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Today I installed a new scroll compressor in a split air to air heat pump... It is 230 volt single phase, I installed a 40 micro-farad capacitor,as the existing one was. After start up I noticed the new compressor required a 50 micro-farad capacitor...What problems could I encounter? Obviously I will return to site and replace it ASAP...should I have left it off in the mean time?
I believe.....
you shouldn't go to a lower microfarad rating... you can go to a higher rated cap. with 10% or so. Was the voltage rating in line?
You can wire caps in series or parallell to increase capacitance?
Parallel I believe is the answer.
40mf + 10mf = 50m
Too low cap rating would be high amps, motor overheating. Maybe the motor protector will keep anything serious from happening (if it were to). You'll probably be good 'til morning.
amps = volts x microfarads/ 2650. You will get about 1.4 amps less through your start winding. Under a low load, the run windings will not pick up that entire 1.4 amps. Even if it did, the amps would probably not be more than RLA. I say very little would happen but the compressor might die a little early.
First, superheat, that was a good answer from the start. I used to know how to use a multimeter to determine microfarads and the number 2650 rings a faint bell.
Second, you are off base referring to RLA on a compressor. It is a calculated rating that has little to do with anything.... it is for wire sizing and breaker sizing mostly.
Third: A capacitor supposedly leads or lags voltage with amperage... and that's where it provides advantage. I always pictured that what it does as similar to blowing up your cheeks before you spit a watermelon seed, or a plumbing device that builds up pressure before releasing a flow to help unclog a drain.
I would enjoy a discussion regarding the voltage ratings of capacitors (why a start has a different one from a run, for example, and is this voltage something we should be able to measure?) and how we can replace a start capacitor with a thermistor, and stuff like that.
I always thaught that RLA is the maximum amps a motor can take for an extended period of time and still live a good long life. The lead-lag current might effect how hot the motor gets I do not know.
As far as start caps and thermister starter: more amps is more power, assuming constant voltage. More start amps is more start torque. Too many amps will burn the windings out, but it takes time to heat the winding up that hot. Add alot of current to start the compressor, but drop the current down a safe level before the winding overheat. That is the secret of all high torque starters.
You are pretty much right. It is a derating of Maximum Continuous Current testing which is a sort of destruction test. But there is much wiggle room how it is derived. It is common to see one RLA stamped on a compressor and a different one stamped on the condensing unit. It is most always significantly higher than the amperage you can expect to see during normal operation.Quote:
I always thaught that RLA is the maximum amps a motor can
take for an extended period of time and still live a good long life
And I don't argue with higher amps at a given voltage providing more power. But isn't amperage draw simply a result of the size and number of turns of wire in the motor?
Do off the shelf hard start kits have capacitance shift changes, or are they merely a start relay (thermistor) that removes the start winding from the circuit once the compressor starts?
Why do we have voltage ranges listed on the capacitor that differ from the applied voltages?
Is it possible that the capacitor actually applies a higher practical voltage to the start winding during the starting process before it takes it out of the circuit?
I honestly don't know. Compressor manufacturers are so specific regarding the start capacitor ratings, yet after-market hard start kits don't even have a capacitance rating, and can be used over a broad range of horsepowers.
Capacitance is just something I am really dense about. Can a below rated capacitor deliver less starting torque than no capacitor at all?
Actually, it is the motor that applies a higher voltage to the capacitor. Think about a two speed motor on a heating unit. When installing something that runs when the fan runs in low apeed (a humidifier, for example) an isolation relay must be used. This is because the low speed windings become an alternator when the motor is spun at high speed, creating a higher volatage than is applied to the high speed winding.Quote:
Originally posted by Dan
Why do we have voltage ranges listed on the capacitor that differ from the applied voltages?
When a voltage is applied to a motor winding, a sort of "counter voltage" is induced in the ajacent windings. This induced EMF (Electro Motive Force) is opposite the applied voltage and creates a form of resistance. This is why Ohm's Law does not apply to motor windings and yes, you can have much higher voltages present than the applied voltage.
I look at it as a transformer effect. More turns in the start winding = more voltage. Capacitors do change the phase angle. I guess the "real voltage" would be a vector sum of the 2 voltages. I have seen several compressors, where the start and run currents do not add up to the common current. Everything else seems to be fine. No ground fault on compressor or capacitor. Thermister device just allows extra current into the motor until the thermister gets hot. Most hard starts are both a start capacitor and a thermister for solid state control.
Thanks for your patient replies, Superheat and JSherril.
That helps to know. I need to look at their labels more closely and get a feel for the mfd ratings.Quote:
Most hard starts are both a start
capacitor and a thermister for solid state control.
I have a fair familiarity and an extremely poor understanding of "back EMF." What you say makes a sense to me and yet doesn't regarding the use and application of capacitors to overcome counter EMF.Quote:
This induced EMF
(Electro Motive Force) is opposite the applied voltage and creates
a form of resistance.
I have read about hysteresis and reactance and think they all play in this park. I know capacitors can be used to improve power factors and that generally it is not worth the trouble on a practical scale.
Would it be at all a useful analogy to use a garden hose with a pulsating water feed? Useful, meaning the analogy holds up for a while before it falters.
Voltage is water pressure
Amperage is flow
The capacitor could be a membrane surge tank that re-introduces built up pressure during those times the pulsation goes below a certain limit? Thus improving overall flow?
:confused: I am doing a term paper on capacitors, and would appreciate any information anyone could give me that would help me out. (actually I am helping my husband out). Thank-you
Memory, your husband is a coward!:)
The questions and answers on this thread, should be quite enough to do a doctoral thesis on.
I would suggest a question and answer format.... FAQ's seem to be the norm these days, so you can keep the paper modern.
I would use analogies to introduce the concept of capacitance... to intrigue the reader, if for no other reason.
After that it is graphs and phase angles and vectors, but do not abandon the reader. Revisit the analogies in the same format.
Alternating current phase differentials affected by capacitance either are similar or different from puffing your cheeks to spit a watermelon seed or they are not. Either way you have the ability to compare or contrast capacitance. Keep simple-minded folks interested. Like me, for example.
Sophisticate the analogies when one breaks down and is no longer valid.
"Okay, it is not really so simple as spitting a watermelon seed, but it is similar to ... advancing the timing on your automobile", or some such.
Use the analogies to help the reader understand what "leads" and "lags" mean.. or what "back EMF" means.
I know I could do it if I had a clue. LOL!
But if you cannot explain it, you do not yet understand it.
You could address why we called them condensers... you could address how the electrons race to one side or the other, you could address how they are so important in DC applications where inductance is not an issue. You might discover that we use the same word for entirely different things... or different words for similar things... for example, reactance, capacitance, inductance, resistance, etc.
The math is clean regarding all these issues. But the failure to communicate the differences is lacking. Uh, that would be the failure to communicate it to me, actually.
If there was never such a thing as an oscilliscope, we would never have envisioned leading and lagging.
Ok... pardon the rave. I feel better.
A term paper. Double spaced and 3 or 5 pages long? Possible titles:
What's the difference between a battery and a capacitor... they both make my tongue sting when I short them. (Do not do this at home)
How to make a capacitor with Reynolds wrap and paper towels. (Do this at home.)
How does capacitance light up your bedroom lamp... are you the capacitor?
"I charged my capacitor with 12 volts. What voltage will it discharge with?"
"What is the best way to spit a watermelon seed the farthest?"
LOL. Thanks for the Memory.:)
Why do we need a capacitor to start our single-phase motors?
What is a sinewave, like AC voltage? It is nothing more than the graph you get when you take a bicyclewheel, and draw the height of the valve over time when the wheel is rotating.
Now, if you were to construct a brushless AC motor, you would use this effect to build a rotating magnetic field in the stator in which the rotor runs. This is exactly what happens in 3-phase motors. The 3 phases are 120° separated from each other, and thus form a rotating field with a known direction. View them as 3 points on the bicyclewheel, each spaced 120 degrees.
When you only have one phase to work with, you never know which way the field is rotating. Thus, you need a bit of help in the form of a start winding to force the motor in the correct direction.
keep in mind:
- with an ideal coil connected in series with an resistive load, current (amps) lags 90° on voltage (volts)
- with an ideal capacitorl connected in series with an resistive load, current (amps) leads 90° on voltage (volts)
Now, consider the resistive start method using a PTC. The inductance of the start winding combined with the resistive PTC generates a sine wave which is a bit shifted from the mains sinewave. Now we have two points on our bicyclewheel, and we now in what direction it rotates. In a motor it is exactly the same: the rotor is pulled to the point where the largest magnetic difference is, which is where the start and run winding both produce maximum field.
After the PTC heats up, it cuts out the start winding. This can be done because the momentum of the rotor is sufficient to keep things rolling. Like a kid on a swing (correct word?): give it a push every cycle, and it keeps going on. At most a little bit of current is required to keep forcing the rotor.
A start capacitor uses the same principle of leading/lagging the sinewave to create a second point on our imaginable wheel. But since the capacitor generates even more phase shift on it's own, and since it can pass much more current without overheating, it produces a larger starting torque. The run capacitor then serves as a weak force to keep things running.
Now, what happens if the start capacitor is too small? The two points on the wheel get close to each other, and the phase-shifted point is weaker since less current passes. So starting torque is lower.
If it is too big, too much current passes through the start winding, so a burnout of the start winding is risked. And the two points on the wheel shift too far apart, so alignment with the other magnets is compromised.
Thus, a bit larger doesn't harm, but when it is too small, the motor won't start. If the motor starts fine with 40uF instead of 50uF under the hardest conditions, then you can just leave it there.
blue :) : :) :) :) :) :) :)
Thank-you both for your prompt response for information on capacitors, I really appreciate all the information you both passed along to me. I am sure this will help me out alot, it also brought some insight to my attention, i had no clue what a capacitor was or what it did. I know that this is not my job to be doing my husbands report but he is just no good at that kind of thing or should I say he is just to lazy:confused: . Thank-you again to both of you. Memory