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Newman Motor Star To Delta?

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William Chitham31/08/2021 15:01:27
125 forum posts
52 photos

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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.

Andrew Tinsley31/08/2021 15:11:51
1485 forum posts

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.

noel shelley31/08/2021 18:20:44
770 forum posts
19 photos

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

john fletcher 131/08/2021 19:17:25
727 forum posts

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

Steviegtr01/09/2021 15:51:13
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2244 forum posts
311 photos

William, may I ask why you are wanting Delta.

Steve.

William Chitham01/09/2021 16:32:38
125 forum posts
52 photos
Posted by Steviegtr on 01/09/2021 15:51:13:

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.

Steviegtr01/09/2021 16:38:14
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2244 forum posts
311 photos

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 done it yet01/09/2021 18:19:15
6350 forum posts
20 photos

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%.

duncan webster01/09/2021 22:48:50
3526 forum posts
63 photos

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

John Haine01/09/2021 22:53:35
4188 forum posts
242 photos

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.

Andrew Johnston02/09/2021 11:29:43
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6283 forum posts
677 photos
Posted by John Haine on 01/09/2021 22:53:35:

So a star connected motor expects to see three phase wires and one neutral wire..........

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

Ian Parkin02/09/2021 12:06:34
avatar
979 forum posts
231 photos

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)

not done it yet02/09/2021 12:14:33
6350 forum posts
20 photos
Posted by Ian Parkin on 02/09/2021 12:06:34:

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).

William Chitham02/09/2021 14:27:21
125 forum posts
52 photos

Well here are the windings:

newman motor windings.jpg

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:

newman motor instructions.jpg

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.

John Haine02/09/2021 15:54:01
4188 forum posts
242 photos
Posted by Andrew Johnston on 02/09/2021 11:29:43:
Posted by John Haine on 01/09/2021 22:53:35:

So a star connected motor expects to see three phase wires and one neutral wire..........

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

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.

Vic02/09/2021 17:52:04
2920 forum posts
8 photos

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.

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