Only post here if you are in expert in an area and are sure that this is something we MUST know before starting a project!

Moderators: robnewyork, bassiclyLouDog, gameboy

User avatar
By gameboy
If you are evaluating motors / doing motor testing and you want to
determine how many Amps a motor draws under load, the thing
to do is to use a LOW Resistance HIGH Wattage SHUNT as
shown in the picture.

The Resistors (R1, R2) are 1 Ohm 25 Watts each, wired in parallel
for a total of 0.5 Ohm 50 Watts. (mine actually came out 0.6 ohms)
These resistors cost $0.99 each. Measure the VOLTAGE across the
RESISTORS (points A, B) and divide that voltage by the resistance
of the resistor pair to obtain a current thru the ENTIRE circuit.

In my particular case, with 12 volts applied to the circuit
1.76 Volts (across resistors)
/ 0.60 Ohms (measured resistance of the pair)
2.866… (amps thru the circuit) No Load

As you increase the load on the motor, you’ll see the current increase.

I wasn’t thinking the other day, (in a hurry) set the meter to read AMPS
and placed the meter in series with the motor. Loaded the motor down,
read a 47 Amp peek and blew an $11.00 fuse in the meter (20 amp, 600 Volt)
BUMMER! Not only was the fuse expensive but it was hard to find locally.

User avatar
By gameboy
Posted - 02/01/2009 : 02:05:23

If you want an inexpensive Amp meter that uses a shunt:

Some of these could even be used as gauges on a scooter. I got one because I can't have long power wires, and using a shunt allows me to still have an Amp gauge on my handlebars without having long power wires that go all the way up (doing that would probably destroy the speed control due to inductance caused voltage spikes).
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