What are quick and easy ways to measure inductance of motors or wires? This becomes interesting for power switches (inductive kick), for doing voltage step up if doing regen mod, for resistive spot welder (need low inductance so current can rise fast), etc. Is there any way to do it without getting LCR meter or scope? Just using multimeter would be neat. Or usb datalogger measuring at up to 1khz...
When measuring the inductance of wires, like in a spot welder, even a bridge will have a hard time because the inductance is so low.
There might be some way to use the sound card on your computer as a signal generator to make a bridge for measuring motor inductance.
You might be able to measure the decay time of a voltage (or rise time of the current) suddenly applied across an inductor, but I don't think 1khz sampling rate would be enough.
1. you can use almost any small transformer. the voltage and current are not very important. you could scrounge one from one of those unused wall adapters that you have lying around the place. but we need the AC output. discard any circuit attached to the transformer and just connect straight to the coils
2. the variable resistor needs to be able to handle the current. so the bigger watts the better.
since we are using motors that commutate at fairly low frequencies the 50 or 60 Hz supplied by your wall outlet will be a good enough sine wave signal source.
first we have to measure the equivalent series resistance (reactance) of the coil. that is why we have the variable resistor. set the resistor to the highest value. plug the transformer into the wall outlet. adjust the resistor until the voltage measured across the coil is exactly the same as the voltage across the resistor. unplug the transformer and using your meter measure the resistance. this resistance will be the reactance of the coil or XL.
using the formula XL = 2 * pi * f * L where f= the frequency in Hz and L= the inductance we can simplify it to:
L = XL / 377 for 60hz outlets or L= XL / 314 for 50Hz outlets.
this is about as simple a way as i can figure out. in use you may find that the 50R variable resistor is too large for your coils you may want to change to a smaller R value.
Neat! For small inductance I suspect equivalent series resistance will start to distort the picture. Do I just measure resistance and subtract it from impedance value i get when using AC?
Also I'm thinking I could easily generate signal waves using my Arduino microcontroller. Square waves easier/higher frequency than sine waves. Does it mater if I use square or sine here?
no the equvalent series resistance is called XL. it is what you use to calculate the inductance by pluging it into the formula. plus it is a much larger value than inductance. if you subtract it you would end up with a negative number.
inductance is always measured using a sine wave. that is why i chose to use a transformer as a simple signal source.
