11.4 A simple d.c. electric motor - an application of the motor effect

How does a simple d.c. DC electric motor work?

The basics

To understand how a simple dc electric motor works consider the diagram above to get the idea.

Instead of a single linear wire, consider placing a d.c. current carrying loop (or many turns of wire loops) in the magnetic field of a permanent magnet (U shaped) or opposite poles from two permanent magnets.

The wire is at 90^o to the direction of the magnetic field - lines of force in blue.

Now we apply Fleming's left-hand rule because the same forces are in operation as for the single wire demonstration.

I've drawn the rule and applied it to both sides of the loop to show the directions the forces produced operate.

The left side of the loop will move downwards and the right side of the loop moves upwards giving anticlockwise rotation.

This produces an anticlockwise rotation movement - and that's quite simply, the basis of an electric motor, but you will not get continuous rotation without some further modifications and added 'bits' described below!

 

Explaining how a simple dc electric motor works

However, as described previously, the above 'diagram' needed a few more bits to be a working electric motor!

The added 'bits' ....

are an axle (spindle) about which the coil can freely rotate between the poles of a permanent magnet,

a split ring commutator that swaps the contacts around every half-turn (swapping the +/-polarity, swapping the direction of resulting force) and keeps the rotation in the same direction, it also enables electrical contact to the external circuit, together with the ...

... brush contacts (of graphite block or copper strip) which enable rotation movement to continue but still maintain a complete electrical circuit - the 'brushes' sweep over the surface of the contacts on the axle,

and of course a frame structure to hold all the components in place!

The way the forces operate was explained in the previous diagram, but I have repeated the application of Fleming's left-hand rule to show the coil will rotate anticlockwise.

theoretically, when the copper wire coil is vertical, the circuit is broken for a split second, but the momentum of the coil carries the rotation a bit further, the circuit is complete again, and continuous rotation is conserved.

You can reverse the direction of rotation either by either ..

(i) swapping the polarity of the d.c. supply to change the direction of current flow,

and (ii) swapping the magnetic poles of the permanent magnet to change the direction of the magnetic field.

A simple, but practical, working model of a simple d.c. electric motor

Notice in the right-hand diagram the rotation is now clockwise, but current flow is opposite in direction compared to the previous diagram - so check it out with Fleming's left-hand rule!

However, there are several sources of energy loss - decreasing the efficiency of the motor

(a) When the electric motor starts running the current decreases a little from its initial value.

As the current flows, the thin wire coils act as a resistance, the coil heats up a little as heat energy is lost: electrical energy ==> thermal energy store of the motor and surroundings.

Since the temperature of the coils increases, its resistance increases a bit more, leading to a greater increase in wasted energy.

(b) Although this machine is acting as an electric motor, simultaneously it acts as a generator!

As the coil rotates in the magnetic field it induces a current to flow in the opposite direction.

See 12. Generator effect, applications e.g. generators generating electricity

 

How can you make a simple dc (or any) electric motor more powerful?

There are three ways to do this, all involve increasing the strength of the magnetic field ...

(i) Increasing the number of turns of wire in the coil.

The magnetic lines of force 'cut' through more wire per unit time.

(ii) By winding the coil on a soft-iron armature to increase the magnetic flux. through the coil.

The ion concentrates the lines of force, so more lines of force are 'cut' through per unit time.

(iii) By making the field magnet as strong as possible.

The stronger the magnet, the greater the magnetic flux - the lines of force are closer together, so more lines of force are 'cut' per time as the armature rotates.

(iv) Increasing the p.d. across the coil to increase the current.

Increase the charge flow will intensify and strengthen the magnetic field around the coil.

These factors apply to any electric motor design.

These factors can be used to increase the speed of rotation of the motor.

To make an electric motor less powerful or slow its rotation down, (i) reduce the current (by reducing the pd across the coils), (ii) reduce the number of turns of wire coils and (iii) decrease the strength of the magnet to reduce the magnetic flux density.

Factor (i) is used to control the speed of an electric motor e.g. an electric car or train. You can't really change any other factor in a working machine!

Practical electrical motors

The d.c. motor described above is pretty simple and very inefficient.

In more practical motors, the magnetic pole pieces are curved in shape to give a more radial magnetic field.

This means the coil is always at right angles to the magnetic field - maximising the resultant force from the interaction of the two magnetic fields.