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3 Phase in a Model Engineers workshop

3 Phase problems

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Anthony Gardner12/06/2020 19:13:26
9 forum posts
22 photos

Hi, my name is Tony and I am a retired design engineer. I have had an interesting career in engineering working on the design of truck mounted hydraulic cranes, a Plutonium facility at Aldermarston, design and construction of super conducting magnets, the JET project at Culham laboratory and the last 15 years working on the Diamond Light Source particle accelerator.

I recently moved from Oxford to the New Forrest. I have a Colchester 1800 Lathe which has a 3 HP 3 Phase motor and a Cincinnati universal milling machine which has a 5 HP 3 phase motor. Back in Oxford I had 3 phase run into my workshop, but I don't have this option in my new home. I therefor have purchase a 7.5KW Rotary Converter. I have connected the lathe to the converter and it runs fine and have been using it for the past year.

I recently connected the milling machine and cannot get it to run. I press the on button on the mill and it tries to run as long as I keep my finger on the button. If I let go of the button it stops. If I keep my finger on the button it runs for a few seconds but then throws the trip on the Isolator.

I would love to hear from anyone who may have had experience of this stuff.

Alan Waddington 212/06/2020 20:13:28
523 forum posts
87 photos

Hi Tony

Guessing the primary coil on the motor contactor is 240v ? if so, make sure it is hooked up to the genuine 240v output leg of the converter rather than a false leg.

Also might be beneficial to start the lathe up first, leave it running and then try to start the mill.



Martin Connelly12/06/2020 20:20:33
1711 forum posts
182 photos

As Alan has pointed out the output from a rotary converter may not be a truly balanced 3 phase supply. Try rotating the input connections to the milling machine around, u to v, v to w, w to u. If there is still a problem repeat the rotation again and see if that works.

Martin C

Les Jones 112/06/2020 20:22:28
2225 forum posts
153 photos

It sounds like the contactor is not holding in. Is the start button just a push button switch or is it the type were the button actually pushes the contactor closed ? (It is probably just a push button as I have only seen the type were the start button pushes the contactor closed on hobby machines.) The start button is normally wired in parallel with an auxiliary contact on the main contactor. WITH THE MACHINE ISOLATED FROM THE POWER connect your multimeter set to a low ohms range to the two connections on the start button (You will probably get a reading of a few tens or hundreds of ohms at there will be a path through other circuits.) Now manually push in the main contactor. The resistance reading should drop to almost zero ohms.If you can post the internal wiring diagram of the machine that would be a great help to suggest other possible causes of the problem. I first thought the problem might be inbalance between phases but the fact that the contactor pulls in while the start switch is held in disproves this theory.


Clive Foster12/06/2020 20:23:26
2675 forum posts
96 photos

+1 for what Alan said about ensuring the contactor coil is connected to the 240 volt incomer leg.

Been there & dunnit a couple of times for other folk.


Neil Wyatt12/06/2020 21:40:23
18585 forum posts
723 photos
78 articles

Welcome to the forum Anthony,

I've moved this thread to general questions where it may get more responses. to the question.


Steviegtr12/06/2020 22:52:18
2026 forum posts
283 photos

Some 3 ph machines are 415v only. In which case the contactor coil will be marked up 400+volts ac. If it has a 4 wire system it will be a 240v coil. Or it can have a 24v A.C or D.C coil if there is a 415/24v or 240/24v transformer in there for the control circuit. Which could be A.C or D.C depending on the control circuit power supply.

I am trying not to post answers on here electrical related because with 415v 3 ph around it could kill you without the correct electrical knowledge to fault find. I have spent my working life as an electrician, I would not like to hear about an accident by a member who is not electric savvy. If unsure find a local sparky to have a look for you.


Martin Connelly12/06/2020 22:59:50
1711 forum posts
182 photos

Steve, from the first post I believe the machine was running on a domestic 3 phase supply before being moved, so unlikely to be a 415V system.

Martin C

Steviegtr12/06/2020 23:10:45
2026 forum posts
283 photos

No Martin it is the machine that dictates what it requires not the supply if that makes sense. We used to install a lot of machinery in factories etc. Some machines would not require a neutral. So they have a power supply internal for the control or have 415v coils in the contactors. Some even had a transformer for the work light. Some machines. require a neutral. So have 240v available. They are all different. These days it is standard practice that all machinery have a 24v D.C control system. It is a minefield.

That is why I would not advice on fault finding if the op is not conversant with this sort of thing. Which from the posting I would say he is not. I am sure lots will give advice, but not a good idea.


Andrew Johnston12/06/2020 23:12:56
6011 forum posts
670 photos
Posted by Martin Connelly on 12/06/2020 22:59:50:

............the machine was running on a domestic 3 phase supply........

What's a domestic 3-phase supply? I've got 3-phase in my domestic garage and it's definitely 415V phase to phase.


Martin Connelly12/06/2020 23:21:04
1711 forum posts
182 photos

UK mains supply to domestic properties is live and neutral (star point of the local transformer) at 240V from a 3 phase system that is 315V phase to phase. each phase feeds one third of the properties to balance the load. If you get 3 phase to your house it will be fed by all 3 phases. If you want 415V phase to phase you would need a suitable step up transformer or have a cable put in all the way back to somewhere with a suitable supply.

Martin C

Steviegtr12/06/2020 23:23:18
2026 forum posts
283 photos

Oh dear.


Stuart Smith 512/06/2020 23:34:55
195 forum posts
26 photos


Sorry, but that is not correct.

Single phase as you say one of the three phases plus neutral, but a three phase supply is 415v between phases. Actually both could be higher or lower depending on where you are on the network and loads locally and in general.


Martin Connelly12/06/2020 23:35:26
1711 forum posts
182 photos

OOPs, sorry I've mixed up single phase peak to peak values with rms phase to phase.

Martin C

Steviegtr12/06/2020 23:37:16
2026 forum posts
283 photos

Ha Ha it's great to see someone else boo boo. It is usually me.


Emgee12/06/2020 23:37:36
2015 forum posts
253 photos
Posted by Martin Connelly on 12/06/2020 23:21:04:

UK mains supply to domestic properties is live and neutral (star point of the local transformer) at 240V from a 3 phase system that is 315V phase to phase. each phase feeds one third of the properties to balance the load. If you get 3 phase to your house it will be fed by all 3 phases. If you want 415V phase to phase you would need a suitable step up transformer or have a cable put in all the way back to somewhere with a suitable supply.

Martin C

Incorrect Martin, 3 phase and neutral supply to UK domestic properties was common in the early years of night storage heaters, the supply is 240v phase to neutral and 415v phase to phase, same whether you are in a domestic property or a factory.


too slow typing again.!!

Edited By Emgee on 12/06/2020 23:39:57

larry phelan 113/06/2020 09:22:02
993 forum posts
14 photos

When I lived in Dublin, I had a 3 phase supply, 415 v from the pole across the road which carried the supply to all the houses along the road.

Now that I live in the Sticks, I get my 3 phase from a Rotary Converter to run my lathe and mill ,each with 1,5 motors

However, my wood planer and spindle moulder both have 4 hp motors, but the converter has no problem handling them. Could it be that your converter is too small, although this seems unlikely.

Andrew Tinsley13/06/2020 09:54:08
1387 forum posts

If the rotary converter still runs the lathe without a problem, then there is nothing wrong with the setup. If it won't run the mill, then maybe the mill requires 415 volts between phases (.ie. the motor(s) is/are wired in star) Maybe if the motor(s) could be rewired in delta, then it should work.

Now sit back and wait for the dissenting chorus.


Edited By Andrew Tinsley on 13/06/2020 09:55:22

Andrew Johnston13/06/2020 10:11:08
6011 forum posts
670 photos

There are rotary converters, and then there are rotary converters. The simplest (but expensive) ones use a single phase motor to drive a 3-phase motor, which generates a true balanced 3-phase output. But there are also ones that use an idler motor to generate a psuedo 3-phase output.

It would help to have more details of the mill. My universal horizontal mill also has a 5hp motor. But it also has two speed ranges. In high range the motor is wired in star and produces 5hp. In low range the windings are arranged to double the number of poles and hence halve the speed. That would normally also mean half the power. But the motor is also reconfigured to be in delta with a corresponding increase in phase current. So the power in low speed is halved due to the speed reduction but increased by root 3 due to the increased phase current giving a power of about 4hp.

If the OPs mill is similar it may be that the converter cannot supply the increased current, although nominally within it's power rating.


Phil Whitley13/06/2020 13:39:29
1328 forum posts
147 photos

We need potos of the mill and the starter fitted to it, Prima facie, if the lathe works but the mill does not, the fault is the mill. If it was previouslt run on normal 3 phase it will not have a neutral or a 240v coil, if the contactor drops out as soon as the start button is released, the holding in circuit is not working. As this is a Cinncinati it is probably pre 1970 when the company became Cincinnati Milacron, and if it is pre 1970, it will not have a low voltage control circuit, these are all assumptions which could be confirmed by pics, especially one of the starter with the lid removed!


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