ECN Forum Forums General Topics of the Trade General Discussion Area phase unbalanced, industrial facilities

This ones for all you industrial electrical guys. Question is how much of a phase unbalance do you guys see in older factories. when plants are set up they are balance fairly well but over time the system can become unbalanced by the adding of single phase circuits. how much is exceptable and how will this effect motors on machines and other equipment. i took some readings from the main panel that feeds our overhead buss bar and this is what i came up with after the figures

it came to 4% unbalance which is equivalent to a 32% raise in temp for motors,

i also took a reading on a motor on one of our machine tools and came up with a 3% unbalance or 18% increase to motor temp.

so how much is exceptable and how can you correct for years of adding single phase circuits with out taking phase unbalance into consideration.

jlhmaint,

What is you method to determine the correlation between %imbalance and temp. rise?

i looked up some information on line and was given the formula. measure phase to phase for example:

480 + 478 +476=

1434/3= 478

480-478=2% unbalance

2squared times 2 = the % temp increase which in this example would be 8%

I can't see where the imbalance adding single phase ckts would effect the motor temp on a 3ph motor, since the neutral isn't being used anyway?

Aldav53,

If the voltage on one phase of a system lowers due to voltage drop caused by higher loading of that phase, that motor phase winding has to draw proportionally higher current to keep doing its share of the work.
Heat increase in a winding is proportional to the the current increase squared.

Ed

Phase Unbalance

Unbalance of a three-phase system is less extreme than a complete loss of phase, but may have similar consequences. On new installations of three-phase power systems, careful attention is given to balancing the loads on each phase. However, as single-phase loads are added to these originally balanced systems, an unbalance may occur. Thermal overloads, magnetic breakers, and other such devices will not detect this gradual unbalance and therefore will not provide adequate protection.

Voltage unbalance of a three-phase system is expressed as a percentage value, and is often defined as the maximum deviation from the average of the three-phase voltages or currents, divided by the average of the three-phase voltages or currents. This voltage unbalance is calculated as shown below:

Voltage Unbalance = 100 x Maximum Deviation from Average Voltage
Average Voltage

With phase-to-phase voltages of 230, 232, and 225, the average would be 229 volts.

230 + 232 + 225
3 = 229

The maximum deviation from the average is 4 volts.

229 - 225 = 4

Therefore, the unbalance is 1.75 percent.

100 x 4
229 = 1.746

Phase voltage unbalance causes three-phase motors to run at temperatures greater than their published ratings. This excessive heating is due mainly to negative-sequence currents attempting to cause the motor to turn in a direction opposite to its normal rotation. These higher temperatures soon result in degradation of the motor insulation and shortened motor life. The percent increase in temperature of the highest current winding is approximately two times the square of the voltage unbalance. For example, a 3 percent voltage unbalance will cause a temperature rise of about 18 percent.

3 squared x 2 = 18

The greater the unbalance, the higher the motor winding temperature and the sooner the insulation will fail. NEMA standards recommend a maximum voltage unbalance of 1 percent without derating the motor. The motor can be derated down to 75 percent for a maximum of a 5 percent voltage unbalance. If the voltage unbalance exceeds 5 percent it is not recommended that the motor be operated. A rule of thumb states that for every 10°C a motor is operated over the rated temperature rise, insulation life (and therefore motor life) is reduced by half.

ok i made a little mistake my unbalance is not that high i forgot to multiply by 100 then divide by my average so in the example it would be
200/478 = .41% instead of 2% so the figures i have at work are within limits sorry about that

It sounds like you are talking about the voltage difference between phases. I'm used to looking at unbalanced loads in terms of amperage, and then I usually concern myself with the neutral for a wye system. I can't say that I've ever really thought about the phase imbalance in older factories, and how it causes temperature rise. Every time I've been called on to install equipment in an older establishment, I would check the phase imbalance, and add my circuits in a way that would best balance the load (if possible). I've seen some 3 phase, 240 volt "high leg" installations where there was almost no load on "B" phase, and as far as I know, there were no adverse affects on the motors. In your situation, I'm not sure how much imbalance is acceptable, but I can tell you what I would consider.
1. Is this problem causing premature equipment failure?
2. Are there transformers with taps that can be changed?
3. Is it cost effective to make the changes?
4. Are there safety hazards involved?
5. Are there any future expansions to the plant that will enable you to correct the load imbalance without changing the circuiting for existing equipment?
I'm sure that there are other things to consider, and there are others on this forum that can give you more advice. IMHO, if the first 4 items don't apply, then I would look to item number 5, and use it help "head off" a problem that may get worse in the future.

Mentally imbalanced,
Doc

well DOC we just wanted to see how bad ours was just in case. We are not having any problems, but before i got there no one had even opened a code book or took things like this into consideration i am trying to change this and make all of our new instalations code compliant and keep our system balanced. After checking my figures agian we are fine on the unbalance but will need to pay attention on future instilations.

I don't blame you maint. If you don't do it now, someone will have to do it later. I don't work in the "factory" environment, but the company that I just started with was, in the past, lacking in the "code compliant" installation catagory, and it looks like I will be the one that repairs it all.

Best of Luck,
Doc