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Simple but clever motor wiring

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DavidA:
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.

Arbalist:
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

Pete.:

--- Quote from: BillTodd 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.

--- End quote ---

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


--- Quote from: DavidA 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.

--- End quote ---

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.


--- Quote from: Arbalist 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

--- End quote ---

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.

Pete.:
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.

Manxmodder:
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.

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