3 Phase Current Flow

kody916

Member
Location
Illinois
Greetings Everyone.
Just trying to get some old stuff revised but cant seem to do it.need help.

When we have a single phase load say a bulb, there are two wires connected to that bulb.
1. Hot Wire 2. Neutral Wire.

the current flows from the hot wire and then returns back via neutral wire. this way current completes its path.

when we have a balanced 3Ph system( STAR or WYE), each phase draws the same amount of current and the neutral current provides the return path.

How is the return path provided incase of DELTA system?there are only 3 phases.so how is return path provided if we have a 3ph load for delta system.

Thanks
 

xformer

Senior Member
Location
Dallas, Tx
Occupation
Electrician
Greetings Everyone.
Just trying to get some old stuff revised but cant seem to do it.need help.

When we have a single phase load say a bulb, there are two wires connected to that bulb.
1. Hot Wire 2. Neutral Wire.

the current flows from the hot wire and then returns back via neutral wire. this way current completes its path.

when we have a balanced 3Ph system( STAR or WYE), each phase draws the same amount of current and the neutral current provides the return path.

How is the return path provided incase of DELTA system?there are only 3 phases.so how is return path provided if we have a 3ph load for delta system.

Thanks
Someone can correct me if I am wrong, but I dont think the neutral carries the same amount of current in a Balanced 3ph star system.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Greetings Everyone.
Just trying to get some old stuff revised but cant seem to do it.need help.

When we have a single phase load say a bulb, there are two wires connected to that bulb.
1. Hot Wire 2. Neutral Wire.

the current flows from the hot wire and then returns back via neutral wire. this way current completes its path.

when we have a balanced 3Ph system( STAR or WYE), each phase draws the same amount of current and the neutral current provides the return path.

How is the return path provided incase of DELTA system?there are only 3 phases.so how is return path provided if we have a 3ph load for delta system.

Thanks
You could also ask how the current flows in a 240 volt load in a 120/240 3-wire single phase system.
It does not flow from L1 through the load to N, back from N into the load and from there to L2. It just flows from L1 to L2 and the neutral is not needed.
If you have 120 volts loads from L1 to N and from L2 to N and they are equal sized, then no current will end up flowing in the neutral. But the neutral must still be provided at each load so that is still sees 120V even if there is an imbalance.

In the same way, with a wye configuration you can have 120 volt loads from L1, L2 or L3 to N. And the current in N will be only the imbalance between the three loads.
But you can also connect a load from L1 to L2, getting 208 volts and not requiring any connection to a neutral at all. Same for L2-L3 or L1-L3.

In the case of a delta supply, the loads must be connected between the line conductors and the current flows out one and back on the other. No need for a neutral. You cannot have any line to neutral loads in a delta because there is no neutral!
A single phase load would be, for example, connected from L1 to L2. A three phase load would connect to all three line conductors and the currents would be flowing among those three wires in different directions at different times during one cycle.
If this were a 240V delta system, the voltage from L1 to L2 would be 240V.
 

fmtjfw

Senior Member
You still need a neutral conductor

You still need a neutral conductor

You could also ask how the current flows in a 240 volt load in a 120/240 3-wire single phase system.
It does not flow from L1 through the load to N, back from N into the load and from there to L2. It just flows from L1 to L2 and the neutral is not needed. ....
You still need the neutral between the two loads. That part of the neutral carries the same amperage as L1 or L2 does. The neutral between the loads and the source carries no current.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
You still need the neutral between the two loads. That part of the neutral carries the same amperage as L1 or L2 does. The neutral between the loads and the source carries no current.
If you look closely you will see that I specified a 240V load. Most 240V loads will not have a neutral connection at all. Those that do are usually really two 120V loads or a mixture of 240V loads and 120V loads.
When I mentioned two back-to-back 120V loads I did say that the neutral was necessary whether it carried current or not.
 

fmtjfw

Senior Member
Sorry

Sorry

If you look closely you will see that I specified a 240V load. Most 240V loads will not have a neutral connection at all. Those that do are usually really two 120V loads or a mixture of 240V loads and 120V loads.
When I mentioned two back-to-back 120V loads I did say that the neutral was necessary whether it carried current or not.
once again I conflated your reply with the original question about neutrals and did not read your reply closely
 

Besoeker

Senior Member
Location
UK
Greetings Everyone.
Just trying to get some old stuff revised but cant seem to do it.need help.

When we have a single phase load say a bulb, there are two wires connected to that bulb.
1. Hot Wire 2. Neutral Wire.

the current flows from the hot wire and then returns back via neutral wire. this way current completes its path.
Agreed

when we have a balanced 3Ph system( STAR or WYE), each phase draws the same amount of current and the neutral current provides the return path.
If the load is balanced, there is no neutral current. Unbalanced loads will result in neutral current. The magnitude and nature of the phase to neutral loads will determine that neutral current.

How is the return path provided incase of DELTA system?there are only 3 phases.so how is return path provided if we have a 3ph load for delta system.
If there is there is no neutral there can be no phase to neutral loads. Call the conductors L1, L2, and L3. Any single phase load has to be connected between two of those conductors. Say L1 and L2. The current flows between those two.

A point you may want to think about. Current flows in a closed circuit with a potential difference (voltage) across it. That's all it needs. It doesn't matter if the circuit has a neutral or is referenced to ground. It just needs to be a closed circuit.

Another. It is convenient to think of or visualise a go and return for live (hot) and neutral with the neutral being the return. It isn't actually like that. For AC, the current oscillates back and forth between the conductors. Which is called neutral is usually determined by how the circuit is configured and which point is grounded. For a star (wye) it is common for the star point to be a grounded (earthed) neutral.

I know you guys commonly use 120-0-120 with the zero understandably taken as neutral and grounded. It limits the shock hazard to ground to 120V.
 

Smart $

Esteemed Member
Location
Ohio
...

How is the return path provided incase of DELTA system?there are only 3 phases.so how is return path provided if we have a 3ph load for delta system.

...
The potential between any two of the three wires is oscillating at 60Hz but not the same 60Hz. The sinusoidal waveform of the voltage and current (assuming a balanced load) is shifted 120 degrees.

If you connected three equal resistors, one end common to the other resistors and the other ends each to one of the three supply wires, the common node will in effect be at a neutral voltage potential to the system. The current through each resistor will be out-of-phase to the other resistors by plus or minus 120 degrees as will the voltage drop across each of the three resistors. Essentially, this demonstrates that three wires of a delta system is no different than three hots of a wye system with no neutral running to the load.

For a balanced three phase three wire circuit, the current of one wire returns to the source on the other two wires, but not equally except for twice in each waveform cycle when the instantaneous voltage potential is equal. The hard part to grasp is that the preceding statement is true of the current through each of the three wires.
 

kody916

Member
Location
Illinois
You could also ask how the current flows in a 240 volt load in a 120/240 3-wire single phase system.
It does not flow from L1 through the load to N, back from N into the load and from there to L2. It just flows from L1 to L2 and the neutral is not needed.
If you have 120 volts loads from L1 to N and from L2 to N and they are equal sized, then no current will end up flowing in the neutral. But the neutral must still be provided at each load so that is still sees 120V even if there is an imbalance.

In the same way, with a wye configuration you can have 120 volt loads from L1, L2 or L3 to N. And the current in N will be only the imbalance between the three loads.
But you can also connect a load from L1 to L2, getting 208 volts and not requiring any connection to a neutral at all. Same for L2-L3 or L1-L3.

In the case of a delta supply, the loads must be connected between the line conductors and the current flows out one and back on the other. No need for a neutral. You cannot have any line to neutral loads in a delta because there is no neutral!
A single phase load would be, for example, connected from L1 to L2. A three phase load would connect to all three line conductors and the currents would be flowing among those three wires in different directions at different times during one cycle.
If this were a 240V delta system, the voltage from L1 to L2 would be 240V.
so let me get this thing right
if there is a delta connected transformer feeding a 3 phase induction motor, when the circuit
is completed, current will flow in all the three phases A,B and C.
so what you are saying is the current that is flowing through phase A will return back via phase B & C. is it so?.
if thats the case, then dont u think that the return current will be opposing the incoming currents in phases B & C.

thanks for your earlier response.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
so let me get this thing right
if there is a delta connected transformer feeding a 3 phase induction motor, when the circuit
is completed, current will flow in all the three phases A,B and C.
so what you are saying is the current that is flowing through phase A will return back via phase B & C. is it so?.
if thats the case, then dont u think that the return current will be opposing the incoming currents in phases B & C.

thanks for your earlier response.
Yes, the currents in the three motor windings (which can be exactly measured separately, since they flow in separate coils) will add up as vectors where two windings come together at a phase line. Currents add and subtract all the time and and increase or decrease the net current. But they do not oppose each other in the sense of running into each other (like two cars trying to go in opposite directions in the same lane would.)
 

Besoeker

Senior Member
Location
UK
so let me get this thing right
if there is a delta connected transformer feeding a 3 phase induction motor, when the circuit
is completed, current will flow in all the three phases A,B and C.
so what you are saying is the current that is flowing through phase A will return back via phase B & C. is it so?.
if thats the case, then dont u think that the return current will be opposing the incoming currents in phases B & C.

thanks for your earlier response.
The line currents are out of phase with each other. At each point in time, their sum is zero.

 

Sahib

Senior Member
Location
India
The line currents are out of phase with each other.
They are not out of phase i.e 180 degree phase difference with each other. If the phase angle of phase 'A' is taken as 0, the phase angle of B is 120 and that of C is 240 for a positive sequence currents.
 

Besoeker

Senior Member
Location
UK
They are not out of phase i.e 180 degree phase difference with each other.
Out of phase means not in phase.
They are not in phase so they are out of phase.
Just look at my diagram.
Simples to paraphrase a television ad here.
 

mivey

Senior Member
If the phase angle of phase 'A' is taken as 0, the phase angle of B is 120 and that of C is 240 for a positive sequence currents.
A is 0, B is 240, and C is 120 for positive sequence currents.

'Out of phase' is usually associated with 180 degree phase difference.
Usually for you maybe. For the rest of us, it means not in phase and can be angles other than 180 degrees.
 

Sahib

Senior Member
Location
India
Are we on the same planet???

If what you say is the usual interpretation where you are from, we are definitely world's apart.
I think you misunderstood me. In technical literature, 'Out of phase' is usually followed with 180 degree phase difference. If the phase difference is other than 180, say 120, it is not usually stated as out of phase by 120 but simply phase difference is 120 degree.
At least that is the way I remember.
 
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