Three phase power

Status
Not open for further replies.

trotter

Member
Need some help with three phase power.

If a piece of equipment is rated to pull in 12 amps at peak performance level with three phase power is 12 amps being drawn through each phase? Or is a total of 12 amps being drawn between the phases?

If not 12 amps per phases is there a way to calculate how many amps per phase is being drawn?

That same piece of equipment is rated for 19 amps using single phase L-L. Does that mean it's pulling in 8 amps per phases for L-L?

Thanks for your help on this?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: Three phase power

The simple answer is: In any single circuit the current flowing in the conductors is always equal to the load current.

More complicated (but barely so) situations occur when multiple circuits are combined.

Possible single circuits examples are:
System - circuit
3 phase 3 wire - phases A, B, and C
3 phase 2 wire - phase A and B
3 phase 2 wire - phase C and neutral
1 phase 2 wire - lines 1 and 2
1 phase 2 wire - line 1 and neutral
 

trotter

Member
Re: Three phase power

"The simple answer is: In any single circuit the current flowing in the conductors is always equal to the load current."

Sorry, I'm not much of an electrician so I'm not following.

So if the equipment needs 12 amps to run does that mean it's pulling 12 amps on phases A, B and C totalling 36 amps between the three when using 3 phase, 3 wire?
 

ron

Senior Member
Re: Three phase power

In each of your examples, the rated current could be measured in the individual hot conductors.
12A in each of the three hot conductors (36A total) in the first example, and 19A in each hot conductor (38A total) for the 2nd.

[ March 24, 2004, 10:23 AM: Message edited by: ron ]
 

trotter

Member
Re: Three phase power

Thanks for the answers. It brings me out of confusion a little bit.

If each phase A, B and C is pulling a current of 12 amps is that a constant draw of current from the source to the load?

I have a 200amp PDU that has 40 amps free on each phase. 40 on A, 40 on B and 40 on C.

Would this equipment reduce the available amperage on this PDU by 12 on each phase making the total available amps 28 on A, 28 on B and 28 on C?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: Three phase power

I'm sorry, you are right sometimes the simple answer can be too simple.

In a single circuit, there is only one current value. Never, ever add the line currents to create a total current, there is no such thing as an "overall current".

The load current is the amount of current the load will pull from each conductor.

Using my examples with your original question;
3 phase 3 wire - phase A=12Amp, B=12Amp, and C=12Amp for a "current of 12Amps 3phase 3wire"
or
3 phase 2 wire - phase A=12Amp and B=12Amp, and for a "current of 12Amps 3phase 2wire L-L"
or
3 phase 2 wire - phase C=12Amp, neutral=12Amp, for a "current of 12Amps 3phase 2wire L-N"
or
1 phase 2 wire - line 1=19Amps and line 2=19Amps for a "current of 19Amps 1phase 2wire L-L"
or
1 phase 2 wire - line 1=19Amps and neutral=19Amps for a "current of 19Amps 1phase 2wire L-N"
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: Three phase power

Originally posted by trotter: So if the equipment needs 12 amps to run does that mean it's pulling 12 amps on phases A, B and C totaling 36 amps between the three when using 3 phase, 3 wire?
No, it means it's pulling 12 amps on phases A, B and C totaling 12 amps. Each of Jim?s examples is showing the same thing, in detail. But the ?simple? fact is that you don?t add amps in the same way that you would add apples. It?s a trigonometric addition. You must account for the relative phase angles, in addition to the magnitudes.

See if this analogy helps: Consider a central train station, with a dozen tracks leading into it (or out of it, if you prefer) from all directions. You observe the station for an hour, and count three trains that enter and three trains that leave. Does that add up to six trains? Not necessarily. Perhaps the train that you counted as it entered the station from the North is the same train that you counted shortly thereafter, as it left towards the West. You don?t count the same train twice. Rather, you count the trains that leave as ?plus 3 trains? and the ones coming in as ?minus 3 trains,? and get a result of ?no change in the number of trains holding at the station.?

Any three-phase machine (e.g., generator, transformer, or motor) will work the same way. Current that enters the machine along one phase will leave along one of the other two phases. You don?t count the same current twice.
 

trotter

Member
Re: Three phase power

Fantastic. Thanks for helping me understand this. But I just have one more question concerning the current.

Going back to my other post about a 200 amp PDU. There's a monitor on it that shows how much current is running through each phase. From that I can determine how much room I have left over on each phase.

Each phase we're allowed 160 amps which is 80% of the 200 amp rating. We're rougly showing 120 amps per phase. Phase A 120, Phase B 120, Phase C 120.

I understand that the total current between the 3 phases would be 12 amps. Since there is a 120 degree of sepearation between the phases.

I'm wondering if the my used current will increase from 120 amps per phase to 132 amps per phase resulting in Phase A 132, Phase B 132, Phase C 132?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: Three phase power

Don't worry about phase angles.

You have a 3 phase system (you don't say if you have a neutral). In your words, this system is capable of supplying 160A per phase and you have used 120A per phase.

So your possible phase loading caused by adding a 12A load is:
3wire L-L-L, -> A=120+12=132A, B=120+12=132A, C=120+12=132A
or
2wire L-L -> A=120+12=132A, B=120+12=132A
or
2wire L-N -> A=120+12=132 (neutral current is a more complicated answer because you are combining circuits)
 

trotter

Member
Re: Three phase power

Yes, it's three phase neutral.

Thanks to you all for helping me with this. Everyone provided input that helps.
 

brian john

Senior Member
Location
Leesburg, VA
Re: Three phase power

Mr. Trotter:

Assuming the PDU is in a data center (critical loads) you state

"Sorry, I'm not much of an electrician...."

for yourself and your customer please take the time to get further educated (and you are doing this by posting here.) There is a wealth of information available free on the web, books are available that are basic but through and of course seminars are offered that can add to your brain power.

Do not under estimate yourself, if your willing to learn the information is available. I'm pretty much a dolt and have to read some things several times before the idea sinks in, but it eventually does (I hope). You'll never damage your brain with information.
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase power

Trotter

I'm not an engineer and do not claim to be one.
Jim and Charlie if I am wrong please correct me I'm sure you will.

Shouldn't the answer to the question be that the total power of the 12 amp. on a three phase system be equal to 12x1.73=20.76 amps. for the full amperage instead of 12 amps. which we would see on a single phase? In other words since this is threephase instead of the three legs equalling 36 amps. the full three phase power in amps. is equal to 20.76 amps. and each phase leg pulls 12 amps. ain't this alfull close to your figure of 20 amps. you give for a single phase?

Ronald :)

[ March 24, 2004, 08:54 PM: Message edited by: ronaldrc ]
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: Three phase power

Originally posted by ronaldrc: Shouldn't the total power . . . be equal to 12x1.73=20.76 amps. . . ?
It may be unintended, but you appear to be crossing apples with oranges. Power = watts (or VA, or VAR); current = amps. The original question started with a load whose rating was given as 12 amps, 3-phase. That means that the current on each of the three legs will be 12 amps. However, to find the ?apparent? power (in VA), you do need the factor of 1.73 (the square root of three). But you also need to multiply by the voltage. Let us assume we are talking about a 120/208 volt system. If you multiply your answer of 20.76 times 208, you get the correct value of three phase apparent power: 4,318 VA. You were correct in stating that the 1.73 would not be used for a single phase system.

Does this help clarify the situation?

(Edited to correct "120" to "120/208," and to correct the resulting VA. Mea Culpa. Charlie)

[ March 25, 2004, 06:47 PM: Message edited by: charlie b ]
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: Three phase power

Ronald,
I am not trying to be condescending but,

Amps is Amps - three phase is no different than single phase

Line to Line volts is Line to Line volts - three phase is no different than single phase
Line to Neutral Volts is Line to Neutral Volts - three phase is no different than single phase (well almost, 1phase L->N = L->L/2 and 3phase L->N = L->L/1.73)

Power is not as easy (commonly called watts these normal formulas really are for VA).
Line to Neutral Power is Line to Neutral power - three phase is no different than single phase Watts = L->N Volts x Amps.
Line to Line Power is different -
3phase 3wire Watts = L->L Volts x Amps x 1.73
3phase 2wire Watts = L->L Volts x Amps
1phase 2wire Watts = L->L Volts x Amps

From the above you can see that the only time we use the 1.73 factor for three phase is when we determine power using all three phase conductors or when we convert from L->N volts to L->L volts.

So six seperate 6A 120V 2 wire loads can be fed: 1 circuit 2wire 120V 36A, 2 circuits 2wire 120V 18A each, or 3 circuits 2wire 120V 12A each. Notice it makes no difference if the system is 1phase 2wire (120V), 1phase 3wire (120/240), or 3phase 4wire (208Y/120V).

And, the power supply requirements for these 6 loads is: 4.32KVA at 120V(1 circuit-36A), 4.32KVA at 120/240V (2 circuits-18A), and 4.32KVA at 208Y/120 (3 circuits-12A)
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Three phase power

Originally posted by ronaldrc:
Shouldn't the answer to the question be that the total power of the 12 amp. on a three phase system be equal to 12x1.73=20.76 amps. for the full amperage instead of 12 amps. which we would see on a single phase? In other words since this is threephase instead of the three legs equalling 36 amps. the full three phase power in amps.
No
Watts for 3-phase:
W = E x I x PF x 1.73
So using 12-amps for I, and .9 for PF we get;
3886 Watts = 208-volts x 12-amps x .9 x 1.73

Watts for single phase:
W = E x I x PF
So using the same 12-amps for I, and .9 for PF we get;
1296 Watts = 120-volts x 12-amps x .9

So if you have a 208/120Y 3-phase transformer with 12-amps on each phase you have 1300 Watts per phase, and 3900 total for all three phases P1 + P2 + P3 = Pt.

So if you raised the current in each phase what happens? If you wanted the same 3000 watts out of a single phase, you need 27.78-amps with a PF of .9

[ March 25, 2004, 04:37 PM: Message edited by: dereckbc ]
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase power

Dereck

My 208 volt transformer at 12 amps. gives me 4318 VA or 4.32 KVA maybe I'm figuring it wrong.

208 volts x 12 amps. x 1.73=4318.08

Please help?

Ronald :)
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Three phase power

You calculated VA rather than Watts. My example included a PF of .9

For 3-phase VA use
VA = E * I * 1.73

Single phase
VA = E * I

Using VA you come up with
4318 VA for 3-phase
1440 VA for single phase.

Does that help?
 
Status
Not open for further replies.
Top