Electric motors · Grade 12
In Grade 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
- 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 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 2
Which change would make a d.c. motor spin faster?
Answer: increasing the current
Example 3
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 4
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%