Motor start capacitors?

I do not have much experience with the nitty-gritty of how motor start capacitors are rated. An electrolytic capacitor of about 140F at 250VAC
failed on a small air compressor. It was replaced with a slightly large capacitance in a smaller package. The motor is used an a 120VAC line After a few starts, the new capacitor failed and expelled some of its contents.
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
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Salmon Egg wrote:

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
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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
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Salmon Egg wrote:

The 250 volt rating is the max RMS voltage applied to the terminals of the capacitor. That is a typical rating for a 120 volt motor, and 330 volt is common for a 240 volt motor. The start windings produce a higher voltage than what is applied to the motor, and that is what the capacitor needs to be rated for.
I am not sure of the relative reactances, but I believe that the capacitor produces a large percentage of the phase shift. That is the reason that electrolytics are used for starting, since an oil-filled capacitor with high enough capacitance would be physically prohibitive.
I agree with you that this sounds like a start switch problem. I refered to a starting relay in my earlier post, but your motor may have a centrfugal switch. If those contacts don't open fast enough, it will kill the capacitor. A typical rating for start capacitors is "3-second start, 20 starts per hour"
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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.
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|
| |> I do not have much experience with the nitty-gritty of how motor start |> capacitors are rated. An electrolytic capacitor of about 140?F at 250VAC |> failed on a small air compressor. It was replaced with a slightly large |> capacitance in a smaller package. The motor is used an a 120VAC line |> After a few starts, the new capacitor failed and expelled some of its |> contents. |> |> 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.
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250VAC
the
not
the
because
cause
shift.
That
That's true but they usually say for motor starting or some other words that imply heavy current.
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|
| |> |> I do not have much experience with the nitty-gritty of how motor start |> |> capacitors are rated. An electrolytic capacitor of about 140?F at | 250VAC |> |> failed on a small air compressor. It was replaced with a slightly large |> |> capacitance in a smaller package. The motor is used an a 120VAC line |> |> After a few starts, the new capacitor failed and expelled some of its |> |> contents. |> |> |> |> 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?
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snipped-for-privacy@ipal.net wrote:

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.
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|> |> 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.
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wrote:

and
words that

current
purpose
their
really
would be

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None, I don't think Digikey stocks capacitors intended for current handling purposes. That's why you can't find current ratings. There are all kinds of high power, high current caps but you won't find them at Digikey, Mouser or any other electronic supply house. Caps for motor starting, motor running, power factor correction, high current RF work, energy storage, and a whole host of power applications are available from suppliers in those fields.
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Bob Eld wrote:

I get most of that sort of thing from a place called Surplus City, I forget the URL but you can google it, they sell a lot of HVAC stuff. Need to email or call for a customer number to get the catalog but anyone can do it. They have a huge selection of dirt cheap motor start and run caps.
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| None, I don't think Digikey stocks capacitors intended for current handling | purposes. That's why you can't find current ratings. There are all kinds of | high power, high current caps but you won't find them at Digikey, Mouser or | any other electronic supply house. Caps for motor starting, motor running, | power factor correction, high current RF work, energy storage, and a whole | host of power applications are available from suppliers in those fields.
Not even for construction of (large) PSUs (AC to DC) and inverters (DC to AC)?
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Hey, if you know the capacitance and the voltage (& frequency) then the current is also known. If a cap is rated for the frequency and voltage then it can handle the current. The reason that the AC electrolytics are rated as either "starting" or "running" is that the "starting" caps can only handle the voltage (& current) for a short time before they start getting warm/hot. "Run" type caps can handle the current indefinitely. IOW: you can safely hang them across the AC mains (with some overcurrent protection in case the cap fails.)

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Except that neither 'starting' or 'running' capacitors are applied 'across the mains'. They are both in series with one of the motor windings and don't 'see' full line voltage.
daestrom
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Actually, they might see voltages even higher than line voltage because they resonate with the inductance of the motor winding. It's up to the folks who make the motor and starter to determine what voltage cap is necessary. But if a cap is rated for a certain voltage, it's designed to take that voltage.
Most "run" caps are rated will above 240 VAC and can be placed across the mains.
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What I am about to say could easily be easily rebutted by someone who actually does some measuring of a real motor.
Assuming the main winding has low resistance compared to reactance at stall, then the start winding current will lag the applied voltage by almost 90 deg. If the starting capacitance resonates with the starting winding inductance, then the current in that winding will be in phase with the applied voltage. If the start winding is oriented a quarter cycle mechanically from the main winding. This is exactly the condition required to generate a rotating magnetic field. The resistance of the start winding must be made large enough to prevent high resonant currents from flowing. Under such resonant conditions, there is sufficient damping to prevent high currents flowing through the reactive components.
Bill
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| | |> |> None, I don't think Digikey stocks capacitors intended for current |> handling |> purposes. That's why you can't find current ratings. There are all kinds |> of |> high power, high current caps but you won't find them at Digikey, Mouser |> or |> any other electronic supply house. Caps for motor starting, motor running, |> power factor correction, high current RF work, energy storage, and a whole |> host of power applications are available from suppliers in those fields. | | Hey, if you know the capacitance and the voltage (& frequency) then the | current is also known. If a cap is rated for the frequency and voltage | then it can handle the current. The reason that the AC electrolytics are | rated as either "starting" or "running" is that the "starting" caps can only | handle the voltage (& current) for a short time before they start getting | warm/hot. "Run" type caps can handle the current indefinitely. IOW: you | can safely hang them across the AC mains (with some overcurrent protection | in case the cap fails.)
Capacitors aren't rated as to frequency, either. So for a capacitor, that is not known. A capacitor used for smoothing DC would have to have a high voltage for the DC, being able to carry a high charge voltage, but yet not have a lot of current due to limited ripple at that point. You can build at capacitor for a given value of microfarads and a given peak voltage, with varying sizes for heavier or lighter conductors (and conducting foil inside).
There are some very high farad capacitors intended to store power in parallel with batteries to allow changing the batteries. They work on CMOS or other circuits that draw very little power. Apparently my universal remote control for the TV has one, as it says I have 10 minutes to change the batteries, but if I press any button without batteries present, the stored codes are lost. The circuits I have seen for these have resistors between the batteries and capacitor to avoid them charging up too quick if batteries are inserted with no charge. Sounds like capacitors with extremely thin plates that cannot take a high current.
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Actually, many caps ARE rated as to frequency. It's a backwards way of rating the cap for current. The higher the frequency at the same voltage, the greater the current and the greater the heat build up in the cap.
Caps designed for ralatively low frequencies will overheat at higher frequencies. Folks who design switching power supplies soon learn this.
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|> Capacitors aren't rated as to frequency, either. | | Actually, many caps ARE rated as to frequency. It's a backwards way of | rating the cap for current. The higher the frequency at the same voltage, | the greater the current and the greater the heat build up in the cap. | | Caps designed for ralatively low frequencies will overheat at higher | frequencies. Folks who design switching power supplies soon learn this.
But I don't see capacitors rated this way in the catalogs from which they can be selected. If I have a projects, such as a PSU design, and need to have a capacitor of a specific current, or need to adjust the design around available capacitors, how can I know what their current maximum is?
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