Motor Star / Delta question

One of the million and three projects I have on the go is to convert the old ML7 over to 3 phase using one of those new fangled inverter things.

I picked up a 3 phase 1/3 horse motor off Peter Neill at the weekend (thanks Peter!) and have been having a look at the windings on it, the motor having three windings which are strapped to operate in star mode. The motor in question is a Brook Gryphon and is marked for voltage as

220/240 & 380/415.

Looking at the specs on inverters, they all seem to output three phase with no neutral at all, so I need a bit of advice from those who have toyed with the inverters.

• Can the motor be left wired as star but no neutral connected? This should in theory still give the right voltage/phase to the windings.

• Should the strapping be removed and the motor wired with all the windings in delta? Just thinking this through each winding will see

415V instead of 240V. Are they rated to handle either voltage or am I going to run into trouble?

Also any tips on the inverters themselves, I've seen the Telemecanique ATV cheaper ones in RS, they can take a speed pot and start/stop buttons and won't break the bank.

Any advice or recommendations on the above welcome :-)

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Telemecanique

Duncan,

Most of the smaller inverters are 240v single phase input and 240 three phase output, making it necessary to wire the motor as Delta. If you have a 415 o/p inverter then the motor will need to be wired Star and the neutral isn't needed.

AWEM

Most or all invertors that have a 240V single phase input will produce 240V phase to phase output. This would mean that you should re-wire the motor to delta to get the right voltage for it. If the invertor did give 415V phase to phase (most three phase input invertors will do this) then there is no problem at all with wiring the motor star and not connecting the star point to the non-existent neutral.

One cunning mathematical reason for not wanting a neutral on an invertor output is that, by avoiding the neutral connection, all harmonics divisible by three (3rr, 6th, 9th ...) are eliminated if you leave the neutral off.

The invertors now seem to be almost getting to be a commodity item nowadays. It's quite nice to use an ML7 without having to keep changing the belts over, although I still find that backgear is necessary for big stuff, just because the belts slip otherwise.

Mark Rand RTFM

Thanks Mark and Andrew. I've just been looking at the pdf for the Altivar ATV 11 on the RS website and it has a parameter on there to match the motor voltage with an adjustment range of 100V to 500V so should be OK on that one.

Although the motor is 1/3 hp, I'm planning to get a 1 hp drive in case I want to use it for something else in future. Is it considered bad practice to oversize the drive by a factor of 3?

Duncan. Swap the links over from star to delta and feed it with the 3 phase wires, usually marked R, S, T, on the invertor. Most invertors applicable to the home shop use live and neutral in at

240v and output 240v 3 phase hence the delta windings.

That's the only neutral needed at the mains input.

In a three phase motor, euro spec not those obsolete US things [ uuugg , spit ] there are three coils and 6 wires, two per coil. Each coil is rated at 220 / 240v volts so in delta you have one coil connected across each phase from the 3 phase supply. In Star you have two coils connected across each phase because they are connected at the star point, so double the voltage and double the connected coils.

The Telemechanique range from RS are very good value for money, in a lot of cases their price to anyone is less than the trade offers.

-- Regards,

John Stevenson Nottingham, England.

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You will find that as sent out most parameters are best left as is. The hardest parameter to find and change is the one that allows you to run on what is called 3 wire operation for remote stop start switch. The invertor manufactures have a contest going at the moment to see who can make the manual as obscure as possible.

I had to set a bit 25 kW Siemens one up the other month as the electricians at this company couldn't understand the 28,000 page book supplied with it. I couldn't either and had to ring Siemens tech support who also couldn't answer the question.

I was told no one runs 3 wire operation anymore. It took 3 hours to find out and what a job.

Somewhere I have a parameter list of the Altivar invertors of just which ones need changing.

It's fine to over rate them as in the parameters there is one that is linked to motor power and this acts as an overload so even if you have a 1hp invertor and tell the invertor the motor is only .3kW after a short period of overload it will trip out.

-- Regards,

John Stevenson Nottingham, England.

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I think that you'll find that that parameter is to help the box translate between hp/kW and amps for the purpose of overload protection. There are variants of the ATV 11 invertors that will give higher output voltage than supply voltage but, unfortunately, RS don't stock them... ATV 11*U**F1 model numbers as opposed to ATV 11*U**M**.

It won't be a problem though, just wire the motor delta and off it goes.

Mark Rand RTFM

It can't be difficult Duncan - I've converted about 5 machines to run on inverters - Never had a problem.

Charles

Duncan

I would be very happy to recommend Newton Tesla, who supplied a Mitsubishi inverter and control box for my Denford shaper# - usual disclaimer, they were very helpful indeed, particularly in respect of how to wire the machine guard microswitches. They sent a drawing and put a loop of wire onto the appropriate inverter terminals with a sticker indicating why it was there. I am a walking advert for their quality service. #This is a six terminal three phase motor switched from star to delta and now running nicely from the domestic supply. I didn't know anything about three phase before (or much now), but this exercise broke the spell and I now have three other items which run off a Transwave phase converter. They were also very helpful, but I realise that isn't what you're after.

Steve

There were a couple of menu items in the handbook linked to current (and overcurrent) ratings. I suspected that it was possible to use a lower spec motor than the max output of the inverter, but TVM for confirming :-)

Mark, you're spot on - I just had a look at the model numbers and it's the ones with M near the end.

Will do!

Hi Colin, I had a look at drives direct, here's the URL for anyone else who wants to check out the website.

For a 1hp drive, they are looking at £120 against the £100 of RS which is odd as I normally associate RS with being expensive....

Hi John, this is the message I'm getting - the ATV 11 seems to have a lot of goodies on it for not much money. It's a safe bet that I'll be ordering the 0.75kw ATV11.

From the info from yourself and Mark, I will have to rearrange the windings to 240/Delta which would be 140V phase to (non-existent) neutral?

Forget neutral, you are complicating the issue. Two mains wires in, three 3 phase wires out on the power side.

-- Regards,

John Stevenson Nottingham, England.

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Hi Charles, I'm getting a lot of good feedback here and a lot more confident about the project than I was 24hrs ago.

That's one of the nice things about this group, plenty of skill and experience on it :-)

Hi Steve, everything I see about Newton Tesla is good press. Their service is obviously A1 quality but having looked at adverts in the model engineering press, the only bad thing I can say is I thought their prices were quite high.

There is probably a very simple explanation which is so obvious that I cannot see it but...the AVT11 series you are looking at doesn't appear to have RUN/STOP buttons or a variable pot for frequency/speed control. (Maybe that's why it's relatively cheap)

For ease of use I would have thought it would be better to buy one with the bits on the front.

John B

Not necessarily, To use an invertor with the bits on the front requires the invertor to be mounted somewhere handy. Most of these are plastic cased with vent slots and if it's handy chips can get in. Second point is that the ones with the bits on the front like the pot are usually only meant for setting purposes and the pot is usually quite delicate, special and part of an internal non serviceable board. So if some ham fisted savant happens to knock this you are either an invertor up the duff or you have to go to plan B anyway.

The ATV11 invertors are designed for a cheap method of providing variable speed, a method to run three phase equipment on single phase or run at a predetermined speed that's then locked, conveyors come to mind here.

To use the ATV in this application you would switch the mains input on / off with a standard motor contactor or decent relay. Speed is increased / decreased by the arrow keys on the front, remember these are primarily setup for constant speed change.

For most home setups these need to be mounted away from chips getting in, either in the machine base or a box above and out the way.

In this case, plan B, you would bring 6 small wires out, small as in they have to handle 10 volts at milliamps so very safe. three wires replace the pot or arrow keys and are wired to a cheap but robust pot that's easily replaced if it gets damaged. The remaining three are common , start and stop and can even be wired into the existing buttons on a machine if so required. If an extra wire is brought out you can have reverse on a separate button.

One word of warning on some of these invertors, READ THE SMALL PRINT as to what's with them and specs. With some of them, Mitsubishi, is good at this, the programming panel is an extra and can cost another £40.00, same for filters, at least the ATV's come complete.

They don't take any wiring and most competent persons can do this, 3 phase motors are cheap, even new, look around and there are some good deals on modern 6 wire Euro motors, you can get everything for doing a conversion, even buying all new and still get change out of £200.

Be wary on secondhand invertors unless buying very cheap, many are already at the £80 mark and new with a guarantee is £100 ? - no contest.

-- Regards,

John Stevenson Nottingham, England.

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Should read remember these are *not* primarily setup for constant speed change.

-- Regards,

John Stevenson Nottingham, England.

Visit the new Model Engineering adverts page at:-