The Coil

The coil is what drives the magnetising flux thru the electromagnet. Its magnetising force is just the product of the number of turns (N) and the coil current (I). Thus:

N = number of turns
I = current in the windings.

The appearance of “N” in the above formula leads to a common misconception.

It is widely assumed that increasing the number of turns will increase the magnetising force but generally this does not happen because extra turns also reduce the current, I.

Consider a coil supplied with a fixed DC voltage. If the number of turns is doubled then the resistance of the windings will also be doubled (in a long coil) and thus the current will be halved. The net effect is no increase in NI .

What really determines NI is the resistance per turn. Thus to increase NI the thickness of the wire must be increased. The value of extra turns is that they do reduce current and therefore the power dissipation in the coil.

The designer should be mindful that the wire gauge is what really determines the magnetising force of the coil. This is the most important parameter of coil design.

The NI product is often referred to as the “ampere turns” of the coil.