Why are 3 phase motors with VFD so popular in the UK?

PS Just checked my drill press. It is actually 970rpm that I leave it set on for 90 per cent of the time, not 800 as I said in earlier post. Seems to be the sweet spot that suits all drills 1/8 to 1/2". For 5/8 I might drop the belt one notch to 800 or so.

Seems like there is a business opportunity for an enterprising fellow in Oz.

As Michael said, the first thing you notice is that the motor runs more smoothly. Single phase motors have an inherent torque ripple as the rotor interacts with a contra-rotating stator field component. This can produce an audible throbbing. Capacitor run motors can be better as they are more true 2-phase. On my Super 7 the highest speed range, i.e. largest motor pulley, was essentially unusable because the motor vibrated so much. I fitted a 3 phase motor and Neston Tesla drive and it transformed the lathe.

Yes, domestic supply is one phase of a 3 phase supply. 3 phase is used throughout the generation and distribution network, mainly because it permits a significant reduction in cost for the same power, using less copper and iron. Also incidentally it enables more efficient 3 phase motors which use less material for the same power.

If you only have a single phase house supply VFDs can generate 3ph for you at much lower cost than having a 3ph mains feed, and incidentally gives variable speed.

To all intents and purposes, VFDs are not applicable to single phase motors. Yes you can kluge them to work, but the speed range will be limited.

3ph motors with VFDs are becoming preferred in all sorts of applications that would once have used "universal" series-wound commutator motors, for example washing machines. They use less material and the VFD permits variable speed. Modern drive electronics can be very robust - though a lot of DC motor speed controllers used in low cost machine tools seem to regularly go wrong due to poor design. Interestingly Tesla cars use induction motors and VFDs but not as we know them.

Your drill press if run from 3 phase would allow you to set the right speed every time. I fitted a 3ph motor to my Myford VMB mill partly because the belt change arrangement was so clunky. I eliminated one step pulley and got variable speed and reverse. That means I change pulleys much less frequently. It also means that I can set the spindle speed very low and use the machine to tap a just-drilled hole in exact alignment; then reverse the tap out of the hole.

When I was young younger, transistors were called 'three-legged fuses' because they were so delicate. Mounted correctly in a circuit they were actually considerably more reliable than valves/tubes, but they couldn't tolerate circuit malfunctions. And because components like resistors, capacitors, insulation, ferrites etc were all relatively unreliable 70 years ago, electronics developed a poor reputation!

Time marches on! Can't speak for what puts US cars on the scrapheap, but over here it's rarely the electronics. Mechanical failure and corrosion do for most UK cars after about 14 years and 120,000 miles. The EMU, radio, and other electronics are more likely to stop working in the crusher than on the road.

Nowadays electronics are everywhere, and generally more reliable than mechanical equivalents, even when high-power is involved. Computers were an early example: although analogue computers got fast results, their complex delicate mechanisms were unreliable and demanded continual maintenance.

Spikes are a concern, but not if a modern motor is permanently connected properly to the VFD. If a wire falls off, the winding inductance is liable to act like an ignition coil and generate a high-voltage capable of puncturing ordinary insulation, but electric cabling of any type should be secure!

Spikes are more dangerous to old electric motors, i.e any made before about 1970. One problem is their enamel wire insulation, which wasn't thoroughly debugged before 1960. A long development requiring improved chemistry and better ways of drying the insulation that didn't leave pores and microcracks. More serious was a design oversight that allowed leakage currents to spark inside the bearings. The fault only showed up when statistical analysis of returns revealed that bearings fitted to electric motors and generators failed much faster than the same bearings fitted to non-electric machines. After a long search it was found sparking left tiny craters in the bearing that became a focus for rapid mechanical wear. For that reason, it's risky to run old motors on VFDs. In hobby use, motors do so little work that the bearings last a human lifetime.

Trouble with staying old-school is it's downright risky. As ways and means age they gradually become out-of-date, ill-informed, and ill-adapted to changing circumstances. And what is 'old-school'. I guess what Pat considers 'old-school' is dangerously avant-garde compared with what went on in 1900, when the 'old-school' methods of 1850 were failing comprehensively!

Bad enough we have to use today's methods to solve tomorrows problems. Positively unwise I think to rely on 20th century methods to fix what future generations are facing.

Dave

Like Hopper, I seldom feel the need to be changing the speed on my drill, and anyway it takes only a moment to change, so no big deal. I like to keep things simple and uncomplicated [less to go wrong ] and if I dont see any real advantage, I dont change. I agree 3 ph motors are smoother running, but I could live with single phase if I had to. Some of my machines are single phase anyway, it,s just that before my last move, I had 3ph for some bigger woodworking machines and when I moved to the sticks, I needed to provide for them, hence a converter.

I get the feeling that for many people, it,s just something they feel they "need" and "must have" in order to fit in.

No doubt I will be told off in no uncertain manner, but then, I,m used to that !

Could be. A very small market the Australian hobby market though. Still, surprising none of our major industrial/home workshop suppliers like Hare and Forbes have not started importing them. I have noticed that things like 3/4HP single phase motors are more expensive here than the same in 3 phase, probably because industry uses 3 phase so more are imported. So it would only be a matter of organising the controller unit side of it. If they were readily available as a plug and play unit, I am sure quite a few guys fixing up old Myfords and Hercuses (Aussie Boxfordy SouthBend clone) would do the upgrade to VFD.

This is my understanding:

A single phase motor produces zero torque twice every AC cycle of the mains waveform, when that crosses zero volts. So there will be a deep torque ripple of 2 x mains frequency.

Compare that to a 3 phase motor; As each phase AC waveform goes "over top dead centre" and torque from that winding starts falling, the next phase - 120° behind it - is approaching "top dead centre", so its torque is increasing, and so-on; So a 3 phase motor will produce torque throughout each rotation and have only a very shallow torque ripple, which will be 3 x the frequency of a single phase motor.

So a 3 phase motor will have much higher torque, delivered much more smoothly.

You can think of this as a being similar to single cylinder 'thumper' internal combustion engine compared to a 3 cylinder engine: the more cylinders, the smoother the torque delivery.

Not being a machinist, I don't know if the torque ripple from a single phase motor is likely to cause more chatter etc. at the cutting bit, compared to a 3 phase motor ?

As an aside, I wonder if the addition of a flywheel onto a single phase motor would help? It would smooth out the torque ripple a bit and provide a more gentle slow start, though not a quick stop of course.

Edited By John Doe 2 on 21/01/2023 13:09:03

My answer would be due to have arthritis in both wrist's the advantage of a VDF is 'reverse'

H

The effect of a flywheel large enough to reduce the acceleration of a single phase motor may be a large current draw until it is up to speed. This is not great for the wiring or the motor unless it has all been designed for such a system. Then when power is cut off the system will act like a generator until the motor is at rest so you may need a system designed to dump that energy into a resistor (a DC brake in effect). Turning a large lump of metal may cause the same effect, they can take a few turns to stop if you have a lathe with no brake and no clutch. That is why larger lathes tend to have both a brake and a clutch.

Martin C

Um. No. A single phase stator generates a static but alternating magnetic field that can be thought of as the sum of two contra-rotating fields of half the strength. Both of these induce currents in the rotor and in turn torque but in opposite directions. Once the rotor is turning the field rotating in the same direction is seen by the rotor as having a lower frequency and since the rotor current at 50Hz is limited by inductance, more current flows at the difference frequency. Likewise the rotor sees the contra-rotating field as having a higher frequency so less current flows. Overall there is a constant torque (once the rotor has reached nearly synchronous speed) on which is superimposed a torque ripple at the sum of the actual speed and the drive (50Hz say). The ripple is much smaller than the torque because the sum frequency is nearly 100Hz and the inductive impedance is much higher, so the current is much less.

Here is a basic example for a 1hp motor and a VFD to suit. This does not include the switches for remote control or wiring, so £35 should be added. Motor and VFD £225 which includes VAT but not delivery. This firm has a "quick start guide" for some VFD's which makes wiring, controls and programming very easy.

**LINK**

If you Google Newton Tesla they do prewired vfd motor control panel packages tailored for many popular lathes. I've bought 3 packages from them, not the cheapest but very helpful.

I went 3ph with VFD on my ML7. Much quieter than the original single phase motor. I can also run up to 2x motor speed but rarely do. Much handier is the ability to run at very slow speeds (down to zero) by just turning a knob - without fiddling with the belts - for such things as threading with tailstock die. (There's always been enough torque to do that for the small threads that I'm generally interested in .... I can always change belts as well if necessary for larger threads.)

Wouldn't be without it.

I’ve lived all these years in this world and never once noticed a torque ripple. I’ll have to start paying attention.

It's what makes the spanners rattle when you leave them in the drip tray of the lathe. My solution is to hang the spanners back up on the rack!

Not noticing isn't the same as not existing! But although single-phase motors are imperfect. they're not obvious rubbish either. The ripple tends to be minimised by spring in the drive belt and flywheel effect, and it's small unless the motor is pushed hard by taking heavy deep cuts. Do that with a difficult metal and take a close look at the surface with a loupe.

If I owned a single-phase Myford in good order I wouldn't bother to change the motor. However, if it needed a new one, I'd go 3-phase for less vibration, better performance and speed control. 3-phase motors deliver more torque too. I don't think anyone who has successfully upgraded to a 3-phase motor has ever regretted it.

3-phase and VFDs are pushing out single-phase motors for many commercial purposes. The motors are cheaper, more reliable, more efficient, and then VFDs save money with controlled soft starts. Big savings whenever a large number of motors stop and start repeated;ly, because the heavy current surge when a single-phase motor starts is a waste of money. A hobby lathe owner is unlikely to notice the waste, but a factory or building manager would.

Dave

Thanks John (Haine); I got most of that except for the constant torque bit at the end. Surely with only a single AC phase applied to the stator, (no capacitors etc) at the instant in time when the AC waveform passes through zero volts, the magnetic field will also - momentarily - fall to zero ? Or is it maintained by induction ?

Obviously the rotor keeps going owing to its own inertia, so it does not actually stop.

Considering all your misgivings it's better you don't have one and that's fine but....

Yes - I did that with an S7 and it's great - would be weary of exceeding speed though.

Very much agree.

Single phase AC motors are resiliently mounted, to reduce vibration.

It has been noted that under certain circumstances, a single phase powered lathe will produce a faint helix on the work. (Presumably caused by ma regular variation in speed )

As a simple mechanical engineer I think of a single phase motor (On 50 Hz mains ) producing 50 impulses per second. A three phase motor fed by the same 50 Hz supply will deliver 450 impulses per second, which suggests a much smoother delivery of power.

Industry has used 3 phase motors for many years, because of the smoother delivery of power

Presumably it could be argued that 5 hp 3 phase motor will draw the same average current as a 15 hp single phase machine. to save on capital and power costs.

A VFD usually delivers 3 phase power (often at 240 volts ) from a single phase 240 volt input.

Mine is fed via a suppressed socket, since I do not wish to cause problems within my household, or any other fed from the same phase, by any spikes being fed back into the mains. After nearly 20 years of use reported, no problems.

The motor concerned is 1 1/2 hp (Replacing the original 2 hp single phase ), and is Australian made!

Howard

Howard, power is volts times amps. For a 15hp 3 phase motor the phase currents will be 1/3 the current of a single phase 15HP motor at the same voltage. This (simplified) means that at full power a 1.2kW single phase motor at 240V motor will require a current of 0.5A in its single phase wiring. A 1.2kW 3 phase motor fed 240V (line to line as in a VFD) will require 3 currents of 0.5A/3 to give the same 1.2kW at full power. The current to the VFD will be slightly higher than the single phase current due to losses in the VFD.

Martin C