Can someone explain how a 1 phase motor is different from a 3 phase one please?

[Domdom]

I know they are different, and I know there are converters of several types, but I just don’t understand any of it.

For example, I have been after a pedestal grinder of old British vintage. All the nice looking ones with some age etc are all 3 phase, which I don’t have. I have no clue what to look for either when it comes to using one on 240v.

I keep reading about star, delta, rotary etc but none of it makes sense to me.

 

[Seadog]

Single phase uses one leg of a 3-phase supply. A 3-phase motor has three sets of windings, a single phase has one set. You need to buy a VFD to convert single phase to 3-phase.

If the motor is dual voltage then a 240v single to 240v converter is the cheapest option.

Most motors can be made dual voltage if you know what to do, bust does involve digging around it to find what is called the star point.

Enough info to be getting on with?

 

[Guineafowl]

Big subject - start with wikipedia.

Single phase can only produce a magnetic field that goes side-to-side, so it can’t make a motor spin. However, if the motor is already spinning, the side-to-side action can keep it spinning. So single phase motors need some way of starting.

Three phase has three wires, which peak in voltage one after the other, in turn. If you arrange these around a motor, they make a mag field that rotates, so a three phase motor will self-start.

Start here, then go down the wiki rabbit hole: https://en.wikipedia.org/wiki/AC_motor

 

[Guineafowl]

For your pedestal grinders, you can run them easily on single phase using an inverter drive, as long as the motor is dual voltage (240/415V or similar stamped on the plate). If they’re not dual voltage, you can get them converted.

First step is a look at the motor rating plate.

 

[minimutly]

So a single phase motor has some sort of mechanism to help it start (the smaller ones a centrifugal switch that opens once up to speed), larger ones a switch and sometimes a capacitor, faster spinning/more powerful ones may have two capacitors.
Vacuum cleaners, drills, grinders - the ones with brushes generally have universal AC/DC motors, ignore all I’ve written above for them, no capacitors needed, other than tiny ones to stop interference.

 

[minimutly]

For your pedestal grinders, you can run them easily on single phase using an inverter drive, as long as the motor is dual voltage (240/415V or similar stamped on the plate). If they’re not dual voltage, you can get them converted.

First step is a look at the motor rating plate.

I would expect even if they are not able to be converted, if you soft start them, once up to speed they should have enough torque to do most things. You’re running a 415V motor at 240V, it will be weaker, but for a grinder does it matter?

 

[Pete.]

I know they are different, and I know there are converters of several types, but I just don’t understand any of it.

For example, I have been after a pedestal grinder of old British vintage. All the nice looking ones with some age etc are all 3 phase, which I don’t have. I have no clue what to look for either when it comes to using one on 240v.

I keep reading about star, delta, rotary etc but none of it makes sense to me.

Obviously you're struggling with the concept so I'll offer you an analogy.

If you think of phases in terms of an engine's cylinders, you might find it easier. Think of that single-cylinder thumper engine, like a moped, or an old dumper or cement mixer. You start it up and it goes pop, pop, pop once every second rotation, it's not very smooth but it's a very cheap and popular format and it gets the job done. That's your single phase motor.

Now swap out that engine for a 3-cylinder one like a kubota or that horrid vauxhall corsa engine. Now you have 3 power pulses every 2 rotations instead of 1, the engine is much smoother, more refined. That's your 3 phase motor. The 3 phase motor has 3 separate windings, all firing separately but connected to the same shaft and timed 1/3 apart to give power pulses 3 times as often. This produces a very smooth torque delivery.

The single phase motor needs only one power input, just like the thumper engine only needs one carb/injector. The 3 phase motor needs 3 power inputs just like the 3 cylinder engine needs 3 injectors firing at 3 evenly-spaced intervals to the other engine's 1.

 

[Guineafowl]

I would expect even if they are not able to be converted, if you soft start them, once up to speed they should have enough torque to do most things. You’re running a 415V motor at 240V, it will be weaker, but for a grinder does it matter?

Yes, you can feed 240V to a 415V motor, just drop the VFD to 29Hz to keep the V/F ratio correct. Motor will run at about 2/3 speed.

 

[GRW]

I need to know more about motor configurations and start types/methods/connections myself.
I'm much more used to Alternators (though the principle, winding arrangements etc are similar basically - so I'm hopeful I could pick it up ok).

I can wire a (brushless, 6 or 12 wire) Alternator in "Delta" config and pick up 2 "corners" and utilise it as a dedicated 1ph machine, or instead wire it in Series-Star (12-wire unit) to get L1, L2, L3 off each of the 3 "Star" points, with Neutral off the centre point.

A 3ph supply has 3 lives, and each Live is 120 degrees out of phase with its neighbour. So rather like the very good engine analogy above - there's a rotational effect as each phase fires 120-degrees after its predecessor, much like the staggered firing order of a multi-cyl engine. This provides the effect of "pushing" the rotor round from one PH to the next, with each phase then benefiting from the momentum & energy input from the preceding phase.

(Also why 3ph motors take a much lower starting current than an equivalent 1ph motor).

 

[Screwdriver]

I know they are different, and I know there are converters of several types, but I just don’t understand any of it.

For example, I have been after a pedestal grinder of old British vintage. All the nice looking ones with some age etc are all 3 phase, which I don’t have. I have no clue what to look for either when it comes to using one on 240v.

I keep reading about star, delta, rotary etc but none of it makes sense to me.

They just ARE different ok.

If you have a 3ph supply then you're good to go.

If you don't have a 3ph supply and just regular single phase household mains, you're better off swapping out the motor.

So you are left with the question of whether or not it is feasible to swap out a motor on a particular machine.

If the motor supplies rotational energy from a belt, a shaft or some sort of coupler, you just need to find a motor with the correct speed/power.

Some machines particularly bench grinders and the big freestanding grinding machines have a dedicated motor built into them. You can't swap them out because the drive shafts or spindles are built into the motor. Those are the type of machine that are available a LOT more cheaply than their "real" value.

Another issue is speed control and switchgear. Sometimes even if you can swap the motor for a single phase equivalent, you discover the speed control won't work, switchgear like e-stop, reversing or even power on off won't work.

Finally I am sure someone will point out how "easy" it is to generate 3ph from single phase. I disagree. It is tricky. Not only is it tricky and/or expensive, there is the issue of voltage because some 3ph systems/motors work best at 3ph/415v (380v for non UK). So you might need to tweak the voltage level. Then you also still run into the issue of compatibility with "real" 3ph switchgear running off a derived "hot made" 3ph. Some of these 3ph converters don't like to be switched off at the machine and some machines don't like being switched off at the power source!

It's a minefield.

 

[Domdom]

OK, I spent a couple of hours reading last night and I think it’s getting clearer, but the issue is I have no clue how these converters work. I have a solid understanding of how electro-magnetism works (always did) but I can’t see where the converter “makes” the extra phases from.

I keep thinking back to my physics lecturer endlessly repeating how you can’t get something from nothing! So I think I’ve identified the missing knowledge, ie, how these converters work. Cos they clearly do work.

 

[daleyd]

OK, I spent a couple of hours reading last night and I think it’s getting clearer, but the issue is I have no clue how these converters work. I have a solid understanding of how electro-magnetism works (always did) but I can’t see where the converter “makes” the extra phases from.

I keep thinking back to my physics lecturer endlessly repeating how you can’t get something from nothing! So I think I’ve identified the missing knowledge, ie, how these converters work. Cos they clearly do work.

Do you mean the rotary converter type or the “digital” vfd type of converter?

 

[Domdom]

Any of them. How do you get 2 non existent phases from fresh air? This is electronics and my HND was 40 years ago. Trying to decipher it all.

 

[DennisCA]

I think Matthias Wandel has a good video on this, he mainly explains single phase but knowing that gives hints as well to how a 3-phase differs.

 

[daleyd]

Any of them. How do you get 2 non existent phases from fresh air? This is electronics and my HND was 40 years ago. Trying to decipher it all.

I can’t answer for the rotary converter types but a vfd uses solid state electronics to convert the incoming single phase into a dc “bus” voltage.

From this dc voltage it then uses more solid state trickery (transistors or mosfets usually) to recreate the 3 phase signal on the output side. It’s not a true 3 phase, it’s more like a digital recreation of that (Similar to pulse width modulation) but it’s “good enough” for motors. The clever stuff is in how it switches the transistors to do this, but for the purpose of a user that’s kind of irrelevant.

 

[Porkypine]

Just get another old single ph motor, not hard to find and far less agro. I bet your pedestal grinder would accept most of the old gryphon, Compton etc type motors as a straight swap. I've done this with lathes etc numerous times.

 

[premmington]

I tried to explain this to someone a week or two ago.

I went for this as an explanation:

For a single phase motor to start it has to create enough magnetic pull to force the centre of the motor round 180 degrees - where with a three phase motor it only has to go 120 degrees. Once moving - it is harder to stop a three phase motor - because it gets magnetic force from three fields - instead of two for a single phase motor. So while you don't get more "power" for the same KW - you do get a more even force!

Not sure - I did a very good job at explaining it? Pick holes it the above (or word it better for me).

 

[Domdom]

Just get another old single ph motor, not hard to find and far less agro. I bet your pedestal grinder would accept most of the old gryphon, Compton etc type motors as a straight swap. I've done this with lathes etc numerous times.

So even in the old machinery with one piece castings, would this be relatively easy? Something like this (Quick search on marketplace) cleary has a separate motor which could be replaced but finding something suitable ??

 

[Hopefuldave]

So even in the old machinery with one piece castings, would this be relatively easy? Something like this (Quick search on marketplace) cleary has a separate motor which could be replaced but finding something suitable ??

View attachment 401171

The entire top end of that is the motor, so no, not going to be practical to swap *the motor* to a single-phase one - a motor swap's only practicable if the motor is "bolted in" and can be removed without disturbing the rest of the machine - a lathe or mill f'rinstance. That grinder in the picture has a big cabinet you could hide a VFD in (about the size of a halfbrick or larger) but either: a) the VFD would convert from 240 to 415 (more expensive) or b) you'd have to run with less torque or c) you'd have to dig out the "star point" - if you're lucky this is 3 of the 6 terminals in a junction box attached to the motor - this means the motor is 240/415*, selected by moving some jumper strips - if you're unucky the motor's 415 only and the star point is buried inside - you'd have to find the (fairly obvious) joint where three winding ends are connected and disconnect, bring the three wires out separately and wire for 240 like a 6-terminal. Doable, not easy for the uninitiated!

Dave H. (the other one)

* Some larger motors, e.g. over 3HP, may be 400/700v instead, depending on the star/delta links

 

[minimutly]

No, impossible to “convert” that.
@daleyd s explanation is correct. The something for nothing thing is covered like this - the “DC bus” must have enough energy to supply three banks of switches (the switches that create the 3 phases). To do this takes a lot of current, so a 3kW motor and inverter drive needs all this from one supply phase, at 240V, so more amps.
The only other thing I’d add is that these sets of switches (in modern drives they are one single module for the smaller stuff) switch the DC into a sine wave, in steps. Better, more expensive ones have more steps, so a smoother sine wave. Steps (if you remember square wave and harmonics theory) are full of harmonics. Harmonics is noise, so cheap drives can be “noisy”. This is helped by a coil in series with each phase on the inverter output, cheaper ones don’t have these.
Edit to add, if it was me I’d just drive it at 50 Hz with 240V and live with less torque - unless I was planning some heavy grinding, in which case it would be dig out the star point in the windings time.