Page 133 - Offshore Electrical Engineering Manual
P. 133
120 CHAPTER 11 Power Electronics (Semiconductor Equipment)
FIGURE 2.11.3
Variable-frequency drive output.
By adjusting the frequency and voltage of the power entering the motor, the speed
and torque may be controlled. The actual speed of the motor, as previously indicated,
is determined by
N s = [(120 × f) /P] × (1 − S)
where N, motor speed; f, frequency (Hz); P, number of poles; S, slip.
VARIABLE-SPEED DRIVE OR VARIABLE-FREQUENCY DRIVE
Both systems have benefits, but technical variable-frequency drives (VFDs) are bet-
ter because of the higher torques available and less maintenance. However, because
of the more complex electronics, quicker obsolescence is likely to be a problem with
VFDs.
