# Changing single phase to 3 phase

#### Fred B

##### Senior Member
Excuse my ignorance but was wondering the "how does it work?" I've seen what looks like a large motor that somehow takes a single phase supply and converts it into 3 phase output. What is the functional operation that accomplishes this?

#### 480sparky

##### Senior Member
A 1-ph motor drives a 3-ph generator.

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
Excuse my ignorance but was wondering the "how does it work?" I've seen what looks like a large motor that somehow takes a single phase supply and converts it into 3 phase output. What is the functional operation that accomplishes this?
You're talking about a rotary phase converter.

#### MTW

##### Senior Member
Rotary Phase Converter, uses a 3Φ motor fed with 1Φ and started with capacitors or a 1Φ starter motor. The un-powered leg of the motor generates a back EMF, electromotive force, that supplies the generated third leg. The manufactured leg is slightly reduced in voltage and is boosted up by the use of capacitors.
Very common in small home shops for machinery. Search engine is your friend.

#### Jraef

##### Moderator
Staff member
Functionally, a spinning induction motor can be thought of as a rotating transformer, so it will cause a voltage to be induced on an unused winding if the other two are energized. It's the same principal that can allow a spinning motor to "fool" a voltage based phase monitor relay into thinking you DON'T have a phase loss. The trick is though, that a 3 phase motor will not start spinning from a dead stop, it has to either be already spinning when you apply single phase power to it, or you have to "trick" it into spinning using capacitors to cause a pseudo phase shift.

#### Fred B

##### Senior Member
Thanks all, this was all very interesting.
So is my understanding correct, single phase L1 and L2 looked to be carried through and the converter actually generates a L3( image from @MTW ). And it seem from the one linked info that Larry gave it shifts the phase a little to put them at 120 deg from each other. It also stated that this way of getting 3 phase was the more stable means compared to straight capacitor use. (Brings up question below)
So @Jraef this way of getting 3ph is actually using a 3phase motor inline to get the 3rd of a 3ph system? That is why I got wondering about it because that is what they look like, just a big motor, but it was getting 3ph output. It seems you could make one yourself out of a 3ph motor.

I've got a customer I've put power cords on some of his new woodworking equipment for him that the mfg install sheet indicated that the motors could be hooked up either by 1ph or 3ph and gave instructions of the connections and where to alter one of the internal connectors. He bought them pre set as a 1ph so my cord connection was simple red-red, black-black, green-green. No neutral connections.
So likely with these peices of equipment they are using a capacitor to "trick" the 3ph motor into spinning on 1ph. (Wow so devious, lol)

This brings up a question, this same customer has had some power issues when using his larger equipment. I'm going to assume they are the same type of motors as I just wired (3ph motor on 1ph supply). When any of these larger motor start there is a momentary flucuation in the power level of the system, but not consistent as to intensity or which phase (L1 or L2)on same motor. Could this be caused by the motors need for or ability to use 3 phase and the capacitor that shifts the phases? It does balance back out once motor starts. But the phenomenon lasts long enough to capture on a standard volt meter though. He wants to install led lighting in the shop so I wonder if this will become a more noticeable issue on the led vs his current fluorescents? I've seen issues with led and poor power quality.

Also, he is planning expanding his shop and with more equipment too. So would he be well served by getting one of these phase converters and using these tools on 3ph? Would that reduce the likelihood of needing to increase the system size all the way back to the utility? I know 3 ph uses a fraction the power as 1ph for the same hp motor (info from spec sheets not experience).

#### retirede

##### Senior Member
Functionally, a spinning induction motor can be thought of as a rotating transformer, so it will cause a voltage to be induced on an unused winding if the other two are energized. It's the same principal that can allow a spinning motor to "fool" a voltage based phase monitor relay into thinking you DON'T have a phase loss. The trick is though, that a 3 phase motor will not start spinning from a dead stop, it has to either be already spinning when you apply single phase power to it, or you have to "trick" it into spinning using capacitors to cause a pseudo phase shift.
I know a farmer who simply put a big wheel or tire on the motor shaft and got it going manually. Was not OSHA approved!

#### Srv52761

##### Senior Member
Thanks all, this was all very interesting.
So is my understanding correct, single phase L1 and L2 looked to be carried through and the converter actually generates a L3( image from @MTW ). And it seem from the one linked info that Larry gave it shifts the phase a little to put them at 120 deg from each other. It also stated that this way of getting 3 phase was the more stable means compared to straight capacitor use. (Brings up question below)
So @Jraef this way of getting 3ph is actually using a 3phase motor inline to get the 3rd of a 3ph system? That is why I got wondering about it because that is what they look like, just a big motor, but it was getting 3ph output. It seems you could make one yourself out of a 3ph motor.

I've got a customer I've put power cords on some of his new woodworking equipment for him that the mfg install sheet indicated that the motors could be hooked up either by 1ph or 3ph and gave instructions of the connections and where to alter one of the internal connectors. He bought them pre set as a 1ph so my cord connection was simple red-red, black-black, green-green. No neutral connections.
So likely with these peices of equipment they are using a capacitor to "trick" the 3ph motor into spinning on 1ph. (Wow so devious, lol)

This brings up a question, this same customer has had some power issues when using his larger equipment. I'm going to assume they are the same type of motors as I just wired (3ph motor on 1ph supply). When any of these larger motor start there is a momentary flucuation in the power level of the system, but not consistent as to intensity or which phase (L1 or L2)on same motor. Could this be caused by the motors need for or ability to use 3 phase and the capacitor that shifts the phases? It does balance back out once motor starts. But the phenomenon lasts long enough to capture on a standard volt meter though. He wants to install led lighting in the shop so I wonder if this will become a more noticeable issue on the led vs his current fluorescents? I've seen issues with led and poor power quality.

Also, he is planning expanding his shop and with more equipment too. So would he be well served by getting one of these phase converters and using these tools on 3ph? Would that reduce the likelihood of needing to increase the system size all the way back to the utility? I know 3 ph uses a fraction the power as 1ph for the same hp motor (info from spec sheets not experience).
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.

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#### retirede

##### Senior Member
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.
A 3 phase induction motor will run on single phase at reduced capacity. But it will not start without help.

You could take a mechanically unconnected 3 phase motor running normally on 3 phase power, remove one phase conductor and probably not be able to observe any physical change - it will keep running.

#### wwhitney

##### Senior Member
So is my understanding correct, single phase L1 and L2 looked to be carried through and the converter actually generates a L3( image from @MTW ). And it seem from the one linked info that Larry gave it shifts the phase a little to put them at 120 deg from each other.
I don't believe it makes sense to say L1 and L2 are phase shifted. If the source voltage system is just 2 wires, there's no sense of phase to begin with. If it's 3-wire split-phase with a neutral conductor N, then the rotary phase converter is creating a high leg delta system L1-N-L2-L3. The phase relationship of L1-N-L2 is unchanged; it's only by shifting the reference point from N to the new neutral point of L1-L2-L3 that L1 and L2 appear 120 degrees out of phase.

Cheers, Wayne

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
I don't believe it makes sense to say L1 and L2 are phase shifted. If the source voltage system is just 2 wires, there's no sense of phase to begin with. If it's 3-wire split-phase with a neutral conductor N, then the rotary phase converter is creating a high leg delta system L1-N-L2-L3. The phase relationship of L1-N-L2 is unchanged; it's only by shifting the reference point from N to the new neutral point of L1-L2-L3 that L1 and L2 appear 120 degrees out of phase.
Exactly. No different than any high-leg delta.

#### hillbilly1

##### Senior Member
Rotophase converters are going out of style, vfd’s have come down in price so much, they are used instead now. Does take a larger feed using a single phase input. Wired a church fellowship hall, elevator was at the far corner of the building, put in conduit and wire in the slab for three phase which was available at the service. 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!

#### norcal

##### Senior Member
Rotophase converters are going out of style, vfd’s have come down in price so much, they are used instead now. Does take a larger feed using a single phase input. Wired a church fellowship hall, elevator was at the far corner of the building, put in conduit and wire in the slab for three phase which was available at the service. 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!
As bad as a private school built some years ago, 208Y/120V service, A/C contractor installed single phase condensing units.

#### hillbilly1

##### Senior Member
As bad as a private school built some years ago, 208Y/120V service, A/C contractor installed single phase condensing units.
That was another thing they did on that job. Put in three phase just for it. The Hvac guy said the single phase units were much cheaper. At least I had a conduit in the slab large enough for them. They were on the far side of the building. They went the cheap route on the duct work too. Cloth plenum. Ever time they started it, the duct would inflate and pop real loud, so they programmed it to keep the fan on when the building was occupied.

#### jim dungar

##### Moderator
Staff member
Rotophase converters are going out of style....
I remember hearing that 25 years ago.

I agree for single motor loads, but VFDs do not always play nicely with multimotor installations like machine shops. There is still a place for phase converters, both rotary and electronic.

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
Elevator had a vfd and three phase motor!
Couldn't the VFD have been removed/bypassed and the 3ph be used?

#### Fred B

##### Senior Member
I don't believe it makes sense to say L1 and L2 are phase shifted. If the source voltage system is just 2 wires, there's no sense of phase to begin with. If it's 3-wire split-phase with a neutral conductor N, then the rotary phase converter is creating a high leg delta system L1-N-L2-L3. The phase relationship of L1-N-L2 is unchanged; it's only by shifting the reference point from N to the new neutral point of L1-L2-L3 that L1 and L2 appear 120 degrees out of phase.
Cheers, Wayne
Exactly. No different than any high-leg delta.
I was thinking if I was to view on scope a 1ph would be 180 deg off between L1 and L2 and believed that by adding L3 it would shift the offset to 120 deg on all 3 or am I off on this? Sorry for stupid question but just never really dealt with 3ph. Your statement seems to indicate L1-L2 stays at 180deg and the Added L3 is off?
Also with the rotary converter would the voltage between L1 and L2 change from the input side, normal 240V? Does the rotary converter create the equivalent of a delta or wye? Your statement of high leg is indicating a center ground delta? These motors on the equipment are listed 230V 1ph or 460V 3ph, but it doesn't say delta or wye, just that you can have it 3ph by changing the contactor out for the 460V. So would a rotophase converter work with this type of motor?
Again reason asking owner is looking at an expansion and was wondering if a change to 3ph by means of a phase converter would be better than adding a lot of additional 1ph from the utility. And if it would help related to the variable (not consistent as to leg, sometimes L1 sometimes L2) voltage drops that the larger equipment is causing on start up.

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
I was thinking if I was to view on scope a 1ph would be 180 deg off between L1 and L2 and believed that by adding L3 it would shift the offset to 120 deg on all 3 or am I off on this? Sorry for stupid question but just never really dealt with 3ph. Your statement seems to indicate L1-L2 stays at 180deg and the Added L3 is off?
Not stupid at all. What an o-scope would show depends on the reference point.

Relative to the actual neutral, nothing changes. Each transformer of a delta is a single-phase unit. The 120 degrees only shows up if you measure from an imaginary central neutral, resembling a wye system.

If you had a delta bank made of center-tapped transformers, like the power company does, you would measure 120/240v from each one, just as you'd expect. Grounding one of them is what creates the high leg.

Also with the rotary converter would the voltage between L1 and L2 change from the input side, normal 240V? Does the rotary converter create the equivalent of a delta or wye? Your statement of high leg is indicating a center ground delta? These motors on the equipment are listed 230V 1ph or 460V 3ph, but it doesn't say delta or wye, just that you can have it 3ph by changing the contactor out for the 460V. So would a rotophase converter work with this type of motor?
The two input lines pass through to the load. The third line is created, and technically speaking, in turn creates a delta supply. To a 3-phase load, only the voltage between lines matter, and not the voltage to ground and/or neutral. The 1ph supply and 3ph output will be the same voltage, so 240v 1ph in creates 240 3ph out.

Again reason asking owner is looking at an expansion and was wondering if a change to 3ph by means of a phase converter would be better than adding a lot of additional 1ph from the utility. And if it would help related to the variable (not consistent as to leg, sometimes L1 sometimes L2) voltage drops that the larger equipment is causing on start up.

#### Fred B

##### Senior Member
Not stupid at all. What an o-scope would show depends on the reference point.

Relative to the actual neutral, nothing changes. Each transformer of a delta is a single-phase unit. The 120 degrees only shows up if you measure from an imaginary central neutral, resembling a wye system.

If you had a delta bank made of center-tapped transformers, like the power company does, you would measure 120/240v from each one, just as you'd expect. Grounding one of them is what creates the high leg.

The two input lines pass through to the load. The third line is created, and technically speaking, in turn creates a delta supply. To a 3-phase load, only the voltage between lines matter, and not the voltage to ground and/or neutral. The 1ph supply and 3ph output will be the same voltage, so 240v 1ph in creates 240 3ph out.