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

There was nothing wrong with your reasoning but you misunderstood why the capacitor was needed.

In some motor applications a 'starter' capacitor is needed and it may be either disconnected or shorted out when the motor reaches full speed but in the case of your fan it sounds more like it is there to provide phase shift. Simplifying the explanation: The motor works by using magnetic fields to push or pull the rotor to a new position. If you fed the AC power directly to a coil and put the rotor inside it, the rotor would try to turn one way and then the other as the AC cycles reversed polarity. The net effect would be the rotor vibrating or given a push it might 'jump' cycles and rotate slowly. You might have found that pushing it in the other direction made it spin slowly in reverse. The capcitor is there to create a second magnetic field in another winding. When AC voltage is connected across a capcitor the current through it is out of phase with the voltage. How much phase shifts depends upon the AC frequency and the value of the capacitor. Because the curent also flows through the second coil, the magnetic 'pull' will peak at a slightly different time to the main coil. The position of the coils is set so the 'peak' is first at one angle then a few degrees in one direction amaking the magnetic fields appear to rotate and hence pull the rotor along.

Motors like the fan you describe are single phase motors, they use two wires from the power source and generate a second phase internally with the capacitor. A three phase motor has three coils and uses the 120 degree shift between the phase voltages to directly rotate the magnetic field so no capcitors are needed.

Brian.

Beautiful explanation betwixt !
Frank

Thanks Frank.

I'm in a somewhat unusual situation here of being a domestic property with a 3-phase supply, a swimmng pool that uses two single phase 1.5HP water pumps and a grid-tied PV inverter. You have to learn about power electrics to survive in the wilderness out here!

Brian.

Hi,
There are a lot of types of Single Phase Induction motors also known as Squirrel Cage Induction Motor. It has got 2 parts one known as Stator (the stationary part) and the Rotor (the revolving part). Here are two pics for your intro only.

The single-phase induction motor is not self-starting. When the motor is connected to a single-phase power supply, the main winding carries an alternating current. This current produces a pulsating magnetic field. Due to induction, the rotor is energized. As the main magnetic field is pulsating, the torque necessary for the motor rotation is not generated. This will cause the rotor to vibrate, but not to rotate. Hence, the single phase induction motor is required to have a starting mechanism that can provide the starting kick for the motor to rotate.
There are CAPACITOR START INDUCTION MOTOR (split phase induction motor), Permanent Split Capacitor (Capacitor Run) AC Induction Motor, Capacitor Start/Capacitor Run AC Induction Motor and so on. few examples :

A lot remaining to explain. This is only a short for your reference.