2. 10-Nov-18

3. converted to Mechanical Energy
DC Motor
Describe how a d.c. motor works:
A current-carrying coil in a magnetic field experiences a turning effect
This effect can be used to produce a continuous rotation (in one direction) by swapping the direction of the current once the coil has turned half a turn. This is done using a split ring commutator
Note the Torque is not continuous (has max & min values)
How can the speed of the motor can be increased ?
Increase the number of turns on the coil
Increase the strength of the magnets
Increase the current flowing in the coil (by increasing the source Voltage, lower circuit resistance)
Electrical Energy is
converted to Mechanical Energy
Key concept:

4. The electric motor
Electric current flowing around the coil of the electric motor produces oppositely directed forces on each side of the coil.
These forces cause the coil to rotate.
Every half revolution the split ring commutator causes the current in the coil to reverse otherwise the coil would stop in the vertical position.

5. Model electric motor
Electric motor - Fendt

7. All of the force is contributing to spinning the motor
Same force, but different torque.
i.e. not all the force is being used to spin the motor F F N S F

9. In the following position, no torque is being produced
In the following position, no torque is being produced. However, the same amount of force is being produced. F N S F

11. Problem? F N S F

12. N S + rotation axis contact brush
Brushes lose contact with the split ring commutator.
Current no longer flows through the motor coil.
The coil will continue to rotate clockwise due to its momentum.
Brushes regain contact with the split ring commutator.
Current flows through the motor coil but in the original direction.
Forces exert a clockwise turning effect on the coil.
Brushes regain contact with the split ring commutator.
Current flows through the motor coil but in the opposite direction.
Forces exert a clockwise turning effect on the coil.
Brushes in contact with the split ring commutator.
Current flows through the motor coil.
Forces exert a clockwise turning effect on the coil
Brushes lose contact with the split ring commutator.
Current no longer flows through the motor coil.
The coil will continue to rotate clockwise due to its momentum. +
split-ring commutator
Electric motor - Fendt

14. Making a Simple DC motor preparation of the wire contacts
The secret…
preparation of the wire contacts
Electrical energy
into
Mechanical energy
Wire cross section!

15. 10-Nov-18

16. 10-Nov-18
brushes & commutator

17. 10-Nov-18

18. Question Choose appropriate words to fill in the gaps below:
The motor effect occurs when a _______ carrying wire is placed inside a ________ field.
The force exerted is __________ when the wire is at 90° to the magnetic field __________ but is zero if the wire is ________ to the field.
The force increases with _________ or current strength, the force __________ in direction if either are reversed.
Applications include the electric motor and ___________.
current
magnetic
maximum
direction
parallel
field
reverses
loudspeaker
WORD SELECTION:
parallel
reverses
loudspeaker
direction
current
magnetic
field
maximum