To cancel out each other's torque, so the room doesn't spin? (Just kidding.)
In fact, though, ceiling fans can rotate in either direction. For example, if you have tall ceilings, it's best to have the ceiling fan move air upwards in winter, to push the warm air trapped up high around to the walls and back down to floor level. In summer, you spin it to keep denser cool air down.
By the way, I was curious if there actually is a standard rotation. This site implies there isn't. The blades can be pitched either way.
http://www.ehow.com/how_6514976_determine-fan-blade-clockwise-ccw.html
I was almost hoping that there was some obscure historical reason why the pitch was always set the same, but evidently not.
I think the reason that fixed the available rotation directions is just the standard, which is followed by the manufacturers. My opinion is that, there is no reason for the a fan to rotate clockwise or anti clockwise but the standard. That is because the wings shape can be made to move the air downwards (for the ceiling fan) or forward (for table fan or standing fan) whether they rotate clockwise or anti clockwise.
May be if motors employed in both type of fans are of same type, in case of the table fan, it's the rotor which rotates, and stator stationary, with respect to the room. But in case of ceiling fan, the rotor is stationary with respect to the room, it's the stator which actually rotates with respect to the room.
Well Ashok, you have put forward a question that has been widely presented into various web forums but never has been answered properly. It is a fact that it does not matter which direction a fan is rotating as long as the blades are designed to push the air into the desired direction. But historically, when ceiling fans first became popular, most supply systems were DC. These DC fan motors used a commutator (and brushes), which is fitted to the non-driving end (upper end for fan with blades fitted to lower end) for ease of servicing as true for any general DC motor even today. Fans were designed to rotate in a clockwise direction, looking from the non-driving end (which is anti-clockwise looking towards the blades), as is true for any drill machine, majority of aeroplane propellers, etc, all probably following the direction of the right-handed cork-screw. Similarly, screws and water taps all normally tighten on clockwise turn. When table fans came later, the protrusion of the carbon brushes at the rear side did not create a sleek look, so the armature had to be placed in the reverse direction, now with the armature (and brushes) towards the blades. This created a reversed direction of rotation of the blades, making them rotate clockwise looking towards the blades. One can see enough evidence of this upon a visit to any Technological museum where such antique DC fans are displayed. Many houses in Kolkata still have such DC ceiling and table fans in operation from the AC supply using rectifiers, since we had DC supply in some parts till about a decade ago. Upon advent of AC supply around the world, the AC fan motors simply followed the same tradition of direction of rotation. When the early AC ceiling fans arrived, the centrally placed rotor (rotor is the part that rotates) rotated the blades and outside placed stator (stator is the part that is stationary) stood still, just as in DC fan motors. Later came the modified AC motor design where the centrally placed stator stood still and the outside placed rotor rotated the blades.
In general the rotor part rotates as in the case of table fan. For ceiling fan the stator part (magnetic part) rotates not the rotor part. Hence the opposit direction.
Shri.Albert Manfredi answer is nearly true, but changing the starting capacitor connections change the direction(AC ceiling fan only)
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