Motor speed control on AC motor?

Hi!

I have this linisher linked below. I am getting on well with it now that I'm working out which belts work best for different tasks. There are times where I feel like running it at a slower speed would be really useful though.

It's 3KW and almost 3000RPM so when it's on, it's really "on".

Does anyone know if it's possible to add some sort of speed control to a motor like this, without damaging it or shortening it's life? I wouldn't want to abuse it in anyway, so if it's not possibly to do safely I'll just have work around it.

Thanks

http://www.axminster.co.uk/axminster-engineer-series-150mm-belt-linishers-ax942736

If your is the single phase version than the speed of that type of motor is not changeable. If you have the three phase machine then it may be possible to wire it to suit a VFD.

How important being able to change the speed probably depends on the type of use the machine is put to, I have a variable speed drive to my small linisher and it is worthwhile for me.

Ian P

Hello Richard,

Sadly the answer is no and since the motor is directly coupled to the drive drum there isn't a chance of pulley reduction either.

Having used a big industrial version in the past, the belt speed sounds about right to me. For metal removal you need rapid cutting to reduce the effects of heating of the job. I think you will get used to it and used intelligently it will dramatically cut down on the need for files.

Regards Brian

Surely it must be possible to reduce the speed of an AC motor? Either by lowering the voltage or using a 1 ph VFD, depending on the motor type. Changing direction is another story but that is not the point here.

NO ! see ten thousand other threads on here on this subject. EVERYTHING is explained painfully, in depth on every one of those threads.

There is one answer, but it means changing the motor to a 4 pole 1 phase 1450 rpm unit; the present one will be 2 pole.

But at 3kW it would be a very expensive change and the frame size and flange mounting could be different to complicate the issue. The shaft is probably extended on the present motor to carry the drive drum so that too would be a further complication.

As I said earlier, Richard will get used to it and could even take in paid linishing work to cover the outlay!

Brian

Thanks Brian. Yeah, I think you're right. I'm still learning to get the best out of it to be honest, and I wish to also make some different jigs and tables/mounts for it to make some operations easier and more consistent.

I've only just tried ceramic belts on it and was amazed and how much quicker and cooler they cut, which is especially important for pieces that are already heat treated. I guess I'll have to learn to adapt!

Slightly off topic

The Alcosa blower on my forge was 1940's with a wall mounted rheostat . 240 ac .On the first stud it would the motor would turn really slowly ,just enough to keep the fire alight at lunchtime, and flat out on the 5th stud, would scream and blow coke out the fire and across the forge floor ..The motor had brushes .. What was regulated , the amps or volts ? The motor bearings died in the end , and i flogged the rheostat on ebay for £££ ! It gave sterling service for 70 +years  used daily 

The current fanset i use is constant speed with a slide air regulator ..the type with a cap , so i couldn't use the old rheostat ..

Now... These brush motors on variable speed angle grinders and routers... they go down to zilch rpm's too ..if i grafted one onto the fanset , would it run week in week out like my old one but using the built in electronic speed control?

Edited By Hacksaw on 23/01/2018 18:25:42

I know the conventional opinion is that single phase induction motors cannot have their speed changed, but my lathe uses the same motor for both 50Hz & 60Hz working with a consequential speed difference of about 20%.

Now, I wonder what would happen if a frequency of 40Hz was also available. Would we get a speed reduction of 20%? Yes, I know about the cooling fan. And possibly the centrifugal switch. But, if it were possible then I could have a total of 27 speeds, although, it has to be said, there would be some overlap which would probably make it a waste of time. Nevertheless, at the extreme ends of the speed range I would have an increase from 2000 to 2400 and a decrease from 125 to 100rpm.

I offer this as a discussion point only.

Peter G. Shaw

I have an old "Variac", I have used it on various motors it just adjusts the voltage to reduce the voltage to give a lower speed, I am not an electrical person but would not this work? Haven't blown anything up yet

Mick

what was regulated

The field current, I expect.

I have varied the speed of a "Universal" (i.e. AC/DC motor with brushes) by using a Variac. Not ideal but bit sufficed for that task.

A single phase, (or 3 Phase) motor speed is determined by the frequency of its supply, so unless you can vary the frequency, I fear that the answer has to No. Reducing the voltage will reduce the power, but not the speed

Those with greater electrical knowledge than mine will agree, I hope.

Howard

Beg to differ but it can be done - there was a single phase induction motor speed controller project / kit designed by silicon chip magazine back in april 2012 that they have recently revised to control the speed of a lathe ( latheboy -silicon chip jan 2018) origionally they stated it couldn'tbe used on motors with start winding switch but they have now re - visited this and shown how to wire the motor up to the controller so it controls the start winding instead of the centrifugal switch .

Whether or not this would be an economic approach would depend on each persons circumstance , availability of the kit and ability to build it , the kit fro memory was around $200au - i personally would go the 3ph + vfd / inverter approach as you can buy a vfd and motor relatively cheap from china .

I can't reproduce any of the pages on this project here - copy- right infringements .

Ian.

Why buy additional equipment to fix the problem of having too large a motor that runs too fast when it's cheaper and more elegant to get a 4 pole motor running at 1500rpm.?

Martin

If you must have variable speed then you need to vary the frequency of the supply.

Martin

Single phase motors are designed to operate most efficiently at the rated frequency. Large deviations are more of a problem than three phase equivalents, one presumes. Motors for either 50 or 60Hz are, I expect, happiest running at 55Hz and any further deviation than ±5Hz would likely compromise the design.

Re buying a 1425rpm motor - likely the OP wants a rather smaller incremental speed change than 50%.

You may be able to identify a VFD (one that doesn't use flux vector control for starters) that can tolerate an imbalanced load on just 2 of the phases. This would be able to run a single phase induction motor between 2 of the phases and thus vary the speed.

One major problem with single phase motors and VFDs is how you would start the motor. Single phase motors don't generate a torque at stall without some additional bits such as a starter winding driven through a phase changing starter capacitor. They won't work so well at low speeds (they weren't designed to), so you'd have to start at base frequency (50Hz) and then vary the frequency from there. But you'd also have to hope the motor starter circuit wouldn't be upset by the different supply frequency.

Lots of unknowns here. By the time you've got a VFD, you might have simply got a lower powered machine in the first place?

Murray

Of course they are. So change the motor to 3phase and fit a variable frequency drive. But I guess the OP doesn't really want to go that route.

regards Martin

.

Very interesting, Ian

**LINK**

http://www.siliconchip.com.au/Issue/2018/January/The+Lathe-E-Boy%3A+high+power+Lathe+Controller+?res=nonflash

Thanks

MichaelG.

You conveniently left of the part off my quote regarding 5Hz being about the most a single phase motor might put up with. An earlier poster mentioned dual Hz motors being OK at either European or American frequencies. Many motors specify only one frequency (ie need a different design for the 10Hz difference), so I would think that more than a 10% frequency change is not permitted for any one design.

Edited By not done it yet on 24/01/2018 09:37:14