Unbalance 3-phase system vs Electrical consumption

Unbalanced voltage creates unbalanced current, at a ratio of around 3 to 5:1, meaning that if you have a 1% voltage unbalance, the current unbalance will be 3-5%, going up as the voltage unbalance % goes up. Unbalanced currents have an energy efficiency effect on motors mainly. "Negative Sequence Currents" will flow in an unbalanced current environment in a motor, which causes corresponding negative torque pulsations. So for a given load where the motor current is unbalanced, more units of power per unit of usable shaft torque will be consumed because the motor is essentially fighting itself. The excess energy is consumed in heating of the motor, which can also cause rapid insulation breakdown and eventual motor damage.

I would be interested in what this means from an efficiency standpoint. First exactly what does voltage unbalance mean? Would it be the maximum difference in voltage between phases divided by the average voltage?

Nesubra stated that if there is a 5% voltage imbalance, the motor has to be derated 20% to avoid overheating the motor. Assume the motor is 90% efficient. Does this mean that a 5% voltage imbalance will decrease efficiency to 88% ?

Your 90% efficient motor cannot be loaded to more than 80% of its nameplate load. This effectively reduces its efficiency far below the 88% you asked about. Even though the motor's load must be reduced by the 20% mentioned, the unbalanced currents are increasing the heat loss within the motor (due to magnetic hysteresis losses) so your system efficiency drops.

For non-rotating loads, such as lighting, many ballasted systems (fluorescent, High-Intensity Discharge, and similar) have compensation for voltage variation built in, so a difference here has a minimal effect on energy efficiency. For incandescent lighting, however, the effect is much more pronounced (as well as shortening the life span of bulbs with higher voltages). I believe that most of the watt-hour meters are OK with modest amounts of voltage imbalances, so your metered energy usage will show the real world usage.

--JMM

One good question: how accurate kWh meters could be in system voltage & frequency and other variations.?

I don't know. My comment assumed that an installed base of literally millions of the meters implies a relatively mature technology which would be capable of handling these variations. --JMM

Un balanced voltage produces un balanced current.This results in negative sequence currents which primarily causes heating in circuits.As rightly pointed out in the reply by member motor is worst affected by Unbalanced Voltage.A 5% unbalance means derating by 20% of the rating to run the motor within prescribed temperature limits .

How could unbalanced system contribute heat in a circuit where infact the excess of resultant current will go directly to the ground. Could this negative sequence currents occur within the circuit aside from the motors or three phase winding device?

Unbalanced AC equals DC. AC motors don't do well on DC. Some single phase AC motors can run on pulsed DC but not recommended.

Brad

Thanks for the effort but i think your misleading.

Ah, from the replies I understand your question. No offence meant but that could be misled. Motor syntax obliviously is not my forte.

Brad

Query, are you the guest from the original question?

Did you mean from the point of view with regards to a motor, then you already have a good answer, or are you wondering about say in a housing area with a big 3 phase transformer and say more lighting (or just load) on one phase, loading that phase up, or something similar?

Your question is not very informatively written!

Wow... so many errors ... DC? Unbalanced current go to Ground? In motors?

The simplest and most accurate answer was given immediately (#1) and it should stop the discussion w/o AC basic knowledge.

So.. the effect of unbalanced three-phase voltage has negative electromechanical effects.

southern123 ,Well said .When some one has given correct and appropriate answer to the thread there is no further place for discussion unless some one elaborates or add to the original comment . The reply by no. 1 is brief and correct.Better not to mislead further .