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Powering a Suds pump?

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Andrew Tinsley06/06/2022 19:58:20
1614 forum posts

I have a Myford tank and suds pump. The pump was made by MG Electrical of Colchester.

At the moment it is connected for 3 phase 440 Volts, Star. The motor can be easily be rewired for Delta and 240 Volt 3 phase. It will take 0.22 Amp in Delta.

It is hardly worth using a VFD for this application. I know that other folk have connected up the motor using a single phase supply (live and neutral) and a capacitor feeding the 3rd phase. I can play about with the capacitor size, but that mean buying in a selection! I have a 440 volt rated 6 microfarad capacitor which will probably work.

If anyone has done a similar conversion, then what size capacitor did you use? There is probably a way of calculating the required size, but I am too lazy to start that , if someone has a working value!



Speedy Builder506/06/2022 20:34:31
2653 forum posts
219 photos

looks like I used a 12.5 Uf 440v on this one.

suds 2.jpg

Dennis R06/06/2022 20:45:30
73 forum posts
16 photos

What horse power is the motor, there is a list of values in the Workshop Practice Series No. 24, Electric Motors in the Home Workshop by Jim Cox.

For .25 hp it's 10 micro farad loaded or 7.5 lightly loaded.


Andrew Tinsley06/06/2022 20:54:03
1614 forum posts

Thanks both, looks as though a 6 microfarad or thereabouts will do the job.


Andrew Moyes 106/06/2022 21:34:39
148 forum posts
23 photos

I have exactly the same three phase suds pump on my Tom Senior mill and wanted to convert it to single phase, too. I played around with different values of capacitor and found 2.5 microfarads gave the nearest to a balanced 230 volts across all three phases.

Chris Crew06/06/2022 22:16:12
235 forum posts

I have done several conversions to single phase of fractional HP motors by just putting a capacitor across one of the windings in delta configuration. As long as no great starting torque is required the motor will start slowly and then run up to speed in a couple of seconds. I have a Clark T&C grinder, a Pollard high-speed drill and a diamond faced wheel grinder configured like this and they all run perfectly well. The capacitors I used came out of old florescent light fittings. I suspect a suds pump requires no great starting torque so you should be OK by doing this. I got the capacitor values from the Workshop Practise series book 'Electric Motors'. There is a table in there that gives the size of run capacitors for motors of various power and also start capacitors if you wanted to add a bit more circuitry for greater starting torque.

ME published a simple electro-mechanical circuit that I designed, circa 1993, which was based on the support circuit given in Electric Motors but which allowed a 3-phase machine to start and stop using the original start/stop buttons. I have a 3-phase Boxford 8" shaper working on single-phase with all original switch gear still in place but re-wired/modified to include my support circuit. I similarly converted a power hack-saw and it worked fine too.

MadMike06/06/2022 22:48:38
237 forum posts
4 photos

I had a suds pump problem about 5 years ago on my 254S. Single phase or 3 phase? I threw the thing away and bought a submersible pond pump which would lift the coolant up to 2 metres and simply popped it in the tank.. Plugged it in and five years on it is still running and pumps coolant at such a rate that I had to turn the volume down using a tap as a restrictor. Like they always taught us as apprentices... KISS.

KWIL07/06/2022 08:37:05
3563 forum posts
70 photos

Is the lathe running on a VFD? The added load of the suds pump would probably not be noticed by a VFD and all would be normal, except if you are running at a low speed. Suck it and see.

Chris Crew07/06/2022 09:22:41
235 forum posts

I have no experience of, or knowledge about, VFD's. I have had a 3-phase Colchester Student with a suds pump running off a Transwave converter for the last thirty-odd years. The only issue I had, and it cost me a suds pump rewind, was that the original Colchester wiring put the suds pump connections to the 'unbroken' side of the NVR. This meant that when the main motor was stopped the suds pump was still connected to the output of the Transwave. Because it is only a fractional HP motor it was not 'powerful' enough to trip out the starting capacitance and, unbeknown to me, merely oscillated and got hot until it eventually burnt out. I cured this problem, once I realised the cause, by putting a small auxiliary contactor between the 'broken' side of the NVR and the suds pump. Thus when the NVR released upon stopping the main motor the suds pump was isolated from the Transwave. There was a reason for this 'belt and braces' solution because I could have just rewired the suds pump to the 'broken' side of the NVR, but I can't just remember what it was after all this time. Anyway, there have been no problems since.

Edited By Chris Crew on 07/06/2022 09:23:50

john fletcher 107/06/2022 09:54:04
805 forum posts

I've used a 4 micro farad at 400 volts ex 4 foot fluorescent light fitting for my Harrison/Colchester friends, that was 5/10 years ago and they are still working OK. For those who don't know, connect capacitors in parallel to increase capacitance value and in series to decrease. So, two 4 in series equals two micro farad and same pair in parallel equals eight. All capacitors should be tested at their working voltage 400, not the 9 volts of most digital multimeter John

Andrew Tinsley07/06/2022 10:19:07
1614 forum posts

Found another 6 microfarad cap so using both in parallel gives 3 microfarads. This works a treat, so thank you Andrew Noyes! It gives almost perfect balance, using my scope (unearthed for this measurement!)

The tank and pump are stand alone items, to be switched to whatever machine is in use, so single phase is the best bet.

Thanks everyone for their input.


AJAX07/06/2022 10:35:52
395 forum posts
42 photos
Posted by Andrew Tinsley on 07/06/2022 10:19:07:

Found another 6 microfarad cap so using both in parallel gives 3 microfarads. This works a treat, so thank you Andrew Noyes! It gives almost perfect balance, using my scope (unearthed for this measurement!)

The tank and pump are stand alone items, to be switched to whatever machine is in use, so single phase is the best bet.

Thanks everyone for their input.


By placing two or more capacitors in parallel you are effectively increasing the plate area and so increasing the effective capacitance. When placed in series, you are increasing the distance between the plates and this reducing the effective capacitance.

Andrew Tinsley07/06/2022 11:20:05
1614 forum posts

Sorry, slip of the brain, I should have said series NOT parallel! Thanks for picking up my error!


larry phelan 109/06/2022 18:30:33
1192 forum posts
15 photos

What,s wrong with using a pump from a scrap washing machine ?

Been using one of those on my bandsaw for the last 10 years or more, simple, cheap, effective.surprise

Andrew Tinsley09/06/2022 19:47:21
1614 forum posts


Why on earth should I go looking for a scrap washing machine pump that will require at least a tank, when I have a perfectly good, professional suds pump and tank, that simply required a change from star to delta and the addition of a capacitor?

I really have difficulty in understanding the logic behind your reply, what am I missing?



larry phelan 110/06/2022 08:36:58
1192 forum posts
15 photos

Good morning Andrew,

You are not missing anything, I simply mentioned this in case it might be of interest to others.

A tank is no problem, on old cooking pot will serve well enough.

Please accept my apologies for any upset, just convert .blush

donkey10/06/2022 10:18:16
83 forum posts
5 photos

thank you Larry

I have a cold saw that needs a pump and had not thought about an old washing machine.

cheers Bri

Andrew Tinsley10/06/2022 10:26:55
1614 forum posts

No problem Larry, I thought there was something more to your post, that I had missed! Washing machine pumps tend to have a higher flow and maybe a larger head than a suds pump, but throttled down they seem a reasonable solution. I have used one in the distant past, but had some trouble with the higher pressure when throttled back to give a reasonable volume. It was however a pump off a commercial Bendix, so the average domestic item should be better in that respect.


larry phelan 110/06/2022 14:16:12
1192 forum posts
15 photos

Thank you Andrew, To err is human, to forgive is Devine!cheeky

duncan webster10/06/2022 14:24:37
4122 forum posts
66 photos

For reasons I never understood, chemical engineers where I worked used to control flow from centrifugal pumps by by-passing some of the output back to the input rather than throttling the output. Then came vfd but the electrical chaps had to be dragged kicking and screaming, their objection being something to do with waveform.

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