Electric motor tech

[bgaidan]

I have an older 5hp 220v Baldor motor on a compressor I bought a while back. I refurb'd the compressor and made everything all shiny again...and now the motor appears to be dead.

It worked very briefly after I first put it all back together...and then it started acting up.

When I flip the switch now, I get sparks at the switch. The motor will slowly turn, but is obviously struggling. The wiring gets warm and eventually the breaker will trip. My initial thought is start capacitor, but my knowledge of motors ends there.

I opened up the box mounted on top of the motor and found what appears to be a capacitor and another cylinder that I can't identify (starter maybe?) and then I found another capacitor mounted to the lower side of the motor.

The cap. on the lower side of the motor looks like it has discharged and leaked quite a bit. The on in the top box is also fairly corroded around the top and appears to have leaked a little at some point.


Am I on the right track thinking it's the capacitor(s) that are causing the problem?
Is it normal to have two capacitors in a smaller motor like this?
Is there any way to test them before spending any more money on new ones?
What is the other silver cylinder thing in the pic?

[bgaidan]

Ok, so a little more research tells me the silver one is a motor run capacitor and the black ones are start capacitors.

I'm also going to take a guess that since there was obvious leakage from the start caps. that they are most likely the problem.

Next question, the number on the label, "161-193" is the micro farad rating. Would there be any benefit to going up a rating or two since it only adds minimal cost...or is the capacitor size specifically matched to the motor?

[Todd W]

If the motor doesn't need the extra juice to start the money is better spent elsewhere.

I have the same problem right now too. Cap leaked, corroded some of the motor housing, and I'm trying to identify the exact problem.

I'll post pics, and hopefully we can fix both problems :)

-Todd

[FullsizeYota]

the capacitors are cheap.. or should be atleast. before you remove them take a screw driver and short out the two connections. a capacitor is nothing more than a battery that blast power to get the motor moving but you knew that

it'll shock the shit out of you if you dont.. think ignition coil strength

[bgaidan]

I dug in to it a little deaper last night. The cap. on the bottom side of the motor has obviously leaked and I found that it had corroded some of the wire terminals connected to it. I replaced a couple of the terminals and it fired right up.

It's still a little inconsistant when it's running though. It's seems pretty likely that at least one, if not both of the starting caps. are shot so I'm probably going to replace them just to be safe. Grainger sells them for about $11 apiece, same for the run cap. For $30 it's cheap insurance for an otherwise good big motor.


FYI - I was doing a little googling last night and found some tech on motors and capacitors. Apparently the caps. are sized and tested very specifically for the motor so you want to stick to the original ratings. You can burn out the windings faster if you go up or down too much.

[fj40guy]

Dan,

Another thing to check on 'start contacts'. When the motor is just starting up, those contacts are closed. In series with the start cap. Pop off the end cap on the motor (ya, power off! :flipoff2: ) and take a look.

Often you will hear a "click" after the motor is turned off and slowing down. Yep, centrifugal switch closing back up. If contacts are bad, motor will hum. If you can "spin" the motor by hand (Depending on the shaft/pulley it is easier to use a loose starter rope... pull to spin, once rope is clear) THEN apply power, and it seems to run normally... either start cap or the contacts.

Tom