A technician explains to a student why a light airplane's alternator can produce full rated electrical output even at engine idle RPM, while an older generator system could not. What is the reason for this difference?
- AThe alternator bypasses the voltage regulator at low RPM
- BThe alternator's rotating magnetic field design allows adequate output at low RPM
- CThe alternator uses a larger battery to compensate for low RPM
- DThe generator produces AC current which requires higher RPM to rectify
Show answer & explanationAnswer & explanation
Correct answer: B. The alternator's rotating magnetic field design allows adequate output at low RPM
An alternator generates AC current using a rotating field (rotor) and stationary windings (stator), then rectifies the AC to DC with diodes. This design allows the alternator to produce rated output at much lower RPM than a generator, which relies on a rotating armature and commutator/brushes requiring higher speeds to reach full output.
Why the other options are wrong
- A. Voltage regulators remain active across the RPM range; they are not bypassed.
- C. Battery size does not change the generation characteristics of the alternator itself.
- D. It is the alternator, not the generator, that produces AC before rectification.
Alternator vs. Generator
Alternators produce AC current via a rotating field and stationary stator, then rectify it to DC, allowing rated output at idle RPM; generators produce DC directly via a rotating armature and require higher RPM for full output.
- Alternators are lighter and more efficient than generators for the same output
- Alternators maintain output at idle, critical for electrical loads on the ground
- Generators use brushes/commutators, which wear faster than alternator slip rings
Memory trick: Alternator = 'always on,' even idling, thanks to its spinning field design.