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Old December 27th, 2005, 07:16 PM   #1
mjkoc518
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Single Phase Motor

This may be a stupid question but why on "Single Phase AC Motor" is there a Capacitor between the Forward and Reverse Leads? I see this a lot and electrically do not understand the theory or purpose behind it.............
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Old December 27th, 2005, 07:38 PM   #2
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Without a capacitor a single phase motor has no starting torgue or explicit direction of rotation. The capacitor shifts the current with respect to the voltage to create this starting torque. If you want to get more technical try this thread.


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Old December 27th, 2005, 08:14 PM   #3
mjkoc518
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Okay, my understanding of why a cap is used for single phase motors is much better..........Now from a previous thread, it indicates that if the cap is bad a CB will trip or a fuse will blow.......This hasn't happened and I am pretty certain the cap is bad. It is rated for 50MFD, when I meter it with my fluke 189 on capacitance it is reading OL...........Does this indicate that the cap is bad? Shouldn't I be reading some sort of farads? Also if it matters, my resistance reading on the cap is .17 ohms. Shouldn't the cap under no AC frequency be infinite/open?
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Old December 27th, 2005, 08:50 PM   #4
aloudermilk
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Troubleshooting single phase issue

Why don't you explain how you came to wonder if the cap is bad? In other words, why did all this come up? Did a single phase 115VAC motor not start? We might be able to help you better if you give more info.
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Last edited by aloudermilk; December 27th, 2005 at 08:52 PM. Reason: More explanation
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Old December 27th, 2005, 10:54 PM   #5
darrenj
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Quote:
Originally Posted by mjkoc518
Okay, my understanding of why a cap is used for single phase motors is much better..........Now from a previous thread, it indicates that if the cap is bad a CB will trip or a fuse will blow.......This hasn't happened and I am pretty certain the cap is bad. It is rated for 50MFD, when I meter it with my fluke 189 on capacitance it is reading OL...........Does this indicate that the cap is bad? Shouldn't I be reading some sort of farads? Also if it matters, my resistance reading on the cap is .17 ohms. Shouldn't the cap under no AC frequency be infinite/open?
A cap acts like a short untill its charged..
a quick and dirty way to check..Short the terminals..did you get a spark? cap should be good..

Put leads from meter on it (Digital is no good for this..use a dial type..) If the needle swings and goes to zero..reverse the leads..it should do the same..More often than not if the cap goes you can tell..it usually swells..

also if it didnt blow the breaker what happens on start??Also is the motor cap start or cap run??

D
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Old December 27th, 2005, 11:59 PM   #6
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A single phase reversable motor actually has 2 phases. When you put power on the forward lead, that's phase 1. The capacitor causes a phase delay. The delayed output goes to phase 2 (the reverse lead).
1,2,1,2... = forward.
If you put power on phase 2 (the reverse lead), phase 2 gets power first, then delayed power goes to phase 1. 2,1,2,1... = reverse.
2 windings, a common, and a cap is all that's needed. This only works for fractional horsepower motors, because the cap is powering one of the phases.

A Fluke (in capacitance mode) will read O/L if the cap is shorted. On Ohms, a good cap will read zero ohms for a split second, then rise to infinity as the cap charges. This is actually how the capacitance test works. It estimates capacitance by timing how long it takes the cap to charge. The longer it takes, the more capcitance. If it never starts charging, then the Fluke reports O/L.

A constant reading of .17 means your cap is shorted. Replace it.
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Old December 28th, 2005, 05:54 AM   #7
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Quote:
Originally Posted by keithkyll
A constant reading of .17 means your cap is shorted. Replace it.
Assuming he disconnected one leg out of the circuit, otherwise it means he's measuring the motor windings.
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Old December 28th, 2005, 11:14 AM   #8
Rod
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Out of circuit testing is very important!!
caps active components and even fuses
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Old December 28th, 2005, 05:09 PM   #9
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Hey guys thanks for all the support.........The reason for the question is this.........At my job we have a single phase actuator motor that hasn't been working. It is critical for the machine to work so it just kind of got swept under the rug. Well lucky old me got a work order for it and here I am. I guessed that the Cap was bad because when I took the capacitive test on it I read OL......I haven't dealt with Caps since school which has been several years so needless to say I was rusty.

One more question......Being that this motor has been hooked up for at least a couple months with a bad cap, is it fair to assume the motor is shot? What is the effect of running a single phase motor without a capacitor. I know when I checked out the motor the other day, it was really really hot.......En fluego..........



Again Thanks, Appreciate all the help
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Old December 28th, 2005, 05:30 PM   #10
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Red face

Sorry, I said in the last post that the motor was critical, I meant, it is NOT critical for the machine to work..........
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Old December 28th, 2005, 05:36 PM   #11
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Two more things...........One, I did disconnect both leads from the cap, I read the capacitance directly across the cap and only the cap.

Second, if reading an ohm value of .17 across the cap indicate that the cap is shorted, why isn't it blowing fuses?
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Old December 28th, 2005, 05:54 PM   #12
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With a shorted cap, both motor phases are energized at the same time. No phase delay means no rotation. The motor will get hot because it's not turning. The same as a jammed motor.
The fuse doesn't blow, because there isn't a short from hot to neutral.
No overload. Just both windings are powered at the same time.
There are many failure scenarios that won't blow a fuse. Trust your Fluke!
The motor is probably okay. Try another cap first if you want.
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Old December 28th, 2005, 06:06 PM   #13
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You need to verify what kind of motor you actually have. Capacitor Start-Induction run motors have a centrifugal switch that takes the capacitor out of the ckt when it gets up to speed. The cap is needed for starting torque so if you can still run the motor and the cap is bad then its probably a PSC motor. It is very rare that a Cap Start motor can start if the cap is bad and there is ANY load at all, the cap can provide up to 300% of full load starting torque.
TIP: When testing a cap make sure it is discharged first.
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