Ok learn me this Run Cap circuit lol???

[JayMan07]

I've known for a long time that my understanding of the compressor circuit is pretty pathetic.  I've been discussing this with HVAC buddies of mine, but I keep having lots of questions.  I need an expert in this to explain it to me! 



A few things I'm very hazy on 

1) If you measure at the cap terminals you're going to see ~370VAC (on a 240V 1Ph unit).  The cap seems crucial to this because you can actually calculate the mF reading from the current and voltage at the cap.  What is that ~370VAC and how is it produced?

2) What is that cap actually doing?

3) What is the "direction" of current flow?  I know AC oscillates but to just to simplify it...
     L2 - R - C - L1
     L2 - S - C - L1
     In other words is it like 2 paths are taken from L2 to L1?  How is that possible since R&S feed off the same line?

Dang obviously I need some real help here 

Thanks in advance!

[CaptDon]

Your schematic doesn't appear to be drawn correctly. The centrifugal switch that opens
after the motor reaches about 50% of run speed would disconnect one of the power
leads. Either you drew it wrong or the chinese manufacturer you copied it from drew
it wrong. A run capacitor generally provides a phase shift which makes the motor always
run in a specified direction and depending on how the motor is wound it will provide
an 'artificial' pole in between the real poles which tends to reduce 'slip' while maintaining
the torque normally produced by current generated in the rotor by slip.

[JayMan07]

Yes the symbol is wrong.  That is actually a thermal overload.  In this kind of circuit the cap stays in circuit.  There is no potential relay or centrifugal switch.

[JayMan07]

So is the phase shift of the capacitor the reason current can flow in the way I listed it in question #2?

[Siwastaja]

Draw XY plane.

Apply a sinusoidal signal controlling the position of your pen in X direction. You have a pulsating force. It won't rotate. It can maintain crappy, torque-ripple-y rotation if this rotation is started somehow. It's like a single cylinder engine.

Apply another sinusoidal signal controlling the position of your pen in Y direction. Again you have a pulsating force but this time, perpendicular to the previous pulsating force.

Now put your pen on the paper and start drawing. You are drawing a circle! It's rotating!

This is called 2-phase electricity (and has nothing to do with the split-phase system of the U.S.A., sometimes incorrectly and confusingly called 2-phase), and it can make 2-phase motors rotate; these motors have two sets of winding, 90 electrical degrees apart. Like a 2-cylinder engine.

Pure 2-phase motors still exist and are driven by inverter/variable frequency circuits, examples include BLDC fan motors and steppers.

Your "1-phase motor" with a start and/or run cap is also basically a 2-phase motor. Kinda. More on that later.

2-phase electricity has never been in wide use for a long time. In residential power, we have only one phase. So we need to somehow generate the 2nd phase to run 2-phase motors.

A capacitor together with the inductance of motor windings introduces phase shift and builds this second phase. Because a capacitor stores energy, it charges from the mains, and supplies this power into the circuit later.

It's impossible to get 90 degrees apart, it will be less, so the motor can be mechanically designed to not expect 90 degree phase difference either; so it's a compromise, but it works. This secondary phase usually doesn't provide as much torque as the "main" non-delayed phase, but enough to make the motor start, and in the case of run cap, increase torque and reduce torque ripple.

Sometimes both start and run cap are used, to provide highest possible torque at start, but then reduce the effect of this secondary phase to prevent overheating it, without completely getting rid of it (which happens when only using start cap).

[james_s]

Your schematic doesn't appear to be drawn correctly. The centrifugal switch that opens
after the motor reaches about 50% of run speed would disconnect one of the power
leads. Either you drew it wrong or the chinese manufacturer you copied it from drew
it wrong. A run capacitor generally provides a phase shift which makes the motor always
run in a specified direction and depending on how the motor is wound it will provide
an 'artificial' pole in between the real poles which tends to reduce 'slip' while maintaining
the torque normally produced by current generated in the rotor by slip.

There is no centrifugal switch in a hermetic compressor, the larger single phase ones use a PSC motor and the smaller ones used in refrigerators and such use an external starting device. Today this is most often a PTC thermistor or electronic circuit but at one time it was common to use a current relay that would pull in from the locked rotor current and then release once the motor comes up to speed and the current drops. 

[james_s]

Sometimes both start and run cap are used, to provide highest possible torque at start, but then reduce the effect of this secondary phase to prevent overheating it, without completely getting rid of it (which happens when only using start cap).

That's what a "hard start kit" is, it adds a second capacitor to increase starting torque. They're mostly needed with reciprocating compressors paired with a non-bleed TXV which will have difficulty starting against the high head pressure. I think most systems these days are using scroll compressors so I don't know how common hard start kits are anymore, I've never used one.

[JayMan07]

Draw XY plane.

Apply a sinusoidal signal controlling the position of your pen in X direction. You have a pulsating force. It won't rotate. It can maintain crappy, torque-ripple-y rotation if this rotation is started somehow. It's like a single cylinder engine.

Apply another sinusoidal signal controlling the position of your pen in Y direction. Again you have a pulsating force but this time, perpendicular to the previous pulsating force.

Now put your pen on the paper and start drawing. You are drawing a circle! It's rotating!

This is called 2-phase electricity (and has nothing to do with the split-phase system of the U.S.A., sometimes incorrectly and confusingly called 2-phase), and it can make 2-phase motors rotate; these motors have two sets of winding, 90 electrical degrees apart. Like a 2-cylinder engine.

Pure 2-phase motors still exist and are driven by inverter/variable frequency circuits, examples include BLDC fan motors and steppers.

Your "1-phase motor" with a start and/or run cap is also basically a 2-phase motor. Kinda. More on that later.

2-phase electricity has never been in wide use for a long time. In residential power, we have only one phase. So we need to somehow generate the 2nd phase to run 2-phase motors.

A capacitor together with the inductance of motor windings introduces phase shift and builds this second phase. Because a capacitor stores energy, it charges from the mains, and supplies this power into the circuit later.

It's impossible to get 90 degrees apart, it will be less, so the motor can be mechanically designed to not expect 90 degree phase difference either; so it's a compromise, but it works. This secondary phase usually doesn't provide as much torque as the "main" non-delayed phase, but enough to make the motor start, and in the case of run cap, increase torque and reduce torque ripple.

Sometimes both start and run cap are used, to provide highest possible torque at start, but then reduce the effect of this secondary phase to prevent overheating it, without completely getting rid of it (which happens when only using start cap).

Jesus Christ that is stretching my brain but I think that is the level of detail I’m looking for!

I drew it out and I see the circle!

So the capacitor is producing some phase shift along with the inductance producing phase shift and the total phase shift is giving about 90 degrees off of mains?  So your X is R&C and your Y is S&C?  So the cap is there just to help make it as close to 90 degrees as possible?  I guess the cap value is calculated by the engineers based on the inductance phase shift?

[Siwastaja]

Yes, you got it right. 90 degrees would be the optimum target.

[JayMan07]

Yes, you got it right. 90 degrees would be the optimum target.

I feel like I need to understand the axis.  I’ve always drawn 240 out in the way shown below.  X axis is time and Y axis is voltage.  The sine waves are 180 degrees apart giving you a total TRMS potential of 240 between them.

What is the X and Y axis in the illusion you gave?

Since we already have 240V with the waves being 180 degrees out of phase why do we need anything else for rotation?

So there are three things being fed into the compressor.
1) L1
2) L2
3) L2 modified by a cap

In the illustration you gave I only see 2 waves, one on X and one on Y, but there are 3 waves going into the compressor?

Sorry to keep asking questions.  I just still don’t feel like I have a really good grasp on everything.

[Siwastaja]

180 degree phase shift won't cut it. They will oscillate on the same single dimension. Basically two electromagnets opposite to each other, one pushes and another pulls, they can't make anything rotate, unless the rotation is started somehow else.

With perpendicular electromagnets, you have one pulling/pushing on X axis, another pulling/pushing on Y axis, and by running one using sine wave another using cosine wave (i.e., sine with 90 deg phase shift), you have your rotating vector - and rotating rotor!

[Siwastaja]

Now a real motor won't of course have just two electromagnets. But four is viable.

Note that you get your 180 degree phase shift "for free": just wind the electromagnet wire the opposite way! But it also doesn't contribute to the rotation, both create forces that run parallel to each other, just opposite direction (which you can arbitrarily change anyway by wire winding direction).

The simplest 2-phase motor will have 4 poles a.k.a. 2 pole pairs: one in each end of X and Y axes. The two electromagnets on the opposite ends of X axis can be wired in series (as happens in practice) or parallel (in theory) or whatever, the point is, they are controlled by the same voltage, just wound the opposite way so that when one pulls, one pushes. The end result is equal to having just one electromagnet, but obviously twice as strong and nicely symmetrical construction.

Now copy the same for Y axis. Two electromagnets driven again by another shared voltage.

The magic happens by the fact that the two axes are driven 90 deg out of phase, this makes the vector rotate.

3-phase power follows exact same line of thinking but has this harder to grasp plane defined by three axes (instead of our familiar X-Y plane) so in 3-phase motor control, the machine is often mathematically transformed to look like a 2-phase motor which is easier to understand and do calculations on.

In induction motor, the rotor does not strictly follow the magnetic field but it's still usable for understanding to start by thinking about a simple magnet following the rotating field caused by the electromagnets.

The single phase (so really internally kinda-2-phase, as discussed) compressor does not need "three things" fed into it, but such motors may have connections for things like overheating protection switches, centrifugal switches, etc.

[james_s]

A single phase compressor has 3 pins, just like a 3 phase compressor, the difference is is that a 3 phase motor has three windings connected between the three pins in either a delta or wye arrangement and the single phase motor has a run winding between the common pin and one of the others, and a start winding between common and the remaining pin. This is why you don't see 2 phase power distribution, with the same number of wires you can transmit 3 phase power which has advantages over 2 phases. Externally a single phase PSC compressor looks identical to a 3 phase compressor, both have the same 3 pin connector.

[Siwastaja]

That is exactly true - in 2-phase system, 3 wires are needed (because you have initially four wires from the two phases of windings; and can combine one pair of wires); and the combined wire carries twice the current! Some types of 2-phase motors (like steppers) do not internally combine the wires, exposing all 4 wires, then the current is same for all wires.

3-phase system is a genius idea to utilize the same number (3) of wires, with symmetrical currents so one wire doesn't need to be thicker. In addition, adding phases reduces torque ripple a tad, similarly how a 4-cylinder engine runs smoother than 3-cylinder engine. For example, 4- an 6-phase motors exist. But anything beyond 2-phase is kind of micro-optimization due to real-world electromechanical constraints. In a simplified world, 2-phase motors is everything that's needed.

[JayMan07]

Ok thank you that helps clear up a lot!