446 Electrical Power Systems Technology
quency. Frequency converters are motor/generator sets that are connected
together, or solid-state variable frequency drives.
For example, a frequency of 60 Hz could be applied to a synchro-
nous motor that rotates at a specific speed. A generator connected to its
shaft could have the necessary number of poles to cause it to produce a
frequency of 25 Hz. Recall that frequency is determined by the following
relationship:
Speed of Rotation (rpm) × No. of poles
Freq. (Hz) = ——————————————————
120
A frequency-conversion system is shown in Figure 16-21. Synchronous
units, such as the one shown, are used wherever precise frequency control
is required. It is also possible to design units that are driven by induction
motors, if some frequency variation can be tolerated. Variable fre-
quency drives are now being used to control many types of industrial ma-
chinery. They are used because they are not as expensive as variable speed
controllers have been in the past. Solid-state variable frequency drives are
used to control overhead cranes, hoists, and many other types of indus-
trial equipment that operate from AC power lines. Variable speed can be
accomplished less expensively, in most cases, with solid-state drives than
with DC motors or electromechanical variable-frequency drives. Solid-
state drives can be used with AC induction motors to change their speed
by varying the input frequency to the motor. There are fewer maintenance
problems with AC induction motors than with DC motors. In addition,
fewer electromechanical parts are involved in the control operation; thus,
equipment has a longer life expectancy.
Figure 16-20. Speed control circuit for a universal motor