I recently investigated a noise complaint, loud transformer hum from a MultiPlus II 5kVA running ESS.
The inverter is installed in the attic and the people sleeping in the bedroom below were complaining about the loud hum transmitted through the building structure every evening. But when I was on site, the hum was not audible.
After a while, mostly by accident, we found that the loud hum appears under very specific conditions: when the dishwasher is running. They were starting the dishwasher before going to bed every evening and that caused the transformer to buzz.
I used an oscilloscope and took a snapshot of the grid voltage and the current absorbed by the dishwasher.
Can anyone guess what is wrong with this picture causing the transformer hum?
It could be a very unfavourable control of the heating element… not constant power but pulsed control… the multi is really happy about that… plus the control delay of the electricity meter…
The current absorbed from the AC grid by the dishwasher has a DC component because all the energy is absorbed in the same voltage polarity.
This DC component (80 - 100 mA) makes a MultiPlus-II buzz like there’s an angry bee stuck inside it.
A DC component can bias or saturate the magnetic core of the output transformer. AC Transformers really don’t like DC through their coils.
The total (AC + DC) RMS current is about 500 mA at 230V, and the power factor is 0.44, although the displacement power factor is 1.
The dishwasher uses a cheap single-diode rectifier at its input and this is used to power everything inside except for the heating element, but that is activated only for about 2 x 10 minutes during a 3 hour washing cycle.
I’ve saw cases where APC UPSes (SUA3000RMI2U) hummed like crazy in the same cases when only one side of the sine wave were used.
The same technology like in your case, MP2-5K: low frequency inverter with classic transformers.
Solution at the time, because it was a case study where anything goes in order to stop the noise: replace the entire case, especially the elements around the transformer, with a non-ferrous material: aluminium.
Result: almost total silence.
As a side note, the HF inverters, like RS range in the case of Victron, make no noise when unballanced loads like above are used.
OK, no audible noise by human ear…
