Calculating Voltage Imbalance

Working on a customer site with Fluke 1750 data at the moment. The customer notes “Error shows udc voltage out of tolerance between stationary and rotating portions of the gantry.” – problems with the main DC bus voltage. Made me think it might be voltage imbalance, causing high DC bus ripple.

Voltage Imbalance Chart 1

Looking at the Fluke 1750 voltage imbalance chart shows a maximum imbalance of 1.1344%.

Voltage Imbalance Chart 2

Looking at the individual RMS voltage measurements, the RMS values are Ph A = 281.34, Ph B = 281.715 and Ph C = 276.725. Voltage imbalance seemed a little high to me; so I decided to double check the Fluke 1750 voltage imbalance.

There’s a good article on calculating voltage imbalance on the ACHRNews (Air Conditioning / Heating / Refrigeration News)¬†Website, here – Three-Phase Motor Voltage Unbalance – specifically, calculate the average of the three phases, take the maximum deviation from average (of the three phases), and divide by the average.

In this case, the average voltage is 279.927 Vrms, and the maximum deviation is Phase C, 279.927 – 276.725 = 3.202 Vrms, Voltage Imbalance = 3.202 / 279.927 = 1.144%.

Close enough. The customer requirement is 2%, so while this imbalance looks a bit large (nearly 5 Vrms between Phase C and the other phases), it seems to meet the requirements.