Garmaine Staff asked 3 years ago

Quick version: Is there a way to reliably identify a short in a motor, such as a garbage disposal? Such as measuring resistance?

Less quick version: My first thought was that since there shouldn't be any switches inside, and making the completely unsupported assumption, that there isn't any transformers or inverters, I thought:

I = E / R      (Thank you Physics I)

So I measured the resistance across the motor Hoping for 8 to 10 ohms, which is where it seemed it should be. Instead, I got 1.8. Hardly open, but much lower than expected. If my theory was right that would be a 67A draw. Then I remembered that resistance was linearly temperature dependant and maybe I could justify the 1.8 ohms like this:

dR/dT = ⍺R => R=[1+⍺(T-Tref)]   (Grrrr.  I hated physics II)

Ultimately, math failed me. I'm glad I wasn't a physics major. Assuming the coefficient for copper I found on the internet of 0.007074 was correct, and plugging eq 2 into eq 1, If that motor is supposed to draw 15 amps, The temperature at the stator would have to be somewhere around 628 degrees celsius above room temperature. So I'm just going to assume that doesn't work. Math has failed me.

Heres the back story. I bought this really nice garbage disposal. Stupid expensive because it's a 1.5hp motor. I was tired of my wife shoving remains of thanksgiving dinner down the drain and destroying the plumbing. It worked great for about a year, then we decided to remodel. Since the goal was to remodel in stages, so that for the most part the kitchen would stay in working order, and I knew I would have to take apart the sink more than once, I un-hard wired the garbage disposal. Yes, I know this is illegal in most municipalities. Yes I know there are many good reasons for not doing this but, well, I did it anyway. At least I'm better with electricity than math. In my defense, I have seem many professionally wired homes that do this.

Regardless, some moron stocking the shelves at Home Depot, put one of those stupid sideways 20A plugs that nobody uses in the wrong spot, and I accidentally bought it. As a result, for a time, I had a working sink with a semi-working drain. While I was at work, My wife, doing her thing as usual, crammed a bunch of food in the garbage disposal, and ran an extension cord from the plug I installed to a wall outlet. Big surprise, it blew a breaker. When I get home I tried a more controlled experiment, thinking that the long extension cord or the arc during the act of plugging it in could have been the problem. I killed the breaker, ran a heavy gauge, shorter extension cord to an isolated outlet on a 20A breaker. After all connections were made I flipped the breaker. It blew again.

I removed the garbage disposal and checked the wiring. After checking the resistance, I pulled out the wires and inspected for damage. Everything looks good. Not sure what to try next.


Installed it last night after cleaning it out and it ran without issue. It must have either been a blocked rotor or excessive draw due to the cable.