Purpose of capacitor on AC Thomas air compressor

[NPloysa]

Went shopping today at the tool surplus store in Denver but got there late, 5 minutes before they closed. Did see a cool AC twin pump Thomas compressor for $50. It has a capacitor on it. What would be the purpose of this exactly? I know what a capacitor is, and does, but the compressor won't run the compressor for more than a few seconds, would it? It was probably about 3x5" inches big.



Thanks, Nick

 

[Rotty]

it is for the start /run windings.

Without it the motor will not start. That is the drawback of single phase.

 

[mgoncalves]

It is for startup only.



Matt

 

[NPloysa]

So it needs the extra juice or amps to start? Cool I guess. Most the ones I have seen are not using capacitors.

 

[drewRoberts]

it's there just to assist in turning the motor initialy. when it turns on, if the capacitor wasnt there, the current draw would be so great the lines to it would overheat. also, the motor might not turn at first thus frying the motor.



if you think of it like a toilet and how it flushes you get the basic principle. the tank is the capacitor, and the normal water lines dont have enough pressure to completely flush, however, the tank does have enough volume and pressure to do just that.

 

[NPloysa]

Sweet thanks. I'm thinking about using this for my onboard air system stepped up through a DC to AC converter. Think it would work? Say, maybe 1000 watts? Should the compressor say how many watts it uses?

 

[AKAMAC]

Think of the cap as a battery. It acts as a buffer and supply for in-rush current. It is there for load when the compressor starts at low setpoint. The Farad rating is for current draw at peak amp value.



If you have the amp or HP value of comperssor wattage is easy to find.

 

[KBennett]

The capacitor on a motor can be there for several reasons depending on how it is wound. In the case of a capacitor start motor that is used on single phase compressors there are two windings, start and run.

The run winding is wired directly to the AC power and the start winding is wired to the AC power through a centrifugal switch and a capacitor. On startup the capacitor causes the start winding to "lead" the run winding and creates a rotating field ahead of the run winding to start the motor rotating based on the fact that current leads in a capacitive circuit and lags in an inductive circuit. The back of the motor armature has a centrifugal mechanism that when the motor gets up to speed it kicks out the start circuit by pushing a switch open. This keeps the start circuit from overheating. If you have used a drill press or similar device with a cap-start motor whenever you turn it off and it coasts to a stop you can hear a click right before the motor stops. This is the centrifugal mechanism closing the switch.



Here are some schematics:

http://www.electrical-contractor.net/ubb/Forum15/HTML/000085.html

 

[NPloysa]

Sweet! Thanks guys. And Kirk, I always wondered what that click was on the drill press!



By the way Kirk, I PMed you!

 

[mikel]

One thing to keep in mind, some capacitor-start motors will start if the capacitor is missing/bad if they are not under a load.



I almost threw out a compressor that would build up pressure if the tank was empty but would trip the breaker once it bled down to the point that it should cycle on and refill the tank. I was actually taking it apart to salvage some of it before trashing it when I pulled the cover off the start cap to see fluid leaking out of it. I replaced the start cap and it works perfectly again.



Just some info to hopefully save someone some time if they run into something similar.



Mike

 

[NPloysa]

Thanks Mike, luckily this dual piston compressor has the capacitor right on top bolted/strapped down and two spade connections. Very easily replaceable if need be.



Nick