Capacitors and Single-Phase Motors

For single phase motors (or any motor for that purpose) you need a rotating magnetic field OK ?

The rotating magnetic field for simplicity - consider a rotating N & S pole.

In the rotor say I have a magnet (stationary N&S poles)

As the N pole of the rotating field on stator comes near the S pole of Rotor, they get locked and as the stator field (not the stator only field) moves, it pulls the rotor pole physically along it.

This is the principle on which motors work. This magnet on Rotor can be a permanent magnet (PM Motors), Electro magnets (DC or Synch Motors) or induced Electromagnets (Ind Motor)

Hope you are aware upto this.

The rotating electric field can be ideally created by at least 3 phases - assume as in the decoration light if the lights blink a-b-c-a-b-c order, it seems as if a light wave is moving.

However if it is only a-b-a-b, it seems it is oscillating back and forth (you might have seen , else you can experiment and see) This is why 2 phases 180 deg apart do not create a rotating field.

Now thing the phases at 90 deg apart (or near about)

The first set shows the 3 Ph motor and the rotating field. (0.5N are the N pole around the tip of the strong N pole and hence it is relatively weak and do for 0.5S)

In the mid is when it is fed with 180 deg apart supply

The last is the single phase Ind Motor (where the feed is by 90 deg or near by supply)

In the single phase ind motor the phase shift is created by the capacitor (which will give a 90 deg leading current)

This phase shift capacitor is used for the starting purpose, since when the motor is already on a sufficent speed, the rotating effect is possibble by other various means.

Pl go through this page and even wiki has a good one on this

http://www.allaboutcircuits.com/vol_2/chpt_13/9.html

Here you have the different type of IMs (Capacitor start, Resistance split, Shaded pole etc)

Normal capacitors do not distinguish between AC and DC , however some capacitors (like Electrolytic) can only block on one direction and are hence used for DC only and have clearly marked + and - terminals.

If it is an electrlytic capacitor for you, and yoy connect reverse, it will become conductor and you will hear boom.

Best is to use a multimeter and measure C on both sides and you will know. Or better put a torch cell and a bulb and charge cap, in one direction it will be virtually open (charging will not even take a sec) in other the current will continue to flow.

Normally the negative lead of this will be shorter than the positive lead.

Hope it helps.

Yes, you're asking a lot ... re : "i mean when its positive on the run the capacitor charge and when its negative on the run the capacitor discharge it on the start winding to make the rotor follows the magnet flux"... you *sort-of* have the idea. But~ in your run-cap case, the capacitor doesn't "charge-&-discharge" in that way, tho. It is always active in the same fashion simply to shift the phase of the current that is flowing through the run winding circuit.

There are many textbooks that cover the "basics" of ALL the different single phase motor designs (there are MANY!). To try and put the theory in a proverbial nutshell, here, would take ... well, perhaps the Lord Himself...

Your dc capacitor can be checked for proper polarity very easily. Have you tried "Google", or Wiki"...? Google search came-up with THIS right-off. Wiki gave THIS for starters... more links in the article...

Best wishes ~