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blown capacitors

blown capacitors

blown capacitors

I am using a dual capacitor single phase 3HP continiuos duty GE motor rated for 115/230, 60HZ, 29/14 amps, SF 1.15 3450 rpm to power a table saw.

I have wired it for 230v, it also says thermally protected.

About every three to four months I blow one of the two capacitors. I am not sure if it is the start cap. or the run cap. When it blows I hear a pop. While the motor continues to run it sounds like it is coming apart. This is when I shut it off and go get another capacitor. The one that I replace ( the smoking oozing one ) is a Phillips
540-648 MFD 110/125VAC 50/60HZ. It seems to blow when I power the machine on and off frequentlty.

Any insight on why this happens and if there is a better rated capacitor I might have better luck with?

RE: blown capacitors

Your capacitor is a start capacitor which helps your motor to start. By the indication it sounds like you need a start capacitor with a bleeding resistor across the terminals. This usually happens when you turn off your motor and there is still power in the capacitor and is shorting out. These type of capacitors come with the resistor already installed in them, just ask the sales person where you purchase your capacitors.  On the other hand, if the capacitor is blowing after you turn it on, your internal rotating or stationary switch may be sticking. Adjustments may be nessasary to adjust one or both switches.  Hope this helps

RE: blown capacitors

Suggestion: Reference
1. Smeaton R. W. "Switchgear and Control Handbook," McGraw-Hill, 2nd Edition, 1987
page 9-6 "Single-Phase Motor Types"
Reference 1 includes a schematic for your type of motor and further references. It appears that the capacitor parameters might also be verified for correctness. You may check with the manufacturer about it. Nowadays, the power supplies are contaminated with a harmonic content that tend to have impact on capacitor functioning. Your power supply is supposed to have 5% of voltage Total Harmonic Distortion (THD) or less.

RE: blown capacitors

Standard Duty Capacitors are only rated for about twenty starts per hour. Try getting a Heavy Duty Capacitor for a higher start per hour rating.

RE: blown capacitors

>". . .I hear a pop. While the motor continues to run it sounds like it is coming apart."       The internal switch that takes the start capacitor out-of-circuit may be not opening after spinup.  

Also, for a lot of dual-voltage capacitor-start motors, the start switch, start capacitor and auxiliary winding are fed from what works out to be the center tap and one of the line connections of the main or run winding; i.e., they operate at half of line voltage when configured for the higher-voltage operation.  (This also is why only a “110V” start capacitor is furnished.)  It’s possible that, instead of being connected to the tap, the start/auxiliary winding and start capacitor have instead been unintentionally connected to both line terminals.  This could drastically reduce he start capacitor’s life.  May be worth carefully checking the terminal-plate wiring matches that of the label.

Some notes about start windings-- In three-phase motors, typically all the windings are of one size magnet wire.  In single-phase motors with capacitors, there are different gauges of wire; making the more delicate lighter-gauge winding harder to protect from thermal damage.  If you detect the unforgettable aroma of burnt enamel, it’s usually too late.

One other comment— sustained high line voltage is hard on all motor capacitors; i.e., >254V for 230V motors.

RE: blown capacitors

HI cabinet maker,
  just one other possibility, and I put this carefully. Are you "working" this machine hard? If the cap is blowing while the machines running it may be (and emphasise the may) that the machine is not getting up to speed to throw the speed switch- 100% start duty. Simply listen to it wind up, you should feel comfortable that it's reached peak speed before you start work.

I did have a acquaintance who burnt out table saws by starting them with big lumps of lumber in them as they were winding up.

Lets us know how it goes hey

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