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Glossary
Appendix A
A–6
Process Variable A physical property of a process that is of interest because it affects
the quality of the primary task accomplished by the process. For an
industrial oven, temperature is the process variable. See also PID
Loop and Error.
PWM Pulse-width modulation: A type of AC adjustable frequency drive
that accomplishes frequency and voltage control at the output
section (inverter) of the drive. The drive output voltage waveform is
at a constant amplitude, and by “chopping” the waveform (pulse-
width-modulating), the average voltage is controlled. The chopping
frequency is sometimes called the Carrier Frequency.
Reactance The impedance of inductors and capacitors has two components.
The resistive part is constant, while the reactive part changes with
applied frequency. These devices have a complex impedance
(complex number), where the resistance is the real part and the
reactance is the imaginary part.
Rectifier An electronic device made of one or more diodes that converts AC
power into DC power. Rectifiers are usually used in combination
with capacitors to filter (smooth) the rectified waveform to closely
approximate a pure DC voltage source.
Regenerative Braking A particular method of generating reverse torque to a motor, an
inverter will switch internally to allow the motor to become a gener-
ator and will either store the energy internally, deliver the braking
energy back to the main power input, or dissipate it with a resistor.
Regulation The quality of control applied to maintain a parameter of interest at
a desired value. Usually expressed as a percent (±) from the
nominal, motor regulation usually refers to its shaft speed.
Reverse Torque The torque applied in the direction opposite to motor shaft rotation.
As such, reverse torque is a decelerating force on the motor and its
external load.
Rotor The windings of a motor that rotate, being physically coupled to the
motor shaft. See also Stator.
Saturation Voltage For a transistor semiconductor device, it is in saturation when an
increase in input current no longer results in an increase in the
output current. The saturation voltage is the voltage drop across the
device. The ideal saturation voltage is zero.
Sensorless Vector
Control
A technique used in some variable-frequency drives (featured in
some other Hitachi inverter model families) to rotate the force
vector in the motor without the use of a shaft position sensor
(angular). Benefits include an increase in torque at the lowest speed
and the cost savings from the lack of a shaft position sensor.