How Does Reversing Start Winding Reverse Single Phase Induction Motor Direction?

I have been reading about reversing single phase induction motor and all articles tell me it is done by simply reversing starting capacitor on the auxiliary winding or the winding itself.  Now I am confused how in an alternating current world polarity of the power supply matters?

Ashish... you're not confused, you are right!  Think about it.  A start capacitor is a non-polarised capacitor, so reversing it is not going to do anything, right?  Correct!   What you do is reverse the start WINDING.  So instead of the start winding attracting the armature one way, it attracts it the other way and the motor starts in the opposite direction.There's a pretty good tutorial on this HERE.You can also reverse the main winding, which will have the same effect.  As long as you reverse one winding relative to the other, your motor will run in the opposite direction.

Hi Ashish- To add to David's comment, a reversible induction motor will always have at least two windings. It's the phase relationship of the voltage fed to the two windings that determines the direction of rotation.  The capacitor causes the voltage fed to one winding to be out of phase (delayed) by 90 degrees as compared to the other winding.-Rick

Thanks for the response.I can understand that if the capacitor is reconnected to the other winding the motor will reverse. But when they say the following, I am not very sure:"If the windings are not the same resistance, you can still reverse it by reversing the polarity of one of the windings, provided that the windings are not tied together inside the motor (as in, more than three wires coming out from the windings). "Can the polarity of a winding be reversed? I understand that winding is nothing but an inductor, again which in AC circuits do not have a polarity.
Regards,Ashish

Thanks David for your response.I understand till the part where it says that the roles of the two windings get swapped i.e. main becomes starter and vice-versa. However, I do not quite get it when we say "reverse the winding". Quoting from the article:
"...If the windings are not the same resistance, you can still reverse it
by reversing the polarity of one of the windings, provided that the
windigs are not tied together inside the motor (as in, more than three
wires coming out from the windings). "How can a winding have a polarity which one can reverse, it is basically an inductor within a alternating current circuit? I understand that broadly inductors and capacitors do not have polarity in ac circuits.
Regards,Ashish

Thanks Rick for your response. Please see my reply to David's comment.

Ashish... OK...an inductor does not have polarity as such BUT it is acting as an electromagnet.  If you connect a voltage one way, it will have north and south magnetic poses one way.  If you connect it the other way, the N and S poles will be reversed, even though it looks the same to the DC or AC that is supplying it.if you look at some transformer diagrams they have dots on one end of each winding to indicate this.Lets say you have your main windings top and bottom, and the start windings left and right.  If you connect the start winding one way, it will add to the field of the main winding on the left.  If you connect it the other way, it will add to the main field on the right.  I don't have time to draw this now but might try later, if this does not make sense.

Ashish.. As a picture's worth a thousand words....The following pics are diagrams of the magnetic fields at ONE INSTANT in time when voltage is applied to the motor.  The armature (rotating part) is considered as a magnet too (from the current induced in it like a transformer, hence "Induction motor".  In the below diagrams only the main and start windings are shown, and this is VERY simplified, there are phase shifts due to the start capacitor, and more poles than are shown, normally, but the theory's the same.  An instantaneous voltage is shown, and the resulting instantaneous magnetic fields.  The magnetic poles of individual windings are shown as small N and S and the overall magnetic field as a large N and S.
Now reverse the polarity of the start winding and see what happens:
You see how the overall magnetic field has changed from one side of the main windings to the other.  The result of this is that the armature will move one way or the other way when the motor is started.As pointed out in the quote of your above post, you need access to both wires of the start winding to do this.  Sometimes the wires are internally tied together and you only have 3 wires coming out (common, main and start). If that's the case then sorry, your motor will only run one way....Does this make it clearer?  Cheers // David

... but would not the polarity be already reversing 50/60 times a second?If this was a DC setup it makes sense, but in ac setup the supply polarities are always reversing.Regards,Ashish

Ashish,  Yes, the polarities are reversing 50/60 times a second.  So when the polarities reverse, so will the N and S poles of the windings on the motor.  But remember that the Armature is also getting its power (and thus magnetic field)  from the windings, so ITS magnetic polarity will also reverse at the same rate.  The net effect is that the armature is attracted in one direction or the other direction, no matter what the instantaneous voltages and magnetic polarities are at any half-cycle of the power.
Remember that with AC the polarities reverse on both the main and start windings, so the net effect will be the same.  What matters are the overall magnetic field strengths - on the diagrams above, the BIG N and S,  may change every half cycle, but their positions won't.I hope you are learning at a college with a practical lab so you can try doing this for yourself.  I had the opportunity once and it was a big help in understanding this.

Ah, that explains it. Thanks a lot for your help and time.Regards,Ashish

Hi David,I got a satisfactory explanation from Rick below. Thanks for your help and time.
Regards,Ashish

OK Ashish, I hope it has helped.  Many thanks for the feedback, much appreciated.