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#12872 - 08/19/02 09:19 PM Compressor motors/Capacitors
BrianSparky Offline

Registered: 01/06/02
Posts: 38
Loc: Lakewood,CO
Hey Scott or anyone with motor knowledge...
A floor guy mentioned to me that he is thinking about buying a new compressor. (pancake style). He asked me if the new ones with capacitors have a initial lower inrush current and wouldn't be so apt to trip circuit breakers. I remember info on caps on motors for power quality and start up for single phase. The one he's considering is 110v.
Would the cap be there to start up the motor or could it also dampen the inrush current.

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#12873 - 08/20/02 03:24 AM Re: Compressor motors/Capacitors
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England

I'm not familiar with a "pancake style" compressor, but in general the capacitor found on a motor is in series with an auxiliary winding. Its purpose is to give enough of a phase shift to provide a rotating field to get the motor started. IN some designs it stays in circuit, while in others a centrifugal switch opens the auxiliary circuit once the motor gets up to speed.

#12874 - 08/20/02 06:51 AM Re: Compressor motors/Capacitors
JBD Offline

Registered: 07/12/01
Posts: 599
Loc: WI, USA
The style of the compressor is not important.
What is important is the horsepower of the motor and if the compressor has an automatic unloader. (Paul, a pancake type is a "flat" tank rather than a cylinder, side view looks like a tire.)

As Paul said the cap is for "kick" starting the motor, not for reduced current. The unloader is probably the most important component to reduce tripping on start up anyway.

I own a 4 gallon unit and have no problem running it on 15A 120VAC circuits.

#12875 - 08/25/02 03:26 AM Re: Compressor motors/Capacitors
Scott35 Offline

Broom Pusher and

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
As mentioned by others, the Capacitor on 1 phase Induction Motors is a Starting Device, not a Starting Aid - the Capacitor, in conjunction with the Auxillary Winding, creates an "Out Of Phase" magnetic field which reduces one side of the Stationary Field produced on the Main Winding - resulting in a "Somewhat" rotating magnetic field. The Auxillary Winding remains active until the Motor reaches >70% full speed.
The Capacitor Start method gives the Motor a higher Starting Torque - which will result in a higher starting amperage draw on loads such as compressor pumps.

The Cap Start/Run Motors will draw lower amperage under load, but not on starting [the run cap corrects the power factor and keeps the auxillary winding in the circuit, making the motor more like a Polyphase motor].

Also as mentioned, unloading the pump is very important! With high head pressure, the motor will draw high current for a long time until either it can move the pump piston[s] or the breaker trips [or the thermal unit[s] open].

Running a compressor with a pump sized 1.5 horse or more should be done at 240 VAC, so the current can be kept lower [starting and running].
Smaller compressors can be driven at 120 VAC, as long as they aren't being "choked" for current by long, small extension cords or branch circuits.

If you give a good sized pump a "Good" dedicated circuit, you should not have tripping problems. If the Motor's FLA [Full Load Amperage] is 12 amps @ 120 VAC and the pump matches the motor's horsepower, a 20 amp dedicated circuit should do just fine. Consider using #10 which will help out on any excessive I2R losses [voltage drop].

Also consider the "State Of health" that a circuit breaker might be in. Breakers which have been tripped a lot tend to either become "Loose" [trip to soon for their time/current charactersitics], or they become "No-Blow" [will not trip no matter how long an overcurrent flows through them!].

Excuse the techno mumbo jumbo! The others have posted better explanations. Only have one day to visit the forum and all this stuff is bottled up for a week!

Scott S.E.T.
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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