NE1 help re dual phase motor please?

These motors will run on a 3 phase 220-250 V supply, normally generated by a

240 v input inverter.

single phase operation with a capacitor is I believe possible, but they have start up problems, and will probably need to be severely de rated.

Regards Jonathan.

Not so, we have a 1/2" pillar drill that came from Holland, it has operated since the 1970's on single phase with a capacitor to fool the motor into thinking it has a three-phase supply, works a treat and we don't see a bad falling off of power, even at full load+ with 9/16" drills etc.

Connect motor in Delta mode with 220V phase to phase and no neutral.

Connections are:

Live to one phase Neutral to another phase Capacitor (about 10mfd 400V PAPER TYPE) from Live to the third phase.

We used a Bosch motor start capacitor, if it is too small then you may have problems, but as the machine starts off-load then it isn't too critical.

Change phase connections for rotation change if you need it.

Peter

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That should work OK, but if all else fails the motors on the Mk1 are very easy to swap for a single phase unit -any similar sized foot mount motor should be able to be made to fit.

Regards Kevin

I would advise against using a motor start capacitor as they are normally electrolytic and rated for very short periods only. Use a paper or polypropylene type (I note that Peter already said use a paper type). Martin

Many thanks for quick & helpful responses. Amongst all the 1ph FHP motors our members have gathering dust under their benches, not one is 2880 rpm - the running speed of the existing

3ph motor. Otherwise, I wouldn't have troubled you guys. Obviously we can buy a new or s/h motor, but the "dual voltage" info on the fitted Brook Gryphon is quite an incentive to give it a try on 1ph. We'll now get a suitable capacitor, wire it up as suggested and stand well back . . . . .

IanC. (Guildford)

It should work ok, this is a small motor and the starting load is pretty low on a T&C grinder.

I'd suggest as a starting point say 15-20uF motor run cap. Get it running and measure the voltage between Neutral and your dummy 3rd phase. This should be as close to your line voltage as possible.

Wayne...

There is a small paperback book called "Electric Motors in the Home Workshop" by Jim Cox. This covers a lot of these issues, especially the capacitor sizes to use to fool a 3 phase motor into running on a single phase. It might be worth investing in a copy, its still current (!) as far as I know.

Steve

Electrolytics are usually polarised, AC caps are non-polarised so either a start or run cap should do the job.

Peter

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I seem to remember a "trick" of connecting two electrolytics in series with one the wrong way round to make up a non-polarised cap. The capacitance halves as a consequence.

Steve

Yes, the capacitance halves in series and doubles in parallel, opposite to the same connection with resistors, but the voltage reverses across the capacitors whatever you do with the connections, and polarised caps go bang when you do that :-))

Peter

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The matter of them being electrolytic is a secondary issue, primarily motor start capacitors are only very intermittently rated so if you use them continuously as a run capacitor they will most likely overheat and fail, not something you want to happen!. Motor run capacitors are continuously rated and hence much larger and more expensive, for the same capacitance and voltage of course.

Greg

This maybe useful in electronics, though not recommended, but it doesn't help here, fundamentally you should not use a motor start capacitor for a motor run application.

Greg

Can you give firm/quoted capacitor manufacturer's figures on which you based that statement?

Peter

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Yes, a whole minute spent on Google found the following info from a very well known group:

From which: "The standard rating is 1.67% or 1/60th full time and corresponds to a maximum duty of 20 starts, each of three seconds duration per hour. It is expressed following BS5267: 1967 and IEC 252 1975, as 3/1.67 (a 3 minute cycle with 1.67% duration during which the capacitor may be energised)."

So they are rated for a whole1 minute per hour...

Greg

Thanks for that, I wondered if there was a defined spec for them.

Peter

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I don't know if every maker follows the same spec but it'll be somewhere near as they've all got to be competitive. It's not the electrolytic/non-electrolytic thing that really matters as people think, it's all down to current rating. Capacitors pass an AC current which heats them and start caps just aren't designed to take the current for long, in practice this generally means that electrolytic is the cheapest way to make them. Some run caps have the kvar and current ratings printed them to reinforce the fact that it's about current.

It's important if replacing a motor capacitor that you ascertain if it's a start cap with a centrifugal switch to disconnect it or a run cap without one.

Greg

somewhere

The heating effect was a major consideration when I was designing and building my prototype small scale induction furnace. I ended up using polypropylene run capacitors in the tank circuit, but had to select within a batch. Running at several hundred amps at 3 to 30 khz the dielectric heating effect was significant and would vary significantly between apparently identical capacitors from the same manufacturing batch. Still at least this way I've ended up with a biggish box of run capactitors suitable for more 'normal' duty

AWEM

Yes that's a fairly demanding application, I would guess there are industrial caps available for the job but at quite a price.

People are often surprised that caps get hot, even in something as simple as a battery charger you can run into problems with the ripple current, and when it comes to wet electrolytic caps people don't realise the temperature has a dramatic effect on the lifetime.

Greg

message

temperature

My full size set up has water cooled capacitors crawling with pressure and temperature transducers. The caps need 8 litres per minute of water at no more than 40 deg C to keep them in limits. I would hate to have to buy new ones, they cost an arm and a leg.

AWEM