Variable-speed drives draw a current that is far from a sine wave, and years of single-phase circuits landed on whichever phase was handy leave the three uneven. Both heat the board and both wear the drives. We meter the board for a week and specify the correction.
Harmonic current from the 6 drives heats the neutral, and single-phase circuits landed on phase C heat its bus. The board runs 17 degrees hotter than its neighbours, from one cause.
A three-phase board is designed to carry the same current on each phase. Single-phase circuits added over the years rarely respect that.
Calder Works, the specimen plant in the scrolled story: A packaging plant with six variable-speed lines. The main distribution board, before and after 3 circuits were moved.
Phase C carries 262 A and phase B 198 A, an imbalance of 14.7%. Office lighting, the break room and the compressor room lighting were all landed on the heavy phases, one job at a time, and nobody rebalanced.
Moving 3 single-phase circuits brings the imbalance to 0.9%. It is a morning's work for an electrician, and the study says exactly which circuits.
A drive takes power from the supply in pulses, and the pulses add up to current at multiples of the mains frequency. A clamp meter reads the total. A power quality meter reads each part.
The same board. Each harmonic order as a share of the fundamental current, before and after line reactors and a passive filter on the drive section.
29.5% of the current on the specimen board is harmonics. The third harmonic from all three phases adds in the neutral instead of cancelling, so a conductor that should carry almost nothing carries 58 A and runs hot.
Filtered at the drives, distortion falls to 9.9% and the neutral to 14 A. The correction is specified for the plant's electrician, and the board is read again after it.
Harmonic current heats the neutral and the transformer, and an imbalance heats one phase's bus. The specimen board ran 17 degrees hotter than the boards beside it, and 2 degrees after the correction.
A drive fed from a distorted, uneven supply runs its own electronics hotter and trips on faults that look random. Two drives failed on the specimen plant in a year.
On a balanced board with clean current the neutral carries almost nothing. Third harmonics add in it: 58 A on the specimen board, 14 A after.
Written for your electrician: which circuits move, what goes on each drive feeder, and what the board should read after.
Move 3 single-phase circuits off the heavy phases: Ckt 7, Ckt 11, Ckt 15. After the moves the phases read 230, 234 and 232 A.
Line reactors on each drive feeder and a passive filter on the drive section, specified to bring distortion under 10%.
Imbalance under 3%, the neutral under 20 A, and the board within a few degrees of its neighbours.
We write the board down phase by phase and put a power quality meter on it for a week.
We read the current on each phase, the harmonics in it and the neutral, and name the cause with its evidence.
We specify the moves and the filters for your electrician, with what the board should read after.
We read the same board again after the work, and a licensed engineer signs the result.
Current and voltage on each phase, the harmonics in the current and the neutral, from a week on a power quality meter.
From the meter on the boardThe imbalance and the distortion, where each comes from, and what each is doing to the board and the drives.
Reviewed by an engineerWhich circuits move to which phase, what goes on each drive feeder, and what the board should read after.
Your electrician makes the changeThe same board read again: imbalance, distortion, the neutral and the temperature against its neighbours.
After the workA power quality meter on the board for a week of normal production, and the panel record for every circuit on it, so each single-phase circuit can be placed on its phase. A clamp meter cannot do this; the harmonics only show on an instrument that reads them.
Moving circuits and fitting reactors and filters is electrical work, made by your electrician. Wattif specifies it, reads the board again after it, and a licensed engineer signs the result under their own licence.
