LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I believe that the connection must be made for a 3ph motor to see the phase angles, but not for a 2ph motor. .
Have you every seen a 120/240V 2PH 5Wire to 120/208V 3ph 4wire transformer?
- Thread starter zemingduan
- Start date
- Status
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
A galvanic connection between the phases is not required for the motor to function. All you need is proper phasing of the current flow.
This could be done with a three phase motor, but you would need 6 wires to make it work.
Jon
This could be done with a three phase motor, but you would need 6 wires to make it work.
Jon
All one would need to do is get a 230V or 200V incandescent lightbulb and connect it between the a and b phases to do a quick test.
I'd bet the POCO in Philly has the phases connected.
I'd bet the POCO in Philly has the phases connected.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
The VFD is pointless. He already can get 3 phase for the motor with the Scott T transformer. What he wants is single phase from the same source.
The most viable option I see is to just get a separate single phase transformer for getting 120/240 split phase, or get a 240D to 208Y120 three phase transformer off of the 240 from the Scott T.
The most viable option I see is to just get a separate single phase transformer for getting 120/240 split phase, or get a 240D to 208Y120 three phase transformer off of the 240 from the Scott T.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I just re-read the original post.
What is desired is a Scott T with the appropriate tap to provide 208/120 directly, rather than one that provides 208V delta and needing yet another transformer to get the wye.
This was answered in post 5. Everything else is fun tangent.
Yes, you can get VFDs which take DC input. One of the canonical applications is elevators, where multiple elevators share the same DC bus so as to provide natural regeneration between elevators. I don't know if this is actually done anymore, given all of the elevator safety codes, but when I needed a DC input VFD for a dyno system, the manufacturer white paper described this exact scenario.
Yes, you can rectify 2 phase to power a VFD. My first instinct is that you would want a 4 leg bridge, but it is probably worth simulating what a regular 3 phase bridge would do with 2 phase 5 wire input.
2 phase 4 wire can have completely independent phases. But I think 2 phase 5 wire has a common grounded neutral in both phases. Though I suppose anything is possible, including something where one phase is center tapped and grounded while the other phase is ungrounded.
Jon
What is desired is a Scott T with the appropriate tap to provide 208/120 directly, rather than one that provides 208V delta and needing yet another transformer to get the wye.
This was answered in post 5. Everything else is fun tangent.
Yes, you can get VFDs which take DC input. One of the canonical applications is elevators, where multiple elevators share the same DC bus so as to provide natural regeneration between elevators. I don't know if this is actually done anymore, given all of the elevator safety codes, but when I needed a DC input VFD for a dyno system, the manufacturer white paper described this exact scenario.
Yes, you can rectify 2 phase to power a VFD. My first instinct is that you would want a 4 leg bridge, but it is probably worth simulating what a regular 3 phase bridge would do with 2 phase 5 wire input.
2 phase 4 wire can have completely independent phases. But I think 2 phase 5 wire has a common grounded neutral in both phases. Though I suppose anything is possible, including something where one phase is center tapped and grounded while the other phase is ungrounded.
Jon
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
If you only connect one pair, it's a single-phase motor 
. The motor windings don't need an electrical connection between the phase pairs because they generate magnetic fields with the 90 deg separation.
(I will admit that the last time I really examined the theory of rotating machines was over 40 years ago...)
. The motor windings don't need an electrical connection between the phase pairs because they generate magnetic fields with the 90 deg separation.(I will admit that the last time I really examined the theory of rotating machines was over 40 years ago...)
drcampbell
Senior Member
- Location
- The Motor City, Michigan USA
- Occupation
- Registered Professional Engineer
The same could be true of 3-phase motors. They'd still run if the motor windings and the transformer windings were not tied together in the "center". It's just a convenience and economy measure enabling 3-phase motors to be hooked up with three wires instead of six.
Why add the 27/7 inefficiency of not one but two transformers humming away when the elevator motor is probably VFD based and probably has a 208--240 input rating?
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
In case a Scott-T transformer with a a neutral tap on the teaser winding is not available (or even a Scott-T without such a tap), another alternative could be the following:
1. First create voltages equivalent to a 240/120V 4-wire high leg delta supply.
If the 5-wire two-phase conductors are a, a', b, b' and n, then a-n-a' can be used directly for the A-N-C high leg delta voltages. Then the high leg B phase can be created by a single phase 240-208V transformer with b-b' driving the primary. One end of the 208V secondary winding would be connected to the neutral, and the other would provide the high leg B phase.
2. Then use a standard 240-208Y/120V delta-wye to provide 208/120V from the A,B,C voltages above.
Not quite as ideal as a Scott-T with the proper neutral tap, but it would likely be simpler, cheaper, and little more efficient than a Scott-T driving the delta-wye. It would also allow the output voltages to be easily balanced using the taps on the delta-wye.
1. First create voltages equivalent to a 240/120V 4-wire high leg delta supply.
If the 5-wire two-phase conductors are a, a', b, b' and n, then a-n-a' can be used directly for the A-N-C high leg delta voltages. Then the high leg B phase can be created by a single phase 240-208V transformer with b-b' driving the primary. One end of the 208V secondary winding would be connected to the neutral, and the other would provide the high leg B phase.
2. Then use a standard 240-208Y/120V delta-wye to provide 208/120V from the A,B,C voltages above.
Not quite as ideal as a Scott-T with the proper neutral tap, but it would likely be simpler, cheaper, and little more efficient than a Scott-T driving the delta-wye. It would also allow the output voltages to be easily balanced using the taps on the delta-wye.
- Status