Motor start capacitors?



If you are only building a handful of units for non-critical applications you might just use the "smoke test." If you are designing for a longer production run it might pay to varify with the manufacturer that the cap is suitable for the application.
When in doubt use a conservative design which in the case of caps mean you a significantly higher voltage rating than you calculate or measure. When you have things working you run an extended "smoke test" than then shut down and give the components the "digital temperature test." If you say "ouch" when you touch a component, it may be running a little hot but not necessarily and that's why you read the spec sheets.
You will often see "hints" in the suggested applications section of the data sheets or catalog description such as "suitable for switching power supplies" or "recommended for capacitor input filters."
You have to pay attention to "frequency" effects as you go to RF as at some point the inductance of the leads will resonate with the capacitance. All this is why when you want a power supply designed you go to a guy who has done it before, etc.
Just for fun, I dug up a "control unit" for our old deep well pump that failed. This is just a motor starter with a current relay. Interesting enough, this cap isn't marked as a starting cap so I suspect that many replacemnet caps are only sold in the "run" version. The label includes: 88-108UF 330 VAC 50/60Hz +65C. Clearly this cap is designed to not fail pre-maturely when it runs a little warm. "Doing the math", 65C is 65*(9/5)+329F. That's up in the "ouch" range but just barely. Above 140F is considered dangerous for domestic hot water.

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| If you are only building a handful of units for non-critical applications | you might just use the "smoke test." If you are designing for a longer | production run it might pay to varify with the manufacturer that the cap is | suitable for the application. | | When in doubt use a conservative design which in the case of caps mean you a | significantly higher voltage rating than you calculate or measure. When | you have things working you run an extended "smoke test" than then shut down | and give the components the "digital temperature test." If you say "ouch" | when you touch a component, it may be running a little hot but not | necessarily and that's why you read the spec sheets. | | You will often see "hints" in the suggested applications section of the data | sheets or catalog description such as "suitable for switching power | supplies" or "recommended for capacitor input filters." | | You have to pay attention to "frequency" effects as you go to RF as at some | point the inductance of the leads will resonate with the capacitance. All | this is why when you want a power supply designed you go to a guy who has | done it before, etc.
The idea I had in mind was designing a DC to AC inverter for whole house use that would support a high enough fault current to ensure the breaker would trip on a short circuit. It would have to be able to deliver the high current briefly, but that would not be a long term expectation.
I suppose commercial inverter designers don't just pick caps out of a Digikey catalog.
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snipped-for-privacy@ipal.net wrote:

Designing a large inverter is not a trivial task, it's a case where you'd be better off buying a commercially made unit, it's a much safer approach at any rate. I'm by no means trying to discourage one from designing and building their own hardware, but this is a case where even I'd try to find something commercial. Maybe pick up a surplus server farm UPS and modify it for your purpose.
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| | snipped-for-privacy@ipal.net wrote:
|> |> | If you are only building a handful of units for non-critical applications |> | you might just use the "smoke test." If you are designing for a longer |> | production run it might pay to varify with the manufacturer that the cap is |> | suitable for the application. |> | |> | When in doubt use a conservative design which in the case of caps mean you a |> | significantly higher voltage rating than you calculate or measure. When |> | you have things working you run an extended "smoke test" than then shut down |> | and give the components the "digital temperature test." If you say "ouch" |> | when you touch a component, it may be running a little hot but not |> | necessarily and that's why you read the spec sheets. |> | |> | You will often see "hints" in the suggested applications section of the data |> | sheets or catalog description such as "suitable for switching power |> | supplies" or "recommended for capacitor input filters." |> | |> | You have to pay attention to "frequency" effects as you go to RF as at some |> | point the inductance of the leads will resonate with the capacitance. All |> | this is why when you want a power supply designed you go to a guy who has |> | done it before, etc. |> |> The idea I had in mind was designing a DC to AC inverter for whole house use |> that would support a high enough fault current to ensure the breaker would |> trip on a short circuit. It would have to be able to deliver the high current |> briefly, but that would not be a long term expectation. |> |> I suppose commercial inverter designers don't just pick caps out of a Digikey |> catalog. |> | | | Designing a large inverter is not a trivial task, it's a case where | you'd be better off buying a commercially made unit, it's a much safer | approach at any rate. I'm by no means trying to discourage one from | designing and building their own hardware, but this is a case where even | I'd try to find something commercial. Maybe pick up a surplus server | farm UPS and modify it for your purpose.
But they don't make them. I've searched. Available currents on the ones I have found online (quite many) have been a small percentage above the rated normal loading.
The largest inverter I found was an 18kVA unit. That would be sufficient to provide my normal needs. But it's short circuit rating was a mere 90A. If you have this feeding a panel with say a 60A or 70A main breaker, how long do you think it would take for that main breaker to trip on a short circuit inside the panel bus bars? How long would it take for a 30A 2-pole branch breaker to trip on a short circuit on that branch?
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If you have a short, then wouldn't the inverter shutdown? The 60A or 70A main is there to protect downstream wiring and devices from overcurrent, not the power supply.
You seem to be saying you want a bigger inverter, capable of supplying a higher fault current because you want to trip the main breaker. Why not just have the correct size breaker on the inverter output? I suspect the manufacturer already has that in their unit. Just because the main breaker in your panel might not always trip before the inverter output, doesn't mean you have less protection.
daestrom
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wrote:
| If you have a short, then wouldn't the inverter shutdown? The 60A or 70A | main is there to protect downstream wiring and devices from overcurrent, not | the power supply.
And that's what I want it to protect.
| You seem to be saying you want a bigger inverter, capable of supplying a | higher fault current because you want to trip the main breaker. Why not | just have the correct size breaker on the inverter output? I suspect the | manufacturer already has that in their unit. Just because the main breaker | in your panel might not always trip before the inverter output, doesn't mean | you have less protection.
The main breaker is the example of what the inverter cannot deliver. The problem exists even for small branch breakers. Even a 20A breaker needs 100A fault current instantly to do a magnetic trip.
My whole point is the "short term" current delivery capability of inverters on the market is terrible. It needs to be similar to that of a transformer rated for your needs with infinite availability on the primary, at least of about a 10% equivalent impedance. These inverters are apparently non-linear. I don't want an inverter that's going to shutdown when I start the garbage disposal motor.
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wrote:

So, if the various breakers are designed to protect the downstream components adequately, and the inverter is not capable of sending enough current to trip the breaker, the inverter is not capable of sending enough current to damage the downstream components. QED.
If the inverter *is* capable of sending enough current to trip the breaker on thermal trip (even if not the magnetic trip), then the thermal trip on the breaker is able to protect the downstream components as it is designed and you still don't have a problem.
About the only thing that can be damaged by a mid-level fault that passes more current than the inverter can supply, but not enough to trip the breaker is the inverter. The downstream equipment is safe because it can handle anything the breaker can pass, so since the breaker passes it, it's safe.
daestrom
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wrote:
| wrote: |> |> | If you have a short, then wouldn't the inverter shutdown? The 60A or |> 70A |> | main is there to protect downstream wiring and devices from overcurrent, |> not |> | the power supply. |> |> And that's what I want it to protect. |> |> |> | You seem to be saying you want a bigger inverter, capable of supplying a |> | higher fault current because you want to trip the main breaker. Why not |> | just have the correct size breaker on the inverter output? I suspect |> the |> | manufacturer already has that in their unit. Just because the main |> breaker |> | in your panel might not always trip before the inverter output, doesn't |> mean |> | you have less protection. |> |> The main breaker is the example of what the inverter cannot deliver. The |> problem exists even for small branch breakers. Even a 20A breaker needs |> 100A fault current instantly to do a magnetic trip. |> | | So, if the various breakers are designed to protect the downstream | components adequately, and the inverter is not capable of sending enough | current to trip the breaker, the inverter is not capable of sending enough | current to damage the downstream components. QED. | | If the inverter *is* capable of sending enough current to trip the breaker | on thermal trip (even if not the magnetic trip), then the thermal trip on | the breaker is able to protect the downstream components as it is designed | and you still don't have a problem. | | About the only thing that can be damaged by a mid-level fault that passes | more current than the inverter can supply, but not enough to trip the | breaker is the inverter. The downstream equipment is safe because it can | handle anything the breaker can pass, so since the breaker passes it, it's | safe.
So you think a short circuit should just sit there and not be opened.
Imagine the scenario of some child pulling a plug out half way and putting some metal object in between the prongs.
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wrote:

Been there actually done that. As a small boy, I 'discovered' that you can take metal staples from my father's desk and they fit just nicely in a wall outlet. Lot's of sparks, a burnt finger, and a scared dog. No blown fuse, no fire, not much else.
Placing a fault of your type across the outlet will either a) pass enough current to trip the breaker (if the inverter can supply enough for that individual branch circuit), b) pass enough current to cause the inverter to shutdown, or c) pass just enough current to get really hot but not trip the breaker or shutdown the inverter.
A) is not a problem, obviously the breaker tripping will stop the event B) is not a problem either C) may seem like a problem, but it is exactly the same problem if you use utility supply. Think about it. You're passing something like 19 amps through the 'fault' for a long time. Bad?? Yes. Related to the inverter supply?? No.
daestrom
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daestrom wrote:

[snip]
If the inverter (or the utility) can supply such a fault for an extended time, it may result in a fire. You are dissipating 2.3kW someplace where it isn't designed to be dissipated.
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Actually, the wattage could be *less* with an inverter if it's output voltage drops.
Yes, it could result in a fire, but that depends entirely on *what* is dissipating the heat. It won't be the wiring, though if the circuit breaker is sized properly.
daestrom
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wrote:
|> So you think a short circuit should just sit there and not be opened. |> |> Imagine the scenario of some child pulling a plug out half way and |> putting some metal object in between the prongs. |> | | Been there actually done that. As a small boy, I 'discovered' that you can | take metal staples from my father's desk and they fit just nicely in a wall | outlet. Lot's of sparks, a burnt finger, and a scared dog. No blown fuse, | no fire, not much else. | | Placing a fault of your type across the outlet will either a) pass enough | current to trip the breaker (if the inverter can supply enough for that | individual branch circuit), b) pass enough current to cause the inverter to | shutdown, or c) pass just enough current to get really hot but not trip the | breaker or shutdown the inverter. | | A) is not a problem, obviously the breaker tripping will stop the event | B) is not a problem either | C) may seem like a problem, but it is exactly the same problem if you use | utility supply. Think about it. You're passing something like 19 amps | through the 'fault' for a long time. Bad?? Yes. Related to the inverter | supply?? No.
If it is short that would draw enough current on mains supply to trip the breaker, I want it to trip the breaker even with an inverter. Marginal cases may not be that important.
Arc fault breakers might fill in a lot of the problem cases.
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You just don't get it...
Either the inverter will shut down, or the breaker will trip, in either event you're safe. I can't think of any reason why you'd want the inverter to be able to supply more current, for the sole purpose of tripping the breaker, that's stupid.
Either way, you're in *WAY* over your head, there's a good reason big inverters are expensive, they're very difficult to design properly.
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| | |> |> If it is short that would draw enough current on mains supply to trip the |> breaker, I want it to trip the breaker even with an inverter. Marginal |> cases may not be that important. |> |> Arc fault breakers might fill in a lot of the problem cases. |> | | | You just don't get it... | | Either the inverter will shut down, or the breaker will trip, in either | event you're safe. I can't think of any reason why you'd want the | inverter to be able to supply more current, for the sole purpose of | tripping the breaker, that's stupid.
I can think of a reason. Just because you can't think of one doesn't mean you have a case to say there are none.
| Either way, you're in *WAY* over your head, there's a good reason big | inverters are expensive, they're very difficult to design properly.
Define "properly". Maybe "properly" means having sufficient fault current? More likely they are expensive because they are larger.
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What is it then?

Properly as in it works as designed, isn't a safety hazard, doesn't emit RF radiation that interferes with other devices, and is reliable. I generally trust my design and construction abilities, but I would not wire up something I built to the service entry like that, it's too risky, it would not be certified, would not meet code, and if the house has an electrical fire, the insurance would not cover it. They are expensive because they are large, complex, carefully designed pieces of equipment full of expensive parts which have gone through safety certifications.
If you want to ignore everyone's advice and go ahead, then by all means go for it, but the risk is yours, and you will likely blow up enough spendy IGBTs in the process to offset any cost savings.
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| Properly as in it works as designed, isn't a safety hazard, doesn't emit | RF radiation that interferes with other devices, and is reliable. I | generally trust my design and construction abilities, but I would not | wire up something I built to the service entry like that, it's too | risky, it would not be certified, would not meet code, and if the house | has an electrical fire, the insurance would not cover it. They are | expensive because they are large, complex, carefully designed pieces of | equipment full of expensive parts which have gone through safety | certifications.
So then explain why it is the smaller sized ones are not so expensive. A 100 kVA inverter should cost 5 times as much as a 20 kVA inverter, if things were consistent. Instead, it's more like 30 times as much. Or are you saying the smaller ones don't meet code?
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Why should it? Much of it is economies of scale. The smaller inverters are made in much higher quantities than specialized larger ones, so the cost is far lower. They're also simpler, use lower power, less expensive components. You can't just take a $30 travel inverter and make it 100 times larger, it doesn't work that way. If you don't agree, don't argue, just build something that works, and then report back how easy it was and how much money you saved. Anything else is just trolling.
As for meeting code, a commercially built unit will have been tested by the appropriate regulating bodies for the region and will meet code if installed in the intended manner. One you build yourself that has not been tested will not meet code, the size is irrelevant.
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| Why should it? Much of it is economies of scale. The smaller inverters | are made in much higher quantities than specialized larger ones, so the | cost is far lower. They're also simpler, use lower power, less expensive | components. You can't just take a $30 travel inverter and make it 100 | times larger, it doesn't work that way. If you don't agree, don't argue, | just build something that works, and then report back how easy it was | and how much money you saved. Anything else is just trolling.
Aren't you the one that says not to build one?
I don't necessarily say I will save money building my own. But at least I will know it will have the ability to do what I want without having extra stuff in it I don't need.
| As for meeting code, a commercially built unit will have been tested by | the appropriate regulating bodies for the region and will meet code if | installed in the intended manner. One you build yourself that has not | been tested will not meet code, the size is irrelevant.
If they don't make what I need, there is no other choice.
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How are centrifugal switches implemented? Descriptions of one or two of the most common ways would be appreciated. I can see that one way would be to have some kind pin moving radially from centrifugal force acting against a against a spring. The pin can then trip a lever operated stationary switch so that the lever moves out of the pin's way after getting tripped.
I have Googled and Wickied, but all I have found so far are haystacks.
Bill
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Salmon Egg wrote:

The ones I've seen have a couple of hinged weights fly outward and cams push back a simple thrust bearing on the shaft, pushing open a contact. You might just pick up and old motor and pull the end bell off for a look, a picture is worth a thousand words.
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