By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

Ac motor 3 wire

All Topics | Latest Posts

Search for:  in Thread Title in  
Buffer06/07/2020 14:12:01
160 forum posts
46 photos


I was thinking of using my old tumble dryer motor for a gadget builder style four facet drill grinder. Only thing is I am not sure how it is wired. I have included a photo because it has three wires going into it and two of those go via a capacitor. The capacitor has four terminals which I think might be because it is a start capacitor for a motor that runs both ways. My question is does anyone know what the connections would be if i just wanted to run this motor one way for grinding. Does anyone know what rpm this motor might be when run on UK mains single phase. I have seen before that some of you can tell the speed of some motors just by looking and knowing the frequency of the mains.

Thanks for any help


John Baron06/07/2020 14:46:23
302 forum posts
122 photos

Hi Buffer,

The motor will run at 2850 rpm. But without the information on the capacitor its difficult to say whether it is simply an interference suppressor or a start capacitor. The way that its wired it could be either.

Maurice Taylor06/07/2020 14:54:00
95 forum posts
9 photos

If you know make and model of drier ,look up service manual/circuit diagram on internet. Just done the same to re-use my old Whirlpool washer motor.

Buffer06/07/2020 15:04:08
160 forum posts
46 photos

John thanks for the info I have just taken a photo of the capacitor.

Maurice, that's a good idea. Even after 20 something years of being on the net I never thought of trying that! I will have a look now.


Ian Parkin06/07/2020 15:17:37
806 forum posts
198 photos

Are you sure that the cap has 4 terminals? not 2 terminals with 2 conections on each terminal?

Edited By Ian Parkin on 06/07/2020 15:17:51

John Haine06/07/2020 16:14:46
3178 forum posts
171 photos

Quick google on the cap confirms it's a start capacitor. Also I think that it has 4 terminals but in 2 commoned pairs. Did the machine reverse in operation? Nothing to say that this is a 2850 rpm motor - could be 2 or 4 pole, if the latter it would be 1425.

Buffer06/07/2020 16:51:26
160 forum posts
46 photos

The capacitor has 2 terminals with 2 wires on each terminal.

I think the dryer turned both ways but I do know that when I took the lid off the dryer i could make it go both ways depending on which way i spun the drum.


John Baron06/07/2020 19:43:22
302 forum posts
122 photos

Hi Buffer,

Thanks for the picture ! Yes its a start cap, and yes the motor will run either way.

Your earlier picture made it difficult to see that there were only two connections.

Effectively the black wire is mains neutral. The live goes to one side or the other of the capacitor depending upon which way you want it to spin.

If you do a resistance check between the black and yellow wires it should be the same resistance black to either yellow wire.

John Baron06/07/2020 19:51:39
302 forum posts
122 photos
Posted by John Haine on 06/07/2020 16:14:46:

Quick google on the cap confirms it's a start capacitor. Also I think that it has 4 terminals but in 2 commoned pairs. Did the machine reverse in operation? Nothing to say that this is a 2850 rpm motor - could be 2 or 4 pole, if the latter it would be 1425.

Hi John,

Definitely a two pole 2850 rpm motor ! I don't know of a tumble dryer that uses a four pole motor to provide direct drum drive. Look at the size of the belt pulley and compare its size to that of a 26" diameter drum. The belt runs around the outside of the drum from that small pulley. Also its only a 1/2 Hp motor.

I actually use a similar motor on my four facet drill grinder.

Buffer06/07/2020 19:54:26
160 forum posts
46 photos

John x2 Maurice and Ian.

Thanks for all the help and advice it seems to be sorted, so now I can work out if it will be good enough for a small grinder.



John Baron06/07/2020 20:10:19
302 forum posts
122 photos

Hi Rich,

I use a similar motor on my four facet drill grinder. I used a switch so that I can run the motor in either direction depending upon which side of the grinding wheel I'm using.

There is a picture of mine on the "Gadget Builder" web site, near the bottom of the page.

John Haine06/07/2020 20:27:51
3178 forum posts
171 photos

If you could reverse the direction depending which way you gave it a spin then the cap is probably gash.

duncan webster06/07/2020 20:30:11
2651 forum posts
36 photos

My 4 facet drill grinder has the motor out of a dishwasher main pump, just the ticket.

Buffer:- make sure you shield those holes in the motor but not to restrict air flow.

Pete Rimmer06/07/2020 20:52:40
734 forum posts
50 photos

I took a motor out of a security barrier once that was wired like that. It's a 3 phase 415v motor but wired to 240v single phase with a capacitor wired across 2 phases. Up and down was provided by sending power to one of the two phases wired to the cap or the other, as required.

For light duty operation it works just great.

Buffer06/07/2020 22:12:28
160 forum posts
46 photos

John B, John H

I just measured the rpm and it was about 1400. I've turned in now but will check again tomorrow with my glasses on!

Do you think the gadget builder design is worth doing? Did you make any changes or would you make any with hindsight?


Kiwi Bloke06/07/2020 23:04:23
443 forum posts
1 photos

Hope I'm not adding to the confusion - there's inconsistent advice in this thread...

The capacitor is, strictly speaking, a run capacitor, in that it is permanently in circuit. The motor almost certainly has two identical windings arranged with one end of each connected together, and brought out as one lead, typically the black one. The other end of each winding is brought out as a lead, and the capacitor is connected across these leads. The identical windings allow the motor to be reversed at will. As has already been said, one wire carrying mains (eg live) is connected to one or other end of the cap, depending on the desired direction of rotation, typically by a change-over switch. The other mains wire (eg neutral) is connected to the winding's common point (the black lead).

You can check this by measuring the resistance between each coloured wire and the black one, and between the two coloured wires (disconnect the cap). I'm assuming the black lead is connected to the two windings' common point. The latter resistance should be twice the former, and the two former resistances should be identical. If that's not the case, it's a single-direction motor, and is wired a little differently - mains across lower-resistance winding (coloured to black), cap across other two coloured wires.

[edited to be even more pedantic...]

Edited By Kiwi Bloke on 06/07/2020 23:07:33

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Latest Forum Posts
Support Our Partners
Eccentric July 5 2018
Allendale Electronics
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest