Motor start capacitors?

Well it should never be placed across the line, but should only be in circuit for a second or two at most. Have you checked the starting gear, maybe its not switching out the cap? Cheers ......... Rheilly

You did not say what the ratings of the new capacitor were or whether or not it was rated for motor start service. Obviously the cap has to be a non-polar AC cap and must be rated for motor service meaning it can take the high currents found in motors.

The replacement cap should have the same capacitance as the original because the purpose of the cap is to generate phase shift in the windings to cause rotational torque. The wrong value may not provide sufficient phase shift. The larger the cap the less the phase shift and the higher the current. That may have been your problem.??

Always replace with the same value cap when servicing motors.

Motor windings and caps are not in resonance.

|> How likely is it that we just got a bad unit? |>

|> Should such a capacitor be able to be placed across a 120VAC line |> continuously? |>

|> Is there typically sufficient inductance in a start winding to bring the |> capacitor toward resonance? I would not think so. |>

|> Bill | | You did not say what the ratings of the new capacitor were or whether or not | it was rated for motor start service. Obviously the cap has to be a | non-polar AC cap and must be rated for motor service meaning it can take the | high currents found in motors. | | The replacement cap should have the same capacitance as the original because | the purpose of the cap is to generate phase shift in the windings to cause | rotational torque. The wrong value may not provide sufficient phase shift. | The larger the cap the less the phase shift and the higher the current. That | may have been your problem.?? | | Always replace with the same value cap when servicing motors.

I've not generally seen current ratings on capacitors ... just farad and voltage.

That's true but they usually say for motor starting or some other words that imply heavy current.

I assume that when you say "electrolytic capacitor" you mean a motor start capacitor. These are non-polarized electrolytics specifically designed for starting motors. They should never be across the line (actually in series with the start winding) longer than 1 or 2 seconds. They will burn up if they are connected continuously. These capacitors sometimes have a 50% tolerance to begin with, so it is unlikely that a small difference in the capacitance value would cause any problems. Be sure the voltage rating is at least as high as the original, as there is a higher voltage on the start winding than the operating voltage. The fact that two failed leads me to believe there is a problem with the starting relay or a short in the winding.

Ben Miller

I havenb't seen a motor cap that was anything but oil and paper. They are usually a few microfarads and fairly large silver cans. The 140uF just doesn't sound right to me.

Those are run capacitors, and can remain connected continuously. Start capacitors are electrolytic, and can have values up to several hundred uF. They are only intended for momentary connection. Either the centrifugal switch or a starting relay disconnects them once the motor is turning.

The design of the motor dictates which type(s) are used, and their values.

They are most often in a black hard plastic case, but not always. All of them should be under a base or in a metallic cover. They sometimes explode and that hard plastic makes for dangerous shrapnel.

Chuck P.

|> |> How likely is it that we just got a bad unit? |> |>

|> |> Should such a capacitor be able to be placed across a 120VAC line |> |> continuously? |> |>

|> |> Is there typically sufficient inductance in a start winding to bring | the |> |> capacitor toward resonance? I would not think so. |> |>

|> |> Bill |> | |> | You did not say what the ratings of the new capacitor were or whether or | not |> | it was rated for motor start service. Obviously the cap has to be a |> | non-polar AC cap and must be rated for motor service meaning it can take | the |> | high currents found in motors. |> | |> | The replacement cap should have the same capacitance as the original | because |> | the purpose of the cap is to generate phase shift in the windings to | cause |> | rotational torque. The wrong value may not provide sufficient phase | shift. |> | The larger the cap the less the phase shift and the higher the current. | That |> | may have been your problem.?? |> | |> | Always replace with the same value cap when servicing motors. |>

|> I've not generally seen current ratings on capacitors ... just farad and |> voltage. | | That's true but they usually say for motor starting or some other words that | imply heavy current.

Would that then mean that for a given capacitor, there is only one current rating needed to make it suitable for motor starting?

Electrolytic? With an AC voltage across it? That's not something I think I've ever seen.

It's not that critical, but generally the current will be pretty similar for a given capacitance. I would just use the correct part and be done with it, they're not expensive.

You've never seen the crossover in a stereo speaker? Horizontal deflection circuit in a CRT monitor? Cap start, cap run motor with two humps? Non-polar electrolytics are everywhere, they're vastly outnumbered by the polarized DC type, but they're not uncommon.

My spa pumps have the start capacitor mounted under the plastic cap on the end bell, the run cap is under a metal hump on top, it's only needed for power factor correction.

James Sweet wrote in news:WZOVj.194$ll1.158 @trndny06:

Why are some capacitors polorized and some aren't. They both pass AC.

Does it only depend on the value of the capacitors? Small capacitors are non-polorized while larger values are?

They're electrically equivalent to two capacitors, with reversed polarity, in series. The effective capacitance is, of course, only one half of each unit, and they are not intended to pass AC current for more than a short time.

They'll pass AC, but put any kind of load on a polarized electrolytic that results in it being charged backwards and it'll blow up.

|> |> I've not generally seen current ratings on capacitors ... just farad and |> |> voltage. |> | |> | That's true but they usually say for motor starting or some other words that |> | imply heavy current. |> |> Would that then mean that for a given capacitor, there is only one current |> rating needed to make it suitable for motor starting? |> | | | It's not that critical, but generally the current will be pretty similar | for a given capacitance. I would just use the correct part and be done | with it, they're not expensive.

Actually my observation about lack of current ratings was for another purpose I was considering quite some time ago that would involve capacitors doing their thing continuously. Not having any idea how much current a capacitor really could handle on a continuous basis, I had no idea if my intented idea would be viable. Things like resistors are readily rated for current/power because they tend to be used in ways that intentionally dissipate power. While dissipation was not my intent for a capacitor, it would dissipate some due to resistance that would be present in the lead wires and the "plate" foils. Electrolyte might even pose an issue (I don't know much of that aspect). So as I pore through the listed capacitors in the Digi-Key catalog, I wonder which ones I might have been able to use, or been able to know I could not use, for this non-motor idea.

On Sun, 11 May 2008 22:47:05 -0500 Steve wrote: | James Sweet wrote in news:WZOVj.194$ll1.158 | @trndny06: | |> start capacitor | | Why are some capacitors polorized and some aren't. They both pass AC. | | Does it only depend on the value of the capacitors? Small capacitors are | non-polorized while larger values are?

The electrolytic can be optimized to work better when one side is going to always be charged positive and the other side charged negative. This is the case for smoothing DC in a PSU. Where the capacitor is going to get AC, then this advantage is not available. I don't know the specifics, so you and your friend Google are on your own to find out. You might want to also get a Digi-Key catalog at

What does the 250VAC on the capacitor really mean? Typically,what are the relative magnitudes of the start capacitor and the start winding inductive reactances?

I have already thought that there may be a problem with the start up switch.

Bill

That is what I would expect. It is possible,however, that there may be some unwanted resonant effect.

Let me ask the question some other way. Is the inductance of the start winding too small or too great to resonate? A quick calculation indicates to me that it takes about 50mH to resonate with a 140µF capacitor at 60Hzm

Bill