PSC motor run winding

[michaele]

In a a PSC motor the current enters the capacitor and travels back through the run winding and through the start winding to reach the other plate of the capacitor. Does this current have an effect on the run winding as well as the start winding? If not , then why?

[nevgee]

In an alternating supply the current doesn't flow "through" the capacitor, there is no "path". The electrical supply pulses, 60Hz in the USA and 50 Hz in Europe. The capacitor in the start circuit causes the current to be out of step (90 deg) with the current in the run winding. Therefore, the magnetic field created by the start winding is slightly out of step with the magnetic field of the run winding . This causes a rotational magnetic field that will drag the rotor round.

[Quality]

I don`t want to sound a clown but they call it out of phase
but you explained it better than I could have anyway

[nevgee]

Yeah you're right I used out of step to keep it simple ... as you realised. :)

[michaele]

In an alternating supply the current doesn't flow "through" the capacitor, there is no "path". The electrical supply pulses, 60Hz in the USA and 50 Hz in Europe. The capacitor in the start circuit causes the current to be out of step (90 deg) with the current in the run winding. Therefore, the magnetic field created by the start winding is slightly out of step with the magnetic field of the run winding . This causes a rotational magnetic field that will drag the rotor round.
I understand all of this, but the current has to flow back through the run winding after leaving the capacitor to go through the start winding to get to the other side of the capacitor. My question is how does this affect the current in the run winding on it's way from the capacitor to the start winding? And if it doesn't, why not?

[Quality]

I understand all of this, but the current has to flow back through the run winding after leaving the capacitor to go through the start winding to get to the other side of the capacitor

The current is constant in each winding not flowing through one but both at the same time but as nev stated they are out of step / time / phase

it all occurs a lot quicker in real time but harder to explain in a few word

[DTLarca]

In a a PSC motor the current enters the capacitor and travels back through the run winding and through the start winding to reach the other plate of the capacitor. Does this current have an effect on the run winding as well as the start winding? If not , then why?

The thicker run winding has a higher inductive reactance than the thinner start winding so current through the run winding lags the applied voltage by about 30°. Installing the capacitor in series with the start winding causes the current through that winding to lead the voltage by about 60°. The run windings inductive reactance and the start windings experienced capacitive reactance gives an overall phase shift of 90°.

The current charging each side of the capacitor each 100th of a second is predominantly from live to neutral and back again. I expect the capacitor to have the effect of slightly delaying the rise and fall of voltage across the motor generally, that is dampening - ever so slightly - so I expect it's presence to somewhat reduce the current through the run winding compared to, say, if the run and start windings were each fed independently all the way from the street transformer (sub-station).

[nevgee]

Thick wire thin wire? come on .... what is that meant to signify? :(

Capacitors have an effect on an AC supply by causing the current to lead the supply voltage by 90 deg. An inductive component would cause the current to lag by 90 deg. This can be confusing if you aren't aware of the properties of an ac supply. It simply means that there is a phase shift between the voltage pulse from + to - to + ( every 1/60th sec) and the current pulse, + to - etc. You need to be able to visualise a sinusoidal wave form to get the whole picture.

Therefore, the capacitor causes a shift in the supply current to the start winding. this creates a difference in the magnetic fields of the start and run windings, such that it causes a rotation effect of the stator's mag field.

The current doesn't "run" back up the run winding from the start winding. It would be best for you to just accept that the flow can only exit from both windings at the common point.

In reality of course there is a pulsing or swaying motion that causes the build up and collapse of the magnetic field in the windings, creating an induced magnetic field. But as stated the start winding "sees" the pulse at a different time, It's this phase shift that causes the field to have a rotational effect.

Something that often confuses people is the fact that current can be at it's maximum when there is no applied voltage.

[nevgee]

The thicker run winding has a higher inductive reactance than the thinner start winding so current through the run winding lags the applied voltage by about 30°. Installing the capacitor in series with the start winding causes the current through that winding to lead the voltage by about 60°. The run windings inductive reactance and the start windings experienced capacitive reactance gives an overall phase shift of 90°.

The current charging each side of the capacitor each 100th of a second is predominantly from live to neutral and back again. I expect the capacitor to have the effect of slightly delaying the rise and fall of voltage across the motor generally, that is dampening - ever so slightly - :eek: so I expect it's presence to somewhat reduce the current through the run winding :eek: compared to, say, if the run and start windings were each fed independently all the way from the street transformer (sub-station).

Tut tut . . . I am so surprised that you actually said that...... " I expect" perhaps, you have forgotten your theory? ;)

[nevgee]

http://www.tpub.com/neets/book5/18d.htm

" . . . Split-Phase Induction Motors
One type of induction motor, which incorporates a starting device, is called a split-phase induction motor. Split-phase motors are designed to use inductance, capacitance, or resistance to develop a starting torque.
CAPACITOR-START. - The first type of split-phase induction motor that will be covered is the capacitor-start type. Figure 4-11 shows a simplified schematic of a typical capacitor-start motor. The stator consists of the main winding and a starting winding (auxiliary). The starting winding is connected in parallel with the main winding and is placed physically at right angles to it. A 90-degree electrical phase difference between the two windings is obtained by connecting the auxiliary winding in series with a capacitor and starting switch. When the motor is first energized, the starting switch is closed. This places the capacitor in series with the auxiliary winding. The capacitor is of such value that the auxiliary circuit is effectively a resistive-capacitive circuit (referred to as capacitive reactance and expressed as XC). In this circuit the current leads the line voltage by about 45° (because XC about equals R). The main winding has enough resistance-inductance (referred to as inductive reactance and expressed as XL) to cause the current to lag the line voltage by about 45° (because XL about equals R). The currents in each winding are therefore 90° out of phase - so are the magnetic fields that are generated. The effect is that the two windings act like a two-phase stator and produce the rotating field required to start the motor.
Figure 4-11. - Capacitor-start, ac induction motor.

http://www.tpub.com/neets/book5/32NE0450.GIF
When nearly full speed is obtained, a centrifugal device (the starting switch) cuts out the starting winding. The motor then runs as a plain single-phase induction motor. Since the auxiliary winding is only a light winding, the motor does not develop sufficient torque to start heavy loads. Split-phase motors, therefore, come only in small sizes.

[DTLarca]

Thick wire thin wire? come on .... what is that meant to signify? :(

Thicker wire = higher XL


The main winding has enough resistance-inductance (referred to as inductive reactance and expressed as XL) to cause the current to lag the line voltage by about 45° (because XL about equals R).

Exactly. However, the phase-shift split is not always an even 45° and 45° and the total is not always 90°.

[michaele]

Thanks for the replies. Electrical is the most facinating part of our job. The capacitor circuit has always been a little confusing to me.

[nevgee]

Although some of the "theory" and explainations given above were not strictly correct, I guess the analogous content does help in furthering an understanding of the principles involved.

[DTLarca]

Although some of the "theory" and explainations given above were not strictly correct, I guess the analogous content does help in furthering an understanding of the principles involved.

Lol :)

Indeed, for instance someone posted that the capacitive reactance shifts the current forward by 45° and the thick wire inductive reactance back by 45°. The problem with this scenario is that were such a motor to gently seize such that a short duration reversal could unseize it - it would in fact not respond to the capacitor being placed temporarily in series with the run winding. The capacitors reactance would cancel the thick wires reactance giving us only pulsing poles and no rotating poles :)

[nevgee]

Lol :)

Indeed, for instance someone posted that the capacitive reactance shifts the current forward by 45° and the thick wire inductive reactance back by 45°. The problem with this scenario is that were such a motor to gently seize such that a short duration reversal could unseize it - it would in fact not respond to the capacitor being placed temporarily in series with the run winding. The capacitors reactance would cancel the thick wires reactance giving us only pulsing poles and no rotating poles :)

Posted by me but not my text it was a quote from a tech site .... I didn't read it thoroughly but then wasn't concerned about the slight inacuracy . . . .

..... I suppose you were just using the thick thin analogy to keep things simple? As with "damping effect of a capacitor" which is quite bazaar.

In a purely reactive circuit the phase shift will be 90 Deg.. I didn't mention pf for simplicity. In a practical scenario then, unity is rarely if ever achieved.

As for seized motors, the bearings would generally be the cause of such, and a "reversal" if at all practical would not really be effective. Adding a cap to the run winding would only cancel the cap on the start winding, ( 90 lead + 90 lag ) would that be "single Phasing" :D
Unless of course a compressor happens to be attached to the motor and we're talking about seizure of that device not the motor.

I was hoping you could explain why the thick wire has a higher XL As you seem to imply the reason is because it is "thick" I feel there may be some predjudice with regard to thick and thin wires ... is there a slip in professional standards here? :)

[DTLarca]

As for seized motors, the bearings would generally be the cause of such, and a "reversal" if at all practical would not really be effective. Adding a cap to the run winding would only cancel the cap on the start winding, ( 90 lead + 90 lag ) would that be "single Phasing" :D

I said "the capacitor" not "a capacitor".



Unless of course a compressor happens to be attached to the motor and we're talking about seizure of that device not the motor.

In the compressor the motor and compressor share the same shaft and bearings.


I was hoping you could explain why the thick wire has a higher XL As you seem to imply the reason is because it is "thick" I feel there may be some predjudice with regard to thick and thin wires ... is there a slip in professional standards here? :)

Why do you think the thicker wire has a higher self inductance?

And from where comes the question "is there a slip in the professional standards here?"?

Did you know that even though self inductance increases with the square of the number of turns, double the turns and the inductance quadruples, and that you can have more turns with a thinner wire, yet still with a thicker wire the run winding has a greater self inductance than the start winding.

Lenz's Law: The induced EMF acts to circulate a current in a direction which opposes the change in flux which cause the EMF.

Self Inductance: A circuit has a self inductance of one henry if an emf of one volt is induced in the circuit when the current in that circuit changes at the rate of one ampere per second.

E = dI/dt = Voltage = rate of change of current. The greater the current changes in a given time the greater the inductance.

E = dr/dt = Voltage = rate of change of magnetic field flux density radius. Thicker wire = greater r.

The most basic self inductor is simply a length of wire - the greater the cross sectional area the greater the inductance - double the radius and the inductance is quadrupled. A typical inductor is simply a coil of wire. Factors which affect the inductance of an inductor are:


The number of turns of wire - double the turns and you quadruple the inductance.
The cross sectional area of the wire or the coil of wire - the greater the cross sectional area the greater the inductance - also a square relationship.
The presence of a magnetic core - when the coil is wound on an iron core the same current sets up a more concentrated magnetic field and the inductance is increased.
The way the turns are arranged - a short thick coil of wire has a higher inductance than a long thin one.

[nevgee]

Thank you ...... I had an idea there may have been some relationship betwixt "thick" and "Thin" but wasn't so sure.

That was easier than me blowing dust off a text book. ;)

[nevgee]

I said "the capacitor" not "a capacitor".

In the compressor the motor and compressor share the same shaft and bearings.



And from where comes the question "is there a slip in the professional standards here?"?




Of course they do with hermetics but not with open drive? But anyway, without wanting to be pedantic, I was infering that the seizure could be within the compresor, ie cylinder(s) etc not bearings.



It was meant as a joke, with reference to thick and thin predjudices ... but sadly it fell onto stoney ground. :)