AC Electromagnets

Question

Could someone help explain the uses of AC electromagnets. Wherever I look it says that DC electromagnets create stronger magnetic fields. I understand why AC electromagnets could be used in transformers but why use them in motors for example? Power stations also use electromagnets in the generator to create the magnetic field, would these be AC or DC? Finally, which type of current would electromagnets in household motors use e.g. food processors, blenders etc?

Answer 1

In motors AC electromagnets can be useful as after half the rotation the magnetic field could be reversed and a permanent magnet placed on rotor could then continue to make a complete turn which would have stopped if the magnetic field was constant. But most of the motors rely on a constant magnetic field in which a coil/rotor of electrical winding is rotated.

As most of the genrators/alternators/dynamos depend on constant magnetic field, I would say that they work on DC electromagents which were a replacement over permanent magnets used in earlier designs.

Household motors usually work by rotating windings of electrical wires in constant magnetic field, so i would say that these contain DC electromagnets.

Answer 2

There are several types of household motor, those with a commutator and those without, those connected directly to the supply and those with complex drive circuitry.

The type without a commutator is by far the cheaper to produce, but has less electro-mechanical efficiency, it could be found in motors that drive the turntable of a record player, but most commonly these days is to be found as the impeller motor in fan assisted ovens, and the fan heater sitting on the floor directly in front of me. See: Here. These motors run exclusively on AC.

The commutator type may have a stator (the material inside the housing that does not rotate) which is built of one or more sets of two poles - these can either be permanent magnets or electromagnets.

In the type with electromagnets, the shaft on which the commutator lies has electromagnets which may have again just two poles or any multiple thereof to match the stator's number of poles. As the motor turns, the magnetic field in the stator always is the identical pole to the corresponding one on the rotor which will be "ahead" rotationaly speaking whereas the field at the place ahead rotationaly will be opposite.(Attracting) This can be achieved using either AC or DC, as the commutator switches the current to the central electromagnets to be in the correct phase with the coils on the rotor to impel rotation in either case. These are commonly found in washing machine motors, food processors and other household items requiring considerable power. It also can be switched to different speeds, and have a starting coil/capacitor which can be used to change direction of rotation, see: Here. These are similar in design to your average car alternator, and on a larger scale generators, they require fluctuating magnetic fields in order to work, and generate ac. (Yes even your car alternator does this and has to be rectified and voltage controlled)

In the type with Ferromagnetic (ie static magnetic fields) either in the stator or in the rotor, these tend to have specialist uses, such as print head drivers, paper feed drivers, hard drive motors and HD head drivers - essentialy anything that requires precision movement, as a high degree of control of speed can be achieved using pulse width modulation. If there is a comutator, it usualy consists of two or more rings, making a permanent connection to the coils on the rotor - but this type is very rare in the modern world. In the type with permanent magnets on the rotor, speed and direction is controlled electronically using pulses of current which can be sequentially positive and then negative, so they are techincally AC - but not in the way that you mean. See: Here. Don't know if you'd call those "common" or "household" though, it's up to you if you include them. There's one in my bedside alarm clock,gets given a pulse every second and drives the arms around with a tic-tic sound, so quite common I guess.