What Causes an Electric Motor Rotate?

An electric motor rotates as the result of the interaction of two magnetic fields. One of the well-known laws of magnetism is that “like" poles (N-N or S-S) repel while "unlike poles (N-S) attract.

This attraction and repulsion of magnetic poles can be used to produce a rotating force. The operation can be summarized as follows.

?The electromagnet is the moving armature part and the permanent magnet the fixed stator part.

? Like magnetic poles repel each other, causing the armature to begin to turn.

? After it turns part way around, the force of attraction between the unlike poles becomes strong enough to keep the permanent magnet rotating.

? The rotating electromagnet continues to turn until the unlike poles are lined up. At this point the rotor would normally stop because of the attraction between the unlike poles.

? Commutation is the process of reversing armature current at the moment when unlike poles of the armature and field are facing each other, thereby reversing the polarity of the armature field.

? Like poles of the armature and field then repel each other, causing armature rotation to continue.

When a current-carrying conductor is placed in a magnetic field, there is an interaction between the magnetic field produced by the current and the permanent field, which leads to a force being experienced by the conductor.

The magnitude of the force on the conductor will be directly proportional to the current which it carries. A current-carrying conductor, placed in a magnetic field and at right angles to it tends to move at right angles to the field.

A simple method used to determine the direction of movement of a conductor carrying current in a magnetic field is the right-hand motor rule. To apply this rule the thumb and first two fingers of the right hand are arranged to be at right angles to each other with the forefinger pointing in the direction of the magnetic lines of force of electron current flow (-to +) in the conductor.

The thumb will then be pointing in the direction of movement of the conductor. Applying the right-hand motor rule. Move upward through the magnetic field. If the current through the conductor were to be reversed, the conductor would move downward.

Note that the conductor current is at a right angle to the magnetic field. This is required to bring about motion because no force is felt by a conductor if the current and the field direction are parallel.

Rotation is the result of the interaction of the magnetic fields generated by the permanent magnets and current flow through the armature coil.

This interaction of the two magnetic fields causes a bending of the lines of force. when the lines tend to straighten out they cause the loop to undergo a rotating motion.

The left conductor is forced downward, and the right conductor is forced upward, causing a counterclockwise rotation of the armature. A practical motor armature is made up of many coils of conductors.

The magnetic fields of these conductors combine to form a resultant armature field with north and south poles that interact with those of the main stator field to exert a continuous torque on the armature.

In general, motors are classified according to the type of power used (AC or DC) and the motor's principle of operation. There are several major classifications of the motor's in Common use; each will specify characteristics that suit it to particular applications.