Myford Motor help

Question for JS really as he seems to be the reisdent expert.

I now have my variable speed system working fine and no longer shutting down the house on RCB nuisance tripping.

The motor is a typical 50/60 Hz .55 kW job ) maker - 'Sever') and is driving a ML7. The inverter is a Eurotherm 610 0.75kW (£46 inluding postage from ebay if anyone is interested)

What is the maximum speed I should set do you think using standard Myford pulleys? I have set 65Hz at present but dare I go higher or perhaps immediately set it lower. I'm as concerned about the Myford bearings as much as motor overpeed

Thanks for any info

Also I now have a spare 1/2HP 'Tuscan' 1 phase motor suitable for Myford if anyone wants it. It is Flameproof Gp 11/111 so weights a ton and a half. Collect from stamford, modest offers in cash or kind invited. (email webmaster (at) vmyg.org. uk)

Right Henry we'll use your motor / invertor as an example. One thing you have missed supplying is speed of motor but knowing what it's used for I'm 99% its a 1425 rpm or similar. Makers get idle marking 'true speeds'

This motor will be running on 12 mm 6201-RS or 15mm 6202-RS sealed bearings. The same bearings it's two pole 2,800 rpm motor will use so we know 2,800 is not excessive. According to the bearing book 6201 is good for 15,000rpm and 6202 is good for 13,000 rpm so bearing are not a worry. Next worry is whether the rotor will explode, well we know they will go to 2,800 or 3,400 if supplied with 60 Hz so we have a known top limit. In truth the top limit is unobtainable in the home shop [ more later] so don't worry.

The actual limit is they way the motor is made and it's ratio of iron to cop per in the stator. Ideally these are designed to run at 50/60 Hz but can go higher, just how higher can only be determined by experimentation.

The motors top limit is determined by this copper to iron ratio reaching saturation point as it's know. To us erks that means it gets egg bound. different motors will have different characteristics, modern light motors like the Sever will go higher that the big old bulking Brookes.

What you have to do is set for say 80 Hz and then take the motor up to top speed via the pot, does it sound Ok? then it is. Gradually go up in 10's until you will reach a point usually between

90 and 120 when going flat out will start to sound rough. back off 5 or 10 and that's you top limit. Going higher will actually cause the motor to run slower, sound really rough and get hot.

Most motors will handle 90 Hz with no trouble.

Now whether the spindle of the ML7 will handle high speeds is another matter. I have heard of the white metal ones running at 1300 with moly oil on production.

As a point of interest I have had some new 1/2 horse 2 pole motors rewound with more turns of finer wire to handle higher HZ. These were done as an experiment to see if we could fit standard motors as high speed motors on wood working equipment. We were able to get up to

12,000 rpm at 200 Hz with no problems. Altogether 5 were done in total and all worked OK, in fact I think they are still working.

-- Regards,

John Stevenson Nottingham, England.

Visit the new Model Engineering adverts page at:-

Curiously the Teco website and associated downloadable manuals suggest that the 7200MA inverter is available as 220V single or 3ph input and 220V 3ph output _OR_ 440V 3ph input and output. There is no 220V single phase input/

440V 3ph output version.

It also indicates that this range is end of line and will be replaced by the

7300CV range, which also use the same input and output voltage combinations.

I don't intend to blow £900 on this anyway, but any prospective bidders would do well to ask the seller for more details :-|

Mark Rand RTFM

Yes, provided your motor came off a fairly modern shelf.

Older shelves can yield motors that are fixed star wired and need 440v 3 phase supply.

Bob

John,

Thank you, that was very interesting and helpful. In practice for my application it means that I need not worry about the motor, just set the maximum so that the lathe spindle speed is 1200 rev/min and with luck all will be well.

Shame though, I quite fancied that 24,000 rev/min on tap.

HS

Replying to my own post (is this the first sign of madness?)

Do I need a special motor wind to use a 240v "output" inverter? Would a 415v motor work at reduced power?

And the other being ... ?

:-)

Mike

Well, thanks to Clockie (John), I now have a new motor which is bein

fitted today. You're a gentleman sir and I much appreciate you generosity.

For everyone's interest, I emailed Tyco/Crompton my woes last week an received the following possible solution this morning:

-"From you explanation it seems that the start circuit is no connecting & is probably due to the internal switch, which is mounte onto the terminal board. If the switch is the problem, the best way t solve it is as follows:-

DIS-CONNECT & ISOLATE THE MOTOR FROM THE MAINS SUPPLY

1) Remove the terminal cover plate 2) Locate the cross-heat screws that mount the terminal board, thes will be at about 2 o'clock & 4 o'clock. 3) Place a cross-head screw-driver in the screw head & firmly tap bot screws to move the board towards the stator. This movement only need to be 1-2mm. IMPORTANT NOTE: Do not move it to far as this will not allow th internal switch to move far enough to open the starting circuit, whic will damage the motor very quickly. The effect will be a very noticabl electrical buzzing causing a high level of vibration. 4) Re-connect the mains supply & re-fit the cover plate & switch on th motor. Does it now operate correctly?

If the motor does not run correctly, please telephone me on 0130

812650 so we can discuss other solutions."-

So if this is a common fault (which relatively speaking, it appears t be) this is worth trying. I might have a fiddle later on just for m own satisfaction but I won't hold my breath

-- DX-SF

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