I thought fan motor was jammed and needed cleaning; but I was wrong!

Hi

I remember once someone told me that a capacitor is used in a motor to give it a 'surge' of power so that it can start revolving by overcoming its inertia, resistive forces such as from grease etc. Electric current alone doesn't have enough strong 'push' to come out of its static position. But once in motion it no longer requires the capacitor as long as it keeps running. I was further told that if you don't have a capacitor in a motor then you can give it a 'push' yourself. Further, if I recall correctly, I was also told that some not-very-large three phase motor can do without a capacitor because three phase electricity has more power per cycle. Do I have it right?

For last some days the exhaust fan of our store room wasn't working properly. Like when I would turn it on, it wouldn't start. Sometimes it would only make a half revolution and that too very slowly, and then it would stop. So, when this malfunctioning started I was giving it a manual push with a stick, and when pushed, it would start rotating but one of the important thing was that it was only rotating almost at half the normal rotating speed. Therefore, from this I inferred that the inside of its motor needs to be cleaned and greased because it seemed as if the motor was pushing against something; in other words it seemed it was jammed and in my opinion that was the reason for its half-speed.

Today an electrician came to fix it. I was arguing with him that its motor needed to be cleaned. But he said that it needed a new capacitor (I think he himself wasn't sure of himself that much and part of the reason he wanted to test a new capacitor was that he was finding it too difficult to open the external cover of the fan. But as soon as he changed the capacitor the fan started running! And I was there surprised and partly embarrassed.

Where did I go wrong with my reasoning? Please help me to figure out. Thank you very much.

Regards
PG

Google and Wikipedia are your friend.
Your fan probably had a run capacitor that had dried out from heat/age.

You can generally tell if it's a bearing/bushing problem by how hard the motor is to rotate when no power is applied, the half speed and hestiation on start are signs it's a run capacitor.

That "someone" doesn't know much about motors.

It has nothing to do with the amount of "push" the electricity provides (it provides plenty). It has to do with requiring a rotating field to make the motor rotor go around. A single-phase AC winding all by itself provides no rotating field, just a field that increases to maximum and zero twice per cycle. To get a rotating field you add an addition winding that has a current phase-shift such as provided by a capacitor or, for very small motors, a shorted turn (shaded-pole).

Three-phase (or two-phase) motors don't require such an extra winding because the multiple phase windings already provide a rotating field.

Many fans are PSC motors which are odd balls. Synchronous motors also require a capacitor, but you don't find them in fans.


The HP rating of the fan would offer some clues. There are start and run capacitors as well.