AC Generators
- If a coil of wire is rotated inside a magnetic field by an external force, an emf will be generated in the wire which causes current to flow within the coil
- The generator effect can be used to:
- Generate a.c. in an alternator
- Generate d.c. in a dynamo
Alternators
- A simple alternator is a type of generator that converts mechanical energy to electrical energy in the form of alternating current
An alternator is a rotating coil in a magnetic field connected to commutator rings
- A rectangular coil that is forced to spin in a uniform magnetic field
- The coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings (or commutator rings)
- The slip rings and brushes provide a continuous connection between the coil and the meter
- When the coil turns in one direction:
- The pointer defects first one way, then the opposite way, and then back again
- This is because the coil cuts through the magnetic field lines and a potential difference, and therefore current, is induced in the coil
- The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins
- This is because the induced potential difference in the coil repeatedly changes its direction
- This continues on as long as the coil keeps turning in the same direction
- The induced potential difference and the current alternate because they repeatedly change direction
Dynamos
- A dynamo is a direct-current generator
- A simple dynamo is the same as an alternator except that the dynamo has a split-ring commutator instead of two separate slip rings
A dynamo is a rotating coil in a magnetic field connected to a split ring commutator
- As the coil rotates, it cuts through the field lines
- This induces a potential difference between the end of the coil
- The split ring commutator changes the connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction
- This happens each time the coil is perpendicular to the magnetic field lines
D.C output from a dynamo - the current is only in the positive region of the graph
- Therefore, the induced potential difference does not reverse its direction as it does in the alternator
- Instead, it varies from zero to a maximum value twice each cycle of rotation, and never changes polarity (positive to negative)
- This means the current is always positive (or always negative)
