In this issue:

Waveform of the Month

~ The waveform below consists of an underground voltage source. Notice the Phase-to-Phase Voltage is good, but the Phase-to-Ground Voltages are floating. This is also called a bad Ground Reference.

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~ Voltage Differential or Voltage Imbalance, as it is more often called, is a problem for feeders and electronic equipment. The feeder is simple, if one phase has more loads on it than the other phases at the same time, than that is a Voltage Imbalance. Since the National Electric Code requires protection, the circuit breaker will trip before any serious problem on the feeder can occur. However, in today’s modern complexed electronics, Voltage Differential or imbalance can cause internal damage.

~ X-Ray generators, Gradient Amplifiers, Laser imaging systems, Complexed printers and more are all vulnerable to Voltage Imbalance issues. What happens is the change in the voltage between the phases causes an uneven current flow internally. If a system requires 100kWatts of power per phase on a 480V feeder, the system is expecting 120Amps on each phase. If one of the phases is 3% lower than the other two phases (14.4 Voltage on the 480V) that means that the current goes from 120Amps on that phase to 124Amps or a 7% increase in losses! The heat losses increase the warmer the components get. This also lowers the resistance, which increases the current flow. It only takes a small change in the voltages to have major effect on the overall system reliability.

~ The phase-to-phase Voltage Imbalance should not be greater than 2% at any time. Sags that occur on the low phase can now have catastrophic effects on the system as the current is extremely high. If there are momentary sags, now the system is in a dangerous condition as the low voltage again creates an increase in the current flow and things can get out of hand quickly. Protection requires a form of an on-line UPS or certain types of voltage regulators. In order to properly diagnosis this, you must monitor both voltage and current feeding the system.