Should I place the LC resonant circuit on the PCB?

[djstrattos]

Good evening mates!

I followed the following application note from ON Semiconductor http://goo.gl/dKly6z, for my Raspberry Pi AC motor direction control project and I have a question.

Should I place the LC resonant circuit on my PCB with all my other components, or should I just use a separate LC component wired between my motor's windings?
In any case how should I determine the values for L (impedance) & C to be used in any of the two cases?

Cheers!

[Richard Crowley]

The capacitor is typically pretty good size, and often doesn't work very well on the PC board, but rather wired directly to the motor leads. In many cases, the capacitor is actually attached to the motor.

The value of the capacitor (and the inductor) are not easily determined (at least IME). Did your motor not come with the capacitor?  If not, it may be an expensive experiment to try a bunch of large motor capacitors (depending on the size of your motor).  In fact, I wouldn't attempt to use a motor that didn't either come with the capacitor, or the specification for the capacitor value.

I suppose you could use a dual-trace oscilloscope (with appropriate safety measures) to look at the waveform on each of the windings, and then select a capacitor value that produced a 90 degree phase difference.

I guess I only play around with small split-phase motors, so I don't recall any of them using that inductor in series with the phase capacitor.

[SArepairman]

how big are these capacitances? Is it unreasonable to use a under-rated cheapo capacitor box to find the correct capacitance, then buy a correctly rated capacitor that will have the longevity that you want. ?

like buying 20 100nF foil capacitors on ebay for 3$, though I don't know what a typical motor capacitor is.

i mean the capacitors will only need to take a beating for a few minutes at most for testing purposes.

[Richard Crowley]

We have no clue how big djstrattos' motor is. But motor capacitors tend to be large. Perhaps as big as a soda can.  I would expect that trying to use a wimpy capacitor even for a few seconds to test a larger motor would just melt or even explode the capacitor.  I would NOT recommend that kind of experiment.



The reason for the large size is that they must carry significant amounts of current to actually turn the motor. Either the capacitor-start kind, or the split-phase, capacitor run variety.

[SArepairman]

I wonder if you can use a capacitance multiplier with a beefy power transistor to tune in on the right value
http://sound.westhost.com/project15.htm

[Richard Crowley]

I wonder if you can do a capacitance multiplier with a beefy power transistor
http://sound.westhost.com/project15.htm

Which would then require a beefy power supply, etc. etc.  SO much easier to use one simple passive component (the capacitor).  Not practical IMHO.

[SArepairman]

I wonder if you can do a capacitance multiplier with a beefy power transistor
http://sound.westhost.com/project15.htm

Which would then require a beefy power supply, etc. etc.  SO much easier to use one simple passive component (the capacitor).  Not practical IMHO.

blargh again with the edits, I need to stop editing so much, I meant just for tuning it, since you know the gain and the initial capacitor.

[Richard Crowley]

Even just to experiment to find the value, you are talking about likely several 10s of amps, and constructing a capacitance multiplier for those kinds of currents is WAY beyond the complexity of djstrattos' original project.  Capacitance multiplication is a technique I've seen only in small-signal (audio, etc.) circuits. I just don't think the technique is applicable to power-level circuits. 

But I could be wrong. I don't troll big-current power circuits much, so maybe I have just never seen the technique applied.

[SArepairman]

I have a big fat BJT sitting around, I wish I bought that 30 amp supply on ebay when I had the chance. i wonder how they categorize them in the factory

[johansen]

you can make a variable capacitor with a variac where the variable ratio changes the capacitance according to the ratio squared.
but this won't work if the circuit sets up a dc voltage across the capacitor.

in anycase, a 15uF cap for a small 1 amp motor could easilly fit on a pcb, such a capacitor would be about an inch in diameter and two inches long.
I don't know why they have a 50uH inductor in series with the capacitor.
btw, this is not an lc resonant application, line frequency is no where near 5.6Khz

as far as finding the capacitor value, its not critical. just set it up so the current flowing through the cap is about the same as the current flowing in the other coil, at full load.

i have a few motors setup like this, one of them uses a 30 or 45 uF motor run capacitor(maybe 1/12hp), the other uses two motor start capacitors in series, each is about  200uf iirc. (chair lift motor, intermittent duty, about 1/8th hp)

[Richard Crowley]

Thanks, johansen.  Great tip to simply equalize the AC current as a rule of thumb.   

And remember that we are talking about pretty high voltages here, also.  I would use a 400V rated capacitor MINIMUM for anything connected to 120V power mains. And I would use a 600V capacitor if we are talking about 230V power mains.

[SeanB]

For motor use you will use a big paper or polyester foil unit, and if the motor is over 500VA it will be definitely over 20uF. I would use proper motor run capacitors for anything over 100VA, as they at least are designed to fail safely as open circuit, and are designed to self disconnect on overpressure or overheating. for a small fan motor of say 50-100VA you will have a capacitor of around 2-5uF, and this is doable with solder in 630V polyester capacitors. Big, and you might need to parallel 2 or more units to get the capacitance value. You can go a little above the value, like 15% or so above, but a lower value than recommended will result in a loss of power ad more motor heating.

[djstrattos]

Good day Gents,

Thanks a lot for your answers.
Sorry for the late feedback as I forgot to enable notifications on this post.

Re the size of the motor I am talking about a small window shades AC motor (http://goo.gl/UqlH1v). Much smaller than those that @johansen describes of course. @Richard, unfortunately I currently have no access to the motor's chasing. I only have access to the up/down switch where I intend to make the connection of my circuit in parallel.
 
So as I understand to be on the safe side you propose a 330V motor start cap (http://goo.gl/0fCZKO) directly connected to the motor windings and no inductor in series needed.

[Richard Crowley]

Are you SURE the capacitor is not inside the case somewhere?  It seems very odd that it requires the capacitor externally.
Does the motor not run when you apply mains voltage to it directly?

Since you have not identified WHERE you are, we have no clue what your mains voltage is.
If you are in an area where you have 120V power mains, then a 330V capacitor is probably OK.
But if you live in a 230V area, then a 330V capacitor is probably inadequate and possibly dangerous.

Since you appear to have a split-phase motor, one of those big "starter" capacitors is not the proper component.

[djstrattos]

Hi Richard,

As I cannot currently access physically the chasi of the window shades were the motor is located I don't have an answer to that.
The motor is currently working since it's already attached to a wall mounted switch.
Do you believe that since I am going to install my circuit in parallel to the already working switch/AC circuit then I shouldn't consider placing a cap?
If not then what would be the proper cap to use in my case.

P.S. I am located in EU (220V)

[Richard Crowley]

If the motor is already running with the switch, then the capacitor is either located at the switch (which should be clearly visible), or else the capacitor is located with the motor. You should NOT be adding any other capacitors if the motor already has the proper cap.  All you have to do is close the contact to the forward motor lead, or the backward motor lead.  The switch already does that. Assuming you are going to be REPLACING the switch?  Trying to use your circuit AND the existing switch TOGETHER in parallel is a fire hazard and should NOT be attempted.