Balanced currents are currents flowing in two conductors and are equal in magnitude and opposite in direction at every point along the line. Sometimes referred to as push-pull currents, they have many applications in electricity. Unbalanced currents, on the other hand, are unwanted and are caused by unbalanced primary voltage, shunted single-phase loads and defective transformers.
Winding Failure
Unbalanced currents and voltage magnitudes depend upon the amount of imbalance. Excessive increases of currents caused by small voltage imbalances can flow in motor windings, resulting in motor burnout when their effects are severe. Temperature increase as a result of these imbalances is approximately two times the square of the percentage voltage unbalance. To illustrate, a 3.5 voltage imbalance and, ultimately, current imbalance will result in a 25 percent increase in temperature rise. From another perspective, a 10 percent voltage decrease would cause a 10 percent amperage increase.
Torque Effect
Breakdown and locked-rotor torques (operational power) are decreased in current imbalances. If this unbalance is extremely severe, the torque might not be adequate for application. Full load speeds are affected when noise and vibration levels are high. Rapid deterioration of the insulation system and eventually premature winding failure occurs when motors are operated under unbalanced voltages for long.
Harmful Harmonic Currents
Heating (due to winding losses) is calculated by the formula (I*I)*R, where I is the current and R is resistance. This can cause additional heating, both in motors and in supply wiring. The percentage of harmonic current increases significantly as third and even-order harmonics are taken into consideration.
Overload Tripping
Frequent temperature changes that will keep disconnecting lever section of the circuit breaker due to induced heat is a common problem with current imbalance. A thermistor or a thermostat to reset the tripped device after the motor cools down may be the only solution. Even where a thermistor is used, the heat sink construction and the motor design may not allow for fast response times, resulting in frequent delays during operation. It is even more challenging where a number of temperature-sensing devices are part of the circuit.
Single-Phasing
This occurs when one phase of a three-phase system is open and the motor tries to deliver full current. This delivery is the worst-case scenario of a voltage and current imbalance since it will continue until the motor burns out.
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