Fan Going Different Ways

[Cloverjo]

Can anyone explain? I work in an amusement arcade shop where we redeem cheap stuff. One item was a small hand-held fan which required 2 AA batteries. Several customers have returned them saying that it "blows backwards". I've found that changing the polarity of the batteries makes it "blow forwards".

I don't understand how this can happen. I am fairly scientifically and technically minded, so you don't have to use words of one syllable to explain this, if you can.
Thank you to anyone who can help.

[jim360]

Hard to answer with total certainty without seeing how the fan works, although on the face of it if you are reversing the direction of the batteries then you would be reversing the direction of the electric field in the wires. Most likely, the fan works based on a simple motor, ie a wire rotating inside magnets. The direction of motion is then sensitive to which way the current is flowing. Reversing the direction of current reverses the motion according to the right-hand rule.

[mikey4444]

Jim has it right ! Just put the batteries in the correct way around and it should work OK !

[Cloverjo]

Thanks, Jim. I must try to memorise your answer to explain to my colleagues who were as baffled as me. Thanks, mikey, too.

[mikey4444]

Only too glad to help Jo.

[jim360]

It might actually be the left-hand rule for motors, but you can switch between the two hands easily enough as long as you get the point that motion, magnetic current and electric field point in the three different directions (up/ out/ left), and stay in a fixed orientation with respect to each other. So, for example, hold your right (or left) hand out with the thumb pointing upwards, first finger pointing away from you and second finger pointing to your left (or right). The magnetic field is your first finger and, depending on convention, the thumb is motion and the second finger is electric current. To change the direction of the current, turn your hand upside-down. The magnetic field is still pointing in the same way, but as the current changes direction so does the motion.

[Togo]

The little fan has probably got a cheap and cheerful DC motor fitted( cheap as chips) they are most definitely supply polarity sensitive, more expensive equipment that intends to utilise this characteristic have what is called an H bridge fitted with whitch the means of a switch will automatically reverse the motor direction. IE Chargeable sucker/ blowers and some vacuum cleaners.

[Cloverjo]

Thanks again, Jim and to Togo. I used to know this stuff, but I had to hold my hand out twisting it for ages to get it all again. God, what has happened to my brain?

[Togo]

Just seen my "whitch" haha which!

[Cloverjo]

I didn't notice, Togo. I think we notice our own mistakes more than other people do.

[Togo]

Only those amongst us who are prepared to be self critical rather than critical of others as a default setting Cloverjo.

[beso]

Togo//
expensive equipment that intends to utilise this characteristic have what is called an H bridge fitted with whitch the means of a switch will automatically reverse the motor direction.//

No, that is not why such devices are polarity insensitive.

The little fan has a permanent magnet field. Reversing the direction of the current reverses the direction of the rotation. Larger motors such as power drills and vacuum cleaners usually have a wound field magnet that is in series with the armature. Consequently, reversing the polarity of the supply reverses the direction of the current in both the armature and the field magnet, resulting in the motor turning in the same direction regardless of polarity.

These motors happily run in the same direction on AC or DC supply.

[Togo]

//An H-bridge is a type of circuit that you can use to get a reversible DC motor to spin both clockwise and counterclockwise.

In other words, this circuit allows you to quickly reverse the direction a motor is spinning by using a switch or controller chip to change its direction.//

http://www.instructables.com/id/How-to-make-an-H-bridge/

//An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards.//
Wikipedia.

[ToraToraTora]

what jim said, try putting the battery in the wrong way in most electric analogue clocks and they run backwards. Used to do that in a club years ago, cranked the steward right up at closing time!