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I have a vintage megawattmeter. It is able to indicate values from -1 to 7 MW. I wish to be able to provoke it to display any value I want, preferably without actually generating megawatts of power.

The meter has these symbols

If I understand this correctly, it means that both power and voltage are transformed down, from max. 1000A to max. 5A, and max. 6000V to max. 110V. The first symbol seems to indicated that two phases out of three are used for the measurement?

This diagram is attached to the back of the meter

There are 7 Connections on the meter, numbered from 1 to 7. If I understand the diagram correctly, this means that connection should be like this:

  1. phase 1 current in (k) - low voltage
  2. phase 1 voltage - low current
  3. phase 1 current out (l) - low voltage
  4. neutral
  5. phase 2 current in (k) - low voltage
  6. phase 2 voltage - low current
  7. phase 2 current out (l) - low voltage

What would be a good way to control this meter to display my desired values?

Update: added some additional pictures and descriptions

I was able to open up the instrument. As far as I can judge, connections 1 and 3 go to one big coil, 5 and 7 go to the other big coil. Connection 4 goes to the chassis of the big coil that has the needle. Connections 2 and 6 go to each of the two smaller coils, and from each of the other ends of those coils, there are wires to the needle mount and the moving element in the other big coil. See pictures below. I'm still not sure how I Shield proceeed to get it to actually show the values I want.

I tried to measure L and R. Wasn't able to find C.

  • Across 1-3 R 0.3 ohm L 0.5 H
  • Across 5-7 R 0.3 ohm L 0.4 H
  • Across 2-6 R 1878 ohm L -4 H
  • Across 2-4 R 225 ohm

The two smaller coild that connection 2 and 6 are wired to The connections, 1 to 7 from left to right The two main coils

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    \$\begingroup\$ Open it up and look for the needle assembly. Disconnect it and test what current you need to make it move. \$\endgroup\$
    – PlasmaHH
    Sep 12, 2017 at 7:57
  • \$\begingroup\$ What do you excpect the needle assembly to look like? Is it likely to have only two connections? Should I supply AC or DC? \$\endgroup\$
    – Magnus
    Sep 12, 2017 at 8:37
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    \$\begingroup\$ Just similar to the needle assembly of any analogue meter, a coil that drives a needle, very likely DC \$\endgroup\$
    – PlasmaHH
    Sep 12, 2017 at 8:45
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    \$\begingroup\$ Clearly you need high current (at low voltage (1)) and high voltage (at low current(2)), both at 50Hz, and 2 phases 120 degrees apart. First, measure some impedances (both L and R with an LCR meter) to establish (1) and (2). Then get hold of an audio signal generator, some passives to make a phase shifter (or Audacity again), a cheap stereo amplifier, and four cheap mains transformers. Step up for voltage, down for current. \$\endgroup\$
    – user16324
    Sep 12, 2017 at 10:14
  • \$\begingroup\$ Appreciated. I ordered an LCR meter and will measure when I next have access to the hardware. The Audacity solution worked well for my frequency meter. In this case, will four 2A transformers with ~20 ratio (intended for 230VAC->12VAC, 24VA) be suitable? I would be able to work from a supply with a little more than 5V and 50mA per phase (ignoring losses)? \$\endgroup\$
    – Magnus
    Sep 12, 2017 at 22:47

1 Answer 1

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I believe you can get full scale output from this meter with 5A/110VAC input (600W). The markings refer to a situation with a 1000:5 CT and a 6000:110 PT (per phase). CT = current transformer PT = potential transformer.

It should also indicate from a single phase. So if you stick a 120V 100W light bulb on there (assuming you have a 120VAC supply available) it should indicate around 1.2 MW. Try leaving the unused phase connections open.

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    \$\begingroup\$ In theory a three phase meter will read the sum of all the power, so if you have no power on one or two phases it will read the power on the single loaded phase. If you are able to take a picture of the meter and the insides it might be possible to offer advice on connecting directly to the indicating instrument, this would be just a few milliamps of current required as mentioned in the other comments. \$\endgroup\$
    – KalleMP
    Sep 12, 2017 at 13:45
  • \$\begingroup\$ I have one phase AC at 110V or 220V. Or I could use a VFD to get three phase power at the voltage I want. Assuming I powered the 100W light bulb with single phase 110VAC, how would I connect the watt meter? (1) and (3) in parallel with the bulb? (4) to ground? How about(2)? On either side of the bulb? \$\endgroup\$
    – Magnus
    Sep 12, 2017 at 22:40
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    \$\begingroup\$ The symbol does not mean a lot to me- I could guess, but... obviously you don't want to put the current measurement terminals in parallel with the load or you'll destroy the internal shunt and probably the wattmeter too. Those terminals will measure a very low resistance, almost a short. \$\endgroup\$
    – Spehro Pefhany
    Sep 12, 2017 at 23:02

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