Making a Motor Move
Electric motors move due to the force created when an electrical current interacts with a magnetic field (Faraday's Law). Depending upon the motor design one of three basic principals are used to move the rotor:
- Electromagnetic Induction
- Magnetic Attraction
- Magnetic Reluctance
For motors with a magnetic stator field, the presence of a current flowing through the rotor will result in a mechanical force acting to move the rotor. The direction of the force is determined by Fleming's "left-hand rule". The size of the force is a function of the current and the strength of the magnetic field.
The current can be supplied directly to the rotor as with a Brushed DC motor or be induced by manipulation of the stator's magnetic field as in an AC Induction Motor. See the page on Types of Electric Motors for more details on how these motors operate.
Motors with windings on the stator and permanent magnet rotors are made to move due to magnetic attraction. When current flows through a stator winding the winding will become an electromagnet. The rotor will move toward the active winding. This type of motion is found in most DC motors.
Motors with windings on the stator and ferromagnetic rotors are made to move due to magnetic reluctance. When the stator winding is activated, the low-reluctance metallic rotor nearest the winding experiences an increase in magnetic flux. The concentration of flux in the rotor forms strong temporary pole which results in a mechanical force moving the rotor toward the stator winding.
While it may appear that motion based on magnetic attraction and magnetic reluctance are similar, there are significant differences in the characteristics of motors using these processes. A detailed description of the differences can be found on the Types of Electric Motors page.