Unbalanced Three Phase Loads

Hello, and the loads are unbalanced. So what? We're just talking about three loads with a wire (neutral) in common. I don't know what you mean by "danger", academic or otherwise. If you're talking about an A.C. power distribution sysem (generators, transmission lines, loads) and grounded vs. ungrounded systems, ine-to-ground faults, fault isolation and related safety issues that's a subject unto itself. Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

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I see what you mean. OK, let's look at it from another perspective. Let's say there are six 500w resistive loads and because of existing wiring it is physically pragmatic to have them loaded 2-1-3. What then, if any, is the danger of that imbalance, and how can it be mitigated? What is the disadvantage?

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I see what you mean. OK, let's look at it from another perspective. Let's say there are six 500w resistive loads and because of existing wiring it is physically pragmatic to have them loaded 2-1-3. What then, if any, is the danger of that imbalance, and how can it be mitigated? What is the disadvantage?

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0

I see what you mean. OK, let's look at it from another perspective. Let's say there are six 500w resistive loads and because of existing wiring it is physically pragmatic to have them loaded 2-1-3. What then, if any, is the danger of that imbalance, and how can it be mitigated? What is the disadvantage?

Hello, and you have to be more specific about what you mean by "danger". As in a threat to life and limb? If so, in what way? Touching a bare conductor?

Or are we talking about energy being dissipated as heat in the transmission medium from source to load? I just don't know what you're asking without more specifics. Perhaps someone more erudite than I in the area of AC power generation, distribution, and utilization can help. Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

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I see what you mean. OK, let's look at it from another perspective. Let's say there are six 500w resistive loads and because of existing wiring it is physically pragmatic to have them loaded 2-1-3. What then, if any, is the danger of that imbalance, and how can it be mitigated? What is the disadvantage?

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John,

Sorry if I'm being a bit dense. I guess at the end of the day, my question is this:

What is the consequence- dangerous or not- of an unbalanced load? Greater heat? Change in Hz? Locusts?

Thanks, again, for your time.

Joe

The danger is that you will worry too much. If installed properly, there should be no problem.

The downside is that you will not be able to use the full capability of your conductors.

Bill

I see what you mean. OK, let's look at it from another perspective. Let's say there are six 500w resistive loads and because of existing wiring it is physically pragmatic to have them loaded 2-1-3. What then, if any, is the danger of that imbalance, and how can it be mitigated? What is the disadvantage?

In the case of resistive loads- if the supply is designed for 3000Watts balanced (1000 per phase) then the phase with 1500 watts is overloaded badly. The others are fine. If the system is such that you can handle 1500 watts on one phase (, then with a 2-1-3 loading, you are not making as good a use of your supply as you could (capable of 4500 watts) but otherwise no problem.

Unbalance in general is to be minimized if possible because you get more efficient use of your supply and also better voltages. With resisitive loads the latter isn't all that much of a problem but with a combination of resistive loads and 3 phase motor loads, there can be some negative effects on the motors. Lets face it, in low voltage distribution with single phase loads involved, there will be unbalance- and all that one can do is try to balance the total load as well as possible- recognizing that with 6 individual loads, the likelyhood of being balanced with a 2-1-3 situation is low- either no load or a load of 500 watts on each phase- other combinations are unbalanced.

So , what are the dangers? overloading of 1 phase resulting in overheating of one leg of the transformer (or tripping of breaker- preferrable) or having excess supply capacity which is not being used . For your temporary situation, the latter is no problem but the first could be. Don't sweat it.

Don Kelly snipped-for-privacy@shawcross.ca remove the x to reply

No problems as long as the feeder is sized to handle the largest phase load, the neutral is sized to handle the maximum imbalance (which would occur when all but one phase's loads are switched off) and the source impedance (positive, negative and zero sequence) is such that the maximum load imbalance doesn't create an intolerable voltage imbalance.

Mostly academic. The power company may have some demands.

If possible, it better to have single phase loads connected phase to phase rather than phase to neutral.

If you are stuck with the equipment then you can't do that.

In the case of motors, you sometimes find nameplates that claim 208-240 volts. Maybe! But you might have starting problem and maybe heating problems at the lower voltage and a drop in efficiency at the high voltage.

This is not a problem for the power company for loads of this size. Not with the loads you're talking about.

Unbalanced three-phase loads *can* be a problem if the imbalance represents a sizeable fraction of the generator. Three-phase generators that are connected to unbalanced loads of significant amount are subject to what's called 'negative sequence currents'. If they are a large amount this can cause rotor-end heating that could damage the machine.

But unless you're supplying these loads with your own, small 3-phase generator, there doesn't seem anything here to worry about.

daestrom