It's true that it's mostly drained, which is exactly what I posted, but like any capacitor, when voltage is applied to it, it does start to charge for the first half of the first AC cycle it sees. That is what I described and stated that isn't how it works because the small amount of energy stored at that point is obviously insignificant.
Does it do that by poking the rotor with a stick? It does it by providing a phase shift, which is what I stated.
In DC system are used as surge or spiking
It's true that caps on DC circuit boards will help limit surges or spiking, but that is not their main function. MOVs are used primarily for that purpose an you will find them there together with a cap where power comes on to a board. The main function of the cap is to provide a close by current reservoir to accomodate the fast changing power requirements of that circuit board. If you have a circuit that is switching on and off, the current needed changes in nano seconds. IF that current has to come through a length of wire that goes to the power supply, that wire has impedance and it will limit how fast the current can ramp up. The local cap doesn't have that limitation and can supply the increased current immediately. Without the cap, the voltage would spike downward and that spike would be seen not only by the components on this board, but other nearby boards in the system. So, in that sense it limits surges, but it does it by serving as a near instantaneous supply of current.
It may not have been the best example. was just the easiest to explain...
The hard start cap is only good for momentary contact. It's to be disconnected as soon as the motor has achieved 'run' rpms. Otherwise, you'd toast the cap. IE; it sends a surge of current to the start windings to get the motor up and going, and within a few seconds, a relay is supposed (in some) to kick it off the start cap/windings and onto the run cap/run windings.
As I said though, hvac isn't my specialty and I fully admit I could be wrong in how the cap is being used in this way.
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This website seems to agree with me though...
What is a Start Capacitor?
Designed for momentary use, the start capacitor is what lets a motor start up instantly instead of taking a long time to come up to speed before it can be used. It stays energized long enough to rapidly bring the motor to 3/4 of its full speed and is then taken out of the circuit. It will briefly increase motor starting torque and allow a motor to be cycled on and off rapidly, but is not meant to be used for more than a few seconds.
What exactly does it do?
Single phase motors will commonly have both a start and run capacitor. The difference between the two is that the start capacitor has a much higher ability to store charge, also known as its capacitance rating, for its size than that of the run capacitor. Essentially, the start cap gives the motor a bolt of energy in order to get it up and running, while the run capacitor keeps the motor going after the start capacitor shuts down.
Everyone seems to be dancing around the hard start capacitor.
It allows the motor to draw lots more current for a very short time to get the motor up to speed faster. As the motor spins up it draws less and less current. When it is at its rated speed , it will draw less current . As most fuses and breakers do not trip at the instant they reach their marked current, but take some time to heat up and trip depending on how much over the marked value the current is. By using the hard start kit the current will be over the marked value by a good ammount, but it will be short enough not to trip the breaker. Where without the kit it will take the motor longer to get up to speed and the breaker will trip during the longer slightly over loaded condition.
Think of it as how long you can hold your hand to a hot surface. You can touch a very hot object for a very short time and not get burnt. YOu can touch a cooler object longer before a burn.
Yes, I know that. The point is the start cap is not holding a charge to send to the motor. As I showed you, all the cap would have in the way of charge is whatever it accumulates in under 1/120 of a second and that's insignificant.
IE; it sends a surge of current to the start
There is no charge to make a surge. It works by creating a phase shift, which is necessary to get the motor rotating. Where is this alleged surge coming from? The start cap has no power to it prior to the motor attempting to start, so it has no charge store. It can't get but a tiny, insignificant charge from the AC, because in under 1/120 of a second, the AC voltage reverseds and what's been charged in one direction is now being discharged in the other. And whatever is going on there, the motor has barely moved, if at all at that point.
and within a few seconds, a
You are.
Sometimes even folks that should know better get it wrong. They're a supplier, but they have it wrong. There is no "surge", it's a phase shift. They only understand the static concept of a cap, ie how it stores charge. They assume it's releasing that energy somehow to make the motor go. They don't understand that a capacitor in an AC circuit causes the current to lead the voltage by 90 degrees. It's that effect that is used to create a second magnetic field that is phase shifted in the motor.
The above is also consistent with the fact that many motors have a run capacitor. If caps work in motors by doing a "surge", then what is one doing in some motors all the time? The answer is those motors need that phase shift to run.
That isn't true. As I said before, a classic symptom of an AC system that needs a HS cap is that it starts to occasionally blow fuses, because it's drawing too much current when trying to start. If the HS cap increased the current, it would make it worse, not better. The cap works by increasing the phase shift in the motor. It's not magically somehow providing more current to the motor.
As the motor spins up it draws less
Essentially what you are arguing is that the HS kit makes it draw more current for awhile, then less. But that is still incorrect. By what mechanism do you claim that the HS kit somehow provides more current? I can explain what it's doing, it's not supplying more current, it's shifting the PHASE of the current to create the proper rotating field to get the motor going. That results in it using less current, less power because the motor is getting started via a better method, as opposed to brute forcing it.
Everyone seems to be dancing around the hard start capacitor.
It allows the motor to draw lots more current for a very short time to get the motor up to speed faster. As the motor spins up it draws less and less current. When it is at its rated speed , it will draw less current . As most fuses and breakers do not trip at the instant they reach their marked current, but take some time to heat up and trip depending on how much over the marked value the current is. By using the hard start kit the current will be over the marked value by a good ammount, but it will be short enough not to trip the breaker. Where without the kit it will take the motor longer to get up to speed and the breaker will trip during the longer slightly over loaded condition.
Think of it as how long you can hold your hand to a hot surface. You can touch a very hot object for a very short time and not get burnt. YOu can touch a cooler object longer before a burn.
Everyone seems to be dancing around the hard start capacitor.
It allows the motor to draw lots more current for a very short time to get the motor up to speed faster. As the motor spins up it draws less and less current. When it is at its rated speed , it will draw less current . As most fuses and breakers do not trip at the instant they reach their marked current, but take some time to heat up and trip depending on how much over the marked value the current is. By using the hard start kit the current will be over the marked value by a good ammount, but it will be short enough not to trip the breaker. Where without the kit it will take the motor longer to get up to speed and the breaker will trip during the longer slightly over loaded condition.
Think of it as how long you can hold your hand to a hot surface. You can touch a very hot object for a very short time and not get burnt. YOu can touch a cooler object longer before a burn.
Not just on power supplies, they are used on circuit boards to protect electronics that has nothing to do with a power supply. Like on a phone jack for a modem for example.
What does it matter? The purpose of the caps on the incoming power to electronics boards use the caps we are talking about not for surge protection, but to SUPPLY DC current to the components on the board due to sudden changes in current needed, eg digital circuits that are switching. These boards have both the caps for that purpose and they may have MOVs to protect from external surges. Two different purposes. The cap is like a local battery, when a circuit on the board suddenly switches, it creates a need for additional current. If that current has to come through feet of wire, it has impedance blocking it. And as the current flows it would possibly have effects on other boards connected to the power supply wires. By having the cap there, it momentarily serves as as source for the extra current, so there is no dip in the voltage.
So why bring it up?
It's doing the same thing as the hard start cap, it's providing phase shift. In the case of the PSC motor, it needs that phase shift all the time, not just when starting.
You can also change power consumption by change the value
Voltage doesn't pass through anything, current can. And yes AC current will flow through a cap, never said otherwise.
The start cap already has a charge, depending on the wiring to the relay. It's waiting to be connected to the motor for startup assistance and when it's disconnected, thanks to the relay wiring, it'll be recharged for the next run.
Oh, but there is.. The start capacitor is energized and patiently waits to be tied into the compressor motor, usually on the start windings.
This is why hvac techs always drain capacitors when they encounter them. Only an idiot would assume a capacitor isn't storing a charge and proceed to screw around with things connected to it. Especially if the device it's connected to has been powered up (or tried to power up) recently.
You'll notice (if you watched the video from the first url I posted) that the start caps usually have bleeding resistors attached to them; that's because they do store current and you wouldn't want to expose yourself to it while servicing the machine.
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Start Capacitors for HVAC Compressors - Potential Relay
The next method to help the compressor on start up is to add a start capacitor. This is added to the HVAC compressor circuit along with a potential relay. The potential relay is necessary. Necessary because we do not want to leave the start capacitor in the compressor circuit after the compressor starts. The potential relay allows the start capacitor to assist the compressor to start. It then opens the circuit between the compressor and the start capacitor after the compressor starts and is running.
The potential relay does this using back EMF or back electromotive force. Otherwise known as back voltage coming from the compressor. As the compressor starts and voltage goes through the system it sends out a back EMF voltage. This voltage energizes the coil in the potential relay for the start capacitor. Start Capacitors for HVAC Compressors | Back EMF
The back EMF is constant while the compressor is running. The relay will hold the start capacitor out of the compressor circuit because of the Back EMF. Until the compressor stops. Then the coil in the relay is de-energized and start capacitor awaits another start up. The start capacitor can assist the compressor on the next start up.
Other methods of taking the start capacitor out of the circuit include a current relay. The current relay opens the circuit between the compressor windings and the start capacitor. The current relay works off a certain amount of current.
I (like the previous website) was summerizing the entire process and saying it 'boosts' (by boosts, it gives the motor a shove of sorts, if you will to get the rotor spinning) the motor for startup; which it does. You're the one who wanted to get all technical about how it does this. :) Which is fine, I'm usually anal about details myself.
Depending on your pov, I suppose. I said it boosts it (it does) but neglected to provide the gory details upon how it's doing this.
You might want to review the videos also included at the link. The first url I shared isn't from a distributor, but a manufacturer of start/run caps. I'd think they have some idea of what their own products do and how they do it.
A run cap isn't the same as a start cap. You can't interchange them and expect the device to run reliably. It won't. You could use a run cap as a start cap, but you can't use the start cap as a run cap; it can't handle being connected for more than a few seconds. And, if you decided to use a run cap as a start cap, you'd probably need quite a few to makeup the difference in capacitance.
The start cap has a different capacitance and is only intended to give the motor a boost (boost is easier to use than explaining EMF, etc to a lay person) to get it rolling. At that point, it's pulled from the motor and the run capacitor takes over. If you left the start capacitor tied into the circuit for more than a few seconds, you'll kill it. It could just vent, leak, and/or release it's internal core from the outer casing (semi violently) OTH, A run capacitor can be tied into the power all day long and you won't hurt it. It will hold a charge though, so, don't be touching it with your fingers when you shut it down. You'll want to use a resistor and bleed it off. Also, make sure you set your meter to test for DC current to verify the cap is dead. Why DC? If you turn the cap off during the period of AC wave form other than zero crossing, the cap will store DC voltage upto dangerous/lethal levels.
On AC motors, they rely on EMF and 'phase shifting'. OTH, on a DC Motor (I know this because I was once banned for cheating on an rc race track) they will give it a boost in the more, legit sense. As in, dumping everything they got at once into your motor, giving it more torque, more rpms, faster car. [g]
I tied a bank of small caps into my car, and when I pressed 'turbo' it dumped the banks entire charge directly into the motor. As soon as I let off the turbo button and/or the caps drained down, they'd automatically kick a small (tiny) relay over to put the motor back on the battery pack and also reconnect them to the battery pack to recharge, quickly. It would take a few seconds and then, I could hit turbo again. [g]
The result? Assuming the drive train held up and I didn't lose traction, off and away it would go, leaving the other cars in it's dust.
I realize the cap is functioning in a slightly different manner when using AC, but the end result is the same; the motor gets a loving push (help to start) without it having to pull a massive shitload of amps to free it's rotor. You know, the infamous 'rotor locked' on startup position.
I'll try to be much more specific with you in the future, ok? Instead of just stating that it gives the motor a push or boost, I should have been more specific on how it's doing this. My apologies for not having done so previously.
The capacitors are not sitting there with a charge on them waiting to dump it to start the motor. The only reason they may or may not be charged is that the alternating current goes from 0 to the peak voltage during the cycle. Depending on where in the cycle the relay or other set of contacts open the voltage left on the capacitor could be anywhere from the peak negative to the peak positive voltage.
The purpose is to provide a phase shift, not hold any charge to provide a bost.
Just trying to asume that what you say is correct, explain how if the charge on the capacitor is the wrong polarity (as they can only store DC) that the voltage would buck the alternating current and the motor would not start correctly.
Neither am I. The above is specific to either a run cap or a start cap. Neither has any charge on it prior to throwing the switch to apply power to the motor. The point is there is no energy there prior to that, no reservoir, to aid in starting the motor. You compared it to a cap at an amplifier in a car. That is very different, because there it's a DC circuit and the cap is fully charged and available to supply instantaneous current needs instead of it all having to come from the battery which is located many feet away, through the impedance of that wire.
That's just flat out wrong. The start cap is on the load side of the contactor in the AC equipment. It's not energized when the eqpt is off. There is no charge in it to provide some kind of energy boost to the motor. Again, the cap works by providing PHASE SHIFT. You compared it to a cap on an amp in a car. It's not like that at all. The cap in the car amp is acting as a charge reservoir because it's a DC circuit. The cap is fully charged and like a small battery. If the amp has a sudden need for more current, it can supply it without the impedance of the wires going back to the battery.
The AC compressor is an AC motor and it uses the phase shift that a cap provides to help create the proper rotating magnetic field. The two uses are very different. That is all that I'm saying.
Wrong again. There is no charging, then waiting. Before the contactor closes, the start capacitor and motor have no power applied. As soon as the contactor closes, the motor and the start capacitor have power applied, they are part of the same circuit.
Assuming the charge isn't drained off by the motor, the cap could have anywhere from no charge, to some charge in one polarity, to some charge in the opposite polarity. It all depends on where in the 1/60 sec AC cycle it was disconnected. Whatever small spark you get is irrelevant, because that isn't what makes the motor start. It's the PHASE SHIFT.
If it has bleeding resistors, where is the alleged charge store that comes into play on starting coming from?
I only got "all techinical" when what you described, comparing it to that DC car amp, was totally wrong. The cap isn't supplying extra current into the motor, it's shifting the phase of the rotating magnetic field.
You specifically compared it to a cap in a car amplifier, which is a DC circuit. The function of the cap there is completely different than how it works in an AC motor.
You might want to review what phase shift is. These videos are targeted at someone who wants to buy a start cap and put it in. They don't describe the physics involved, they just use vague terms like the cap assists the motor in starting.
The
Who ever said it would? All I said was they are both caps, they both work by providing phase shift. That they work by providing phase shift explains how they can both be there. If the cap just serves as a charge reservoir, like it does in the car amp, then what is the run cap doing there all the time? Your surge, charge assist explanation would make no sense. The phase shift and correct explanation does explain it.
You could use a run cap as
It's actually giving it a phase shift to provide the correct rotating field. It's not really what I'd call a "boost".
At that point, it's pulled from the
And what is that run cap doing? If the purpose of caps in these motors is to give a "boost" from some mysterious stored charge, then what is that run cap doing? Answer: both the run and start cap work by providing phase shift. In a cap, current and voltage are out of phase by 90 degrees.
If you left the start capacitor
I don;t think you do, because it's in a totally different manner.
but the end result is the same; the motor gets a loving push
Which it does by phase shift and which occurs with the cap first seeing current when the switch or contactor is closed, not having some pre-stored boost charge.
Like I said, and keep saying, my only point was that the cap doesn't work like a charge reservoir, using stored charge to give a kick to the compressor. It doesn't work like the cap in the DC car example you used. You're not the only one here who has it wrong. Maybe you don't care, don't want to learn, but I think some people want to know how it really works. The incorrect version leads to other misconceptions, like Ralph thinking that having the HS cap means that it pulls more surge current initially to get the motor going. It doesn't, it pulls less. And that is again explained by the fact that what it really does is provide phase shift to get the right rotating magnetic field that gets the motor started easier, faster, and with less current.
Without seeing the circuit diagram of the motor in question, I can only assume that said motor has a 'start winding' which tells the non-rotating motor which might have stopped in a position lined up with a pole (or exactly between poles) to move off (in a certain direction) the pole it is currently lined up with (or between) so that it "knows" which direction it is meant to rotate in.
The *uncharged* capacitor presents an instantaneous short which effectively connects the power to that 'start winding'. Once rotating, the regular rotating poles take over and the start capacitor is switched out by a centrifugal switch or other mechanism/device and if needed there is a 'run capacitor' still in the circuit which provides whatever shift is needed if any.
There are too many variations of motor types for me to know exactly what kind is used in modern air conditioners, but a little reading indicates that both start and run caps are often housed together in a single unit to save space. Each "side" of such a capacitor would have a different value indicated on the can.
A 'bleeding' resistor is American for what the Brits call a 'bloody' resistor. :) I think what you meant was a 'bleeder' resistor which a lot of equipment uses to bleed off any residual charge left in a capacitor when the equipment is turned off.
I agree, bad comparison on my part. When using AC it's phase shifting. When using DC, it's holding and dumping it's charge. For simplicty sake, I just say it boosts them and don't go further into the specific details. One (DC) actually does boost and the other assists it with a phase shift (so not technically a boost, but, it does cause less of a startup drain on the panel and the motor comes up to speed quickly)
I also clearly messed up the explanation when I said the start cap holds a charge (I was confusing it for the DC type which is what I have more hands one experience with) and dumps it into the AC motor. My apologies. That's what I get for playing on usenet on little sleep. [g]
Understood.
I agree. it was a piss poor comparison on my part. And I should have been much more specific. I should also have started that when used in the AC start method, it's phase shifting; so, you're right, not technically a 'boost', but essentially in laymens terms, it is. The motor comes up to speed quickly and doesn't draw down an excessive amount of amps to do it, with the start caps help.
I know what phase shifting is, thanks. And, I was completely off base with the comparison as I said, several times now. It wasn't the best comparison I could have used for the purpose, but the idea behind the selected comparison is this: Your mains (battery for car example) or panel for your house isn't being taxed hard to supply the required startup power when a start cap is put into play. That's really all I intended to convey with the poorly chosen example.
Fair enough. Is it easier to just say it gives it a boost, or try to explain what it's actually doing to a lay person? By lay person, I mean someone who can't even find the travelers on a 3way switch without wiring it up several different ways, several times.
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