Electric Motors - Technical Information

Motor Basics for the beginner

The electric motor is the prime mover in most dynamic systems.

How does an electric motor work ?

It all relates back to magnets and electrical current.

Firstly, in the year 1819 Hans Christian Oerstad, a Professsor of Physics, discovered that a wire conveying a current was able to deflect a magnetic needle,due to the magnetic flux surrounding the electrified wire.

Secondly, We all know that magnets generally have a North and South pole and that opposite poles attract and equal poles repel

The above graphic illustrates the construction of a simple electric (d.c.) motor. It consists of a rectangular coil of wire mounted on a spindle so that it can rotate between the curved pole pieces of a U-shaped permanent magnet. The two ends of the coil are soldered respectively to the two halves of a copper split ring or commutator.

Two carbon brushes are caused to press lightly against the commutator by means of springs, and when these are connected in circuit with a battery and rheostat the coil rotates.

Suppose the coil is in the horizontal position when the current is first switched on. Current will flow through the coil in the direction shown, and by applying Fleming's left-hand rule it will be seen that the side ab of the coil experiences and upward force and the side cd a downward force.

These two forces form a couple which causes the coil to rotate in a clockwise direction until it reaches the vertical position. In this position the brushes touch the space between the two halves of the commutator and the current is cut off. The coil does not come to rest, since its momentum carries it past the vertical, and when this has occured the two commutator halves automatically change contact from one brush to the other.

This reverses the current through the coil, and consequently also reverses the directions of the forces on the sides of the coil. Side ab is now on the right-hand side with a downward force on it, and side cd on the left-hand side with an upward force. The coil thus continues to rotate in a clockwise direction for so long as the current is passing.