0195136047.pdf

16.1 POWER SEMICONDUCTOR-CONTROLLED DRIVES 749

Source

Power

semiconductor

converter

Motor

Sensing

unit

Control

unit

Command

signal

Load

Figure 16.1.1Essential components of an

electric drive.

0

Forward

braking

Forward

motoring

Reverse

braking

Reverse

motoring

III

Speed ωm

Torque T

IV

II I

Figure 16.1.2Four-quadrant opera-

tion of drives.

are shown in Figure 16.1.3 for speeds below and abovebase speedωmb, which is the highest drive
speed at the rated flux.
Motors commonly used in variable-speed drives are induction motors, dc motors, and
synchronous motors. For the control of the motors, various types of converters are needed,
as exemplified in Table 16.1.1. A variable-speed drive can use a single converter or more
than one. All converters have harmonics in their inputs and outputs. Some converters suffer
from a poor power factor, particularly at low output voltages. The main advantages of con-
verters are high efficiency, fast response, flexibility of control, easy maintenance, reliability,
low weight and volume, less noise, and long life. The power semiconductor converters have
virtually replaced the conventional power controllers such as mercury-arc rectifiers and magnetic
amplifiers. Most of the drive specifications are governed by the load requirements, which in
turn depend on normal running needs, transient operational needs, and needs related to location
and environment. Other specifications are governed by the available source and its capacity,
as well as other aspects like harmonics, power factor, reactive power, regenerated power, and
peak current.