3PH Speed control, what Pulley?

I bought a small wood turning Lathe some time back from Axminster Tools. It came fitted with a three phase motor, inverter, digital readout and stop/start forward/reverse speed control pendant. It also has three pulleys on the spindle and motor to provide a speed range of 0-800, 200-1750 and 400-3600. I have so far only used the machine in the middle range. It's not very convenient on this particular machine to swap belts so I'd like to avoid it. It would like to make use of the highest speed at times though so I'm thinking of setting the belt on the highest range. Will it really be detrimental to run the Lathe at speeds lower than 400rpm on this pulley setting? Folks keep saying how useful three phase speed control is but I'm not so sure if I have to continue slipping belts?

Your motor is likely to be running slower when turning large heavy stuff which puts the most load onto it so you will have to watch that it does not get too hot assuming it has the fan on the motor shaft.

It may also have a job starting if you have a big lump of green wood in the lathe as there is a lot of weight to get moving compared to when doing spindle work. If it were mine I'd select the best speed range for the job in hand eg 0-800 if doing a big green wood bowl to the fastest range if doing a pen

Edited By JasonB on 04/12/2015 13:32:05

The worry for people using low speeds with inverters and 3Ø motors is motors getting hot. This is only likely to happen if they are running slowly and under a high load. The other worry is low torque at low speed. I would try using the motor at low speeds and see if it gets warm under normal operation. If it stays cool then don't worry about that aspect of slow running. As for low torque, if you are manually operating the cutting tool you control the forces on the motor, if it starts to slow down you will know straight away and can back off to get the speed back up. If the torque is too low to be used then you will have to revert to the slower pulley set-up.

In summary try it and see how it works for you.

Martin

As Jason has already said, keep on eye on the motor temperature when its revolving slowly, the fan won't be pulling in the necessary cold air to keep it happy. Is it possible to fit an ex computer fan to help things along ? I have an old inverter on my lathe and the motor has never got very hot when I've had it running on slow speed screw cutting etc. I do know that industrial machine when fitted with inverters often have an extra built in fan to keep things from over heating when the motor is running at slow speeds. Give it a try and keep popping your hand on the motor. Modern motors run very hot compared to the older ones. John

In general terms it's the torque that causes the heating, not the power, as torque is proportional to current. Magnetising current and eddy current losses are generally small. Unless you are bogging the motor right down for extended periods, I doubt you'll damage anything.

The windings of most modern motors are usually Class F (155C) or H (180C) these days. Pain threshold for most of us is around 50C ie only 25-30C rise above room ambient. Although the case will be quite a bit cooler than the winding hot spot, it could still be filthy hot to touch but entirely happy inside.

If you plan to make a "massive green bowl", it may be worth changing the belt anyway.

Typically when people show the revs being slowed right down they only do work with it for 10 of seconds rather than many minutes.

While talking to Brook's and Mitsubishi they did admit that some people do burn their motors out. They are a bit cagey about it. Some of the inverter retailers say everything will be ok. TEC produce charts in one of their catalogues that show just what anyone who knows about the subject would expect.

I did get one interesting but unofficial comment from Brook's. They doubt if one of their 1400 rpm motors would burst it's armature at 2,800 rpm. Many people who convert lathes from single phase fit a more powerful motor so that the torque is increased from what it would be when the speed is reduced. This doesn't help with overheating other than a bigger motor may take longer. I up'd 1/2hp to 1hp and also went for a 6 pole motor. I also changed the motor pulley to suite what I need and set the inverter for 40 to 80 Hz.

Hopefully it's bomb proof. It is very easy for me to change the countershaft belt setting though - if I need to also put the back gear in.

John

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Edited By John W1 on 04/12/2015 15:31:47

Thanks for your thoughts so far. Its not so much the size of the item being turned - I've seen some pretty large bowls turned at high RPM, but balance. Out of balance workpieces can be turned at slow speeds until material removal improves the balance. I'll try swapping the belt and keep a check on the motor temp.

Vic:

I'm a woodturner, too, and I sympathise with your wish to avoid changing frequent pulley changes. Forgive the obvious question but is there anything you can do to make changes easier? It would help to know what model lathe we are talking about.

Hi Vic

Forum members have pointed out the pros and cons of a variable speed drive.

As john indicates that a six pole motor will give better performance at it's lower rated speed of 950 rpm over a 2 pole 2850 rpm motor.

On any variable speed motor be it AC or DC an Auxiliary fan is needed if running on low speed.

A large computer fan run from a 12 volt DC plug pack is a good place to start.

As ega requests that more information will help and photos are invaluable.

My wood lathe is a Bitsa with a DC motor and variable speed controller .

Have a look at my Album "Lathe Clutch" it may be of help.

Eric

Not had many natural edge green oak blanks around the 15kg mark on your lathe then John

Takes a lot more than 10 seconds to get them any where near balanced and roughed out

As a point of interest the motor plate says 230v 3PH 1450rpm.

Must be a six pole motor then, the natural speed of a 3 pole motor with 50Hz supply would be a small percentage below 3000.

Martin

Errr, poles are specified per phase. The theoretical speed for a two pole induction motor (at 50Hz) is 3000rpm. The motor referred to by Vic will be a four pole machine. Being three phase it will have 12 actual windings, ie, four per phase.

Andrew

Is that good Andrew? Can you tell I know very little about such matters!

Oops, you are of course correct, I'm mixing up the motor in my lathe (S&B model M) which is six pole with speed when supplied with 50Hz is somewhere around 1000rpm (speed plate figure without backgear and centre pulley about 3:4 ratio is 752rpm). The one I put on my mill which was 4 pole to try to keep the speed similar to the single phase one on it that it replaced having an rpm about 1500.

Martin