22-06-2012, 01:08 PM
Design and Implementation of PLC-Based Monitoring Control System for Induction Motor
INTRODUCTION
SINCE technology for motion control of electric drives became
available, the use of programmable logic controllers
(PLCs) with power electronics in electric machines applications
has been introduced in the manufacturing automation [1], [2].
This use offers advantages such as lower voltage drop when
turned on and the ability to control motors and other equipment
with a virtually unity power factor [3]. Many factories
use PLCs in automation processes to diminish production cost
and to increase quality and reliability [4]–[9]. Other applications
include machine tools with improved precision computerized
numerical control (CNC) due to the use of PLCs [10].
To obtain accurate industrial electric drive systems, it is necessary
to use PLCs interfaced with power converters, personal
computers, and other electric equipment [11]–[13]. Nevertheless,
this makes the equipment more sophisticated, complex, and
expensive [14], [15].
PLC AS SYSTEM CONTROLLER
A PLC is a microprocessor-based control system, designed
for automation processes in industrial environments. It uses a
programmable memory for the internal storage of user-orientated
instructions for implementing specific functions such as
arithmetic, counting, logic, sequencing, and timing [23], [24].
A PLC can be programmed to sense, activate, and control industrial
equipment and, therefore, incorporates a number of I/O
points, which allow electrical signals to be interfaced.
CONTROL SYSTEM OF INDUCTION MOTOR
the block diagram of the experimental system is
illustrated. The following configurations can be obtained from
this setup.
a) A closed-loop control system for constant speed operation,
configured with speed feedback and load current
feedback. The induction motor drives a variable load, is
fed by an inverter, and the PLC controls the inverter
output.
b) An open-loop control system for variable speed operation.
The induction motor drives a variable load and is fed by
an inverter in constant control mode. The PLC is
inactivated.
c) The standard variable speed operation. The induction
motor drives a variable load and is fed by a constant
voltage-constant frequency standard three-phase supply.
The open-loop configuration b) can be obtained from the
closed-loop configuration a) by removing the speed and load
feedback. On the other hand, operation c) results if the entire
control system is bypassed.
CONCLUSION
Successful experimental results were obtained from the previously
described scheme indicating that the PLC can be used
in automated systems with an induction motor. The monitoring
control system of the induction motor driven by inverter and
controlled by PLC proves its high accuracy in speed regulation
at constant-speed-variable-load operation.
The effectiveness of the PLC-based control software is satisfactory
up to 96% of the synchronous speed. The obtained efficiency
by using PLC control is increased as compared to the
open-loop configuration of the induction motor fed by an inverter.