Type of magnetic material used for small motors.
Answers
In very small electric motors which run on dc (direct current) a permanent magnet made of iron - or of some other alloy of elements capable of being magnetized - is used to create the two poles of the stator.
The rotor is built-up using winding made of insulated copper wire wound onto poles made of layers of well-insulated thin cast iron sheets which are called "laminations".
The thin sheets must be well insulated from one another to minimize power wastage caused by internal eddy currents that are induced as the rotor's poles continually break the stator's magnetic field.
In larger motors, which may run on either dc or ac (alternating current), the stator's magnetic field is created by field windings made of insulated copper wire wound around pole pieces which are not permanently magnetized.
For use with ac the stator, like the rotor, must be constructed either:
using laminations of thin cast iron sheets, well insulated from one another, to minimize power wastage caused by internal eddy currents that are induced into the stator by the field winding, or using a non-magnetic material, such as aluminum, which cannot have eddy currents induced into it.
Explanation:
ELECTRIC MOTOR
Permanent-magnet motors
The magnetic field for a synchronous machine may be provided by using permanent magnets made of neodymium-boron-iron, samarium-cobalt, or ferrite on the rotor. In some motors, these magnets are mounted with adhesive on the surface of the rotor core such that the magnetic field is radially directed across the air gap. In other designs, the magnets are inset into the rotor core surface or inserted in slots just below the surface. Another form of permanent-magnet motor has circumferentially directed magnets placed in radial slots that provide magnetic flux to iron poles, which in turn set up a radial field in the air gap.
The main application for permanent-magnet motors is in variable-speed drives where the stator is supplied from a variable-frequency, variable-voltage, electronically controlled source. Such drives are capable of precise speed and position control. Because of the absence of power losses in the rotor, as compared with induction motor drives, they are also highly efficient.
Permanent-magnet motors can be designed to operate at synchronous speed from a supply of constant voltage and frequency. The magnets are embedded in the rotor iron, and a damper winding is placed in slots in the rotor surface to provide starting capability. Such a motor does not, however, have means of controlling the stator power factor