Which statement best describes the difference between DC motors and AC motors in shipboard use?

Prepare for the Engineman EN A School Test 1. Utilize flashcards and multiple choice questions with detailed explanations to boost confidence and readiness for the test.

The key idea is how speed is controlled and what power type each motor uses on ships. DC motors run on direct current, and their speed can be varied by changing the voltage (or current) supplied, often with dedicated DC controllers. AC motors run on alternating current, and in shipboard practice their speed is typically adjusted by using a variable frequency drive to change the supply frequency while maintaining torque as needed. Since ships usually have AC power distribution, AC motors with VFDs are common for adjustable speeds, while DC motors are used where direct DC control is straightforward or where a DC supply (or rectified power from AC) is available. This makes the statement describing controllable speed with direct current for DC motors and AC motors’ use with AC power and variable frequency drives the best fit. The other ideas are not accurate because DC motors can be powered from rectified AC or DC buses, AC motors can be speed-controlled with drives, and DC motors do require control systems to vary speed and torque.