Power Factor, Inductance, explained

[Don P]

We touched on this a few months ago and then a week or two ago my partner asked "what does a run capacitor do?" This is a good description of what is going on with power and inductive loads like motors.

[Hilltop366]

I was wondering the same thing the other day as I was looking for a capacitor for a bench grinder. Some were listed as "start" others were listed as "run" others had none just a capacitor is there a difference.

The bench grinder only had one cap (with the end blown out of it) and it won't start without turning the grinder so I figure it is a start cap. Off to Google I go.

[Don P]

If it has just one cap it is usually a start. They are the smaller, usually Bakelite wrapped dry capacitors. They have a lot of punch but no bench. A run is usually when you have a pair and it is the larger metal bodied oil filled one. They have lower uf ratings but can run all day. They are to absorb and release that out of phase reactive load and get it back in line with the voltage wave, correcting the power factor.

I grab capacitors off junk whenever possible. The planer blew a start awhile back but I had a run with enough microfarads (uf) to get it turning, so it has 2 run caps under the hood. You can go that way but not vice versa.
The free lathe I got going the other day only has  start and it was shot. I had several too small that wouldn't do it and there was one that is double sized compared to what it had, it does start it and once it gets to speed the start cap gets switched off so it works fine. A start is not trying to balance anything it is just a kick in the pants to get it spinning.

Edit, especially with grinders, if you lean into it and bog it a lot to where the start is engaging repeatedly, "She can't take much of this cap'n". A neighbor blew up the start cap on his rotary phase converter starting and stopping repeatedly in a short time.

[SawyerTed]

The video is a great explanation.

It took me back to the 1980's when I was teaching electricity and electronics at East Carolina University. 

I used to tell the students in the intro class, "You have to have a good imagination and trust to understand electricity and electronics because you can't normally see electricity."

[moodnacreek]

That was good Don even for a dummy like me with no imagination and poor at math. I have enough trouble figuring out the last board thickness. But I never knew what a capacitor did only help a motor start. So I understand it converts power lost back to where it is needed.

[Larry]

I've found the primary problem with a dead motor is the start capacitor, followed by the run cap, than the centrifugal switch. When I need a motor for a project I go to the junkyard and sort through the pile of motors. Most will only need a cap and maybe bearings, both a cheap fix.



A box for one of my shop built rotary phase converters. I'm using a bunch of capacitors to balance the voltage output of my converter to my intended load. A important step that a lot of commercial phase converter builders forget to do.