unbalanced three phase
- Thread starter ckann
- Start date
- Status
- Location
- Massachusetts
Re: unbalanced three phase
You can start with this
http://www.bacharach-training.com/norm/protecting.htm
You can start with this
http://www.bacharach-training.com/norm/protecting.htm
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: unbalanced three phase
Lets look at an extreme case. Consider a 100 amp, 120/208 volt panel. Connect single phase loads from phase A to neutral until you get a total load resistance of 1.2 ohms. The current in phase A will be 100 amps (from the formula 120V / 1.2 ohms). Now connect the same amount of load from phase B to neutral, and the same amount from phase C to neutral. You have a balanced system, with each phase carrying 100 amps.
Now go disconnect all loads from phase B and from phase C, and connect them all to phase A. When you connect these loads, you are placing the loads in parallel. So the original 1.2 ohms of phase A loads are now in parallel with 1.2 ohms from phase B and another 1.2 ohms from phase C. This will cause the total resistance to be deduced to one third of its original value, or 0.4 ohms. When you impose 120 volts across a 0.4 ohm resistor, the total current is 300 amps.
NOTE: You don?t have to tell this whole story to the non-electrical people. Just tell them the total current in phase A goes up to 300. But I wanted to make sure you knew how I arrived at that number.
In this situation, do you think that the phase A conductor will be able to handle a 300 amp load? Probably not. Do we size conductors at three times the expected load, just in case someone puts all the load on one phase? We do not.
In this extreme example, the phase A conductor will overheat. If you followed the link that Bob provided, you can read about wires in a motor overheating. The reason is the same in both cases. You get overheating because one phase will have higher current than the others, and the flowing of current is what causes the wire to heat up.
Lets look at an extreme case. Consider a 100 amp, 120/208 volt panel. Connect single phase loads from phase A to neutral until you get a total load resistance of 1.2 ohms. The current in phase A will be 100 amps (from the formula 120V / 1.2 ohms). Now connect the same amount of load from phase B to neutral, and the same amount from phase C to neutral. You have a balanced system, with each phase carrying 100 amps.
Now go disconnect all loads from phase B and from phase C, and connect them all to phase A. When you connect these loads, you are placing the loads in parallel. So the original 1.2 ohms of phase A loads are now in parallel with 1.2 ohms from phase B and another 1.2 ohms from phase C. This will cause the total resistance to be deduced to one third of its original value, or 0.4 ohms. When you impose 120 volts across a 0.4 ohm resistor, the total current is 300 amps.
NOTE: You don?t have to tell this whole story to the non-electrical people. Just tell them the total current in phase A goes up to 300. But I wanted to make sure you knew how I arrived at that number.
In this situation, do you think that the phase A conductor will be able to handle a 300 amp load? Probably not. Do we size conductors at three times the expected load, just in case someone puts all the load on one phase? We do not.
In this extreme example, the phase A conductor will overheat. If you followed the link that Bob provided, you can read about wires in a motor overheating. The reason is the same in both cases. You get overheating because one phase will have higher current than the others, and the flowing of current is what causes the wire to heat up.
rod
Member
- Location
- North Carolina
Re: unbalanced three phase
Let me take a crack at this using a real world example in very simple terms. Right before I deployed to Afghanistan with the 82nd Airborne Division, we did a practice set up our headquarters; tents, generators, the whole thing. While I was picking up the generator my people were setting up the operation. They set up the DICE boxes (nothing more than 120/208 single phase circuit panel with 60 amp mains). Everyone and his brother plugged their little extension cords into whatever receptacle they saw first. Now of course, you can infer that the generator is a 120/208 1ph/208 3ph generator, typical army generator. I was extremely unhappy and apprehensive when I got back and saw this spider web of extensions and cables but the show had to start. As soon as we cranked things up main breakers starting tripping. The division operations officer told a sergeant, upon my request, to go out and get me an ammeter so that I could get a reading on the feeder cables to these panels. Want to guess what I found? -- leg A: 8A, leg B: .6A, leg C: 59A and periodic demand increases caused the main to trip. The short answer is, your loads must be balanced as well as possible because they're protected by an OCPD. One leg that is extraordinarily loaded will stop the show. It's almost impossible to balance loads exactly given differing demands. We got it figured out and as a result of that lesson learned we had flawless command and control of our forces both in Afghanistan and Iraq.
Let me take a crack at this using a real world example in very simple terms. Right before I deployed to Afghanistan with the 82nd Airborne Division, we did a practice set up our headquarters; tents, generators, the whole thing. While I was picking up the generator my people were setting up the operation. They set up the DICE boxes (nothing more than 120/208 single phase circuit panel with 60 amp mains). Everyone and his brother plugged their little extension cords into whatever receptacle they saw first. Now of course, you can infer that the generator is a 120/208 1ph/208 3ph generator, typical army generator. I was extremely unhappy and apprehensive when I got back and saw this spider web of extensions and cables but the show had to start. As soon as we cranked things up main breakers starting tripping. The division operations officer told a sergeant, upon my request, to go out and get me an ammeter so that I could get a reading on the feeder cables to these panels. Want to guess what I found? -- leg A: 8A, leg B: .6A, leg C: 59A and periodic demand increases caused the main to trip. The short answer is, your loads must be balanced as well as possible because they're protected by an OCPD. One leg that is extraordinarily loaded will stop the show. It's almost impossible to balance loads exactly given differing demands. We got it figured out and as a result of that lesson learned we had flawless command and control of our forces both in Afghanistan and Iraq.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: unbalanced three phase
Rod: Thanks for the practical real-world example, for it confirms my theoretical discussion.
Rod: Thanks for the practical real-world example, for it confirms my theoretical discussion.
charlie tuna
Senior Member
- Location
- Florida
Re: unbalanced three phase
rod,
keep your head down over there---and thanks for you time in service!
.
the drawback i see concerning balanced loads is the problem with using the system to it's full capacity. as described above--100 amp three phase panel and the loads are:
a phase - 6 amps
b phase - 4 amps
c phase - 58 amps
you could have a problem adding a 25 amp three phase load to this panel that is originally loaded to about twenty per cent of it's capacity! if you can balance the original load, you could add twice as much load with no problem!
rod,
keep your head down over there---and thanks for you time in service!
.
the drawback i see concerning balanced loads is the problem with using the system to it's full capacity. as described above--100 amp three phase panel and the loads are:
a phase - 6 amps
b phase - 4 amps
c phase - 58 amps
you could have a problem adding a 25 amp three phase load to this panel that is originally loaded to about twenty per cent of it's capacity! if you can balance the original load, you could add twice as much load with no problem!
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