Jargon buster: Motors

Chris Meynell cuts through the jargon to explain how mini power plants work.

Traditional electric motors comprise an array of permanent, or ‘field’ magnets, arranged around the inner rim of a cylinder, ring or open frame. Placed within the field magnets, yet not in contact with them, is a set of three or more electromagnets. Electromagnets consist of ferrite (iron) cores, or ‘poles’, each wrapped with fine coils of copper wire and mounted onto a common spindle. This assembly is known as the armature, which is free to rotate within a set of bearings and forms the output shaft. Power, in the form of Direct Current (DC), is applied to the armature via a pair of carbon brushes. These rub onto a set of contacts on the spindle - the commutator - and each contact being connected to one of the electromagnets on the armature. The electric current thus activates the electromagnet, which is deflected sideways by the outer field magnets due to the natural repulsion or attraction of magnetic poles (see below). As the spindle turns, the next section of the commutator is brought into contact with the brushes, energising the next electromagnet on the armature. The process repeats itself, keeping the spindle rotating until the power is shut off. The direction of rotation is governed simply by swapping the positive and negative polarities applied to the two carbon brushes.

ADVANTAGES

What was good enough for the Victorians is good enough in 2016. Cheaper… or they used to be.

DISADVANTAGES

The brushes wear out, leaving their remains on and between the commutator plates. Inefficient - they require more current than coreless motors. Heavy, but surely this is an advantage in a locomotive? Not always!