Homebuilt Ammeter Accuracy

[McLuvin]

I am testing a brushless motor and need an accurate measure of amps while the motor is running. My multimeters measure amps, but the wires have too much resistance, so I am using a homemade shunt of wire with a DC voltmeter across it. Will this measure the true amps when the ESC is drawing pulses of current? The motor appears to have the desired static torque, but the dynamic torque while running is a bit less than I expected. That is why I am concerned that my measurements are not right. How would you guys suggest doing this?

[Unterhausen]

it seems that you need some piece of calibrated equipment. I'm working through this myself, but I have a calibrated power supply, so that takes the guesswork out of it.

[ghoti]

Measure the voltage drop across a fuse. Calibrate it with a known DC current. Bill

[Acetronics]

Hi, Gothi

Fuses are well known for being the worse shunts you can find ... not linear at all !!!

Alain

[McLuvin]

I was using some thin wire to measure the voltage drop, but realized that it was heating up and changing value, so I will use thicker wire next time. I am comparing the current against another multimeter that I hope to be accurate.

[weber_alejandro]

Maybe the easiest way is using a Current Transformer, just make the primary coil with some thick wire and the readings in secundary will be proportional to the turns relationship (n).


Alex

[JohnMuchow]

Will this measure the true amps when the ESC is drawing pulses of current?Assuming the shunt is accurate...
Your multimeter will only give you the average, or perhaps, depending on the meter and it's frequency response, the RMS value of the current. It will not tell you the current flowing into the motor when the current is on. Only the avg/RMS value of the on and off periods of current flow.

Unless you're running at 100% PWM duty cycle and the motor current is always on.

[AndyKunz]

I am testing a brushless motor and need an accurate measure of amps while the motor is running. My multimeters measure amps, but the wires have too much resistance, so I am using a homemade shunt of wire with a DC voltmeter across it. Will this measure the true amps when the ESC is drawing pulses of current?

Yes, but it's pulsing. You need to filter the output. An analog meter is good at that.

Andy

[bnrusso]

I started out using an ammeter out of a battery charger (0-30A). When I finally broke down and bought a wattmeter I found that my homebrew setup of 20A was really only 12A. Big difference. I never used the homebrew ammeter again.

[AndyKunz]

If you're using an Astro WhattMeter, it has a shunt inside it and the software filters it for you.

Andy

[JRuggiero]

IIRC, a 4 inch length of #14 copper wire will drop .001 volt per amp of current passing through it. Or maybe it's .01 volt per amp. As long as you keep the wire cool, you can use that as a shunt and get consistent readings.

Jim R

[Richard Ingram]

Jim
That's pretty close. My quicky reference says #14 @ 149F is 2.97 ohms per 1000 feet. So 4 inches would be 0.00099 ohms, and close enough to .001 volt per amp.

Richard

[vollrathd]

I've done this many times over the past 40 years. Problem is, copper wire is not stable with temperature. Also, you need to connect the meter in a "4 wire bridgestone" configuration. By the time you've done all that, the Astroflight Whattmeter starts looking pretty good. I've got one, and stopped using all my own home brew ammeter set ups.