bound and bonded with epoxy it would have simply  broken the copper wire if I had tried to pull it apart.  In the end I choose to drive it in star mode at 415vac from the main motor's VFD 

In the case of my Hardinge 2 speed (dual windings) main motor, the rewiring would have been so complicated by extra wire, that it just isn't worth it; Stepping up the 240 mains to 415 with a transformer and using a high voltage (=cheaper on ebay) VFD was easier and meant I didn't need to touch the, awkward to get at , motor at all (nor did I need to disturb the motor's 0.0003" balance!) .

Stepping up the mains voltage and using a high voltage VFD is easy :)

Bill

 Worse yet, my Holbrook has a 3-speed 415V motor with 9 connections (a Dahlander-wound BTH with a separate medium-speed winding - the 3-speed switch is a thing of Frankensteinian beauty)... I too chickened out of a rewound stator (and the local motor rewinders quoted rather more than I'd paid for the complete lathe) but rather than step up the mains I went with a "voltage doubler" mod to the 415V-input VFD (find the common point between the two banks of DC bus capacitors, move the Neutral there, away you go) and derated the VFD somewhat (5HP to 3HP as suits the motor).

 The motor (precision balanced like those in Hardinges, Harrisons etc.) seems fine up to 75Hz (giving an useful spindle speed increase from 1500 to 2250 RPM) and down to 25HZ, below which torque seems to start dropping off a bit - I've taken it as low as 5Hz (the "jog" speed I configured), which gives 2.2RPM in the lowest-speed gear/backgear/winding but wouldn't expect it to do much of a cut! Handy when setting up or toolpost grinding, though.

 The VFD I use (an old ABB ACS300 series) has a relay output that can be configured to indicate above/below a preset frequency and I've used this to switch *off* an external fan when it goes above 30HZ so it keeps cool at low speeds and cools down in idle periods when the motor stops - seems to work OK!

Dave H. (the other one)

Rules are for the obedience of fools, and the guidance of wise men.

Offline BillTodd

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Re: Rewiring old 3 phase motor for low voltage
« Reply #18 on: November 01, 2012, 02:24:04 PM »
Quote
The VFD I use (an old ABB ACS300 series) has a relay output that can be configured to indicate above/below a preset frequency and I've used this to switch *off* an external fan when it goes above 30HZ so it keeps cool at low speeds and cools down in idle periods when the motor stops - seems to work OK!

Neat :)

I've a ABB ACS300 series VFD on my mill - very nice device (albeit with an almost impenetrable manual! - although I did manage how to get it to measure [with an external sensor] and display spindle speed in RPM )

Bill