Convert motor from 3ph to single phase

[Colin HeseltineParticipant @colinheseltine48622]

• I have a bench top drill which is currently 3 phase. I would like it converted to single phase and son in law will have it. Can this motor be run as a single phase. The picture below shows the motor plate. Many thanks and Merry Christmas to all.
  Colin

[Robert Atkinson 2Participant @robertatkinson2]

It can with a VFD or fixed speed single to 3 phase converter.

[FatgadgiParticipant @fatgadgi]

Hi Colin

You can (probably) run your three phase motor by building a module with relays and run capacitors, which I still have on a couple of machines.  They run OK, bit lumpier than proper 3 phase, and occasionally I’ve had the relays stick (needing a good wack to free them).

But, as Robert says, a VFD is definitely the way to go now.

I bought a couple off fleabay couple of months ago that were just £60 each.  They seem to work perfectly, and hopefully will not fail early, so I’ll slowly be converting the remaining machines to VFD.

Cheers Will

[not done it yetParticipant @notdoneityet]

Don’t bother to try to run it as single phase is my advice.  Converters are expensive and VFDs are cheap.  Both options use a single phase supply to produce three phase output.  VFDs are programmable for several optional settings.

It is more likely that the motor will be configured for ‘star’ operation.

It would definitely need to be ‘star’ for operation via the converter.

With a 220V output VFD, It would run in star but at lower power, so needs to be configured as ‘delta’ for full power output.

415V output VFDs are available – but too pricey to be a sensible option.

The connections, for each, should be shown under the terminal cover.

 

[Andrew TinsleyParticipant @andrewtinsley63637]

A VFD and running the motor in star configuration will certainly work. Just a word of caution, the motor is probably around 70 years old and the insulation will not be up to modern standards. Running it on a VFD just may give you problems with the insulation breaking down. Probably be perfectly OK if you don’t use the drill at maximum power for long periods.

Depends a lot on whether you have a good electric / electronics knowledge. If not, then the simplest option is to replace the 3 phase motor with a single phase 1/2 HP motor  A good second hand one or even a new one would not cost a great deal more than a basic VFD, by the time the VFD has been safely installed in an enclosure and stop / start buttons installed the single phase motor option would likely work out cheaper. Before everyone weighs in with ” A 3 phase motor runs more smoothly and you don’t have to worry about start windings overheating, with a lot of stop / start use” etc etc. Just realise that the OP may not have the knowledge to rewire the motor and install a VFD safely. If that is the case then a single phase moor is a safer and better bet.

Just to illustrate  the variety of pitfalls that can be encountered. I have a small BTH  3 phase motor which will not run on a VFD (Tried several different VFDs). I have checked the insulation resistance with a 1000 volt megger, checked each coil to earth and between coils. All fine! I have a genuine 3 phase supply and the motor runs on that with no trouble. I am not suggesting for a minute that this will happen with your motor. I am totally dumfounded with this, never seen anything like it!

Andrew.

[Robert Atkinson 2Participant @robertatkinson2]

To run that motor off a basic 1 phase in 3 phase out VFD on 230V mains the motor will have to be connected in DELTA (220 -230 V phase to phase) not star.
From the data plate it is connectable for either configuration. If running off 3 phase mains it will be currently wired as star (380-440 V).

Robert.

[Andrew TinsleyParticipant @andrewtinsley63637]

Robert you are quite correct, not reading what I had written is my excuse! Delta connected is the correct answer!! Have probably imbibed too much of the Christmas spirit.

Andrew.

 

 

 

 

 

 

[FulmenParticipant @fulmen]

Do you need  full power and starting torque? If not you can wire in a run cap (steinmetz wiring):

https://www.homemadetools.net/forum/how-connect-3-phase-motor-single-phase-98311

 

 

[not done it yetParticipant @notdoneityet]

Motors configured in star CAN be run on a 230Volt VFD.  They will only develop ~58% of the rated power.  They require delta configuration to run at full rated power from the same inverter.

There is no question of suggesting otherwise.  My Centec 2B mill IS running with a 2HP 415V three phase motor via a 230V VFD.  It could develop about 1.2HP, which is a bit too much power (IMO) for the Centec – but I know its limits and don’t ever use its full output.

The power feed on my Centec is running on its original motor (star config) but driven from a 230V VFD.  It has never stalled while operating yet.

The use of the 2HP motor was simply due to me having it available when I changed out the single phase motor, which struggled to start the mill when the gear oil was very cold.  It was only a star config motor.  I would have rewired it to enable delta operation if I had needed it to deliver 2HP.

Anything above, which states it cannot be done, is wrong.

 

[Dave HalfordParticipant @davehalford22513]

Colins motor is dual voltage so no need to go digging for the star point just strap it for 250.

Fulmen’s Steinmetz capacitor is how some people convert a suds pump to single phase. It might work for a drill with low mass to start.

[FulmenParticipant @fulmen]

A separate starting cap will provide more torque, but that will require a bit more wiring. The proper way is of course to use a centrifugal switch, but a simple workaround is to run the motor through a self latching relay and a starting button that activates the cap for as long as it’s pushed.

[not done it yet]

On not done it yet Said:

Motors configured in star CAN be run on a 230Volt VFD.  They will only develop ~58% of the rated power.  They require delta configuration to run at full rated power from the same inverter.

There is no question of suggesting otherwise.  My Centec 2B mill IS running with a 2HP 415V three phase motor via a 230V VFD.  It could develop about 1.2HP, which is a bit too much power (IMO) for the Centec – but I know its limits and don’t ever use its full output.

The power feed on my Centec is running on its original motor (star config) but driven from a 230V VFD.  It has never stalled while operating yet.

The use of the 2HP motor was simply due to me having it available when I changed out the single phase motor, which struggled to start the mill when the gear oil was very cold.  It was only a star config motor.  I would have rewired it to enable delta operation if I had needed it to deliver 2HP.

Anything above, which states it cannot be done, is wrong.

 

While the current is reduced to 58% in a resistive circuit that does not apply to a motor due to back EMF. Even in a resistive circuit the power is reduced to 34%. For example a 100V ph-ph Delta with 100R load is 1A and thus 100W per phase. With Star connection this is reduced to 58V (100/√3) and 0.58A so 33.6W per phase with the same resistor.
In practice the output power is reduced to about 25% when running a 440V motor on 240V.

Robert.

 

[larry phelan 1Participant @larryphelan1]

Might be cheaper and simpler to pick up a single Phase motor and be done with it.

[larry phelan 1]

On larry phelan 1 Said:

Might be cheaper and simpler to pick up a single Phase motor and be done with it.

Single phase motors are cheap because they suck!  So many advantages of a three phase motor, with VFD.  Merry Christmas.

[duncan webster 1Participant @duncanwebster1]

On a pillar drill? My lathe and milling machine are 3 phase, but I won’t be converting the drill any time soon

[Clive SteerParticipant @clivesteer55943]

This subject comes up fairly often so I wondered if this site could have Wiki type pages for the more common asked about subjects that could combine our collective wisdom.

Clive

[Chris CrewParticipant @chriscrew66644]

I have to say that probably, for me at least, ignorance is bliss. I just follow the instructions of more knowledgeable people without really understanding the theory and principles behind them. About thirty years ago I acquired an ex-school 3-ph. Boxford shaper. Not knowing anything very much about motors, let alone converting one to run on single-phase, I bought a copy of the Workshop Practice book ‘Electric Motors’. This provided some easy-to-understand information on running 3-ph. motors on single-phase, giving configuration and capacitor values etc., and a couple of circuit diagrams for the switching-in and out of the starting capacitance. I couldn’t get the one using thyristors to work but the ‘manual’ one got my machine up and running.

Following on from this, I devised a circuit using an auxiliary relay (which has already been mentioned above), along with the machine’s original NVR and the original start and stop buttons integral to the machine’s stand. Press and hold the ‘start’ button, which switches in the ‘start’ capacitor, until the machine is running, then release it and the motor continues operating on the ‘run’ capacitor. Pressing the ‘stop’ button breaks the supply lines and releases the NVR. I actually sent a copy of this circuit to ME which the Editor of the day was kind enough to publish but I can’t recall which edition it appeared in and have since disposed of my ME collection so I can’t now reference it.

One thing the book did mention was that if the motor is not configurable from ‘star’ to ‘delta’ by moving straps you had to dismantle it and excavate the ‘star-point’ of the windings and this proved to be the case with the Boxford’s motor. It wasn’t really a ‘big deal’ to do this, or was that just my ignorance in not fearing this operation? Anyway, the shaper has been running to all intents and purposes as a single-phase machine for the last thirty-odd years and has coped with every job I have given it with no detectable loss of power. I did go on to similarly convert a power hacksaw with equal success but that has since been sold.

[Clive FosterParticipant @clivefoster55965]

Further to what NDIY says about running a star connected motor on a 230 V motor Inverter Drive Supermarket have an application note on the subject.

Summarising things the base motor frequency should be set to 29 Hz and the full load current to nameplate value.

The motor will then run at full torque up to 29 Hz and become progressively under fluxed as higher frequencies are demanded for higher speeds. In general the motor is unlikely to be able to exceed 50 Hz with useful power.

Around 3/5 th the power at (theoretically) 835 rpm for a 1440 rpm motor.

It’s well over a decade since it became cheaper to buy a low power VFD than make proper job of a basic Steinmetz  type static converter with automatic start capacitor disconnection and appropriate electrical safety measures. I’ve made and sold a few in the past.

Back in the last century I ran a pillar drill successfully for many years using an ultra simple version of the Steinmetz with the start contactor connected to a normally open switch lmechnically linked to the start button. Briefly holding down the start button putting the start capacitor into circuit bought things up to speed. Helps that a drill starts pretty much unloaded.

Clive

 

 

 

 

[not done it yetParticipant @notdoneityet]

The modern-day VFDs have all sorts of features to maintain torque at lower speeds as well as the usual options of rotation speed choice, ramp, etc.  All I know is that my Centec came with a tiny VFD on the power feed and that appeared to work OK.

 

After asking around and then checking on the internet, a couple of ‘electrical’ friends told me that fitting my 2 HP 380V star configured motor would run but deliver less power (the 58%).  Fitted my requirements and has worked OK for me over the ensuing years.  It most certainly avoided the poor cold starting problems with the single phase motor – that of never reaching full speed when attempting to start on high speed under particularly cold conditions.

The single phase motor would either not attain full speed or dropped back until the start windings gave it another power boost – only for the cycle  to repeat itself.  I used to run it for several minutes on low speed before attempting to use the highest speed.  The change to the nominal 2 HP 3 phase motor has given no problems at all, for me.  End of.  I am completely satisfied with both 230V VFD drives on my Centec 2B.  The power feed is clearly under-powered but works OK and the main drive motor has not screwed the gearbox to shreds, either.

The main point is they will obviously run on a 230V VFD while still configured as star connection.  QED.  You can argue amongst yourselves as much as you want.  I’m out of this argument and sitting smugly with all my 3 phase drives. operating successfully.

 

[Robert Atkinson 2Participant @robertatkinson2]

NDIY,

Not sure why you are still pushing the “it will be OK wired as star” position when the OP’s motor is dual voltage.
Just because your situation, a small mill with multi-speed gearbox with a motor 2-4 times the size of the original motors fitted and “and don’t ever use its full output”, works for you does not justify a blanket statement that running a star wired 3 phase motor on the voltage rated for delta operation “works”.
Yes, you can in theory get 57.7% of rated power at 57.7% of rated speed but that assumes no resistance in the windings and needs a VFD. Loosing tht amount of power AND speed is likey to severely limit the usefulness of a pillar drill. Practically, just connecting to the lower voltage gets between 25 and 33% of rated power.

Robert.

[noel shelleyParticipant @noelshelley55608]

The issue of single and 3 phase electrics always draws interesting comments ! The fact is that single phase has been with us and been used by many for decades, that it has a capacitor or a centrifugal switch, and may vibrate is true but it is also simple technically. When I bought my Myford S7 in 1971 I knew nothing of all of this, it had a 1Ph motor – it just worked and has continued to do so for over 50 years. The lathe and drill have lived in a damp shed for the last 40 years without issue. Time passed and when I needed 3 Ph I built a diesel generator, there is no 3Ph in my area and I needed 17Kw for one machine and 11Kw for the other.

Static converters are not cheap and rotary ones even dearer. In the main their plug and play, no need to find star point Etc or convert to 220v. They will run multiple motors with simple adjustments to the control. The steinmetz circuits are fine and work if you understand how to do it.

Then there’s the VFD –  Cheaper than converters but mostly need 220v motors and may need extra work to make them comply with some regs, technically complex and whilst very clever, to get the best out of them one needs to understand how to set them up. Variable speed is a big bonus. How well would a cheap one stand up to a damp shed, never mind salt air.

It comes down to what you’ve got, what you need, how much you understand electrics and how much money you can spend.

That single phase motors are cheap because they suck, I beg to differ !

As to the original question, change to delta, run a steinmetz circuit, Buy a cheap VFD or find a single phase motor of 1/2 or 3/4Hp. I fancy that there will not be much cost advantage to any of the options.

Good luck and best wishes.  Noel.

[Andrew TinsleyParticipant @andrewtinsley63637]

I have 3 or 4 spare quality VFDs on the shelf. I also have several1/2 HP single phase motors too. If I were in the OP’s position. I would swop the motor out for one of my single phase motors and be done with it. It is after all, only a pillar drill and doesn’t need all the fancy options that a VFD provides.

It would also be considerably less hassle to convert to a single phase motor. A VFD would require an enclosure to be mounted on the drill and possibly new on off push buttons to be installed. For the drill application, a single phase motor is a no brainer solution, both in less complexity and work and the cost would be approximately the same for both solutions.

I am fortunate in having a genuine 3 phase supply, so don’t have any axe to grind on this question.

Andrew.

[Clive Foster]

On Clive Foster Said:

ultra simple version of the Steinmetz with the start contactor connected to a normally open switch lmechnically linked to the start button. Briefly holding down the start button putting the start capacitor into circuit bought things up to speed. Helps that a drill starts pretty much unloaded.

This should suffice for a simple drill press. Shooting from the hip I would guess it would take one relay, to caps and to push buttons (one two pole NO and one single pole NC).

[Andrew TinsleyParticipant @andrewtinsley63637]

I suspect the OP has little knowledge of electrics / electronics. Otherwise he would not be asking such a basic question.

If that is so, a swop to a single phase motor is even more of a no brainer for the drill application.

Andrew.

[Gary WoodingParticipant @garywooding25363]

My Centec 2B came with 3Ph power and feed motors, driven from a rotory converter. I found the converter cumbersome and the noise bothersome so I converted both motors from delta to star and fitted 2 VFDs. What peace, just the variable speed was well worth the effort.

I also converted the 3Ph motor of the drill press to use a VFD – it’s amazing how useful it is to be able change the speed for different sized drills and counter-sinks.

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