Motors are the workhorses of appliances. By making a motor shaft spin, engineers can harness that mechanical work to do all kinds of things in appliances, such as move air across an evaporator and through a dryer or an oven plenum; and move water through a washer or dishwasher.
As appliance techs, we deal with various kinds of electric motors in every appliance we work on. Yet many techs do not understand how electric motors work. Electric motor theory is a deep rabbit hole. Although we don’t need to understand all the theory, we do need to appreciate that motors offer a special opposition to current flow (electron movement in wires) that we CANNOT measure with our meters. That special opposition is called inductive reactance. In a running motor, inductive reactance is the lion’s share of current opposition. The resistance of the copper windings that we measure with our meters is minuscule compared to inductive reactance.
Ohms Law still applies! But instead of resistance, the total opposition to current in any inductive or capacitive load is called impedance. Impedance is denoted by the letter Z, as in “LoZ meter” which indicates a meter with low input impedance. The total impedance, Z, is the vectorial sum of the real resistance, R, which we can measure with our meter, and the inductive and capacitive reactance.
Z = R + XL + XC
Where
Z = impedance (ohms)
R = real resistance (ohms, the resistance of the copper wire that we can measure with our meter)
XL = inductive reactance (ohms)
XC = capacitive reactance (ohms)
These are added together using vectors, not simple arithmetic.
I am not saying we have to actually do calculations like this. But we have to understand that this is what is going on so we can optimize our use of specifications and electrical measurements to troubleshoot accurately.
The take-away point in all this is that in a running electric motor, resistance, R, is a tiny fraction of the total impedance, Z, to current. Now do you see why relying on ohms to see if a motor is able to run in spec or not is utterly inadequate?
So what should we do?
Loads are all about watts. No one I know, myself included, carries a watt meter. But we all carry an amp clamp and amps are an excellent proxy for watts because P = I x E and I = P / E. So if you can measure amps, you know watts. Similarly, if you know watts (like from the motor nameplate) then you know the expected corresponding amp reading.
The manufacturers perpetuate tech myths and over-reliance on ohms testing by giving stupid specs for motors like the ohms of the copper windings. I think they do this because many techs today do not understand motors, impedance, or reactance. Worse, techs will make diagnostic conclusions about a motor based solely on an ohms test. But since everyone knows how to do an ohms check and, since those are the only specs the manufacture gives, it must be the full story, right?
Wrong!
Watch this short excerpt from a Live Dojo workshop to see this in action in a real-world troubleshooting problem on a Whirlpool dishwasher.
Ready to really understand what you’re doing and how to troubleshoot? We’ll teach you all about motors, circuits, reading schematics, troubleshooting and more at Master Samurai Tech. We’re open 24/7 and available anywhere you have a computer, whether a desktop, notebook, tablet, or phone. Come and check out our courses.