Single phase speed control - VFD?

Hi All

I'm sure this subject must already be covered somewhere but can I find it????

Basically what I want to do is reduce the speed of a recently acquired bandsaw

It's fitted with a small 0.7kw 2 pole single phase motor and runs far too fast (even on slow speed) for cutting mild steel. There doesn't appear to be a start capacitor fitted and a mechanical solution or change of motor would be quite difficult.

Looking on Google and U-tube it appears that it should be possible to run a single phase motor from a 220v 3 phase output VFD by splitting the start/run windings across the three output phases. Has anyone tried this and can offer some advice?

KenL

Theoretically it might be possible to start a single phase motor by connecting one phase from a three phase inverter to the main winding and another to the start winding. This will give a 120° phase shift between start and run plus whatever "natural" phase shift is built into the motor winding arrangements.

Sounds to me that it would be way too much and you'd need to put a capacitor or inductor in series with the start winding to adjust things to something more reasonable.

Oversimplifying things. Some phase shift between start and run windings is needed to generate a rotating field in a stationary single phase motor so it will run. Too much shift will limit how fast the motor can go with the start winding energised so the main winding won't take over and accelerate the motor up to speed. Too little phase shift and starting torque will be impractically small.

Once running the thing ought to keep going on the main winding fed by one of the VFD phases.

In practice a party trick with things set-up just so to do a demo. I imagine the VFD is going to be seriously unhappy at start-up time with one output floating and wildly different loadings on the other two let alone dropping onto one phase once all is going.

This style of idea has been floating around almost as long as VFDs have been around. To the best of my knowledge no one has made a commercial product of this type but I'd be unsurprised to discover that special, dedicated, motor / VFD pairs haven't been made for some applications. It would appear to be contender for things like multi-speed washing machine drives.

Clive

Clive

You can get single phase VFDs but they are uncommon. This makes them relatively expensive. Most are not suitable for capacitor or centrifugal start motors. Proper details of the motor would be required before anything else can be decided. Are you sure it's not a universal (brushed) motor? that would open up a lot of possibilities. They are common on machines with the motor integrated into them.
Reducing the speed will also reduce the available power.
Unless it is a particuarly special machine it's probably not worth the effort.
Why can't you change the motor to a 3 phase one? As I note above if it's highly integrated it may be a universal.
Just because someone on on the internet says you can use a 3 phse one does not make it so. The output would be severly inbalanced and would, or should trip the protections.

Robert.

I had a similar, but not the same problem, with my Arboga pillar drill. I don't have three phase power and the motor was two speed, three phase. The two speed motor made fitting an invertor drive difficult. I decided to fit a single phase motor but discovered that the existing motor has a shaft about a foot long with a gear machined on the end.

Problem was solved by swapping the existing motor fan for a pulley and then adding a single phase motor to drive the pulley. The new single phase motor now drives the machine using the old motor as a kind of layshaft.

I got the idea from work where we used a similar trick to drive oil pumps with failed bespoke motors with replacement standard motors.

You could possibly do the same but use an invertor powered three phase motor to drive your single phase motor.

Martin

Precisely my reason for posting here - thanks for your contribution, well informed advice is often advisable when considering propositions from sources such as U-tube.

I haven't yet dismantled the motor so can't be certain of its construction but it doesn't sound like a brush motor and although it's only what I would describe as partially integrated (custom flange fitted) it would involve quite a lot of work to fit a different unit.

I've ordered a bimetal blade to try which should withstand a higher cutting speed so I'll see how that performs and then investigate the motor more closely if needs be.

Any further observations would be welcome.

KenL

Just thinking out loud, and I don't have much motor knowledge, but would a 240V mains lighting dimmer work on a single phase motor, once it had started ? (probably have to be a pro theater lighting dimmer to handle a 750W load).

How do 240VAC domestic cooling fans with, say, three fixed speeds work ? Different, selectable windings in the same motor, or some electronics?

.

Edited By John Doe 2 on 21/09/2023 23:11:19

VFDs are (usually) used on induction motors. A single phase induction motor needs a phasing capacitor and a second set of windings. (O.K. a shaded pole motor doesn't but that only gets used on low power applications.) But you don't have a phasing capacitor!

So I think you have a universal motor with commutator and series field windings. A dimmer control should slow things down for you while generating significant RF interference in the locality for anyone still using an AM radio.

I guess you have a high speed wood saw.

Good luck, Colin

Adding extra pulleys is the way this guy attacked it - Pulley mods

Question- would a stepper motor be a suitable replacement ? Yes, it may be a bit "lumpy" at slow speeds, but would that matter ??

Nope. Dimmers effectively reduce the voltage applied to the lighting (or any other - even things like 3kW immersion heaters, for instance). The frequency is not altered and an induction motor is frequency controlled for its rotational speed.

Your existing motor is probably a split-phase type where the start winding has a considerably higher resistance than the run winding, so the current in it leads in phase to provide some directional starting torque. These motors often don't switch out the start winding, I guess the induced voltage in it once the motor is running reduces the current anyway so it doesn't get too hot. I briefly owned a bandsaw driven my one of these.

Fans in cooker hoods are driven by induction torque motors, which have a relatively high resistance rotor giving highest torque when stalled. You can speed control these by varying the drive voltage, they are very inefficient. As NDIY says you can't generally vary the speed of an induction motor by changing its drive voltage, only frequency.

A stepper motor powerful enough to drive a bandsaw would be expensive and incredibly noisy.

I'm afraid that you are stuck with the current motor if you can't physically change it. It's just possible that you might be able to find a 3-phase unit with the same basic frame and fit the end-plate, of even just transfer the stator and its windings (the rotor won't know if it's single or 3 phase). Or use the end plate to adapt a DC motor.

Ah yes, I forgot about the frequency.

So how do simple domestic fans - the reciprocating type you have on the office desk - change speeds? Do they have electronics - or just a plain three way switch? How do they achieve three different speeds with an AC motor running on a fixed AC frequency ? The motor in my fan looks like a shaded pole motor - no commutator or brushes. Since the motor - directly connected to the fan - is changing speed, it cannot be staying in step with the mains frequency. 

Would a single phase VFD work for this bandsaw, (or a 3Ph VFD, with two phases feeding dummy loads) ?

.

Edited By John Doe 2 on 22/09/2023 10:21:05

RE cooker hoods etc. Induction motors driving fans are a special case. Because the drag of the fan is proportional to speed squared, it falls off rapidly as speed reduces. Within a range of "slip v torque" of the motor characteristic, reducing the voltage does indeed allow a range of speed control..

This obviously does not apply to other types of load, where for more than a certain drag, the motor will stall (or not start).

Kenl

if you want to try a single phase inverter i have a few secondhand units in as new condition

Hmm, those appear to be 3 Phase output, single phase input VFDs. The OP needs a single phase OUTPUT VFD.

They are both single in and single out

Running significantly slow will reduce the effect of the cooling fan, watch out for overheating

Many thanks for that offer Ian.

Maybe you could message me with a price?

In the meantime the bimetal blade has arrived and performs much better than the carbon ones at the lowest speed. Only time will tell if these blades last a reasonable amount of time, they are substantially more expensive than the standard metal cutting blade supplied by the manufacturer of the saw (Elu/Dewalt).

I had a similar problem and wanted to control the speed of a lathe motor in as simple and inexpensive way as possible.

I bought one of those very cheap Pulse Width Modulator units from Amazon, it's rated at 230v input and 4000w output and cost about £5.

Wired a 13 amp plug and lead into the input and and a 13 amp socket into the output . The unit worked flawlessly, controlling the speed of the motor with no noticeable loss of torque, as it doesn't drop the voltage just the frequency.

I'm currently using it on a Sealey bench grinder for delicate tasks, just slow it down to tick over and it's fine.

David

I am very curious as to how a variable frequency 4Kw device can be made and sold at £5 and work flawlessly on (two different) motors.

What are the type of motors is it controlling?

Ian P

Ian,

I don't think it actually controls the frequency, it just chops the electricity up into variable length pulses.

I had it on a Unimat SL lathe with original 90w motor first, then same lathe with a sewing machine motor.

Now it's on a Sealey BG150WL bench grinder.

All motors 230/240 vac single phase.

I don't want to get into where and by whom, and under what conditions, it was made.

David