Changing single phase to 3 phase

[gar]

210220-1551 EST

Fred B:

You need some basic understanding on motor theory.

If you have two different sources of magnetic fields, then where these fields interact there is a force developed between the two field sources. Depending on field polarity the force is either attractive, or repulsive.

In a DC permanent magnetic field motor with a wound rotor and commutator, there is a magnetic force that pushes the rotor away from its position (rotation) by current and its magnetic field in the rotor wire. Because of the brushes and the commutator (a switch) the first rotor wire has current removed, and another rotor wire is connected producing another push in the same direction. So the rotor builds up speed in one direction to some steady level.

Now go to an AC motor. For simplicity make the rotor a permanent magnetic.

First, assume there is one stator winding (coil of wire) fed from an AC voltage. The magnetic field from this coil will oscillate plus and minus some equal amount, and the rotor will not rotate. It may slightly oscillate in one nominal position.

Based on the frequency of the voltage there will be points in time, equally spaced, where the stator magnetic field will be positive. Rotating masses have inertia. So once a mass is started it wants to keep going. So start the mass rotating at synchronous speed to the AC voltage, then apply the AC voltage to the coil in the correct phase relationship, and you will pulse energy into the rotating mass keeping it going at synchronous speed. Now we have a single phase motor running at a speed.

Second, an AC induction motor is essentially the same, except it has rotor slip.

What we don't have is a self starting rotating motor. This is because the AC magnetic field is just oscillating up, down, and reverse.

Third, add a second stator coil positioned 90 degrees from the first coil in space. To this apply a voltage that is 90 degrees shifted in phase from the phase of the first coil. Excite both coils. Now we have a magnetic vector in space that rotates thru 360 degrees. Put a permanent magnet in that rotating field, and that magnet will rotate in space in sync with the rotating magnetic vector. Now you have a self starting motor.

A 3 phase motor simply adds another coil and you make the phase differences 120 degrees.

In the 2 phase motor you could not use 180 degrees because the two phases are in line with each other. So no rotating vector in space.


A single phase motor to be self starting must include some way to get a second magnetic vector shifted in space and electrical timing that is turned on for starting. Usually because of the way it is generated it will draw a high current and needs to be turned off when speed is reached, means a switch. A pure single phase motor will never be self starting.

There is insufficient information on your motor to know how it works. Going from one voltage level to another by itself is not going to produce a single phase motor vs a three phase motor.

There are possibly ways for you to test the motor, and determine how it actually works. But you need a fundamental understanding of motor theory.

.

 

[Fred B]

I've never heard of such a motor.

I pulled up a different manual on it. Must an issue in the translation from Chinese. The other manual indicated different motor # between 230V 1ph and 230V 3ph. The 230V conversion to 460V is in 3ph only based on the other manual. I too thought it odd but don't know much about motors especially 3ph.

 

[retirede]

I pulled up a different manual on it. Must an issue in the translation from Chinese. The other manual indicated different motor # between 230V 1ph and 230V 3ph. The 230V conversion to 460V is in 3ph only based on the other manual. I too thought it odd but don't know much about motors especially 3ph.

The 230/460 is a dual voltage 3 phase motor which means is can be connected to operate on either voltage. It does not perform a voltage conversion.

 

[hillbilly1]

Couldn't the VFD have been removed/bypassed and the 3ph be used?

Not without violating the listing, vfd was part of the controls too.

 

[Fred B]

Gar. Thanks. Now trying to visualize, familiar with the DC motor. Familiar with windings. 2 coils at 90deg got it. Now see if I got this. 1ph 1 leg you will have point in time that you have 0V as it cycles thru positive/negative, add in leg 2, this is where I'm getting a little fuzzy. Trying to visualize from an o'scope pattern, L1 wave form oscillates +,0,-, 0,+. Question is laying over the L2, independently it too will cycle similarly but the interaction on the scope is where I'm unsure and where the difference of is it 180 deg, 120deg, or 90deg. At 180deg you will have a common point in time where both legs will be at 0V, but either of the other 2 options the 2 legs will lap in such a way that you will not have point in time of 0V as one leg will be climbing past 0 as the other is declining thus maintaining a certain minimum voltage. I'm I correct so far?

 

[paulengr]

Gar. Thanks. Now trying to visualize, familiar with the DC motor. Familiar with windings. 2 coils at 90deg got it. Now see if I got this. 1ph 1 leg you will have point in time that you have 0V as it cycles thru positive/negative, add in leg 2, this is where I'm getting a little fuzzy. Trying to visualize from an o'scope pattern, L1 wave form oscillates +,0,-, 0,+. Question is laying over the L2, independently it too will cycle similarly but the interaction on the scope is where I'm unsure and where the difference of is it 180 deg, 120deg, or 90deg. At 180deg you will have a common point in time where both legs will be at 0V, but either of the other 2 options the 2 legs will lap in such a way that you will not have point in time of 0V as one leg will be climbing past 0 as the other is declining thus maintaining a certain minimum voltage. I'm I correct so far?

If your reference point is only L1 and L2 then you are correct. But let’s talk for a moment about a high leg delta system which is what you will see. In this case you have 3 phases, 120 degrees apart, and let’s say 240 V between phases. Now if I have a single phase center tapped transformer between say L1 and L2, I will see 120 V from L1 to the center tap to either L1 or L2. With L3 I will see 208 V to the center tap because it’s the “high leg”. If you draw it out and do the trigonometry pythagoras tells us if the hypotenuse is 240 and one side is 120 the third side is 208. Now in high leg delta we ground that neutral. So we have a neutral and a ground like normal, it’s just not dead center in the wye like it would be in a 480/277 or 208/120 system. The only tricky thing to remember, and the reason L3 MUST be orange by Code is that if you lose track of your phasing and attempt to connect a single phase 240 load to L3 you will have low voltage problems, and if you attempt to connect a 120 load between L3 and neutral you will have bigger problems.

 

[kwired]

Couldn't the VFD have been removed/bypassed and the 3ph be used?

Or possibly even had the drive supplied with three phase or swap drive for one that does accept three phase input.

Somebody needs to eat some cost regardless. Or pass it on to the owner. Should at least start with who specified what, and did they get that information to all parties, then - who changed it and why didn't everyone else involved get notified?

 

[winnie]

They bring out a single phase elevator! Conduit, wire and breaker too small now, had to run overhead 350’. Elevator had a vfd and three phase motor!

Kind of insane to have a VFD that can only take single phase input. The additional pair of diodes adds very little to the cost and then you have a single stock item that works for single or 3 phase.

Jon

 

[winnie]

Another point on phase angles: In a high leg delta system there are 4 terminals and thus 6 pairs of terminals that you can measure.

A-N, B-N, C-N, A-B, B-C, and C-A. (And their 6 inverse versions.)

The 3 L-L pairs are what a motor uses, and these have the expected 120 degree phase offset.

The center tapped pairs are exactly like a single phase system. A-N and C-N are inverse of each other, 180 degrees apart on a scope. The high leg B-N is at _90_ degrees to the A-N reference. Of course the motor isn't connected to the neutral so it never 'sees' this 0, 90, 180 degree system.

Way back there were some posts by 'hill Billy sawmiller' or some similar name. This person described running a bodged sawmill with a single phase service. They had a rota phase to get the third leg, but it wasn't big enough for the largest machines. So the rotaphase was used to run small machines, and the small machines were run unloaded to reinforce the third leg for the larger machines.

Jon

 

[Frank DuVal]

So these motors that indicate 230 on single but 460 on 3ph a rotary converter wouldn't accomplish this. It just changes like for like on voltages From your statement. So that indirectly answers my other question as to whether adding 3ph converter might help vs adding capacity.
But it brings up a different question, why would the 3hp motor that wire as 1ph 230V require 460V as a 3ph?

???
I want to see THAT electric motor!

 

[Frank DuVal]

Way back there were some posts by 'hill Billy sawmiller' or some similar name. This person described running a bodged sawmill with a single phase service. They had a rota phase to get the third leg, but it wasn't big enough for the largest machines. So the rotaphase was used to run small machines, and the small machines were run unloaded to reinforce the third leg for the larger machines.

Works just fine! The starting ability of a roto phase derived service in horsepower is just less than the connected and running horsepower. So to start and run a 10 HP motor, you need 10 or more HP worth of motors already spinning. I've seen installations of 5 HP converters running 20 HP worth of motors. BIG wire! Sawmills. It was the next step above diesel powered saws.

On another note, "they" also learned when three phase 480 service came available, not to use full voltage starters (460 volt coils) and push buttons on rural sawmills here in the east. Why? Because it is not good to come to work in the dark hours and see the saws already running! Humidity will conduct enough current through the open push button switch at 480 to pull in the coils, which then lock on through the auxiliary contact.

 

[retirede]

On another note, "they" also learned when three phase 480 service came available, not to use full voltage starters (460 volt coils) and push buttons on rural sawmills here in the east. Why? Because it is not good to come to work in the dark hours and see the saws already running! Humidity will conduct enough current through the open push button switch at 480 to pull in the coils, which then lock on through the auxiliary contact.

I’d have to see that happen to believe it.

 

[AdrianWint]

Kind of insane to have a VFD that can only take single phase input. The additional pair of diodes adds very little to the cost and then you have a single stock item that works for single or 3 phase.

Except that the rectified & smoothed voltage across the capacitors will be higher for the three phase unit (assuming 120V SP & 208 3P)

If you connect to 208 Line/line 'single phase' the DC ripple will be higher than if you connect true 208 3P.

 

[AdrianWint]

Mmmmm.....
I have a rotary converter that uses a 3ph motor. I have never encountered a 3ph motor that could “run“ at single phase. Most dual rated motors must stay within its phase domain; 120/240 @ single phase, 220/440 at 3ph, or something like that. I am interested in the brand and model. That would be convenient.

Wrt using less power, Power is power. 3 hp generates 3 hp. The power is the same. 3 phase allows you to spread that power over another leg, allowing for smaller conductors.

Isn't that the 'Steinmetz' connection? Wiki is your friend

Only really works for fractional HP motors & then they have to be downrated to 1/3 I believe.

Its sometimes seen here in the UK to run a 400/230V fractional HP motor (perhaps a blower motor), configured into delta and using a capacitor across one winding.

 

[winnie]

Except that the rectified & smoothed voltage across the capacitors will be higher for the three phase unit (assuming 120V SP & 208 3P)

If you connect to 208 Line/line 'single phase' the DC ripple will be higher than if you connect true 208 3P.

Agreed that 208V three phase to the rectifier would result in higher DC voltage than a 120V supply. But I think that it would also be insane to supply a large system with 120V single phase. If it were a single phase system it would be designed for a 208V or 240V supply.

-Jon

 

[AdrianWint]

Agreed that 208V three phase to the rectifier would result in higher DC voltage than a 120V supply. But I think that it would also be insane to supply a large system with 120V single phase. If it were a single phase system it would be designed for a 208V or 240V supply.

-Jon

Ok, I don't disagree.... but a single phase 208V connected drive will require bigger smoothing caps to supply the same DC current and have the same ripple than a 208V three phase connected drive would. Three phase has the big advantage that, when the three waveforms are combined using the diodes alot of the smoothing is done 'automatically' due to the way the phase waveforms overlap = less smoothing for the caps to do.

Bigger caps = greater cost to the manufacturer.

 

[kwired]

Works just fine! The starting ability of a roto phase derived service in horsepower is just less than the connected and running horsepower. So to start and run a 10 HP motor, you need 10 or more HP worth of motors already spinning. I've seen installations of 5 HP converters running 20 HP worth of motors. BIG wire! Sawmills. It was the next step above diesel powered saws.

On another note, "they" also learned when three phase 480 service came available, not to use full voltage starters (460 volt coils) and push buttons on rural sawmills here in the east. Why? Because it is not good to come to work in the dark hours and see the saws already running! Humidity will conduct enough current through the open push button switch at 480 to pull in the coils, which then lock on through the auxiliary contact.

I’d have to see that happen to believe it.

Me too. Irrigation wells around here typically have 480 volt control circuit and this is not a common occurrence. I have seen it happen more frequently, even at lower voltages, when indoor equipment was being used outdoors.

 

[GeorgeB]

I want to see THAT electric motor!

What I HAVE seen is a single phase brake. Motor nameplate please ...

 

[kwired]

What I HAVE seen is a single phase brake. Motor nameplate please ...

Or even separately powered cooling fan. Though the ones I've seen happened to also be three phase motors, but only like 1/8 HP or less.

 

[Fred B]

What I HAVE seen is a single phase brake. Motor nameplate please ...

Evidently the manual for this Chinese tool had a translation issue, when using downloaded manual it indicated that it was indeed 2 different motors that could be in the product, 1 3pase or 1 single phase. The conversion process in the printed manual left that fact out. In the online version the conversion was to change from 230V to 460V 3ph, on the 3ph motor only, and not a conversion from 1ph to 3ph.