NEIEP Mechanics Practice Exam

For synchronous machines, how is speed typically controlled?

1. By varying load resistance

2. By adjusting excitation

3. By modifying alignment

4. By changing the phase sequence

The correct answer is: By adjusting excitation

In synchronous machines, speed is intrinsically linked to the frequency of the AC supply and the number of poles the machine has. Unlike induction machines, where speed can be varied by changing the load or adjusting resistance, synchronous machines run at a constant speed determined by these factors. One of the key methods to control the operation of synchronous machines, particularly under varying load conditions, is by adjusting the excitation. When the excitation level is altered, it affects the magnetic field strength within the machine. This change impacts the voltage output and ultimately helps maintain synchronism with the grid frequency or can be used to manage reactive power. When excitation is increased, the machine can operate at a leading power factor, while a decrease in excitation leads to a lagging power factor. This control mechanism is fundamental in applications requiring fine regulation of output characteristics or when coordinating with other power system elements. Thus, adjusting excitation serves as the primary mode of managing a synchronous machine's operational dynamics in relation to speed.