Options for powering a three phase machine

[Guineafowl]

TL;DR Summary

I have a 1960s mortiser powered by a three phase motor. I can get a three phase supply to it, but is there another way?

The motor from this fine, old and very heavy machine is designed to take UK 3ph, 415V. There are only three connections inside, and the nameplate says it’s wired in star. I have stripped it down and cannot locate the star point.

I was hoping to rewire it to delta, and run it with a 1ph/240V to 3ph/240V inverter drive. This doesn’t seem possible, so I think I have three options:

1: Send the motor to a rewind shop to bring out the star point. Is this feasible?
2: Find an inverter drive that converts 1ph/240V to 3ph/415V. I can’t seem to find one; do they exist?
3: Run a 3ph supply to the machine - the simplest, but it may be costly.

I imagine 3 is the best, but even so, I’d be interested in hearing about the other two options.

 

[anorlunda]

It is possible to have a Y connected star, but to not ground the center point (star point). We call that ungrounded Wye. So I do not see that as forcing your choice. The cost of providing a 3 phase supply is a bigger problem.

You can do it with transformers instead of inverters. Three single phase transformers in the primary and a three phase secondary. But that may also be expensive.

Rotary converters do the job, but that is considered an obsolete method.

Variable Frequency Drives VFD, are also used for that purpose. Still not cheap.

https://www.precision-elec.com/convert-single-phase-to-three-phase/
Is there any chance to buy three phase power from your power company?

 

[Dr.D]

You can do it with transformers instead of inverters. Three single phase transformers in the primary and a three phase secondary. But that may also be expensive.

@anorlunda Doesn't this assume that the OP has 3-phase power available? If not, I don't see how this could work. Perhaps you might elaborate?

 

[anorlunda]

@anorlunda Doesn't this assume that the OP has 3-phase power available? If not, I don't see how this could work. Perhaps you might elaborate?

You're correct. My bad. I was thinking of three single phase transformers to make a three phase primary. But that does need a three phase supply.

Thanks for catching my error.

 

[Guineafowl]

It is possible to have a Y connected star, but to not ground the center point (star point). We call that ungrounded Wye. So I do not see that as forcing your choice. The cost of providing a 3 phase supply is a bigger problem.

Yes, the motor is ungrounded wye. It expects to have 3 phase connections, with 415V phase-phase, but...

Variable Frequency Drives VFD, are also used for that purpose. Still not cheap.

... I can find plenty of cheap (£60-70) inverter drives which only put out 3 phase power with 240V phase-phase, not 415V. My thinking is, if I were able to bring out the star point into three new connections, I could reconfigure the motor to delta and run it with such a drive.

Is there any chance to buy three phase power from your power company?

Yes, it is nearby. But the cost might exceed that of a suitable inverter, and will I always live somewhere with 3ph power? On the other hand, it would enable just plugging the machine in and using it.

 

[Guineafowl]

I’ve found this quote:
“If you are unable to change the connections or windings to 240V 3 phase [that’s me!], then read on for the next best solution... The speed of an Electric Motor is determined by Voltage and Frequency. So, a 400V 50Hz Motor will run at rated speed at 400V x 50Hz and half rated speed at 200V x 25Hz. Provided this Voltage x Frequency ratio is maintained, the Motor will operate at full Torque – ideal in a Lathe application where speed must remain constant even when a load (the tool) is applied (to a work piece).

An Inverter Drive is not only able to convert a 230V single phase supply to 230V 3 phase but it also controls both the output Frequency and Voltage to maintain the correct ratio. It therefore follows that a 400V x 50Hz Motor will operate normally at 230V x 29Hz, just at two thirds the speed (eg. 1000rpm instead of 1500rpm).”

From this link:
https://inverterdrive.com/HowTo/240V-Supply-to-a-400V-AC-Motor/
It’s the only place I’ve found this information. Is the physics correct? The machine would work just as well at 2/3 speed.

 

[Guineafowl]

I’ve made some progress...
I re-opened the motor, and had a gentle dig in the top of the windings. I found three wires joined together, which when separated showed continuity with only one of each existing terminal:

I extended each wire with flex, the joint being covered with doubled heat-shrink tubing, and brought out the connections to the paxolin panel:

Epoxy to repair the work:

Now the motor can be configured in delta:

I verified the winding designations, then tested for insulation resistance between phases and from each phase to Earth at 1000V. All readings were over 100 MOhm.

So hopefully I can now use it with a standard VFD putting out 3ph/240V.

 

[Mark44]

A good friend of mine has a 3-phase milling machine running in his single-phase shop, wired to (US) 220 volts. He uses an electric motor that outputs 3-phase, and starts the motor using some capacitors. I'm not totally clear on his setup, but I can ask him and report back.

 

[anorlunda]

That sounds like a motor-generator set. Perfectly valid, but it has higher losses and thus adds cost to your electric bill.

 

[Dr.D]

M-G sets are a common way to convert AC to DC and vice versa. I saw a lot of these when I worked for the US Navy.

 

[Mark44]

That sounds like a motor-generator set. Perfectly valid, but it has higher losses and thus adds cost to your electric bill.

I don't think the higher cost is relevant in my friend's situation. He doesn't run the milling machine that much, so it's more a question about being able to run 3-phase without having 3-phase wiring coming to the shop.

 

[gmax137]

A good friend of mine has a 3-phase milling machine running in his single-phase shop, wired to (US) 220 volts. He uses an electric motor that outputs 3-phase, and starts the motor using some capacitors. I'm not totally clear on his setup, but I can ask him and report back.

That sounds like a motor-generator set. Perfectly valid, but it has higher losses and thus adds cost to your electric bill.

Or, a rotary converter.

The home shop machinist forum is full of advice on these issues. The professional (or "practical") machinist forum has an entire subforum for VFDs and rotary converters. I'm not sure if links to these would be frowned on here or not?

Regarding getting 3-phase from the local power company: at least in the US this is either impossible or hideously expensive in a residential setting. Maybe if your shop is in an industrial building, or an old dairy farm but otherwise I don't think they will accommodate you. I think the story may be different in the UK?

 

[Guineafowl]

Or, a rotary converter.

The home shop machinist forum is full of advice on these issues. The professional (or "practical") machinist forum has an entire subforum for VFDs and rotary converters. I'm not sure if links to these would be frowned on here or not?

Regarding getting 3-phase from the local power company: at least in the US this is either impossible or hideously expensive in a residential setting. Maybe if your shop is in an industrial building, or an old dairy farm but otherwise I don't think they will accommodate you. I think the story may be different in the UK?

There is, in fact, a 3-phase distribution board in the workshop. Most farms and commercial properties in the UK will already have it, and many rural homes will not be far away. The distribution is 3ph delta, then this is stepped down and converted to star (415V ph-ph, 240V ph-star point), each group of homes receiving one phase.

The problem is, the size of breaker needed in this particular board (max. Breaking capacity, 10kA or whatever) means the cost will actually exceed the price of a VFD, if only I could convert the motor from star (needing 3ph 415V) to delta (needing 3ph 240V). As you can see above, I managed to bring out the star point from this motor and so convert to delta.

 

[anorlunda]

A MG set would do the job, but in this case we are talking about MGM. A motor to run a generator to run a motor. You have to ask yourself if it doesn't make more sense to use the first M in that chain to replace the third M in the chain and skip the G.

 

[Averagesupernova]

A good friend of mine has a 3-phase milling machine running in his single-phase shop, wired to (US) 220 volts. He uses an electric motor that outputs 3-phase, and starts the motor using some capacitors. I'm not totally clear on his setup, but I can ask him and report back.

I have this exact setup in a shop with a Bridgeport mill.
-
ONE 3 phase motor with starting and running capacitors. The start capacitors allow the motor to spin up using a single phase source. Running capacitors (oil filled) are used to help balance the voltage on the manufactured phase. I have posted about this type of convertor here on PF before. Do a search.

 

[Baluncore]

@Guineafowl.
You talk of three phase power but you do not specify power.
Does your mortiser require 1 HP, 3HP or 10 HP?
Low cost single phase motors are also available.

 

[Guineafowl]

@Guineafowl.
You talk of three phase power but you do not specify power.
Does your mortiser require 1 HP, 3HP or 10 HP?
Low cost single phase motors are also available.

Yes, sorry, I forgot to state the power, which is only 500W or 3/4hp. The motor is a custom fit to the machine (no belts/pulleys) so replacing with a 1ph might be tricky.

It is now sorted and running with a 2.2kW inverter drive. I’ll show a pic of the final setup in case it helps someone else.

 

[Guineafowl]

Pics as promised:
Power comes in direct to the inverter, and the old DOL starter is bypassed and just used as a junction box.

The machine makes short work of mortices, even through knots!

 

[Klystron]

Thanks to the OP for the great pictures. Good to see older technology re-used.

M-G sets are a common way to convert AC to DC and vice versa. I saw a lot of these when I worked for the US Navy.

Ditto for the USAF. Also used amplidynes coupled to motor-generators for max DC.

With the problem solved I should mention another solution also from the USAF.
Refurbish a triple-phase 400V 50hz diesel generator.

A radar tech friend rebuilt a salvaged generator purchased as scrap in Northern Montana. When municipal power finally reached his land, I understand he replaced the noisy diesel with some sort of electric drive but kept the 3-phase 400V generator section.

 

[Baluncore]

It is not usually a problem with three phase input to a VFD, but when converting single to three phase you may find that there is insufficient internal reservoir capacitance to hold charge through the low voltage parts of the one input cycle.
Some VFDs have terminals that allow additional external reservoir capacitance. That may allow greater output power.
Look at the terminal assignment to identify if external capacitance is supported. Read the manual to identify the capacitance selection and the limits.

 

[Guineafowl]

It is not usually a problem with three phase input to a VFD, but when converting single to three phase you may find that there is insufficient internal reservoir capacitance to hold charge through the low voltage parts of the one input cycle.
Some VFDs have terminals that allow additional external reservoir capacitance. That may allow greater output power.
Look at the terminal assignment to identify if external capacitance is supported. Read the manual to identify the capacitance selection and the limits.

I see what you mean - where the single phase crosses zero the three phase does not. With this sort of thing in mind, as well as longevity, I bought an over-rated, 2.2kW drive to power the 0.5kW motor. It seems to work nicely, and the speed ramp-up is a nice feature.