4″ x 6″ modern Belt/Disc sander motor

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4″ x 6″ modern Belt/Disc sander motor

This topic has 16 replies, 9 voices, and was last updated 25 August 2017 at 16:58 by DMR.

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9 August 2017 at 16:32 #311190
DMR
Participant
@dmr
Attached picture is for the electrics in a modern disc and belt sander. Associated capacitors say its 2 years old. Relay coil is effectively in series with the start winding and what does that 10 M.Farad capacitor do?
Now then.
The motor is synchronous, rated at 240V 370W and 2850 RPM. It has 24 stator sections and two windings and its cooked. Fault was a short circuit of half the (M) Main windings due to the lead out wire of that winding being pulled tight across the turns.
As an exercise I am looking at rewinding the thing myself. Replacement costs stated as the same as a new complete unit. Damn sure I can make a better job of it than the (ex?) Chinese farmers first attempt. My time is my own and I just fancy spending time doing it. Well I've never do it before! I have part disassembled the remains, determined the number of turns in each segment, assessed winding resistances and generally prepared to have a go at it.
But then I hit a snag. The thing is wired in aluminium and I don’t seem to be able to purchase enamelled Aluminium wire in any gauge in the UK. So I plan for Copper, but Copper isn’t the same and I don’t seem to be able to gauge how I should go about any changes to the SWG, number of turns, etc. I don’t even know the spec for the various aluminium wire grades, although the conductivity doesn’t vary significantly.
1. Aluminium carries about half the capacity of copper.
2. The existing aluminium terminations onto the copper wire tails seem to be steel crimped tubes. another reason to use copper.
3. Aluminium wire has nearly twice the area to give it the same resistance as copper wire.
4. I have been through lots of websites but don’t find anything specific to comparing the two motor windings types. There is a wealth of info on http://www.electricaltechnology.org. as an example, but nothing specific to what I want.
I need someone with better motor knowledge than I ever had. The aluminium windings approximate to 22SWG for (M) and 24 SWG for (S). I would like to use copper but what would be correct.
Relevant data as found:
M winding 175 metres and 12.8 ohms
S winding 69 metres and 12.8 ohms. Any other required data can be supplied.

As an aside, there is no – what I will call – characteristic smell of burnt electrics in this motor. There was a little puff of white smoke which smelled a bit, but these windings just simply have no smell. Motto appears to be don’t trust your nose any more in modern second hand motors. More pictures in the album.

Seeking someone who knows.

Dennis

Edited By DMR on 09/08/2017 16:35:00
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9 August 2017 at 16:32 #33049
DMR
Participant
@dmr
Rewind considerations
9 August 2017 at 17:15 #311195
larry Phelan
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@larryphelan54019
Where I worked [I mean was employed ] many years ago,we made transformers for the E,S,B,and all the coils were in copper wire. At one stage they built one and one only using aluminium wire but it must not have been a success because it was never repeated. One thing I did notice about it was that the the entire unit was much bigger than the usual copper coil type,so either the wire gauge had to be bigger or the number of turns had to be more. I suspect the gauge had to be bigger since I think the number of turns is fixed.

The transformer was dumped but the tank was used again for a copper coiled unit. These transformers were in the range 400 kva to 5 mva,with a few 10 mva,s.

Not sure if any of this is any use to you,but it,s all info.

PS I have a 250 amp welding plant with ali coils,no problem ! [fan cooled ] Also have a 180 amp oil cooled. Big difference in the weight !
9 August 2017 at 17:29 #311196
John Haine
Participant
@johnhaine32865
This not a synchronous motor, it's an induction motor. If it was synchronous it would run at 3000 rpm on 50 Hz or 3600 rpm on 60Hz. A slightly odd capacitor arrangement, I think it may be an attempt to avoid a centrifugal switch for the start cap by having a relay that senses the high start current and switches in (or out?) the 10 uF capacitor. Are you sure that circuit is correct?

As to using copper wire rather than ali, just use the same gauge and number of turns which will fit the same space, I'd have thought. 10uF at 50Hz is ~300 ohms reactance which is more than 10x the winding resistance, so I doubt that the lower resistance of using the same gauge copper would affect anything except it should run cooler!
9 August 2017 at 18:00 #311204
Mike Poole
Participant
@mikepoole82104
I think you will need to match the ampere turns to make the motor have a similar performance to its original spec.

Mike
10 August 2017 at 07:10 #311293
John Haine
Participant
@johnhaine32865
Yes. Amps depends mainly on inductance which is determined by the number of turns and the core reluctance. So wind on the same number of turns on the same core and near enough you'll get the same AT.
10 August 2017 at 11:59 #311328
Swarf, Mostly!
Participant
@swarfmostly
Hi there, Dennis,

You might find this thread of interest:

**LINK**

My circuit diagram differs a bit from yours. I didn't need to discover the values of the capacitors or to investigate the motor, my brief was only to replace the faulty NVR.

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 10/08/2017 12:00:34
10 August 2017 at 12:34 #311329
Andrew Tinsley
Participant
@andrewtinsley63637
As a light aside. Gold is very much better than copper when it comes to conductivity. During the Manhatton atomic bomb programme, the need arose for a lot of isotope separators. The amount of copper needed, impinged upon the supply of copper for the American war effort, so impasse. Until someone realised there was a lot of gold in Fort Knox, doing nothing! Yes you guessed it, Fort Knox supplied the material for the isotope separators electromagnets!

Andrew
10 August 2017 at 15:18 #311349
John Haine
Participant
@johnhaine32865
Material ρ (Ω·m) at 20 °C σ (S/m) at 20 °C Temperature
coefficient^{[note 1]}
(K⁻¹) Reference

Carbon (graphene) 1.00×10⁻⁸ 1.00×10⁸ −0.0002 ^[17]

Silver 1.59×10⁻⁸ 6.30×10⁷ 0.0038 ^[18][19]

Copper 1.68×10⁻⁸ 5.96×10⁷ 0.003862 ^[20]

Annealed copper^{[note 2]} 1.72×10⁻⁸ 5.80×10⁷ 0.00393 ^[21]

Gold^{[note 3]} 2.44×10⁻⁸ 4.10×10⁷ 0.0034 ^[18]

As this table of resistivity/conductivity shows, gold actually has about 2/3 of the conductivity of copper rather than being better. Only silver is a better metallic conductor than copper. But in wartime, copper is needed for lots of things, gold is pretty useless. In the end, magnetic separation (which my father worked on briefly during the war) proved to be much slower and less efficient than diffusion separators, so only a limited number of magnetic separators were in operation.

And a quick reference to Wikipedia shows that actually they used silver from the US Treasury rather than gold. I think the gold story is an urban myth.

Edited By John Haine on 10/08/2017 15:32:11
11 August 2017 at 17:07 #311506
DMR
Participant
@dmr
Its humble pie time. I was not reading my own circuit sketches correctly, which is why I queried it on here, Very glad I did, although my main reason for the post was for the copper versus aluminium windings query, which is not meant as my excuse.

John H, you are of course correct as well; it's an induction job and my circuit was incorrect. I must try harder in my old age.

Thank you Swarf for your input and the previous post. Our sanders are identical apart from the motor fixing and mine is essentially nameless, but I know where it came from. Your motor is on extension pillars to the 3 (of 4) motor fixing bolts, but mine are on 3 separate cast-in frame extensions (See album). There is another apparent difference in that your motor windings look neat! Our connecting wire colours are identical and I can tell you that the white cap is 10Mf and the grey cap is 100Mf. The capacitors are even fixed in the same way, rather awkward to get at the single fixing screw. How on earth did you twig that the white wire from the relay contact went into the motor and came straight out again as a black wire (no internal motor connections) without taking it apart? My rewind will not include that feature! I will retest what is left of my windings again as you have the start winding at 6.6 ohms. Incorrect factor of two on my part as well it seems. I second your observation of the relay ID, I have the same single "up" help marking.

Its going to be a while for me to complete this little job in hand, amongst others, but I will let you all know how it goes. Thank you all for your input.

Dennis
11 August 2017 at 18:23 #311515
larry Phelan
Participant
@larryphelan54019
Hi DMR,

I have to eat humble pie all the time,it,s rough is it not? But I suppose it,s better than making a King sized cock up !

At this stage,I,ve made so many mistakes that there can,t be many more left,but I regard it as all part of the "Learning Curve" I just hope I dont fall off at the bend !

Just as well there are guys out there to pick us up.
11 August 2017 at 18:28 #311516
Mikelkie
Participant
@mikelkie
Note that the core laminations of chinese motors are mostly recycled steel and very seldom proper silicon iron is used for making cores. This results in poor performance and to make matters worse aluminium (loominum)wire is used as well which does not have the same electromagnetic properties as copper. The company that our business uses for rewinds absolutely refuses to rewind these motors. One can usually judge by the weight of any motor what the quality is. Some info from the winders: 50 hz x 60 sec x 2 = 6000 . 6000 devided by the number of poles times pf = rpm. (friction & other losses excluded)
11 August 2017 at 19:44 #311526
Muzzer
Participant
@muzzer
A little knowledge is dangerous.

The motor current is primarily a function of the load torque, with a (relatively) small magnetisation current due to the (relatively high) inductance of the windings. Induction motors are a bit less straightforward, as the slip frequency is proportional to the torque applied and the rotor acts as a sort of variable frequency transformer with an almost shorted secondary but you can ignore that for this purpose. But it's not something you can intuit easily.

To get roughly equivalent performance with a different supply voltage, you need to end up with similar volts per turn. So if you double the voltage, you'd want to double the turns. It's the same basic theory as using constant V/f, ie keeping the flux constant.

Murray
14 August 2017 at 10:07 #311917
Swarf, Mostly!
Participant
@swarfmostly
Posted by DMR on 11/08/2017 17:07:41:

SNIP

Thank you Swarf for your input and the previous post.

SNIP

How on earth did you twig that the white wire from the relay contact went into the motor and came straight out again as a black wire (no internal motor connections) without taking it apart? ……… I second your observation of the relay ID, I have the same single "up" help marking.

SNIP

Dennis

Hi there, Dennis,

Careful use of a hand-cranked Megger on the 'Ohms' range and a bit of timely inspiration!

Best regards,

Swarf, Mostly!
25 August 2017 at 16:07 #313907
DMR
Participant
@dmr
I have to come back to this for a bit of clarification, and apply a corrected circuit diagram while I’m at it. I have now done the sums and moved towards the copper wire ordering stage.
The existing aluminium windings are as follows:
Main (run) winding 12.0 ohms. Approx 180 metres. Approx 22SWG.
Start winding 6.6 ohms. Approx 73 metres. Approx 24 SWG.
Equivalent copper windings will come out at around M = 7.75 ohms, S = 3.14 ohms.
That is significantly less and I fail to see how such a difference will make the motor run cooler. I fully realise that the related resultant inductance makes a lot of difference but the starting current is obviously going to be higher.
Would it not be sensible to go down an SWG (yes, I know that’s a bigger number) or even two to put the straight winding resistance back up? The smaller wire/s would be a free fit in the stator segments, but I could add more windings or use a (heat proof) filler. I favour going down one SWG and add more windings to fill the space or just use a smaller start winding wire, and using more turns. Has anyone in the know got any advise on that?
The enamel on the existing windings is hard to remove, possible due to overheating. Anyway I use the term “approx” for the SWG as the aluminium is soft and it is difficult to say if the wire involved, once scrapped, is to AWG, SWG, or just something the maker had to hand!

There is another point here. I discovered that the likes of RS, Farnell, etc only stock the AWG spec now, and not in a good (enough) range of lengths either. Wires.co.uk seems like a good bet.
Don’t blind me with science please. Just convince me on the right way to go; I'm not going down this path twice.

Thanks in anticipation.

Dennis

Edited By DMR on 25/08/2017 16:15:25
25 August 2017 at 16:48 #313912
John Rudd
Participant
@johnrudd16576
Dennis,

I have bought copper wire,for rewinding alternator rotors for petrol gennies, from Brocott Electronics…..google them…

They will sell you what ever your needs are, weight or length.

As an aside, I rewound such a gen rotor whose windings were aluminium. I rewound it based on length of wire x number of turns, then went for an equivalent amount based on the weight of copper. Once rewound/reassembled, the gen performed faultlessly. A very satisfying feeling.
25 August 2017 at 16:58 #313914
DMR
Participant
@dmr
OK John. That's a different angle. Thanks for that.

Dennis
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