Newman Motor Star To Delta?

I have bought a 3 phase Meddings Driltru which I want to run on single phase. I'd like to use the original Newman motor with an inverter but unlike the motors on my lathe and mill which I have already converted the Newman is not marked as dual voltage and there are only three terminals under the plate. I have read that in such a case the necessary connections can sometimes be accomplished by dismantling the motor to get at the windings but sometimes this is not practical. Before I take mine to bits I thought I'd ask if anyone has experience of converting this particular type.

Thanks, William.

I have done the star to delta mod on quite a few motors that are not preconfigured for star or delta. Unfortunately not on a Newman motor. I have yet to find a motor that can't be converted.

Most times the conversion is straightforward, but sometimes it can get quite difficult especially if more than 3 joints are to be seen!

You need to take care with insulating any mods you make to bring out the 3 wires of the star point to the connection box. If you are not completely comfortable in modifying the mains wiring, then I recommend you to leave well alone and get someone do the job who is!

Andrew.

Shown as a single voltage on the plate it may not be easy to change to delta. You need to be able to get at star point and sort the tails out. As Andrew has said IF you are not used to working with mains voltages safely then get it done for you or change to single phase or replace with a dual voltage motor that can be easily changed to delta. Good luck, Noel

A couple of years or so ago some on here posted a series of picture on how he did the conversion and it explained in detail of he did, very informative. I've done a few for friends and have 500 volt DC insulation tester (Megger) to test the final result. You need to take the motor completely apart to locate a LUMP in the windings, mark each end the the motor and take a lot of picture as you go, to enable you to reassemble the motor on completion. Have you look on Youtube some must have done the conversion on there. John

William, may I ask why you are wanting Delta.

Steve.

Normal 3 phase is 400-440v. The "synthetic" 3 phase from a VFD is 230v. Most 3 phase motors can be configured to run on either voltage - star for 400v or delta for 230v ("dual voltage" motors) and this is usually made clear on the spec plate. Motors that are not intended as dual voltage, like this one, may or may not be easily converted depending on whether the node of the star is accessible or not.

However, I had a conversation with the fellow at the Inverter Supermarket who pointed out that it the motor may run satisfactoraly at the lower voltage as is. Here is a link to an explanation of how that works:

**LINK**

Basically the motor can be considered as a 230v 29Hz motor so will run at about 60% of normal RPM. Above 29Hz torque will drop off but that may not matter because one normally uses higher speeds for smaller drillbits which require less torque to turn. Definitely worth trying I think.

William.

Interesting read. As it is a question that comes up a lot on this forum.

Shame those paragraphs could not be put on here for members to readily access.

Steve.

Not quite what the man told, I think. The power will be about 60% for 230V star as if driven at 400V star from the mains supply. But that power would be at 50Hz and normal motor speed. Power is Torque x RPM and, as that line on the graph is straight, the power will be about 60% of the 400V rating, power will drop as RPM decreases if the torque remains constant.

With a 230V output VFD, the motor power will be the same (at 50Hz) - as if the motor were wired star @ 400V @50Hz.

Power of the motor @230V single phase in star configuration would be nominal motor power divided by the square root of three, so just under that 60%.

I once converted a bench grinder from 3 to single phase. The motor manufacturers were very helpful, but as it was an old motor they advised against running it off a vfd, something to do with insulation. Didn't matter to me as I used a steinmetz circuit (capacitors). This might be good enough for a drill machine, and a lot cheaper

Whoever wrote that web page for Inverter Supermarket should hang their head in shame! A awful bit of explanation.

And could I just repeat, yet again, that a "normal" 3 phase supply is ~400V phase-to-phase, but nominally 230V phase to neutral. When single phase is delivered to our houses the neutral comes from the STAR point of the substation transformer and we get one of the 3 phases as the live. So a star connected motor expects to see three phase wires and one neutral wire, and each winding sees 230V. However ordinary VFDs can't generate that much voltage because of the way they work, but can generate 230V phase to phase, which is why you need to reconnect the motor into delta so each winding sees 230V. Just to repeat, a "415V 3-phase supply" from the mains is just 3 "230V single phase supplies" where the supplies differ in phase by 120 degrees.

None of my machine tools, running on a 3-phase supply, have a neutral connection; just the three phases and earth. In theory at least for a motor running in star there would be no current flowing in a neutral wire connected to the star point, which is why it is not connected.

Andrew

I have run quite a few 3 phase motors in star connection straight from a VFD with no ill effects maybe a bit down on power but they ran satisfactorily ( disc sanders ,drill presses,tool post grinders)

Quite. 42.3% down, on power (to be exact) but at exactly the same speed (at the same Hz).

Well here are the windings:

and here are the instructions which emerged when I opened the case - these are the two sides of an oil soaked piece of paper so rather transparent but legible:

I wonder why they would make two versions of the motor, with and without dual voltage. Some small cost saving i suppose.

In a spirit of enquiry I think I will reassemble it and try at as is. If it is too feeble I'm pretty sure I'll be able to dig out the node and rewire it.

The Steinmetz circuit looks interesting but I do want the VFD extras (reversing, speed control, jog) to make power tapping easier.

William.

Yes, that's quite correct you don't need a connection to the motor star point, I should have said that you could connect the neutral to the star if you wanted to. Of course if you want to run a single phase pump, or axis drive, or worklight, then either you need a neutral connection or a 415:230 V transformer.

I ran my old three phase mill on 240V single phase. I just swapped the wires in the connection box from star to delta and put a large capacitor across the live and neutral. It ran fine like that for a couple of years until I sold it on. I was told at the time that the motor power would drop by 20% from that marked but I never had any issues. I believe this topic is covered in one of the workshop practice books on motors.