Another 3 phase issue

If the motor is not marked as star/delta and it was already working off 415V then it is reasonable to assume that it has been wound for 415V delta service. If it pops the fuses when you use it in that way, then you might assume that someone has been fiddling with things and rewire it to star.

My 0/2d worth

Mark Rand RTFM

Mark I think that you're right. In one of Jim Cox's books he has a chart that shows power, winding resistance and voltage.The intersection of 3HP and 415volts is indeed at 7 ohms. Therefore I assume that 415volts should be across the full winding which would suggest that the motor should indeed be wired as delta - otherwise in a star configuration, with no neutral, the

Charles

Charles

I did record on here, I think, that when I set out to fit an inverter to my 'newer' CVA, I took the motor to the rewind shop to ask them to convert it from star to a 6-wire configuration. They reported back that, despite the rating plate saying it was star wound and there being no evidence of previous alterations, it was already in delta and had apparently run that way since the lathe was new (on 440V). I must admit that I can't remember what the DC winding resistances were, though I might have noted them here. I ended up swapping the motor for a genuine 6-wire job, but that was in the middle of struggling to get the inverter properly configured. It's a fairly fancy vector jobbie, it's running really well now but rather surprisingly (to me) it wasn't at all happy with either motor with just a basic setup. Whether it would work ok with the original motor with all the fancy parameters set, I'm not going to swap it back to find out!

Probably no help to you at all

Tim

Not at all Tim. Useful stuff.

I've been testing it today and with 4HP transwave and a 1.5 HP pilot I can get the lathe to work fine but it won't start in top speed (2,220 rpm). It just blows a 20amp fuse. The options are:

The last one is the least attractive. The other two require a replacement motorso if anyone has a 3HP dual voltage motor with a 1 inch shaft please drop me a line

All other comments welcomed

Charles

it will blow a fuse ...the suerge for start up is probably greater than 20 amps ...you need, i think D-rated mcbs D16s perhaps

these take up to 16 amps but allow a starting surge through.without tripping.

my 5.5 hp rotary converter is on a D25

all the best.mark

My 8/10 hp Mawson Special converter with 10hp idler struggles to start my other CVA (4hp) at top speed (3krpm), there's a fair bit of clicking of contactors in & out until it gets up to speed. This one was much the same until I fitted the inverter. On test, it would start & reach full speed via a 13A plug, though I now have it on its own 32A spur.

If you can afford it (and the time to do a proper job of the conversion) I'd strongly recommend the inverter/VFD route. I reckon it took me at least 3 full days to get mine converted, but I did build the inverter onto the original switchgear panel and wire the original lever forward/reverse switch in so it did the same job, made a new small panel for the controls on the base casting, modified the motor mounts for the replacement motor, etc etc. Now the other, unconverted, CVA is definitely 'second choice' when I have a job to do.

Tim

I had the same problem on one of my lathes and got by it by driving it from another.

-- Regards,

John Stevenson Nottingham, England.

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You're a very, very sick man :-)

Mark Rand RTFM

Interesting.. Steviewonder had me going for a minute. :-) Good one of the late Fred Dibnah too.

Archie

For those still interested I have reached a satisfactory solution.

I obtained another 3HP motor from Andrew Mawson (thanks). This is a modern finned dual voltage one. The difference between this an the original 3HP motor is best shown (aside from size) by the "spin test". If you give the old one a spin with a flick of the wrist it takes twice as long to stop spinning as the newer one. Whether this is significant I don't know.

With this new motor installed, wired for 415volt in star (unlike the old motor which is 415 volt delta) the lathe starts in top gear without blowing fuses or taking an indecent time to reach working speed. Quite why is beyond my electrical knowledge (explain of you want/can) but for the record, it's worth a try if you have a similar problem. Now all I have to do is sort the rest of the wiring.....

Regards

Charles

Not quite in response to Charles' posting, but I wandered down to the workshop during the week to look at the two-speed motor on the ward 2A. That has a ginormous axial change-over rotary switch in a big cast-iron box on the from apron. Inside there are a profusion of contacts and the big drum assy that makes the change-over.

The motor starts immediately (we have 3-phase power at home) and with the gears on the basic lathe we have 12 speeds IIRC.

The change up to high-speed can be done without turning the motor off, but the lathe has to be disengaged, and it does have a most satisfying noise to it, with the large drive belts and the very large motor running like a well-oiled tram.... :-))

That size motor would probably be 15 or 20hp these days!

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

well-oiled

I've got a ginormous brand new three phase motor if anyone wants to play with REAL power. 15 kW iirc so that's 20 HP - it weighs 70 kgs and is a lovely bit of kit. I got is to make a huge three phase rotary converter, but never did, and now I'm moving it needs to go ! To go with it to make the three phase rotary converter I also have one of the ex Peter Forbes whopping big 240 - 110 isolating transformers that with a bit of simple re-jigging can be used as a 240 to 415v auto transformer to generate genuine 3 phase 415 v from single phase mains. email me if interested

AWEM

Those trannies are pretty good for boat anchors as well..... :-))

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

Am new to this group, but this may be usefull: Recently built a 500v 10a ish dc psu to drive 3 phase input inverters. (They are much cheaper) If you try to drive them across two phases with a 110v transformer in series with the mains, most complain about phase loss, but internally they are just a rectifier driving an inverter, so if you run them from DC, they work fine.

The psu is just a 10a 110v isolation tranformer, in series with the mains, driving a 25a bridged rectifier, with a couple of v large electroytics in series across the output. The two caps have voltage sharing resistors and a time delay relay + resistor for soft start. Have been able to drive a 10 hp motor for the first time using this setup from standard house mains, though the workshop lights dim a bit on full load.

One thing to note is that 500v dc is *seriously lethal* and needs utmost respect and care. Any such psu should be in an all enclosing box, with hrc fuses on the op and non standard connectors so that nothing else can be plugged in...

Chris

To right.

-- Regards,

John Stevenson Nottingham, England.

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internally

Chris,

Even easier is to apply your 415v single phase AC from a transformer between 'two phases joined', and the other phase of the inverter input, then the phase loss sensor still sees volts on all phases! Don't forget to slightly downrate the inverter as the input rectifier is really only using four of the six diodes in the three phase bridge.

AWEM

Hi, Chris. Still got that little diesel in the garage? Wanna buy a VAX?

Regards Mark Rand RTFM

Hi Mark,

Name rings a bell, but can't place it ?.

The McLaren diesel genset went some years ago, if that's what you mean by "little diesel". Couldn't get in the garage to get any work done :-)...

Chris

Didn't think of that, but doubt if it would work, since the phase loss detect usually senses ripple on the rectified dc and doesn't monitor phases on an individual basis. At least, all the inverters i've seen work that way. If a phase is dropped on 3 phase, the ripple frequency would be lower, so all they need to do is measure the period of the waveform to detect phase loss on any single phase...

Chris