08-02-2013, 10:36 AM
AC Motor Speed Control
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Introduction
An important factor in industrial progress during the past five decades has been
the increasing sophistication of factory automation which has improved productivity
manyfold. Manufacturing lines typically involve a variety of variable
speed motor drives which serve to power conveyor belts, robot arms, overhead
cranes, steel process lines, paper mills, and plastic and fiber processing lines to
name only a few. Prior to the 1950s all such applications required the use of a
DC motor drive since AC motors were not capable of smoothly varying speed
since they inherently operated synchronously or nearly synchronously with the
frequency of electrical input. To a large extent, these applications are now serviced
by what can be called general-purpose AC drives. In general, such AC
drives often feature a cost advantage over their DC counterparts and, in addition,
offer lower maintenance, smaller motor size, and improved reliability.
However, the control flexibility available with these drives is limited and their
application is, in the main, restricted to fan, pump, and compressor types of
applications where the speed need be regulated only roughly and where transient
response and low-speed performance are not critical.
Thyristor Based Voltage Controlled Drives
Introduction
During the middle of the last century, limitations in solid state switch technology
hindered the performance of variable frequency drives. In what was essentially
a stop-gap measure, variable speed was frequently obtained by simply
varying the voltage to an induction motor while keeping the frequency constant.
The switching elements used were generally back-to-back connected
thyristors as shown in Figure 15.2. These devices were exceptionally rugged
compared to the fragile transistor devices of this era.
Basic Principles of Voltage Control
The basic principles of voltage control can be obtained readily from the conventional
induction motor equivalent circuit shown in Figure 15.3 and the
associated constant voltage speed-torque curves illustrated in Figure 15.4. The
torque produced by the machine is equal to the power transferred across the
airgap divided by synchronous speed.
Speed Control of Voltage Controlled Drive
Variable-voltage speed controllers must contend with the problem of greatly
increased slip losses at speeds far from synchronous and the resulting low efficiency.
In addition, only speeds below synchronous speed are attainable and
speed stability may be a problem unless some form of feedback is employed.
Thyristor Based Load-Commutated Inverter
Synchronous Motor Drives
The basic thyristor based load-commutated inverter synchronous motor drive
system is shown in Figure 15.7. In this drive, two static converter bridges are
connected on their DC side by means of a so-called DC link having only a n
inductor on the DC side. The line side converter ordinarily takes power from a
constant frequency bus and produces a controlled DC voltage at its end of the
DC link inductor. The DC link inductor effectively turns the line side converter
into a current source as seen by the machine side converter. Current flow in the
line side converter is controlled by adjusting the firing angle of the bridge and
by natural commutation of the AC line.
Torque Capability Curves
One useful measure of drive performance is a curve showing the maximum
torque available over its entire speed range. A synchronous motor supplied
from a variable-voltage, variable-frequency supply will exhibit a torque-speed
characteristic similar to that of a DC shunt motor. If field excitation control is
provided, operation above base speed in a field-weakened mode is possible and
is used widely. The upper speed limit is dictated by the required commutation
margin time of the inverter thyristors.
Constant Speed Performance
When the DC link current is limited to its rated value, the maximum torque can
be obtained from the capability curve (Figure 15.10). However, operation
below maximum torque requires a reduction in the DC link current. When the
field current is adjusted to keep the margin angle D at its limiting value, the
curves of Figure 15.12 result. It can be noted that the torque is now essentially
a linear function of DC link current so that the DC link current command
becomes, in effect, the torque command.