415V motors

[catgate]

I have for a number of years run, quite succesfully, 415/440v motors on 230v by the usual trick of turning them from "star" to "delta", and providing the third phase by using suitable capacitors. I now have a need to run a 1.5 hp three phase 415v motor in the "star" confuguration and I have obtained a transformer to boost the 240v voltage to 415v. What I am not so sure about is the value of capacitors to use. The rule of thumb for the 240v route is 40mf per hp + booster capacitors (x 3 or 4) for starting with load, but this does not seem to be applicable. Can anyone help, please?

[Malcolm]

I can't help directly, since like you I've only used the 230v method, but why not try the capacitance calculated for 230v and divide by root3 and give it a whirl? You can monitor the voltages to see if you are somewhere near.At worst it's only a blown fuse or total failure to accelerate. Best of luck!

[catgate]

Thanks for that Malcolm, I shall ponder the matter. Since my original post I have spent several hours doing it the hard way, by trial and error. From the 240v I.5 hp requirement, of 60mf for "run" reqirement, the 415v need seems to be about 10/12mf, with the addition of a start capacitor of the order of 12mf, depending on load. I am running an old Progress 4 speed pillar drill with this set up, and it has a fairly wide speed range. I have it set up with a voltage sensing relay/contactor cutting out the start cap. when I get 415v on the artificial phase. Sadly I have not yet got it spot on, as this later device tends to hunt occasionally in the top speed. I have ordered some more small value capacitors and will have another session when we come back from our touring, in "Der Vaterland", in about 10 days time.

[jackrae]

Alternatively , why not fork out some of that which the chancellor has yet to steal from you and buy an inverter unit to "convert" the single phase to 3 phase. Since getting one of them (about£150) all my 3-phase motor rig-ups have gone and I have the benefit of infinitely variable speed.

Jack

[catgate]

and buy an inverter unit

Thanks for that Jack. I suppose you are the sort of helpful soul who, if asked for directions as to how to get to Salisbury, would tell the enquirer he should not bother going to Salisbury, and point him in the direction of Leeds. Very constructive.

Did you write many songs for Frank Sinatra??

[jackrae]

Not for Frank I didn't so he didn't "do it my way"

But I was taught to have an open mind and think laterally

Sometimes the answer to a question isn't immediately relevant to the actual question but it, in turn, opens up a whole new train of thought for the original enquiree.

Ever happy to help

jack

[catgate]

Sometimes the answer to a question isn't immediately relevant to the actual question

Many years ago, before I retired, I was with a very large multinational. At one of our monthly management meetings the Work Study manager proposed a foolish cost cutting scheme which I, as Quality Assurance manager, vetoed, on the grounds that it was a retrograde move and degraded the integrity of the product. His reply has lived with me since and illustrates beautifully much of todays thinking. He said, "Well you must remember, all improvements are not necessarily for the better."

Was he a member of your songwriting team??

[Malcolm]

Interesting that the capacitance requirement on 415v is only one-fifth of that for 230v. I can't think of an electrical reason, off hand, why this should be so. If you find out, let us know.

[catgate]

Having got back from a delightful few days in the Mosel valley I have had a couple of further sessions with the drill, and it has finally succumbed, after some fine tuning. For those of you who may find the information of some use, at some time in the future, here are the details. Don't expect a rational explanation, because I am not sure that one exists!
It is an old Progress 4E pillar drill with 4 spindle speeds .... 120, 235, 395, 703 r.p.m. It is a 3ph. 1.5 h.p. delta motor. The capacitance now in use is Run...30mf, Start....20mf ,with the voltage on the voltage sensing relay (connected across the artificial phase) set to switch out the start capacitors at 400v. If it is set any higher the relay starts to "hunt" when "full" speed is reached. I hope this is of some help somewhere.
I shall now after start again, because I have just bought a fresh bigger lathe (Colchester Triumph) and it has a 5hp 3ph motor on it. Sadly I cracked the three jaw Taylor chuck on my anonymous old faithful about a week ago, and I think trying to find a replacement would be an impossible task. Sad really because I have had it for over 25 years and I thought it would see my time out..but I have out lived it. (and to be fair I think I am in better shape than it is.) Still it has served me well and I suppose a younger model will not need all the "allowances" to be made!!!