Generator coming-in speed

Aviation Mechanic Powerplant (AMP) practice question 5 of 13 on Engine Electrical Systems. ACS element AM.III.F.K1 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONMost aircraft generators begin to produce their normal voltage at a "coming in" speed of about how many rpm?

A500 rpm
B3,000 rpm or higher
C1,500 rpm

Show the answer and explanation

Correct answer: C. 1,500 rpm

A — incorrect This figure is far below the stated coming-in speed; at such a low armature speed the generator has not yet reached normal output voltage.
B — incorrect Normal voltage is produced well before this speed; the handbook places the coming-in point at roughly half this value.
C — correct The handbook gives the coming-in speed, the speed at which the generator starts producing its normal voltage, as usually about 1,500 rpm.

BACKGROUND

Engine speed varies from idle to takeoff rpm, but for most of a flight the engine turns at a steady cruise setting. The generator drive is usually geared to turn the generator between 1 1/8 and 1 1/2 times engine crankshaft speed so that useful output is available across that range. The speed at which a generator starts to deliver its normal voltage is called the coming-in speed. The handbook states that this speed is usually about 1,500 rpm.

STUDY TIP

Remember the pair of drive numbers together: roughly 1 1/8 to 1 1/2 times crankshaft speed, with output coming in near 1,500 rpm.

KEY POINTS

Coming-in speed is where normal voltage first appears.
That speed is usually about 1,500 rpm.
Generator drives turn 1 1/8 to 1 1/2 times crankshaft speed.

SOURCES

amp-powerplant/ch04.txt "Generator Ratings"

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