4.7.2.3 Electric Motors (HT only)

Question 1

Electric motor:

Answer 1

a coil of wire carrying a current in a magnetic field tends to rotate

Question 2

Why will a current carrying coil in a magnetic field stop rotating if it does not have a split ring commutator?

Answer 2

• once loop is at 180 degrees it will stop rotation
• direction of current is now the opposite so the forces acting one another side of coil is in opposite direction
• causes coil to turn 180 degrees opposite direction instead
• coil would keep flipping back and forth on itself as forces kept switching around - coil would never turn 360 degrees

Question 3

What is a split ring commutator and how does it work?

Answer 3

• split ring commutator is a split metal ring which is connected to conducting brushes
• the brushes allow the electric current to pass onto the ring
• this shows current producing a turning force on the motor
• the force makes the motor rotate in the clockwise direction

- changes the direction of the current every half turn
- split ring commutator swaps the positive and negative current connections at every half turn so direction of current swaps every half turn so forces acting on sides of coil will act in same direction so will allow coil to continue to rotate in same direction
- commutator ring rotates with the coil but the positive and negative terminals do not change so current always flows in same direction

Question 4

How does an electric motor work and how does the force on a conductor in a magnetic field cause the rotation of the coil in an electric motor?

Answer 4

- coil with a current is placed in a magnetic field
- (current is running in opposite directions on either side of the loop - on left hand side current is running back to front and on right hand side current is running front to back
- wire experiences force on both left and right side of the coil - using Flemings left hand rule the force on left hand side acts upwards and the force on the right hand side acts downwards
- have moment on left hand side and moment on the right hand side about the centre axis - means that the loop will rotate in the clockwise direction)
- the sides of the coil parallel to the magnet experience a force in opposite directions (as current running in opposite directions)
- the forces cause moments that act in the same direction (clockwise/anticlockwise)
- at each half-revolution the 2 halves of the rotation commutator swap from one carbon brush to the other
- so at each half-revolution the commutator reverse the current in the coil
- this keeps the forces in the same directions causing the coil to continue rotation in the same direction

Question 5

Why does the rotating coil (armature) often have a soft iron core?

Answer 5

• as has greater MFS as iron is magnetic
• greater force on coil
• coil rotates faster

Question 6

Methods to decrease force acting on wire:

Answer 6

• decrease current in the wire
• decrease magnetic field strength
• rotation of field so magnet no longer perpendicular to the wire

Question 7

How to increase speed of rotation of coil?

Answer 7

• increase the current
• add more turns to the coil
• increase the magnetic flux density by using stronger magnets