Author Topic: Simple but clever motor wiring  (Read 4146 times)

Offline Pete.

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Simple but clever motor wiring
« on: October 14, 2015, 05:41:22 PM »
I have this little 3 phase motor that I took out of an automatic barrier. The barrier runs on single phase but I couldn't work out at first why the use of a 3-phase motor and capacitor, then it dawned on me. I knew that you can run a 3-phase motor on single phase by putting a capacitor to the spare leg but these guys have gone a step further and configured it so that powering one leg of the capacitor for forward (barrier up) and the other for reverse (down). Similar to reversing two of the three phase supplies on a 3-phase motor for reversing.

I thought I'd post here just for interest.

Offline awemawson

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Re: Simple but clever motor wiring
« Reply #1 on: October 15, 2015, 03:24:27 AM »
A nice bit of lateral thinking.

Thanks for sharing  :thumbup:
Andrew Mawson
East Sussex

Offline Arbalist

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Re: Simple but clever motor wiring
« Reply #2 on: October 15, 2015, 06:23:17 AM »
Interesting, thanks for the picture.

I read some time back that it is not at all uncommon for manufacturers to fit three phase motors in products intended for use on a single phase supply, a simple capacitor being all that's required for operation. The reason given was the low cost and availability of electric motors on the open market.

I ran my three phase mill on single phase with just a large capacitor for several years without issue.

Offline Will_D

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Re: Simple but clever motor wiring
« Reply #3 on: October 15, 2015, 06:26:16 AM »
So just to be totally clear:

Am I correct: Grey is the Neutral and either Brown or Black is the live (depending on direction you want)?
Engineer and Chemist to the NHC.ie
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Offline DavidA

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Re: Simple but clever motor wiring
« Reply #4 on: October 15, 2015, 12:02:12 PM »
Arbalist,

The only real problem with that approach is that the efficiency depends upon the capacitor being reasonably close to the ideal for the load.

Running a three phase motor on a single phase supply with a badly matched capacitor will result in the motor overheating.

It's quite easy to work out the correct value; as long as you know the load.

Dave.

Offline Pete.

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Re: Simple but clever motor wiring
« Reply #5 on: October 15, 2015, 01:01:28 PM »
So just to be totally clear:

Am I correct: Grey is the Neutral and either Brown or Black is the live (depending on direction you want)?

Hi Will,

Yes, that would be right.

Offline Pete.

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Re: Simple but clever motor wiring
« Reply #6 on: October 15, 2015, 01:05:00 PM »
Arbalist,

The only real problem with that approach is that the efficiency depends upon the capacitor being reasonably close to the ideal for the load.

Running a three phase motor on a single phase supply with a badly matched capacitor will result in the motor overheating.

It's quite easy to work out the correct value; as long as you know the load.

Dave.

David,

I'd like to know how because my tool & cutter grinder has a capacitor and it struggles to come up to speed when it's cold. The motor also runs quite warm.

Offline DavidA

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Re: Simple but clever motor wiring
« Reply #7 on: October 15, 2015, 06:39:56 PM »
Pete,


Capacitance = Amps/(2*Pi* Frequency*Volts)

I'm assuming that you  have a three phase motor running off a single phase supply.

If you measure the voltage of each phase when your motor is running, and the current, then you should get an indication of which way to adjust your capacitor.  In the ideal world the voltages will be the same.
Maybe you need an extra starting capacitor.

Dave.

Offline Pete.

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Re: Simple but clever motor wiring
« Reply #8 on: October 16, 2015, 01:38:58 AM »
Easy as that eh? Thanks David I'll get my multimeter out.

Offline BillTodd

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Re: Simple but clever motor wiring
« Reply #9 on: October 16, 2015, 04:33:14 AM »
That's not an unusual configuration for a  capacitor start/ motmotor. If the start and run windings are the same then a simple change over switch is all that's needed to reverse it.

(just so happens that my dad made me a toy with one in and showed me how it worked while i was at primary school! :-)  )

The diagram below shows a typical configuration and how the third phase of a 3ph motor would be in parallel with the start/run cap.
« Last Edit: October 16, 2015, 05:07:18 AM by BillTodd »
Bill

Offline DavidA

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Re: Simple but clever motor wiring
« Reply #10 on: October 16, 2015, 06:26:25 AM »
Pete,

I would strongly recommend getting the book

'Three Phase Conversion' by Graham Astbury.  It is Number 47 in the Workshop Practice Series.

He goes into the practicalities of this and you will get a much better understanding from it than you will from me.

I got mine via Amazon, it's quite inexpensive and worth every penny.

Dave.

Offline Arbalist

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Re: Simple but clever motor wiring
« Reply #11 on: October 16, 2015, 02:58:44 PM »
There used to be a website page by a capacitor manufacturer detailing the formula for working out the capacitor size for a given motor. I just followed the instructions and as said, it worked fine for several years before I sold the machine due to a house move. They stated that there would be some reduction in rated motor output but my old mill never showed any signs of being under powered. Total cost to get the mill up and running on single phase was about 12.  :D

Offline Pete.

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Re: Simple but clever motor wiring
« Reply #12 on: October 18, 2015, 04:33:35 AM »
That's not an unusual configuration for a  capacitor start/ motmotor. If the start and run windings are the same then a simple change over switch is all that's needed to reverse it.

(just so happens that my dad made me a toy with one in and showed me how it worked while i was at primary school! :-)  )

The diagram below shows a typical configuration and how the third phase of a 3ph motor would be in parallel with the start/run cap.

Very interesting, I had never come up against it before, except done deliberately by home users.

Pete,

I would strongly recommend getting the book

'Three Phase Conversion' by Graham Astbury.  It is Number 47 in the Workshop Practice Series.

He goes into the practicalities of this and you will get a much better understanding from it than you will from me.

I got mine via Amazon, it's quite inexpensive and worth every penny.

Dave.

Thanks David. I already have some of the workshp practice books and they make easy reading. I'll look out for that one now too.

There used to be a website page by a capacitor manufacturer detailing the formula for working out the capacitor size for a given motor. I just followed the instructions and as said, it worked fine for several years before I sold the machine due to a house move. They stated that there would be some reduction in rated motor output but my old mill never showed any signs of being under powered. Total cost to get the mill up and running on single phase was about 12.  :D

I've seen all kinds of figures quoted even up to 80% rating (which surprised me), by using the proper size capacitor. One page said that adding an extra start capacitor (on a timer or momentary switch I guess) would help to bring the 3-phase motor up to speed then it could be disconnected to allow it to run on the one cap without over-heating.

This motor came off an entry-system barrier with a counterbalance so I guess it was never under very high loads at all, and only used for a few seconds at a time at that.

Offline Pete.

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Re: Simple but clever motor wiring
« Reply #13 on: October 18, 2015, 04:39:42 AM »
I went to hook this up to a small VFD and suddenly realised that it only has the three wires coming from the stator. So it seems they have either used a 415V motor wired in star on a 240 supply or it's been specially would for the application. I can't believe they'd custom wind a cheap motor when they could have just used a 6-wire one so I guess it's the former. Anyway, I hooked it up and the VFD runs it just fine giving around 1420rpm at 50hz. It is a little buzzy though which I wasn't expecting.

Offline Manxmodder

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Re: Simple but clever motor wiring
« Reply #14 on: October 18, 2015, 02:43:58 PM »
All very interesting stuff.

The motor on my Alba shaper is the original 3 phase 960 rpm,3/4 horse power. It has been modified by the pevious owner to run on 240v single phase,but it needs a bit of help to start by manually hand spinning the large pulley wheel.

Sometimes it will start unaided when the machine has been run for some time and everything is warmed up.

I've been thinking maybe the capacitor that is fitted isn't quite up to the job,so I think I 'll pull the motor out and see if matters can be improved with a larger capacitor,or maybe a second switched booster cap....OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline Arbalist

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Re: Simple but clever motor wiring
« Reply #15 on: October 18, 2015, 04:20:09 PM »
My mill was the same in winter, once warm it was ok but the first start needed some coaxing. It was fine in summer though and would happily start by itself. I wonder if the motor bearings were a bit gummy?

It's been suggested that motor orientation can make a difference as well but I've no experience of this having only run the mill.