Phase angles and a few transformers

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parallel

Member
Location
california
I made a rookie mistake while i was doing an estimate walk-through for a friend.
he wanted to hook up a machine at 240v that included a big load coil, and a small conveyor motor.

I was told a previous tenant in the same building was running a 240v air compressor out of the panel
that the client had shown me.

The panel tested 240v between legs so i told him it was what he thought.

i acquired the job specs from him:

the machine is scheduled to be on for many hours at a time
The machine requires 22A fed by two phases of 120v equaling 240v, and has a twist lock plug.
I sized it for 30A on a standard two pole breaker,
running (x1)#12 ground and (x2)#8 conductors going through about 75 feet of 3/4 EMT to the panel.

i installed the circuit and went to test the socket before anything was to take power from it.
It was reading 240v from phase to phase, then it read 240v phase to ground.
I inspected the panel and found 1 of the 3 legs with 0 potential to ground.
I set my breaker to the live legs and plugged the machine in.

it did not start.

Now i am here,
knowing now that i'm dealing with a corner grounded 240v Delta system, the wrong type of breaker,
and having only this panel to work with.

My Questions:

1. if i use (x2) single phase 2 kva (pri 480v/240v-240v/120v sec) transformers, will it work, and if so,
will the phase's angular difference cause more wear on the machine than 2 phases 180* apart?

2. hypothetically, if i wanted to take the output of those transformers(120v and 120* out of step) and balance the phases 180* out of step, would i be able to use the grounded b phase through a third transformer and use it like an isolated ground?
reason being that while Vab|Vag=0*, Vcb=60*, Vca=120* and Vcg=-120*.

connection at machine
--------------
A phase
C phase
b(isolated ground)
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
If you take any two legs from your corner-grounded panel, you will have 240V 1-phase, 2-wire.

You can use a single 'isolation' transformer connected 240V 1-phase 2-wire on the primary to 120/240V 1-phase 3-wire on the secondary.
It might be possible to do this with an autotransformer, (e.g. buck-boost), but I don't normally think of them for 2X voltage changes.
 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
If you hook up a transformer the way you propose, you won't really have a "phase angle difference". It'll be single phase, 3-wire on the secondary (some call it "split-phase".)

You need one transformer with 240V 1Ø primary, 120/240V 1Ø, 3W secondary, 7.5 kVA.
 
kwired

kwired

Electron manager
Location
NE Nebraska
parallel said:
I made a rookie mistake while i was doing an estimate walk-through for a friend.
he wanted to hook up a machine at 240v that included a big load coil, and a small conveyor motor.

I was told a previous tenant in the same building was running a 240v air compressor out of the panel
that the client had shown me.

The panel tested 240v between legs so i told him it was what he thought.

i acquired the job specs from him:

the machine is scheduled to be on for many hours at a time
The machine requires 22A fed by two phases of 120v equaling 240v, and has a twist lock plug.
I sized it for 30A on a standard two pole breaker,
running (x1)#12 ground and (x2)#8 conductors going through about 75 feet of 3/4 EMT to the panel.

i installed the circuit and went to test the socket before anything was to take power from it.
It was reading 240v from phase to phase, then it read 240v phase to ground.
I inspected the panel and found 1 of the 3 legs with 0 potential to ground.
I set my breaker to the live legs and plugged the machine in.

it did not start.

Now i am here,
knowing now that i'm dealing with a corner grounded 240v Delta system, the wrong type of breaker,
and having only this panel to work with.

My Questions:

1. if i use (x2) single phase 2 kva (pri 480v/240v-240v/120v sec) transformers, will it work, and if so,
will the phase's angular difference cause more wear on the machine than 2 phases 180* apart?

2. hypothetically, if i wanted to take the output of those transformers(120v and 120* out of step) and balance the phases 180* out of step, would i be able to use the grounded b phase through a third transformer and use it like an isolated ground?
reason being that while Vab|Vag=0*, Vcb=60*, Vca=120* and Vcg=-120*.

connection at machine
--------------
A phase
C phase
b(isolated ground)
Click to expand...
Can you be more specific on the information I bolded in your quote? That might mean you need two 120 volt circuits, it might mean you need one 240 volt circuit, or it might mean you need both 120 and 240 volts at the machine. What you have doesn't have 120 volts at all without additional transformation being involved.
 
P

parallel

Member
Location
california
kwired said:
Can you be more specific on the information I bolded in your quote? That might mean you need two 120 volt circuits, it might mean you need one 240 volt circuit, or it might mean you need both 120 and 240 volts at the machine. What you have doesn't have 120 volts at all without additional transformation being involved.
Click to expand...

im sorry if i caused any inconsistencies,
the machine is a silk screen print dryer.
the nameplate specifies 120/240v
its hooked up to receive 2 phases of 120v(split phase)
 
P

parallel

Member
Location
california
JoeStillman said:
If you hook up a transformer the way you propose, you won't really have a "phase angle difference". It'll be single phase, 3-wire on the secondary (some call it "split-phase".)

You need one transformer with 240V 1Ø primary, 120/240V 1Ø, 3W secondary, 7.5 kVA.
Click to expand...

this is basically center tapping correct?

and there's nothing special about the deltas corner ground versus a fresh grounding electrode?
 
kwired

kwired

Electron manager
Location
NE Nebraska
parallel said:
this is basically center tapping correct?

and there's nothing special about the deltas corner ground versus a fresh grounding electrode?
Click to expand...
yes it is 240 volts, with 120 from each "end" to the "center tap".

Both will get bonded together anyway, whether intentional or not in many cases, and you want that to prevent any voltage differential between adjacent exposed bonded components anyway.

When equipment nameplate says 120/240 it usually means it has 120 as well as 240 volt loads within it.

Does the supply terminals/leads have L1, L2, N? If so did you initially connect anything to N? leaving N alone could have saved you from burning something up if it has 120 volt controls, because if you had connected anything to it you would have applied 240 volts to the control circuit.
 
kwired

kwired

Electron manager
Location
NE Nebraska
topgone said:
Q: why does the OP keep mentioning "two phases"?
Click to expand...
Somebody doesn't fully understand what a center tapped winding really is, or for that matter why you have 120 volts to the neutral if you have a 208/120 wye system. Now throw in the corner grounded delta and you have no 120 whatsoever on the system and are even more confused.
 
P

parallel

Member
Location
california
kwired said:
yes it is 240 volts, with 120 from each "end" to the "center tap".

Both will get bonded together anyway, whether intentional or not in many cases, and you want that to prevent any voltage differential between adjacent exposed bonded components anyway.

When equipment nameplate says 120/240 it usually means it has 120 as well as 240 volt loads within it.

Does the supply terminals/leads have L1, L2, N? If so did you initially connect anything to N? leaving N alone could have saved you from burning something up if it has 120 volt controls, because if you had connected anything to it you would have applied 240 volts to the control circuit.
Click to expand...

all control equipment, relays, and switches inside machine are rated for 240v ac
machine came wired, didnt look too far into it once i figured out the panel voltage wasnt what i thought.

machine was tested prior to buying so all connections are good inside.
 
A

Adamjamma

Senior Member
Location
Jamaica and london
topgone said:
Q: why does the OP keep mentioning "two phases"?
Click to expand...

Because some of us have been taught that 120/208 or 120/240 is two phases of a three phase system, not a center tapped single phase system as used in 120 volt transformers in the UK. There is some differences in how the power is calculated and used. Actually, I am slightly wrong in this. If you have a 110/220 or 120/240 type system where you measure line to line as double the voltage of line to neutral, then you more than likely have a center tapped single phase system. However, if you measure and have line to line 208 or 230 and line to neutral 110 or 120, then you more than likely are on a three phase system using two phases. The variance is within the normal operations of most consumer equipment and most people would not ever know the difference in use. But, the harmonics on the neutral are different, and the calculations are different. Not installation calculations but conversion of power calculations.

However, not too sure any of this would help the OP. But, I was taught to call center tapped as two phase as you have a positive and negative phase at same time, compared to three phase. Currently it is simply single phase then three phase as teachings go. But other countries still teach single phase - two phase - three phase.
 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
Adamjamma said:
...Currently it is simply single phase then three phase as teachings go. But other countries still teach single phase - two phase - three phase.
Click to expand...

Here in the US we still have vestiges of the two-phase, three-or-five-wire distribution from 100 years ago. (Hence table 430.249) It's still in use in some parts of Philadelphia, PA.
With a three-phase system, the "angle" between phases is 120°. The old two-phase angle is 90°. That's why we avoid the more common meaning of the term "two-phase" in some parts of the country.
 
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kwired

kwired

Electron manager
Location
NE Nebraska
parallel said:
yes, 120/240 and wondering if there are any vector differentiating qualities produced between (the grounded corner delta's a & c phases) when testing between b phase and a freshly sunk grounding electrode.
Click to expand...
Nothing any more significant than the fact that grounded conductor often is also electrically the same thing as the grounded conductor for the POCO's medium voltage distribution system, or grounded conductor of some other system in your plant if what you have is separately derived on site.
 
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