Two days ago I conducted the first successful resistive load test of a 3 phase power meter that can measure voltage, current, power consumption, harmonic distortion …
I wanted a small unit to plug into, or between two, mains feed cables to measure voltage, RMS current, power factor and harmonic distortion.
I found this Merlin Gerin PM700 power meter at a fair price on ebay and decided to find some current transformers and hopefully be able to fit it all in a small box.
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I wanted to build the unit for continues 32 A load, so that it could be used in larger installations. So I had to make some 16 A to 32 A CEE adaptors in order to use it with my more regularly used 16 A fed items.
|Merlin Gerin PM700||True rms up to the 15th harmonic
on three-phase (3P, 3P + N)
two-phase and single-phase AC systems
32 samples per cycle
|Current transformer||30 A / 5A (2 turn)|
29th March 2016
Finding a box among those I had in stock turned out this aluminium enclosure where everything would barely fit. I really wanted this to be as small as possible, so I took the challenge of fitting it all into it.
It is a wonderful tool that provides accurate measurements of voltage, current, power consumption, power factor and harmonic distortion. It will make a great addition to bettering the power factor of Tesla coils, to find out what kind of corrections that work the best at different loads.
Unfortunately it requires externally power from 230 VAC, I could just have built this in to take from phase and neutral, but that would render the unit useless with a variac, at low voltages, in front of it.
30th August 2016
First demonstration of how it is built and a test with a pure resistive load from a 9 kW electric heater.
This is chapter 5: PFC of the DRSSTC design guide
Power factor correction
It would be optimal to feed all DRSSTCs from a boost converter with PFC front end, but it can be a complex task to undertake for the power ratings we need. There is three ways to deal with power factor problems in regard to the average Tesla coil experimenter.
1. Do nothing about it. This just means that AC side components like wires and rectifiers will have to have higher ratings and that you do not get as much real power on the DC link side as you are drawing in apparent power on the AC side, due to the reactive power drawn from the bad power factor caused by the capacitive load. This is the easy and cheap solution. Expect power factor to be 0.5 to 0.6.
2. Passive power factor correction with a AC side choke / inductor can help a little. It is however heavy, bulky and might not be easy to find cheap. If such a choke can be found at a scrap yard, it is a easy and cheap solution, if it has to be bought its more like easy and expensive. If you use a variable autotransformer / variac to adjust the input voltage, it will also act as a choke and give you better power factor. Expect power factor to be 0.6 to 0.8.
These two simulations show the difference in current waveform (yellow) from no passive inductor and a 5 mH inductor.
3. Active power factor correction with a boost converter. Complexity and cost is high and designing a multi kW PFC unit is not an easy task. 3-phase active PFC is even more complex. Very complex task, expensive and time consuming. Expect power factor to be 0.95 and above.