Question

why only magnets are used for making motors and generators ...? why not other material...?plz answer my question

Answer 1

The field of a machine is the part that generates the direct magnetic field. The current in the field does not alternate. The armature winding is that which generates or has an alternating voltage applied to it.

Usually, the terms "armature" and "field" are applied only to alternating current generators, synchronous motors, DC motors, and DC generators.

Alternating current generators. The field of a synchronous generator is the winding to which the DC excitation current is applied. The armature is the winding to which the load is connected. In small generators, the field windings are often on the stator, and the armature windings are on the rotor. Most large machines, however, have a rotating field and a stationary armature.

Always make sure that the brush pressure is set to the manufacturer's recommended level. If wear is still excessive, you should investigate the type and size of brush being used. Remember, current density (amperes per sq in. of brush) must be correct for the application. Proper current density is required to make certain that a lubricating, conductive film forms on the commutator or slip ring. This film is composed of moisture, copper, and carbon. Insufficient current density inhibits the formation of this film and can result in excessive brush wear.

Also, very low humidity environments do not provide enough moisture for the formation of the lubricating film. If excessive brush wear is a problem in such an environment, you may have to humidify the area where the machine is operating.

Question No. 3: What is service factor?

Service factor is the load that may be applied to a motor without exceeding allowed ratings. For example, if a 10-hp motor has a 1.25 service factor, it will successfully deliver 12.5 hp (10 x 1.25) without exceeding specified temperature rise. Note that when being driven above its rated load in this manner, the motor must be supplied with rated voltage and frequency.

Keep in mind, however, that a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the 10-hp motor is operated continuously at 12.5 hp, its insulation life could be decreased by as much as two-thirds of normal. If you need a 12.5-hp motor, buy one; service factor should only be used for short-term overload conditions.

Question No. 4: What is a rotating magnetic field, and why does it rotate?

A rotating magnetic field is one whose north and south poles move inside the stator, just as though a bar magnet, or magnets, were being spun inside the machine.

Look at the 3-phase motor stator shown in the accompanying diagram. This is a 2-pole stator with the three phases spaced at 120 [degrees] intervals. The current from each phase enters a coil on one side of the stator and exits through a coil on the opposite side. Thus, if one of the coils is creating a magnetic north pole, the other coil (for the same phase) will create a magnetic south pole on the opposite side of the stator.

At Position 1, B-phase is creating a strong north pole at the upper left and a strong south pole at the lower right. A-phase is creating a weaker north pole at the lower left and a weaker south pole at the bottom. C-phase is creating an overall magnetic field, with its north pole in the upper left and its south pole in the lower right.

At Position 2, A-phase is creating the strong north pole at the lower left and a strong south pole at the upper right; thus the strong poles have rotated 60 [degrees] counterclockwise. (Note that this 60 [degrees] magnetic rotation exactly corresponds to a 60 [degrees] electrical change in the phase currents.) The weak poles also have rotated 60 [degrees] counterclockwise. This, in effect, means that the total magnetic field has rotated 60 [degrees] from Position 1.

With a more detailed analysis, we can show that the magnetic field strength rotates smoothly from Position 1 to Position 2 as the currents in each of the phases varies over 60 electrical degrees. 

The speed at which the magnetic field rotates is called the synchronous speed and is described by the following equation:

S = (f x P) / 120 where S = rotational speed in revolutions per minute f = frequency of voltage supplied (Hz) P = number of magnetic poles in the rotating magnetic field

If a permanent magnet were placed into this stator, with a shaft that allowed it to rotate, it would be pushed (or pulled) along at synchronous speed. This is exactly how a synchronous motor operates, except that the rotor (field) magnetic field is created by electromagnetism instead of a permanent magnet.