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Old 01-31-2015, 01:04 PM
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Using a 25 cycle AC meter on 60 cycle current....

I was at an estate sale this morning, and saw a bunch of antique meters there. I wanted one for my variac, as it just has a dial, and I came across this monster. It's identical to the one in the pic, except that it's a "B" and not a "C" style. It weighs about 10 pounds!

Tried it out on AC line current to see that it pegged as soon as it made contact. Then, I realized that THIS one is made for 25 cycle current as opposed to 60 cycle (the pic isn't the meter I have, but other than that is identical).

Question is, can I use this thing on 60 cycle current, and if so, what modifications do I need to make to the line voltage going in?
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Old 01-31-2015, 02:30 PM
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It might work with a power resistor voltage divider. I'd run it on the variac, and see what input voltage range it is accurate over, and measure it's current consumption and input voltage at min and max dial reading (to determine it's impedance which will be needed in the calculation of the divider).
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Old 01-31-2015, 06:45 PM
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If I remember right....25 cycle was common in rural areas. Don't know why though. Found this though...

Quote:
The first generators at the Niagara Falls project, built by Westinghouse in 1895, were 25 Hz because the turbine speed had already been set before alternating current power transmission had been definitively selected. Westinghouse would have selected a low frequency of 30 Hz to drive motor loads, but the turbines for the project had already been specified at 250 RPM. The machines could have been made to deliver 16⅔ Hz power suitable for heavy commutator-type motors but the Westinghouse company objected that this would be undesirable for lighting, and suggested 33⅓ Hz. Eventually a compromise of 25 Hz, with 12 pole 250 RPM generators, was chosen. Because the Niagara project was so influential on electric power systems design, 25 Hz prevailed as the North American standard for low-frequency AC.
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Last edited by Celt; 01-31-2015 at 06:50 PM.
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Old 01-31-2015, 07:15 PM
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33.3 Hz would have made DD synchronous phono motors viable from square 1...At least for LP based machines...

To expand upon what I said earlier...If it works at lower input voltage (as I suspect it should) then I think what you will need to do is find an voltage at which it registers full scale, note that voltage and the current it draws. Then design a voltage divider to reduce the input voltage from full scale mark value to the actual value needed to get full scale needle deflection. Divide the voltage you measured earlier by the current it draws to find it's running resistance. It's running resistance will be in parallel with the bottom resistor of the voltage divider unless you scale the top resistor properly (then you don't need a bottom resistor).
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Old 01-31-2015, 08:00 PM
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The lower part of the meter face says "45-65 cycles," and "110 volts + resistance = 110 volts on dial."
It should work just fine with 60 cycles, provided you add the needed series resistance. If you have a decade resistance box, start at 1 meg, and dial down until you get the proper reading on the meter. The decade box then indicates the proper series resistance needed for the meter. Switchboard panel meters commonly had an external resistance to compensate for the line frequency. Higher frequency = higher readings by a few %, so they used an external resistance to accommodate the wide frequency range - yours was probably part of a dual use (50Hz and 60Hz) application.

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Last edited by Findm-Keepm; 01-31-2015 at 08:03 PM.
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Old 01-31-2015, 11:40 PM
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Just noticed the pic isn't what you are asking about. 25 cycle meter might work with the proper dropping resistor - hard to tell. I know 60 cycle transformers will work (very inefficiently) at 400hz, but not knowing the internals of the meter, it would be hard to say.
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Old 02-01-2015, 06:39 PM
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Can you open it to see if it is using an internal capacitor to drop the voltage. (I know nothing about these, but as you indicate it is reading high, either it needs an external resistor or it has an internal reactive impedance that would vary with frequency would be my guess. An internal capacitor would go along with Brian's observation that they used external trimming resistors to make up for a variation with frequency, and your experience that it's reading high.)
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