Page 130 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 6/111
and the acceleration compensator. The sum of these two pre-set reference parameters and help to implement the
is the output signal that is fed to the torque reference required precise adjustments in the system’s parameters
controller (section 5). instantly by providing corrective command signals to
the switching circuits of the inverter unit. This in turn
Section 7 adjusts the system variable parameters within the required
This is the flux reference controller which provides the limits by adjusting V andfas in a simple Vlfcontrol or
absolute value of stator flux to the flux comparator (section I,,, andf as in a flux phasor control, or I,, I,,,, p and f as
3). The value of this absolute flux can be varied to fulfil in a field-oriented control or the torque phasor control as
many functional requirements from the inverter unit such in a direct torque control technique etc. As a result of
their accuracy and speed, they are capable of achieving
;IS
prompt corrections, high reliability. better flexibility and
hence a high dynamic performance of the drive.
Field strengthening - to obtain speed variation below There are many types of sensors used to feed-back the
the base speed process operating conditions to the switching logistics
Flux braking - to carry out braking duties of an inverter unit. They can be in terms of temperature,
Ficld weakening - to obtain speed variation above the pressure. volume, flow, time or any activity on which
base speed. depends the accuracy and quality of the process. Direct
sensing devices used commonly for the control of a drive
and used frequently in the following text are speed sensors.
6.5 Use of phasor control for as noted below.
flux braking
6.6.1 Speed sensors
It is possible to perform braking duties by the motor by These are closed-loop sensing devices and are mounted
raising the level of magnetization (field strengthening). on the machine or a process line. They are able to sense
By raising flux, the speed reduces (Figure 6.7(b)) and the operating parameters and provide an analogue or
the stator current rises. This is an apparent advantage in digital feedback input to the inverter switching logistics.
this kind of a speed control. as the heat generated by the For example:
motor during braking appears as thermal energy in the
stator rather than in the rotor. Also it is easier to dissipate Tacho generator (TG) -This is an analogue voltage
heat from the stator than from the rotor due to its (stator) feedback device and can provide a speed input to the
bulk and its outer surface. which is open to the atmosphere. inverter control circuit. It provides only a low level of
speed regulation, typically i 0.4% of the set speed.
Pulse encoder - This is a digital voltage feedback
6.6 Control and feedback devices device and converts an angular movement into electri-
cal high-speed pulses. It provides speed and also the
The control and feedback circuits are also solid-state angular position of the rotor with respect to the stator
devices and offer high reliability and accuracy. The output field when required for field-oriented control to the
from these devices can be interfaced with a microprocessor inverter control circuit to achieve the required speed
to carry out the required corrections in the system’s control. It is a high-accuracy device, and provides
parameters through the inverter controls. A microprocessor accuracy up to k 0.001% of the set speed. (See the
is a semiconductor device and consists of logic circuits simplc feedback control scheme shown in Figure 6.12.)
in the form of ICs (integrated circuits), capable of
performing computing functions and decision making. 6.7 Application of solid-state
These capabilities are used in carrying out process
corrections by providing the necessary timing and control tech no1 ogy
corrective signals to the switching logistics of the inverter
unit. This field is very large and a detailed study of the subject
A microprocessor is capable of solving complex is beyond the scope of this handbook. We will limit our
mathematical problems very rapidly and can analyse a discussions to the area of this subject that relates to the
system more closely and accurately. It can be fed with a control of a.c. motors and attempt to identify the diffcrcnt
variety of measuring and control algorithmic software solid-state devices that have been developed and their
that may be necessary for system monitoring and control. application in the control of a.c. motors. Only the more
They are also capable of performing supervisory and common circuits and configurations are discussed. The
diagnostic functions and carry out historical recording brief discussion of the subject provided here, however,
io store information related to process and drive conditions, should help the reader to understand this subject in general
io facilitate reviews of data for continuous process terms and to use this knowledge in the field of a.c. motor
monitoring. fault analysis, diagnostics, trend analysis, controls to achieve from a soft start to a very precise
etc. speed control and, more importantly, to conserve the
The control logistics such as PWM or frequency controls energy of the machine which would be wasted otherwise.
are diFitd1 circuits and are microprocessor based. They For more details of static controllers see the Further reading
can compare the actual inverter output parameters with (Sr. nos. 2, 4. 5, 8 and 12) at the end of the chapter. To