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Seeking Motor Start Capacitor Wisdom

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Forum topic by MacSteveT posted 04-09-2012 06:09 PM 3898 views 0 times favorited 21 replies Add to Favorites Watch
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MacSteveT

55 posts in 1388 days


04-09-2012 06:09 PM

Topic tags/keywords: start capacitor electric motor jointer

Hi all,

I have recently acquired an old (1960’s) Craftsman 6” Jointer. This machine still has the original Emerson 3/4 hp motor 110/220v. The person I got this from had the unit wired for 220v, and had been having issues with the motor not starting. The motor was FULL of sawdust and chips. I completely disassembled the motor and thoroughly cleaned it out, including dressing the contacts on the capacitor cut-out switch. I only have access to 110v so I followed the wiring diagram from the dust cover on the motor, and wired it for 110v, and reassembled everything.

I plugged the motor into a surge protected strip, and switched the strip on – the motor spun up immediately, much to my satisfaction, and then about 15 seconds later the surge breaker tripped. I carefully felt for hot spots on the motor, and the start capacitor. The motor was not hot, but the cap was very hot. I disconnected the cap from the motor, and plugged the motor back in to the strip, and turned it on again, and got the loud buzz I expected, I gave the pulley a yank and the motor started up and ran fine. I tested the AC voltage across the leads to the capacitor, and it was 178 VAC… here is where I am lost. I have no idea what, if any voltage should be across those leads. I DID verify that the centrifugal cut-out switch is working, and this is a start capacitor, not a run capacitor.

My thinking is that perhaps I have wired something incorrectly, and I have searched in vain for a internal wiring diagram for this motor. I have two questions:

1. Should there be voltage across the start capacitor leads after the cut-out switch throws? If not, what would cause that?

2. There are only 5 terminals, so if I have something connected wrong, would the motor run at all?

Any and all suggestions would be greatly appreciated!

Steve T.

-- "Do, or do not. There is no try." ~Yoda


21 replies so far

View Bill White's profile

Bill White

3507 posts in 2656 days


#1 posted 04-09-2012 07:07 PM

I’m not spendin’ a lot of time tryin’ to recreate electrical nirvana. I take any motor probs to a motor shop. They’re the experts (I hope). Sometimes it just gets tooooo complicated.
Bill

-- bill@magraphics.us

View MacSteveT's profile

MacSteveT

55 posts in 1388 days


#2 posted 04-09-2012 07:16 PM

Motor shop is my last resort Bill, but I may be there sooner than later. I was hoping to tap into the vast LJ knowledge pool first, for my own education if nothing else :)

-- "Do, or do not. There is no try." ~Yoda

View MoshupTrail's profile

MoshupTrail

296 posts in 1176 days


#3 posted 04-09-2012 07:33 PM

Could it be the starter coil is not disengaging? I believe in those old motors it might be a switch that is activated by centrifugal force. If that’s not happening, then the starter coil will stay energized. That would result in an overload very likely. Maybe you can google an old wiring diagram to see what happens to the starting cap. I’m suspecting a mechanical issue based on your description of the condition and the re-assembly of the motor.

Try this...

So in the CSIR diagram you will see a little relay. If that doesn’t work it might be the cause of your problem.

Sorry. I missed the line where you said you DID check this. Maybe the diagram will help.

-- Some problems are best solved with an optimistic approach. Optimism shines a light on alternatives that are otherwise not visible.

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MacSteveT

55 posts in 1388 days


#4 posted 04-09-2012 08:35 PM

Problem solved – I think…

@MoshupTrail – thanks very much for your response, and for the diagram. This motor is too old for a relay, but does have the centrifugal switch. The diagram did confirm that there shouldn’t be any significant voltage across the cap once the switch is open, so it occurred to me that the even though the centrifugal part of the switch was working, the contacts were remaining closed.

And then I had an epiphany – when I put the tail cap back on the motor, I might have had a wire that was pressing against the contact spring, keeping it closed – I pulled the tail cap off, and sure enough that was the issue. I made sure to keep all the wires free from the switch spring, replaced tail cap, and checked the voltage across the cap leads – its still not 0, but it is very low, only 17 VAC (a factor of 10 lower than it was). I reconnected the capacitor and it starts right up and runs, without getting hot.

I will definitely keep an eye on it, and bench test it for a lengthy run to make sure the cap doesn’t burn up – I am not sure why there is ANY voltage across the cap leads once the switch is open – perhaps it is residual from the start winding?

And finally, I really like your tag line – I attempt to approach everything with a reasonable degree of optimism :)

-- "Do, or do not. There is no try." ~Yoda

View RussellAP's profile

RussellAP

2959 posts in 982 days


#5 posted 04-09-2012 08:46 PM

Start capacitors are just coils that store energy and discharge to start the motor. Say you have a motor that runs on 3 amps, but it needs a 7 amp jolt to get started, the capacitor ramps up the amps to get the motor spinning then the centrifugal switch throws out and disengages the capacitor and lets the motor run on the regular voltage.

-- A positive attitude will take you much further than positive thinking ever will.

View MoshupTrail's profile

MoshupTrail

296 posts in 1176 days


#6 posted 04-09-2012 10:28 PM

So when you measure the voltage across the cap, are you using the AC or the DC scale? A DC measurement would measure any voltage stored in the cap and that might not be zero although most old caps will quickly discharge any voltage through internal resistance. But the AC scale might be measuring the phase difference across the cap. That is, a cap will only pass AC. So if you’ve got 120VAC on one side, the other side will have 120VAC (measure to ground, don’t touch it!) but it will be slightly out of phase. Try measuring each side to ground on the AC scale. I bet they are both about the same. So 120 – 120 = 0 :)

-- Some problems are best solved with an optimistic approach. Optimism shines a light on alternatives that are otherwise not visible.

View MacSteveT's profile

MacSteveT

55 posts in 1388 days


#7 posted 04-10-2012 01:43 AM

@MoshupTrail – I am measuring AC voltage (AC scale on meter) across the wires that connect to the capacitor, WITHOUT the capacitor connected. My goal was to see what voltage was being fed into the cap after the cut-off switch opened.

Before I fixed the issue I noted above, there was 178 VAC between the wires that feed the cap – after resolving the cut-off switch issue, I am getting 17 VAC between the cap wires without the cap connected. My assumption was that there would be 0 VAC with the cut-off switch open, but perhaps the 17v is residual in from the start winding… when I run the motor with the cap connected, the cap doesn’t get hot – at least it hasn’t yet for the short tests. I will test it for a longer duration run this evening.

Do you have any idea why there would still be voltage between the two cap wires (without the cap) once the cut-off switch has opened?

Thanks for your help,

Steve T.

-- "Do, or do not. There is no try." ~Yoda

View hhhopks's profile

hhhopks

564 posts in 1073 days


#8 posted 04-10-2012 02:53 AM

Congra on resolving your motor problem.
It is amazing how a schematic help solve the problem.
Too often people triying trouble-shoot a problem without ever looking at a schematic. It creates a lot of confusions & waste a lot of time.

Job well done.

-- I'll be a woodworker when I grow up. HHHOPKS

View TopamaxSurvivor's profile

TopamaxSurvivor

14901 posts in 2371 days


#9 posted 04-10-2012 03:50 AM

I was going to tell you your Klixon switch is stuck, but you got it ;-))

Next, any capacitor without a discharge resistor can store a charge up to the highest voltage it was charged.

178 volts issue. A sine wave generates voltage 1.414 times higher than the utilization voltage. The utilization voltage is the root mean square of the sine wave. Generating 178 volts is what gets a utilization voltage of 120. Hope that makes sense ;-)) If not, do not worry about it; just the facts of life.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View TopamaxSurvivor's profile

TopamaxSurvivor

14901 posts in 2371 days


#10 posted 04-10-2012 03:52 AM

By the way, since that Klixon switch is properly functioning, DO NOT touch it!! They can be very difficult to get to operate properly. You got lucky ! ;-))

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View MacSteveT's profile

MacSteveT

55 posts in 1388 days


#11 posted 04-10-2012 09:01 PM

@TopamaxSurvivor – Thanks for the info. You are correct, I believe the switch and the thermal protector are both from Klixon.

I did follow the bit about the utilization voltage… I think. Now what I don’t understand is why there is still voltage between the capacitor wires (without the cap) once the centrifugal switch has opened – it measures 121 VAC.

Here is how it seems like it should work:

1. Plug motor into 110v outlet
2. Centrifugal switch starts off closed, allowing 110 (+/-120 VAC) to flow into start capacitor
3. Start capacitor charges, and boosts output voltage / current to start winding?
4. Energized start winding spins up motor
5. Centrifugal switch opens, which should prevent voltage from flowing into start cap…???
6. Motor continues to spin from main winding

I am not sure what actually happens at steps #3 and #5, and I would really appreciate any clarity you might be able to provide. This is primarily for my own education, because I like to know how (and why) things work. The motor is currently working well, and the start cap is not overheating, even though there appears to be voltage going into even after the centrifugal switch is open.

Thanks very much for your time!

Steve T.

-- "Do, or do not. There is no try." ~Yoda

View ajosephg's profile

ajosephg

1854 posts in 2256 days


#12 posted 04-10-2012 10:10 PM

Some misinformation on a previous post. The purpose of the capacitor is not to increase voltage, but to provide a phase shift to get the at rest motor turning. Here is a link that explains it better than I can: http://www.brighthub.com/engineering/electrical/articles/44951.aspx

The reason you see voltage across the capacitor wires (without the capacitor connected) is there is a small amount of leakage current across the open switch contacts. Since there is no load without the cap hooked up you see some voltage. If you would measure the same points with the capacitor attached the voltage should be nill because the starting circuit leakage would bleed off the voltage.

-- Joe

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TopamaxSurvivor

14901 posts in 2371 days


#13 posted 04-10-2012 10:55 PM

When he got 178 volts on the capacitor it had to have captured the peak voltage some how. The capacitor did not raise it.

The capacitor just provides a little kick to get the motor spinning in #3. #4 The start windings are lower impedance (think resistance) and higher amperage to start the load. #5 The switch opens and the motors runs on the run winding. The capacitor is 2 plates with a insulator between them. The capacitor still has any voltage that was on it when the switch opened.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View MoshupTrail's profile

MoshupTrail

296 posts in 1176 days


#14 posted 04-10-2012 11:03 PM

@josephg – if the cap were energized on one side (connected to 120VAC) and disconnected on the other side, and you put an AC voltmeter across the cap, wouldn’t you see the voltage due to the phase shift created by the cap?
In the image below I show two sine waves, slightly shifted and difference. I’m thinking he’s measuring the difference across the cap-the yellow line. No current, just voltage.

-- Some problems are best solved with an optimistic approach. Optimism shines a light on alternatives that are otherwise not visible.

View Dragonsrite's profile

Dragonsrite

136 posts in 2092 days


#15 posted 04-10-2012 11:10 PM

I could be wrong, but I’m thinking there’s an induced voltage in the start winding.

-- Dragonsrite, Minnesota

showing 1 through 15 of 21 replies

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