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Motor Capacitor Replacement ?s

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Splat

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#1
Guys, I need some help here please. My G4003G lathe's motor started slowing down while taking a really light cut the other day. Yesterday I go to start the motor at the 1400rpm setting and it wouldn't spin up unless I helped it. I spins up fine on slower settings tho. I'm thinking it could be either both start and run caps going bad or the motor itself. The motor seems to run and sound fine so I'm hoping it's the caps. The start cap is the one I'm unsure which to go with. The start cap is 150uF, 250V. McMaster has two ranged caps that might work, this 130-156uF cap, and this 145-174uF cap. I don't know which is the better bet to go with. Thanks.
 

markba633csi

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#2
Sounds like just a bad run cap- very slim chance both run and start caps are bad at the same time unless one of them was on the way out and the other one just caught up with it...
Mark
ps just checked the G4003 manual and the run cap is a 20 uF 450 volt unit.
As to your question about cap sizes, I would pick the larger one of the two. Make sure the voltage rating is equal to the original or larger.
 
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Splat

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#3
Thanks Mark. I'll try the run cap then first and see what happens.
 

markba633csi

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#4
OK. Check physical size to make sure the replacement will fit in there
Mark
 

Ulma Doctor

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#5
you beat me to it mark! ;)
 

Splat

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#6
OK. Check physical size to make sure the replacement will fit in there
Mark
I'll duct tape the darn thing to the side of the motor if I have to. :)
 

Ulma Doctor

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#7
if you have a hard time gettin' a cap, i got one i can send to you brother
 

Splat

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#8
Thanks Mike. I ordered one from McMaster. Yet another thing to work on this weekend.
 

mksj

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#9
I would replace both as they both deteriorate over time, as others have said check the size as they are often not a uniform size. I believe the ones from McMaster may be too large for the G4003G motor. You can also check pool repair shops, often the pump capacitors may be smaller. I had another machinist bring over his G4003G motor that stopped running, pulled the start capacitor and tested it DOA. The electrolytic capacitors go with time, the start is most common from overheating with frequent start/stops. Usually the lower uF higher voltage are film or oil caps. I have replaced my share in various motors capacitors out here in the hot desert, nothing seems to last with the heat.

On the G4003G motor capacitors on the one I saw the wires went directly into the motor, so you will need to cut the wires and use wire nuts or some form of insulated connectors. A number of people have also replaced the motor.
G4003G motor caps.jpg
 

Splat

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#10
Well, I could make another set of covers for them. I just looked up the caps part #'s on Grizzly's site and the one's from McMaster are a little bigger so I may have to make covers for the new ones. We shall see.
 

darkzero

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#11
I agree with Mark, might as well replace both while you're at it. I read that the start caps are usually the ones to go out first. Cheap ones don't have good duty cycle so if you are doing a job where you have to turn & off the machine repeatedly you eventually blow the start cap.

Sure enough, that's what happened to my mill some years ago. Start cap blew during a production run I was doing. My lathe has the same motor as my mill & it's older but surprisingly it hasn't blew a cap yet. If you have a multimeter (even better if it can measure capacitance) you can check if the cap is bad. Sometimes the caps will also show physical signs of failure, like bulging or cracks, depending on the type of cap.

I don't know much about AC motors so I wasn't sure about ordering caps from Mcmaster at the time since they listed a uF range. Don't see that in DC & is what I'm used to. I ended up finding the exact values & physical size at Grainger. Weren't too expensive & I was able to just drive down to pick them up the next morning.

They were their house Dayton brand & at first I was worried cause they were made in China. But they looked nice & had metal cases. As mentioned, I had to cut the wires on the old ones & I installed spade connectors as that's what the new caps had. That will make future changes quick & easy.

Weird thing is the stock start cap was made in China & the stock run cap was German, both with plastic cases. I replaced both & haven't had a problem since.
 

darkzero

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#12
Ah well I take that back, memory not too good anymore. I found the pics I took. Only the replacement run cap I got was a metal case but I have no idea it that even matters. Looks like my caps are the same value as yours. Here's what I had, again they fit inside my covers just fine. The stock caps barely lasted me a year & it's been 4 years since I replaced them. (I forgot how ugly that motor was painted white, yuk, I painted the motor black when I changed the caps).

Img_8723.jpg

Img_8727.jpg

Img_8724.jpg
 

Splat

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#13
Thanks Will. They're probably the exact same caps I'm gonna get from McMaster, but branded otherwise. I know about caps via DC (I've built circuits in the past) but wasn't too sure of them pertaining to AC motors. But, this is how we learn. I'll get mine on Friday. We shall see.
 

darkzero

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#14
Thanks Will. They're probably the exact same caps I'm gonna get from McMaster, but branded otherwise. I know about caps via DC (I've built circuits in the past) but wasn't too sure of them pertaining to AC motors. But, this is how we learn. I'll get mine on Friday. We shall see.
Yup same here, I worked in automotive & electronic repair. I'm not very familiar with AC motors. But at least when that happened I now know what the centrifugal switch looks like & how it works. And good thing there's plenty of guys here that are knowledgeable & helpful.

I assume the caps from Mcmaster will be good quality & they don't sell China stuff that is junk. As long as they fit you should be good.
 

darkzero

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#15
Oh BTW, be careful when changing those caps. Make sure they're denergized first! Search online on how to do so or the guys here can recommend a safe way. Don't do like I do with a screwdriver.... ;)
 

Splat

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#16
Oh yes, I learned that cap drain lesson long time ago. It was a ....shocking moment. :p You definitely do not forget those lessons quickly. BTW, I first thought the centrifug switch was the culprit. It still could be but I doubt it. I thought of going to 3ph VFD setup but it'll have to wait. I'll be happy to have the lathe running properly again.
 

Blackjackjacques

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#18
Guys, I need some help here please. My G4003G lathe's motor started slowing down while taking a really light cut the other day. Yesterday I go to start the motor at the 1400rpm setting and it wouldn't spin up unless I helped it. I spins up fine on slower settings tho. I'm thinking it could be either both start and run caps going bad or the motor itself. The motor seems to run and sound fine so I'm hoping it's the caps. The start cap is the one I'm unsure which to go with. The start cap is 150uF, 250V. McMaster has two ranged caps that might work, this 130-156uF cap, and this 145-174uF cap. I don't know which is the better bet to go with. Thanks.

Either of the two will work fine, particularly since the tolerance of standard electrolytic capacitors are 10 to 20%. The more important paramter is the working voltage - if the old cap is 250V, you will want a cap 250 V or more -- but never less.
 

Splat

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#19
Yep. I've always went by the 20% rule with caps, though that's for general use. For tight tolerances in a circuit I use 10% or less.
 

Doodle

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#20
If the motor has enough age to have a cap failure, if it isn't a big effort, good time to check bearing lube, smell inside the motor to see if windings got hot. Sometimes the windings change color from the heat too.
 

Ulma Doctor

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#21
Thanks Will. They're probably the exact same caps I'm gonna get from McMaster, but branded otherwise. I know about caps via DC (I've built circuits in the past) but wasn't too sure of them pertaining to AC motors. But, this is how we learn. I'll get mine on Friday. We shall see.
Believe it or not, there is no storage of AC voltage possible in a capacitor.
the capacitor stores and releases the voltage as a DC conductor only.
so when we use capacitors in an AC circuit, the caps charge is DC and we introduce DC power into the AC system.
 

Splat

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#22
Finally got the time to replace the caps. There's no schematic or circuit drawing on the cap or inside the cap covers so I can't tell which lead is which. I guess I'm used to DC polarized caps, guys. Are these run and start caps non-polarized (doesn't matter which lead goes to which cap pole)? Thanks guys.
 

darkzero

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#23
AC, non polarized so it doesn't matter.
 

Splat

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#24
Sweet. Thanks Will!
 

Linghunt

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#25
I replaced the motor on my compressor a while back and left the old one in the corner. Local scrap guys picked it up a month ago or so.

I should have pulled the capacitors off of it 1st, but didn't even think about it till now. Windings were all smoked on it so not a cap failure. Anyhow just a note to think about that if you are tossing a motor.
 

Janderso

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#26
Oh yes, I learned that cap drain lesson long time ago. It was a ....shocking moment. :p You definitely do not forget those lessons quickly. BTW, I first thought the centrifug switch was the culprit. It still could be but I doubt it. I thought of going to 3ph VFD setup but it'll have to wait. I'll be happy to have the lathe running properly again.
I have worked at a Ford Dealership since the mid 70's. Back in those days we had car parts called, points and condenser. If you charge the condenser and lay it on the break room table, you are sure to hear some cussing in a short time.
Yeah, I got bit once.
What does, "can't win for losing", really mean anyway?
Can't help with the capacitor/centrif. though, I am as ignorant about electrical motors as the next guy.
 

Bob Korves

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#27
I replaced the motor on my compressor a while back and left the old one in the corner. Local scrap guys picked it up a month ago or so.

I should have pulled the capacitors off of it 1st, but didn't even think about it till now. Windings were all smoked on it so not a cap failure. Anyhow just a note to think about that if you are tossing a motor.
Electrolytic capacitors do go bad over time. Make sure you test an old one properly before using it. A motor shop can test it quickly, and usually for free.
 

TonyRV2

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#28
Generally speaking, electrolytic capacitors (which motor caps are) are polarized devices meaning that they have to be put in a circuit with a specific polarity. Motor starting caps are an exception. They are considered to be nonpolarized but technically, internally they're two polarized caps wired in parallel with one another but opposite in polarity. I bring this up just to point out the importance of choosing the right kind of capacitor. For instance, you may run across a 150uF cap rated at 250V and think you can use it on your motor since it has the right rating. This is not necessarily so unless the capacitor is made specifically for starting an AC motor.

One other thing....these caps, even though they're AC can still hold a charge of one polarity or the other. The amount of charge really depends upon when in the AC cycle the power was turned off. So be careful when swapping them out. The purpose of the starting cap really isn't to hold charge, but to cause a phase shift of the AC voltage that's being applied to the motor. One set of motor windings gets straight AC and the other set gets a phase shifted AC. This gets the magnetic field within the motor to start revolving around the rotor. The magnetic field then drags the rotor along with it do to magnetic forces acting on the rotor windings. When the cap goes bad, the magnetic field stops rotating and the rotor stalls. Thats the gist of it anyways....probably more than you wanted to know, LOL. Count yourself lucky....my students would have had to endure this for an entire semester. :oops:
 

markba633csi

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#29
Hi Tony- I believe the non-polarized caps you mentioned are actually two caps wired in series, you see these in loudspeaker crossovers and such; they have a plus sign on both ends. A 50 uF unit would consist of two 100 uF caps in series.
The ones for ac motor use are truly non-polar oil filled units, they are physically large given their relatively small capacitance values for this reason.
Mark
 

TonyRV2

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#30
I think the confusion here is that yes, the cap is wired in series with the winding, but internally what you have are two caps wired in parallel but with opposing polarity. I've not actually taken one apart but I would imagine that there are diodes involved in order to protect the caps for the opposite half cycle that they'd be in the wrong polarity.
 
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