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Motor trials and tribulations (long)

Hi all,

Last night I had chance to investigate my 2.5 hp motor in more depth. There's good news and bad news, so I'll tell the story the way it happened.

First I removed the bearing covers and fan cowling. The ball races appear sound and are still packed with grease. Peering into the stator the windings look clean and shiny. There's a little rust on the rotor, but this could easily be brushed off and painted over. The inside of the motor is coated with this orange-red paint or resin, which looks a bit like red oxide paint but isn't. I've seen it in motors before but I don't know what it's called. Anyone know?

Next I opened the terminal box and removed all the links. I measured the winding resistances with a multimeter. All are 6.5 ohms. There was no measurable leakage between any winding and the motor casing. So far so good. I now planned to connect a run capacitor and power up the motor with no load, just to check everything worked. So I worked out the correct link positions for a delta connection.

Aargh. Problem! The motor had originally been wired in a delta connection. Of course 240 V is the line-to-neutral voltage of the UK's three phase system, so I wondered (hopefully) if this motor had been wired for 240 V three phase. But then I realised that there was no neutral wire entering the terminal box, so it must have been wired for

415 V. Apologies Bob, you were right. Damn!

I've never seen a motor of this size which uses a delta connection for

415 V before. In fact, I don't think I've seen a motor below 10 hp which uses a delta connection for 415 V. But I've never seen a motor of this size with a fan cowling which weighs 20 lbs, or a rotor which is fabricated from individual pieces of stock before, so I guess I shouldn't be surprised. I guess in 1931 there may have been less export, and it seems LSE specialised in large motors, so maybe a delta connection was their standard?

On the plus side, this motor is a really cool piece of work. Had I known it was 415 V only I would most likely have rescued it anyway rather than letting it go to the landfill. I've got rather keen on the phase converter idea, so it looks like I've now got the challenge of finding a

240 V -> 415 V transformer with a rating of around 3 kVA. Anyone know of likely sources? Jim Cox's books "Electric Motors" and "Electric Motors in the Home Workshop" (which are a goldmine of practical information about motors - Igor might like them if he hasn't got them already) suggest modified old-fashioned welding transformers. I'm usually pretty good at scrounging industrial junk, so if anyone can think of equipment containing likely transformers please let me know!

A 415 V output is actually more useful than a 240 V output in the UK, as I could then just wire a plug onto any equipment I buy and plug it into the convertor. Almost all three phase equipment in the UK is wired for a

415 V line-to-line voltage. Jim Cox's books also note that smaller capacitors are adequate for 415 V operation, so this setup has its attractions. I doubt I'd want to run loads over 1.5 hp, so a carefully designed convertor using a 2.5 hp idler should be okay. Food for thought, anyway.

Sorry for the long post. I'd be interested to hear people's thoughts.

Best wishes,

Chris

Reply to
Christopher Tidy
Loading thread data ...

SNIP

Red Stuff is called Glyptol. Comes as paint or in a spray can. Made in the US and Canada under General Electrics name.

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Reply to
Brian Lawson

Thanks. I will look out for some.

Chris

Reply to
Christopher Tidy

It would be exceedingly uncommon to connect the neutral point of a Wye-connected motor to neutral. In almost all cases, it is just taped off.

The data plate (earlier message) says 400 V, but seems to also say "Star Delta" on the connection line. If you have 6 wires, you should be able to (re)connect it in the star configuration for 230 V.

Jon

Reply to
Jon Elson

That's what I thought, but I'm puzzled by the fact that the motor was delta connected in its original factory installation. I would have expected a motor of this size to be star connected for 415 V operation, but I think it's unlikely that a factory in the UK would have a supply with a 240 V line-to-line voltage. The only thing other thing I have to go on is the winding resistance (6.5 ohms) but I can't see how this is a help.

Apparently Laurence, Scott and Electromotors donated their early records to the IEE library in London, so I could try there if all else fails.

Best wishes,

Chris

Reply to
Christopher Tidy

Chris Rather than use it in reverse, I would suggest you use it as an auto transformer by wiring 240V to the 240V on the primary winding and pick up the 480V off the higher tap on the primary. I wouldn't want to run 240V into the 120V winding if that's what you were planning to do as it may well overheat. My RPC has a 415V delta wound motor that can be wired 'Y' for a 660V supply but it's a 10HP motor, although considerably smaller than your 2.5HP motor. I had a transformer wound just for this RPC but it's fairly costly (£140 for an 8KVA unit); you may well be in luck and find that the transformer on ebay is multi-tapped and does have a 415V or 440V tap. I had to go the 415V route as my mill cannot be rewired for 240V. Good luck, Martin

Reply to
Martin Whybrow

OOps, I think I got this backwards! I should have said : If the STAR voltage is 400 V L-L(from the plate, not 415) then the actual voltage across the individual windings would be 400/1.732 = 231 V. (1.732 is the square root of 3.) So, if this motor only has 3 windings, your only connection choices are delta or star. (Most motors in the US are Wye only, with nine wires. There are 2 windings for each phase. One set has an inaccessible center point, the other is wired totally separately, so it can be connected in series with the other set for high voltage, or parallel for low voltage. This is generally for

240 or 480 V systems. In the special case of 230/400 or 240/415 V systems, you have the additional trick of the Delta/Wye conversion.

Jon

Reply to
Jon Elson

Chris

Since this is a fun/hobby project for you, I wonder if you'd consider it worth the complexity to find a choke to put in series with the transformer. If the choke was the right size to drop about 15 percent of the incoming

240vac, the 2 to 1 transformer would provide the required 415vac for your idler. It would be possible that a choke like this could be wound 'at home' fairly easily, depending on your interest.

Jerry

Reply to
Jerry Martes

But, an inductor will drop a voltage proportional to current. So, if the load changes, the voltage drop will change, too. I really don't think this will work, except under conditions of constant load.

Jon

Reply to
Jon Elson

Small 24 or 28v transformers are pretty common items. A 24 or

28v secondary, connected in buck with the 240 in will give a consistent 10 or 12% voltage drop which is probably near enough.

Jim

Reply to
pentagrid

Thanks for the suggestion. It's an interesting idea and I'll give it some thought. I'm taking some time to think over the options anyway while I work on other things - fortunately there's no hurry! An interesting development is that I have the option of two 2 kVA 230 V ->

110 V transformers. These are ex-factory units designed to run continuously and are enclosed in a steel box about 12" cube. The core fills most of the box. I think these things are pretty conservatively rated. Anyway, they are the old-fashioned type which can be dismantled, and I was wondering about removing the primary and replacing it with a 185 V additive overwind to give me an autotransformer with an output of 415 V. Any thoughts?

Thanks for the help,

Chris

Reply to
Christopher Tidy

snip

The rewound secondary in auto transformer connection should result in 4KVA continuous rating which is more than enough for your application.

With 2 transformers, 110v from one, 75v from the other in auto transformer connection yields over 7KVA. It bit wasteful but it's now only necessary to remove a few turns and doesn't need a full secondary rewind

You shouldn't need this, but an often overlooked kludge is that quite a small fan will substantially uplift the long term rating of a power transformer - something like 50% is possible.

Jim

Reply to
pentagrid

Thanks for the advice. It sounds like these transformers would be ideal for the project.

I studied transformers at university but my recollection of the theory is now a little fuzzy. Why does the rewound secondary in auto transformer connection permit a 4 kVA rating? I'll dig out my notes and refresh my memory before starting the project in earnest.

Best wishes,

Chris

Reply to
Christopher Tidy

Look at it in a slightly different way.

For the 1:1 ratio case think of a 1KVA 100V secondary that sits on top of a 100V supply live. For its 1KVA rating this delivers 10A. The return connection for this 10A current runs through the supply which adds 100V to the delivered output so the load sees 200V at 10A = 2KVA.

An easy way of working out the autotransformer connected rating is to simply add the secondary volts to the supply volts and multiply this by the rated secondary current.

Jim

Reply to
pentagrid

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