1 Ph to 3 Ph converter

designer82

Member
Location
Boston
I need to install a 3 PH food processing machine on a one phase system and need a converter. The food machine requires a neutral for the onboard electronics.

1st queston: Can I wire the machine as shown on the picture and run the neutral directly from the main panel, will this work?

2nd question: If the machine is pulling about 40 amps on the single phase panel, will it be pulling about 23 amps (40/sqrt 3) downstream of the converter? (therefore smaller cables downstream of converter)


Thank you

Note, lastly.. can you recommend any good quality 1 to 3 ph converters?
 

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synchro

Senior Member
Location
Chicago, IL
Occupation
EE
As far as I know, most if not all rotary phase converters generate a third phase but the other two phases just pass from the input to output with a direct connection. So that would not constitute a separately derived system. A motor-generator could be used to create a SDS but that would be a more costly and less efficient option. That being said, as Ron mentioned the neutral needs to run along with the phase conductors.

To use the neutral as shown by the OP, the two phase conductors from the single phase panel must come from a 3-phase supply so that they are 120 degrees apart relative to the neutral. If instead they came from a 120/240 split-phase system then the output phases of the rotary converter and neutral conductor would look just like a high-leg delta, which may not be good for the electronics in the machine.

A small but not negligible amount of extra current should be allocated because the phase converter is not 100% efficient. In the FAQ at the link that Ron posted they say:
"As a general rule, determine your 3 phase load amps and double for your single phase feeder amps." This is about 15% more than the ideal sqrt(3) factor.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
How big is the machine and what inside of it needs 3 phase? If the 3 phase is just for AC motors, separate out their circuits and use BFDs on them that accept single phase input to give you 3 phase output. Then for whatever needs the neutral, nothing changes.
 

wwhitney

Senior Member
Location
Berkeley, CA
The OP specifies that 120/208 single phase is available. So can't 208Y/120 be recreated using transformers? I think it is called open wye-wye. Are rotary converters or VFDs cheaper or more efficient?

Cheers, Wayne
 

kwired

Electron manager
Location
NE Nebraska
I also agree if you already have two phases and neutral of a 208/120 system that building an open delta system will give you three phase and probably will be about the best option in most cases.

If driving a single three phase motor a VFD might be right there or the next best option.

rotary phase converter is likely going to be more expensive but might be the way to go if there are multiple motors.

static phase converters (basically a capacitor bank to derive the third phase) generally will not get full output rating of the motor, other than for fairly short durations. Current is too unbalanced to run anything continuously via this method and motors need to be oversized compared to if they were installed on a true three phase system - doesn't work so well when the machine is already designed for true three phase instead of designing the machine to a limited supply.

All rotary or static converters I ever seen bring L1 and L2 of the source to the motor and the third derived phase is created by the phase converter. Voltage from that derived phase is seldom even close to 120 volts to ground and will possibly vary as load conditions vary, so one needs to make sure any 120 volt loads associated with the supplied equipment are connected to the source leads and not the derived third phase.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
... if you already have two phases and neutral of a 208/120 system that building an open delta system will give you three phase and probably will be about the best option in most cases.
Agree with that. What I showed above effectively has an open delta on the secondaries of the two transformers, with one corner tied to neutral. This delta corresponds to the triangle of phasors -V[SUB]A[/SUB], -V[SUB]B, [/SUB]and V[SUB]C [/SUB]in the upper left quadrant of the diagram. Since we don't have to transform to a different 3-phase voltage we can use phases A and B as is, and use an open delta to create just phase C. Phases A and B will then have more stable voltages and a lower source impedance. If a SDS was needed then the open wye-wye mentioned by wwhitney would be more suitable.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
How difficult would it be to pull in the third phase of the 208/120V from wherever the other two are coming from?
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I'm confused- I always thought it was physically impossible to turn single phase into 3 phase with transformers alone.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I'm confused- I always thought it was physically impossible to turn single phase into 3 phase with transformers alone.
What the OP has is L1, L2, and N of a 208V/120V 3-phase source, but is missing the third phase conductor. So it's not really just a single phase source, which as you say cannot be used to create 3 phase using transformers alone.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
What the OP has is L1, L2, and N of a 208V/120V 3-phase source, but is missing the third phase conductor. So it's not really just a single phase source, which as you say cannot be used to create 3 phase using transformers alone.
Ahhh, ok, 120* displacement and a neutral. I was thinking only two hots. :ashamed1:
 

GoldDigger

Moderator
Staff member
Ahhh, ok, 120* displacement and a neutral. I was thinking only two hots. :ashamed1:
And that is one reason some people do not like to call 2-out-of-3 208Y/120 single phase. Whatever else you might say about it, it can be used to produce a full three phase supply using only transformers.


Sent from my XT1585 using Tapatalk
 

kwired

Electron manager
Location
NE Nebraska
And that is one reason some people do not like to call 2-out-of-3 208Y/120 single phase. Whatever else you might say about it, it can be used to produce a full three phase supply using only transformers.


Sent from my XT1585 using Tapatalk
2 out of 3 has 180 degree phase angle and is still single phase.

2 out of 3 plus a neutral can be three phase.
 
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