The second half turn
Now consider what happens when the coil and the two split rings have turned through 180°, so that the positions of the loop and the split rings are reversed. This position is shown in figure 13.16.
Figure to be added
Following the current again
- Now brush P touches half ring Y and brush Q touches half ring X. The current enters half ring Y from brush P, travels in the direction DCBA, comes out through half ring X and leaves through brush Q.
- The current now flows in the sides DC and BA.
- Fleming's left-hand rule gives a force upwards on AB and downwards on CD. The couple keeps turning the armature clockwise.
- If the battery terminals are swapped so the current enters the other way, the forces reverse and the armature turns anticlockwise.
Figure to be added
The energy change
While a DC motor is working, the electrical energy supplied to it is converted into mechanical energy.
Why the turning never stops
Compare the two figures. In both, the side on the left is pushed down and the side on the right is pushed up — it is just a different side each time. The commutator makes sure that whichever side is on the left carries current in the same direction as the left side did before. So the couple always turns the coil the same way, and the motor runs on for as long as the current is supplied. Only at the moment the coil stands upright is there no turning effect; the coil's momentum carries it past.
Two ways to reverse a motor
Swapping the battery terminals reverses a motor, and so does swapping the magnetic poles. Doing both at once leaves it turning the same way as before.
Electrical energy becomes mechanical energy
After half a turn the commutator reverses the current, so the armature keeps turning the same way. Reversing the supply reverses the motor.