Inverter

Can you tell me your opinions please on constantly stopping and starting 3 phase motors run on modern inverters , e.g. lathe with no manual clutch, I thought they would be ok but a motor rewind specialist told me no motors, single phase or three please like being stopped and started constantly, maximum 10 - 15 times per hour. But thinking about it aren't most modern three phase lathes stop and start motors without an electro magnetic or manual clutch ?

Edited By Paul Scholey on 12/03/2019 13:15:32

The three phase motor on my lathe (running on true three phase) gets stopped and started more frequently than 10-15 times an hour. And the motor on the repetition lathe gets reversed and swapped between numbers of poles (and hence speed) hundreds of times per hour. All with no apparent problems. Ironically my only machine tool with an inverter doesn't get stopped/started very frequently, as it's on the CNC mill.

I would have thought you'd be even better off with an inverter as it should control the inrush current, unlike starting direct on line.

Andrew

There is an element of truth in the number if starts for a motor....1ph or 3 ph.....This is because of winding temperature......we had some large motors (110kw) driving large fans, these were limited to 3 starts per hour...but were dol motors....the large starting currents causing the motir windings to heat...

In our applications where the motors are driving basically unloaded, via a vfd, the issue doesn't raise an issue....personally speaking...

Other users may have a differing opinion, but I wont lose any sleep worrying sbout my motors..

I think your rewind specialist's comment is in the 'not wrong' category. What he says is true of all machines, lights and electronics - starting and stopping is far more stressful than cruising. Leave a car engine running continually in a test-rig at 3000 rpm and it will do the equivalent of a million miles before failing.

In the real world starting and stopping is unavoidable and equipments are designed to cope for a reasonable amount of time. Nothing lasts for ever, maintenance is required, and it might be necessary to replace damaged parts. Electric motors are good at start/stop running compared with the alternatives, and being simple 3-phase motors are particularly reliable. An inverter further reduces stress because the electronics can start the motor up gently.

Inverters are all good good news except pre-1970 motors may not take kindly to them. Older designs are vulnerable to pitting of the bearings due to allowing current to flow to earth through them. Also their insulation is less resistant to voltage spikes.

Dave

Your motor rewind friend is correct, stopping is no problem, its that sudden rush of current when starting which cause the heating up of the windings. Over heated motors (cooked) keeps him in business. I've seen on some motor manufactures publicity information, where they stipulate maximum starts per hour. I note that most home work shop lathes of Chinese extraction haven't got a clutch. I think some of the smaller Colchester lathe were the same. I'm surprised that they don't damage the gear box internals with the sudden ON. Clutch takes up the drive gentle, no sudden ON. John.

Can be less of an issue with inverter drives as they generally implement a 'soft start'. On the other hand if you run the motor at low speed without additional cooling, it may be more prone to overheating - some inverters have a 'low speed protection' setting where the 'I squared t' trip threshold gets automatically reduced at lower speeds.

So an inverter may well make overheating less likely, but you should bear the issue in mind and perhaps review the config settings.

Thanks for your replies, I have a Harrison 140 with a top speed of only 750 rpm and would like something as well as the Harrison with a higher speed, I have been looking at Warco but comments seem to vary on the satisfaction,

What make of inverters would you recommend if you have fitted them and are satisfied, bearing in mind I am not a professional for wiring it up, does forward and reverse cause a headache wiring it up ?

I have had an inverter fitted to my Myford ML7R for 5 years with no problem.

Currently it is set up so the motor runs at full rated speed [to keep the air flow cooling] and I use the belts to change the spindle speed. The inverter is set up with a soft start [5 sec to speed] and a dynamic stop [1sec] .

When doing threading there is a lot of stopping / starting / change of direction with no ill effects.

The inverter is an IMO Jaguar CUB

Steve

In the real world of a production facility motors can be stopped and started hundreds of times an hour, motors on conveyors and hoists are very commonly run with inverters to get accurate positioning and smooth mechanical operation. Obviously the equipment is specified for the task and duty cycle but I think the motors were standard continuously rated machines. Inverters now produce quite nice sine waves and do not heat the motors with poor waveforms so with a current limited soft start they stay smooth and cool. I doubt that a decent quality motor with a properly set inverter could be worked hard enough in the home workshop to run into problems. Starting single phase motors is another ballgame and it could be rather easier to push it beyond its limits.

Mike

Paul,

I've been very happy with the HuangYang VFD that I purchased last year and the TEC 3-phase motor that it drives (admittedly on my mill, not lathe, but the priciple is the same). Being an eletricity numpty, I also purchased a ready-made forward/reverse switch with speed pot incorporated. Was a doddle to wire it all up, just a couple of niggles with the VFD settings, but nothing that wasn't easy to fix. Mill is now a delight to use and I wouldn't hesitate to fit a similar set-up to my lathe as well, when funds permit.

John

P.S. I made the most of the motor change by taking the opportunity to fit a more powerful motor. Something I'll do with the lathe, too.

If you feel you might need help getting things going, either stick with a main-line European brand (they tend to have real people that you speak to in the UK who will help if you get stuck - adjust wording for your part of the world), an alternative is to buy inverter and control pendant as a package from one of the specialists who'll potentially do the configuration before shipping the inverter to you.

Thank you for all your replies coming in, very much appreciated

Paul, you say your Harrison 140 has a top speed of 750 rpm, is that the limitation of the lathe or that of the drive motor on your lathe. I fitted a HuangYang VFD to my friends Harrison 140 and I'm sure his went faster that 750. Anyway, providing the lathe will stand higher speeds than 750 you can alter the inverter program quite easily to alter the speed upwards and we used the existing controls. On John's 140 it was a 10 minute job to alter the motor from Star to Delta. I fitted a capacitor to the pump so that it now runs from a 13 amp socket. Nice lathe I'm envious but can't fit one in. John

John, I fitted a single phase motor to my Harrison some time ago, it struggles a bit at 750 rpm when it's cold because I put a thicker oil in than it should have because the gears are a bit noisey

Hi Paul,

I currently have 2 Warco lathes (have had others and a mill from them in the past), a GH1330 which I've owned for about 14 years, fitted a 3 phase motor and and IMO inverter. Been very happy with the lathe, no problems with reliability or accuracy. I also have one of their newer WM250V lathes which come standard with 3 phase motor and Delta inverter. Again very happy with the machine.

I buy a lot from Warco as they are quite close to me and I have always been very satisfied with their service

Thanks for that Gray, interesting !

Paul I have a ( very noisy ) gearhead lathe and likewise have thicker oil in the gearbox, at least now I can run it without having to wear ear defenders, mine struggles also when the oil is cold. In cold weather I run the lathe for a few minutes at high speed with no load just to warm the oil up.

Dave W

The manual for the Jaguar Cub inverter I have on my Myford says,

'For up to 10 starts (max) per hour the inverter may be controlled by switching the unit ON & OFF by a line supply contactor only, for up to 10 starts (max) per minute the electronic RUN / STOP input on terminhals 10 & 11 should be used. In either case the motor will freewheel to a standstill and ramp up to set speed following a 300 millisec (approx) delay.

John

I have made literally dozens of starts an hour on my lathe and milling machine when cutting threads, with no ill-effects. That's on single-phase, 3-phase and DC motors. I would not worry about it at all. If you have a VFD then set a gentle start ramp and you'll see on the display that the amps don't go much above the no-load running figure.