What is it that reverses the poles of electromagnets?

Question

In electric motors and speakers and other devices using an electomagnet or solenoid, what is it that switches the polarity of the magnet?

What changes the direction of the DC?

Answer 1

Motors and speakers are different devices, mechanically, but lets look at them one at a time.

We have to begin with alternating current, AC. Alternating, means it goes in one direction, then reverses itself and goes in the other direction.

Think of a playground swing. It swings higher and higher toward the top of its arc, but (if you measure its distance of travel against the level ground, it moves a shorter and shorter distance with each successive small increment of time until it reaches the farthest extent of its travel.)

Or, think of the ocean waves: they roll in, they roll out. At the top of their travel (the highest extent of the dampness on the sand), they have spent all their power. They periodically reverse direction, which is the same as alternating current does.

Because the current reverses direction, the magnetic field it generates is also reversed. In order for the loudspeaker to generate a tone of A above middle C, the current must reverse itself 440 times a second, because that is what 440 Hertz means.

So a solenoid will also reverse itself when the current reverses itself, because it is basically a coil, just like the voice coil of the speaker. The solenoid is made to hande a lot more current and the 'throw' is a greater distance (instead of the little fraction of an inch travel of the voice cone of the speaker) but otherwise they are coils.

If a solenoid is powered by DC, it will go in one direction only and requires something like a spring to mechanically return it to its original position. Or an electronic device called a diode or rectifier can be used to send pulses of current in only one direction through the solenoid. Think of a doide as a one-way gate for current.

A motor is slightly more complicated. Because we want to spin in one direction (instead of just vibrate back and forth) we have a set of electrical contacts in the motor. The motor starts with current going through two sets of coils, a stationary coil and a moving coil.

The stationary coil generates a magnetic field. The moving coil generates a magnetic field. The two fields are set up so that like poles are presented to each other, and they repel.

[Opposite poles attract and like poles repel; it's a law of nature and there's nothing we can do or say that will make it change. There are no 'gay' magnets.]

Okay, so they repel. The moving coil moves away from the fixed coil. What happens next...? Well, there is more than one set of moving coils in an AC motor. When one set moves away, the contacts come apart and there is no longer current going through the moving coil. Where is the current going...? Another set of contacts come together and the current goes through the NEXT moving coil, attached to the same metal shaft in the center of the motor!

So, in the typical electric motor, there is one stationary set of coils (one pair) and many sets of moving coils.

So each successive moving coil has current passing through it at a certain time, and each moving coil is repelled by the stationary coil in turn. The motor keeps turning. The polarity of the coils with respect to each other is always the same, since we always want them repelling.

(Actually I have simplified this somewhat. You can have one moving coil on one side being attracted to the stationary coil and another moving coil on the other side repelled by the stationary coil, as long as the motor keeps turning the same direction. But I wanted to show the simplest setup first.)

The sets of contacts are connected to the shaft of the motor (on an insulated sleeve) so they always stay in the same relative position to one another, and so the motor always spins the same speed. The amount of torque (turning power) it has is due to the strength of the magnets. If you have a brownout (lower than usual voltage) or bad bearings, the motor can lose torque.

I hope this is clear enough to help.

19 NOV 06, 0034 hrs, GMT.

Answer 2

Electromagnets have a magnetic field generated by current flowing through a conductor, with the conductor wound into a coil to concentrate the magnet field for greater strength. An iron (ferro-magnetic) core such as a nail or iron or steel rod works well, and a model train transformer and some #22 gauge copper wire with a really thin insulator (such as lacquer) works great.

You can determine the orientation (polarity) of the magnetic field in space using the left-handed rule: If the conductor were stretched straight, and you reached out to wrap the fingers of your left hand around the condutor, leaving your thumb sticking out, and were careful to make certain that your thumb points in the same direction as the electron flow through the counductor, the direction your fingers curve around the conductor (toward your fingertips) would indicate the orientation of the magnetic field.

Consequently, if you swap the connectons of the conductor on the power supply, positive for negative, you can see that you'd have to change your grip around to show then new direction in which the electrons are travelling -- and you'd notice that the field polarity has also reversed!

Here's a site with a good explanation of the left-hand rule: http://www.sea.siemens.com/step/templates/lesson.mason?motors:2:5:2

Answer 3

boy, those confused me - anyway, a solenoid is spring loaded, when power is applied to open a car door for instance the coil draws in the metal rod, when power is stopped the spring at the bottom of the rod shoots the rod back out (just take one apart). an electromagnet only works with ac - you can not have a transformer winding work unless alternating current is flowing through it. for dc to ac you need an inverter, for ac to dc (to electroplate something , i plated a penny with a nickel) all you need is a rectifying diode from radio shack. some handle up to 600 volts, put 110 v. household current through it and you'll have 110 volts dc (it goes negative to positive)

Answer 4

the direction of the DC(direct current)

the design of the circuit, who ever did it and what was the purpose

Answer 5

the direction of the current flow