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Do I need a new start capacitor?

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Forum topic by Ripthorn posted 11-06-2012 01:28 PM 980 views 0 times favorited 19 replies Add to Favorites Watch
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Ripthorn

797 posts in 1733 days


11-06-2012 01:28 PM

My table saw is an older Grizzly saw (early 80’s vintage) and as far as I know has the original motor and start capacitor. My shop is in an unheated garage and I noticed last year that when it was cold, the saw would trip the breaker on startup once or twice and then start up fine the 2nd or 3rd time. It has started getting cold here again (40’s in the shop or so) and the saw now trips the breaker every time and simply will not start up when it’s cold. The motor runs fine if it gets started, so I am thinking maybe the capacitor is not holding a charge, resulting in a large inrush current when I engage the power switch, causing the breaker to trip. Has anyone had a problem like this before and does this sound about right for a capacitor that is on it’s way out? Thanks.

-- Brian T. - Exact science is not an exact science


19 replies so far

View SCOTSMAN's profile

SCOTSMAN

5586 posts in 2333 days


#1 posted 11-06-2012 02:23 PM

TRY this upgrade your breaker a notch to a slightly larger one seems you might be on borderline with the one fitted at present.Alistair

-- excuse my typing as I have a form of parkinsons disease

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MrUnix

655 posts in 947 days


#2 posted 11-06-2012 02:45 PM

Typically, when a cap goes, it goes quick.. but you could pull it and test with a meter pretty easily. I’d be more inclined to think that it might be a bearing problem given the age of the machine.. try turning it by hand to see if you feel resistance or hear noises.. take the belt off if it has one and try that way. Scotsman also brings up a good question.. what size circuit are you running that saw on and how much does it draw and do you have anything else running on it?

Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

View Fred Hargis's profile

Fred Hargis

2038 posts in 1241 days


#3 posted 11-06-2012 02:55 PM

I think upsizing the breaker is asking for problems, it’s probably sized to the wiring and putting a higher capacity one in may lead to problems. But those caps can be easily tested…or if you don’t want to test, simply buy a new one. They are fairly cheap….I think I paid less than $10 for the last one I bought. Or, a motor shop could test it for you. I do think starting it up without the belt(s) on may help with the diagnosis.

-- Our village hasn't lost it's idiot, he was elected to congress.

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muleskinner

740 posts in 1185 days


#4 posted 11-06-2012 02:58 PM

If it was me, I’d suspect either capacitor or bearings. Bearings are easy to check (does it spin easily?). You can check your capacitor with an ohm meter. It should start with a low resistance and ramp up to infinity. (make sure you short it out when you’re done). If it’s questionable, they’re pretty cheap to replace.

-- Visualize whirled peas

View Tennessee's profile

Tennessee

1556 posts in 1262 days


#5 posted 11-06-2012 04:12 PM

You can tell if it is the capacitor if you hear a humming noise in the motor just before the breaker trips, but the motor is not really turning.
Caps are designed to give the magnetic field a push in one direction, and are wired to give a large inrush of one half of an AC sine wave voltage over and over to get the rotor moving. In a lot of capacitor start, induction run motors, the capacitor is used for the first 75% of run-up rotation, and then the switch changes the motor over to standard induction run, taking the cap and it’s start windings out of the circuit.

My guess is either you have a weakening start winding which would raise the inrush amperage, or your breaker is weakening.
Do NOT upgrade the size of the breaker. It is probably sized correctly, after all these years of working well.
Changing the cap may help a little, but I doubt it. Also, if your motor has an internal switch, dirty contacts can raise the amerage, as well as stiff bearings and overall general load bearings. Just do not put in a bigger breaker. Worst case, bad windings, causing you to lose the motor. Best case, weakening breaker. In the middle, bad bearings and machine load.

-- Paul, Tennessee, http://www.tsunamiguitars.com

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Ripthorn

797 posts in 1733 days


#6 posted 11-06-2012 04:20 PM

The motor is a TEFC motor, 1.5 HP, so running current should be less than 11 amps. The breaker is 15 amp and recently installed (put in a dual breaker in the place of the single that was there). Again, the motor starts fine when it is a little warmer, so it has to do with the cold somehow. As for dirty internal switch contacts, how likely is that on a TEFC motor? It is very likely that no motor maintenance has been performed in the last 25 years, motor is original, etc. The saw has worked quite well except for the cold startup issue. I was able to get it started about midday over the weekend, but in the evenings or early morning, it doesn’t like ot start.

-- Brian T. - Exact science is not an exact science

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JohnChung

281 posts in 822 days


#7 posted 11-06-2012 05:27 PM

When is it cold. Resistance is lower. From what I gather, you need to check the inrush current for the motor.
If the capacitor was bad to start with…. Why after a few attempts it starts up the motor? Each attempt slowly charges the capacitor? It is unlikely. I would suspect that the age of the motor is the issue. Anyway to be sure you would need to check the inrush current first. Use a clamp current meter.

http://en.wikipedia.org/wiki/Inrush_current
http://www.amazon.com/Sinometer-MS2108-True-RMS-Current-Measurement/dp/B0017WRH7O //I have not tried this product before. It is a suggestion

View muleskinner's profile

muleskinner

740 posts in 1185 days


#8 posted 11-06-2012 05:43 PM

A 15 amp circuit is kind of marginal for a 1.5 horse motor. The starting current inrush could be several times full load.

-- Visualize whirled peas

View oldnovice's profile

oldnovice

3868 posts in 2116 days


#9 posted 11-06-2012 05:58 PM

Ditto to what muleskinner said!

-- "I never met a board I didn't like!"

View dhazelton's profile

dhazelton

1269 posts in 1045 days


#10 posted 11-06-2012 06:07 PM

If a motor is slow or hums on startup and the motor is fine if you give the pulley a hand (WATCH YOUR FINGERS!) then it is the start capacitor. Before I dug into the motor I’d take the switch apart, maybe just wire in a temporary new one (heavy duty 20 amp switch from hardware store is fine) and see if that is the problem.

View EEngineer's profile

EEngineer

906 posts in 2361 days


#11 posted 11-06-2012 06:56 PM

so I am thinking maybe the capacitor is not holding a charge
are wired to give a large inrush of one half of an AC sine wave voltage over and over to get the rotor moving
Each attempt slowly charges the capacitor?

No, no, no! I refer you to wikipedia for an accurate but brief description of what the cap is for.

Troubleshooting: you can disconnect the belt, turn on the power and if the motor starts with a quick twist of the motor pulley in the right direction, bet on a bad cap.

If the motor is having problems when it is cold, I would be more inclined to look at the bearings. Interfernces all get tighter when cold. However, I wouldn’t rule out a bad cap.

-- "Find out what you cannot do and then go do it!"

View Tennessee's profile

Tennessee

1556 posts in 1262 days


#12 posted 11-06-2012 07:17 PM

Ok, I phrased it slightly wrong. The capacitor is there to give an inrush in direction, which it does over and over as it charges and discharges. It also keeps the magnetic field slightly ahead of the rotor, causing it to “chase the magnetic field”, so the motor comes up to speed. If the rotor cannot keep up due to faulty bearings, excessive load, etc, the current rises, and the breaker trips. That is why when the cap is bad, you can still make the motor go one way or another by giving it a spin. (In some cases) You are advancing the magnetic field in a particular direction and the rotor will follow the magnetic field, since we’re dealing with AC.
But it is responsible for the first directional current inrush to make the motor go one way or another, depending on how the cap is wired in the circuit.
Also in my experience, caps don’t go bad slowly. Once they short across their any of their plates internally, the party is pretty much over at those amperages.

-- Paul, Tennessee, http://www.tsunamiguitars.com

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Ripthorn

797 posts in 1733 days


#13 posted 11-06-2012 09:46 PM

So it is possibly the bearings that are going? I am curious as to why this would happen only in the cold. Also, I took the belt off and it is still tripping the breaker even with no blade to spin. I suppose that if the caps are not bad, then what is my path forward? I am supposing I can’t replace the bearings myself. Would this mean that the motor is starting to die a slow death?

-- Brian T. - Exact science is not an exact science

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Ripthorn

797 posts in 1733 days


#14 posted 11-07-2012 12:23 AM

Okay, so some more info. I had the belt off of it, gave the pulley a spin and hit the power button and it spun up just fine. Put the belt back on, hit the power button (no spinning help, I’m not that dumb) and it tripped the breaker. Does the extra info shed any extra light on the situation?

-- Brian T. - Exact science is not an exact science

View dhazelton's profile

dhazelton

1269 posts in 1045 days


#15 posted 11-07-2012 12:27 AM

Start capacitor.

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