Electric motors · Year 12

In Year 12 you get fluent with: how a d.c. motor works and motor efficiency.

A wire carrying a current in a magnetic field feels a force: the motor effect. In an electric motor, the forces on the two sides of a coil push in opposite directions and turn it, and a split-ring commutator keeps it turning the same way. Loudspeakers use the same effect to make a cone vibrate.

Key facts

  • The motor effect: a wire carrying a current across a magnetic field feels a force. Fleming’s left-hand rule gives its direction.
  • Motor: electrical → kinetic energy. Generator: kinetic → electrical energy.
  • A d.c. motor’s split-ring commutator reverses the coil current every half turn, keeping it spinning one way.
  • A motor spins faster with more current, more turns, or a stronger field; reverse the current to reverse it.

Worked examples

Example 1

What energy transfer happens in an electric motor?

Answer: electrical → kinetic (plus some heat)

Example 2

How can you make a d.c. motor turn in the opposite direction?

Answer: reverse the current (or swap the magnet’s poles)

Example 3

How does a loudspeaker make sound?

Answer: An alternating current in a coil in a magnetic field makes the coil and cone vibrate

Example 4

A loudspeaker is fed a 440 Hz alternating current. How often does its cone vibrate?

Answer: 440 times a second

Example 5

A small electric motor lifts a 1 kg load through 4 m. How much useful energy does it transfer to the load? (g = 9.8 N/kg)

Answer: 39.2 J

Example 6

What is the job of the split-ring commutator in a d.c. motor?

Answer: it reverses the current in the coil every half turn so it keeps spinning the same way

Example 7

Which change would make a d.c. motor spin faster?

Answer: increasing the current

Example 8

A 12 V motor draws 2.0 A for 5.0 s while lifting a 4.0 kg mass through 1.5 m. Find its efficiency. (g = 9.8 N/kg)

Answer: 49%

Example 9

A 12 V motor draws 3.7 A for 8 s while lifting a 2.5 kg load through 3 m. Find its efficiency, as a percentage to 1 decimal place. (g = 9.8 N/kg)

Answer: 20.7%

Other years