FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A carbon-pile voltage regulator controls a DC generator's field current using a stack of carbon discs whose resistance is varied by spring pressure and an electromagnetic coil connected across the generator output. As generator output voltage rises above the set point, current through the sensing coil increases, pulling the plunger away from the carbon stack and reducing compression on the discs. What is the resulting effect on generator output voltage?
- AArmature current reverses, tripping the reverse-current relay
- BField current remains unchanged since the regulator only limits current, not voltage
- CField current increases, driving output voltage higher
- DField current decreases, lowering output voltage back toward normal
Show answer & explanationAnswer & explanation
Correct answer: D. Field current decreases, lowering output voltage back toward normal
Reducing compression on the carbon pile increases its resistance, which is in series with the generator field circuit. This reduces field current, weakening the generator's magnetic field and lowering output voltage back to the desired setting, forming negative feedback control.
Why the other options are wrong
- A. Reverse current relates to generator failure, not normal voltage regulation feedback.
- B. The carbon pile regulator's entire purpose is voltage regulation through field current control.
- C. This is backwards — reduced compression increases resistance, decreasing (not increasing) field current.
Carbon Pile Voltage Regulator
A DC generator voltage regulator that varies field circuit resistance by mechanically compressing or releasing a stack of carbon discs in response to output voltage changes.
- Sensing coil pulls plunger against spring pressure as voltage rises
- Less compression = higher carbon pile resistance = less field current
- Provides continuous negative-feedback voltage control
Memory trick: Squeeze the carbon less, resistance grows, field current slows, voltage goes low.