Reactive power - Reactive Generator research from GotoLuc - discussion thread

[tim123]

Ok, hopefully I've done this right...

Attached is a schematic of the test setup.
- Mains is 250v - measured
- MOT is UK 230/250v 600w (I think).
- Resistor is standard 1/4 watt carbon film - 148 Ohms - According to my DMM

Measurement:

- I used the wattmeter to read overall power in
- The DMM (I presume) is showing RMS volts AC,
- The 'scope is using just CH1 to show RMS Volts AC

Note 1: The scope is battery-powered (DSO203) - so I can connect the two ends of the probe directly over the load.

Note 2: The scope is not calibrated... Note the DMM & Scope show different values.

Note 3: I cannot connect my mains 'scope probe's earth leads to the Neutral line - even at the end of the circuit. It still trips the RCDs...

Results:

5 uF:
- Watts : 1 W (approx - it wanders within 0.3 for all the tests...)
- DMM   : 6.47 V
- Scope : 5.8 V
- V2/R  : 0.23 W

6.5 uF:
- Watts : 2 W
- DMM   : 8.1 V
- Scope : 7.3 V
- V2/R  : 0.36 W

7.5 uF:
- Watts : 3 W
- DMM   : 10 V
- Scope : 8.4 V
- V2/R  : 0.48 W

Note: Power factor remained at 0 throughout - according to the wattmeter.

Also attached is
a) A pic of the test bench.
b) A scope shot - showing the waveform - at 7.5uF

Another note: I put a solenoid coil around one of the secondary's wires - as a basic current probe. I couldn't get much amplitude with the small currents, but it did seem to work. The signal from the solenoid coil & the voltage probe were out of phase... I'll test that some more if folks are interested...

Let me know if I've done anything wrong, or need to do more...

Regards, Tim

[poynt99]

Thanks Tim.

Wow, you've got some harmonics going on there. Is your grid power clean?

Can you measure the resistance and inductance of both the primary and secondary of your MOT?

The difference between the scope and DMM could be from the harmonic noise we see there. The scope should be the more accurate of the two.

Also, it occurred to me that these power meters were not really designed to be that accurate down around the 1W range. I am amazed that yours even has tenths of a watt!

Anyway, it would appear in all cases that your output power is below the input power, even using the higher DMM measurement.

Obviously the power factor computation by the meter is incorrect (i.e. it can not be 0) if the resistor etc. is dissipating power. You can confirm the actual phase angle (hence PF) using the second channel of your scope. See my attached diagram. NOTE: You will also be able to get the true input power with this probe arrangement, provided your scope can do the required math.

[tim123]

Wow, you've got some harmonics going on there. Is your grid power clean?

The harmonics go away with higher capacitance / power throughput. See pic below... I don't know what causes them (harmonic reactance form small caps?). (EDIT: They only go away on the DSO scope - the other one still shows them)

Can you measure the resistance and inductance of both the primary and secondary of your MOT?

Primary:
- 2.4 Ohms
- 0.27 Henries
- with 2ndary shorted = 0.05 H

Secondary:
- 133.5 Ohms
- over 20 Henries
- with 1ary shorted = 3.8 H

The difference between the scope and DMM could be from the harmonic noise we see there. The scope should be the more accurate of the two...

I've done another test. Pic attached. A few changes this time:

- 10 Ohm, wire wound, variable power resistor (100w) as the load (on 2ndary), instead of the 148 Ohm...
   I wanted to try a lower resistance and see what happened...

- I included my mains scope - to take the same measurement - so I now have 3 things all reading the voltage across the load.
   I wanted to have another source for comparison.

Test run is depicted - using 12.5uF.
- Watts : About 5 W
- DMM : 1.116 V
- DSO Scope : 2.515 V Pk, 0.899 V RMS

- Mains Scope : About 2.5V Peak, Probably... This takes a little explaining - and I have a potential problem...

  1) To read the voltage using this scope, I have to put probe A on one side of the resistor, and probe B on the other. I then have to use MATH A-B to get the true voltage. BUT my scope doesn't display the calculated wave's peak or RMS - so it has to be judged by eye. lol.

Yellow = Chan A, Blue = Chan B, Red = MATH A-B
 
2) For some reason my scope is displaying 10 times more than it should...
- Probes are set to 10x. (Max scale is 5v per div on scope)
- BUT Scope has to be set to 'Probe 1x' to match the other 2. If I set it to 10x - it's 10x too big!

I don't understand this..... Will engage tiny brain, and see if I can figure it out... I may be some time...

[nilrehob]

I just did a first test:
http://www.youtube.com/watch?v=xc056vnuIYU

/Hob

[poynt99]

The harmonics go away with higher capacitance / power throughput. See pic below... I don't know what causes them (harmonic reactance form small caps?).

Well it makes sense actually. We are high-pass filtering the grid voltage by using a relatively small capacitance. So we are seeing the noise present in the grid voltage. As you increase the cap value, the voltage increases and there is more of the 60Hz signal drowning out the "noise".

Primary:
- 2.4 Ohms
- 0.27 Henries
- with 2ndary shorted = 0.05 H

Secondary:
- 133.5 Ohms
- over 20 Henries
- with 1ary shorted = 3.8 H

Thanks for the specs.

Test run is depicted - using 12.5uF.
- Watts : About 5 W
- DMM : 1.116 V
- DSO Scope : 2.515 V Pk, 0.899 V RMS

The signal is still a bit noisy so the DMM may be giving an error here.

- Mains Scope : About 2.5V Peak, Probably... This takes a little explaining - and I have a potential problem...

  1) To read the voltage using this scope, I have to put probe A on one side of the resistor, and probe B on the other. I then have to use MATH A-B to get the true voltage. BUT my scope doesn't display the calculated wave's peak or RMS - so it has to be judged by eye. lol.

Sounds like you are doing a differential measurement A-B. That's fine. It's not recommended, but you could try lifting the earth gnd temporarily on your grid scope to see if that would permit use of your probe gnds. But be careful, your scope chassis could become line voltage...potential danger!

Try the phase and input power test with your DSO203. It will be interesting to see how it compares to the watt meter PF indication and power.