'Anybody know anything about 3ph electric motors?

[Dave]

I'm in the process of finally getting a rotary 3 phase converter going with a BIG pilot motor but I'm confused by the motor rating. It's a 4kW 2850RPM Brook Crompton motor to BS5000, frame 3D112MD, serial D102709, 415 delta (symbol), rating MC, design 2E.

The bit that I'm not sure about is whether to leave it delta or re-connect it as star (there are 6 terminals so it's easy to change). My limited understanding was that: smaller motors would normally be wired star for 415 and delta for 230 (eg with an inverter); larger ones would normally be started in star and then switched to delta (or the other way around if max starting torque was required). With a converter there is negligible starting torque needed so I would guess that star would be the answer BUT when the load motor is started I assume a lower source impedance would be good and therefore a delta pilot might be better. Aaargh - onset of brain fade.

The motor is marked "4kW, 415 delta" will it still act as a 4kW motor if wired in star? Which is best for a converter, star or delta? Can anyone decode the rating, design and frame codes?

Dave S

[Andy Wade]

You need to match the (running) voltage rating of the motor to the output of your converter. The voltage quoted for 3-ph systems is always the line voltage (voltage between phases) unless stated otherwise.

If the rating plate says 415V delta, the motor would require a line voltage of 720V if wired in star. So leave it wired in delta unless you're planning to use a star-delta starter. If your converter only produces 230V line - as some variable speed drives with single phase inputs do, I believe - then you're stuffed (or need a 3-ph auto-transformer).

Try

and look for technical support.

[Dave]

I'm using a single phase auto to generate 415 and then capacitors and the pilot motor to generate the 3rd phase. It seems to run OK in star but when I connect the load motor (2HP) it has a marked effect on the pilot, when I run the pilot in delta it pops a 13A fuse at start up.

I looked here earlier but couldn't find any decodes (or even a tech support section)

Dave S

[Andy Wade]

I can't really follow that, but it sounds as if you haven't got a proper converter and are trying to use some sort of lash-up with a second motor(?). Some success at doing that was reported here a while ago, but it all sounded rather hairy. If you do a Google groups search with the search string as "3-phase motor group:uk.d-i-y author:andy author:pugh" you'll find plenty to read.

I suggest getting either a single-phase motor or a proper static converter.

BTW, if you're trying to run a 4 kW (3 HP) motor via a 13 A fuse it's not surprising the fuse blows at start-up! For motors of that size you want a dedicated circuit with (say) a 32A fuse for s/c protection, and an overload relay in the starter for overload protection.

No, but under "worldwide service" you can find a UK telephone number to ring.

[Dave]

I'm converting a static converter to a rotary converter by adding a large pilot motor.

Thanks for the pointer, I'll have a look

Single phase isn't an option (the primary load motor is 2HP, 3 speed and buried deeply in the bowels of a lathe, there's also the coolant pump); I'm converting a static converter to a rotary converter by adding a pilot motor.

The pilot motor is running at zero load, just acting as a rotary transformer; running current (admittedly in star) is just under 1A/ph.

[Dave]

If anybody is interested in following this I asked on uk.rec.models.engineering and there's been a good discussion

Dave S

[Andy Wade]

Yes sorry, your last post (here) made me realise that I'd completely mis-read your original article and just though that you were trying to run the 4kW motor from a converter.

I've no experience of these rotary converters and can't help much more, although in the meantime I did come across

which might be of interest.

Sorry for the confusion.

[Dave]

No worries, thanks for the help.

Dave S

[Andy Dingley]

Have you read the Old Woodworking Machines site yet ?

[Andy Wade]

and some of them are in the wrong order only makes it slightly more challenging to read :-)

[Dave]

Thanks Andy, useful pointers. I'd seen the first but not the second. Is there anything else on files.owwm.com? When I try to poke around anywhere other than the the phase converter FAQ it tells me that I'm not authorised. Dave S

[Andy Dingley]

Probably - but they're a very disorganised site. Try the top level at oldwoodworkingmachines.com or

and look for the FAQ.

Incidentally, they won't talk to _you_ because you have a Wadkin, and they don't talk to people with non-American machines !

[Dave]

...snipped

Thanks again, it looks like there are some interesting topics there. All I need is the time to read them. The Wadkin machinery is all sold now so maybe if I mention the Shopsmith (boo hiss) they'll accept me. ;-)

Dave S