BALAJI INSTITUTE OF TECHNOLOGY AND SCIENCES
BALAJI NAGAR ,LINGAPURAM,
INTRODUCTION TO DRIVES:
In recent years, there has been a trend toward using variable speed to operate
motors for equipment and systems that provide heating, ventilation, and air
conditioning for buildings.
One of the largely untapped cost-saving features of variable-speed drives is built-in
The internal control logic of variable-speed drives is microprocessor-driven.
COMPARISION BETWEEN A.C & D.C DRIVES:
The higher cost of the AC inverter compared with the DC converter is offset by
the much lower cost of the standard induction motor compared with the DC
There is a clear implication in much of the promotional material that the
inverter-fed system can perform at least as well, or even better, than the DC
This is true for some applications such as fans and pumps where high
torque is only needed at high speeds.
Whereas a DC drive will invariably be supplied with a motor which is provided
with through ventilation to allow it to operate continuously at low speeds without
overheating, the standard induction motor has no such provision, having been
designed primarily for fixed-frequency full speed operation.
The stator phases are energized in sequence to produce torque on the rotor.
The rotor shape allows it to act as a fan for cooling the motor.
The controller structure is similar to that of AC and DC drives.
The biggest single difference is that the current magnitude and wave shape must be
MICROPROCESSOR CONTROLED DRIVES:
In a power-electronic system, the microcomputer functions can in general be
categorized as follows:
1) Control of feedback loops
2) Gate firing control of phase-controlled converters
3) PWM or square-wave signal generation of
4) Optimal and adaptive control
5) Estimation of feedback signals
6) General sequencing control
7) Protection and fault overriding control
8) Signals monitoring and warning
9) Data acquisition
10)Diagnostic Miscellaneous computation
Microcomputers are having a major impact on industrial applications including
the areas of testing, control, instrumentation, data acquisition, numerical
machine control & even robotics.
The microprocessor control a.c drives are widely used in following industrial
•Travel& hoists drives in cranes and conveyors
•Roller tables in rolling mills
Depending on the process requirements, environmental conditions and
financial objectives of a company, choice of a drive can be made .
The drive systems can be divided into two groups, d.c drive systems & a.c
1) D.C DRIVE SYSTEM
2) A.C DRIVE SYSTEM
1) D.C DRIVE SYSTEMS
The invention of power semiconductor saw the advent of d.c drive systems for
most of the early variable speed requirements based on the advantages of simple
construction & easy of control , this technology continued to be taken up for further
improvements allover the world till very recently .
1) sharing or precision coordinated speed and tension control
2) Fast control response& rotational accuracy even at very low speeds
3) Speed No start -up problems with high torque Large speed setting range without
4) Multi motor operation with load stability high
5) Insensitivity to system voltage drops
6) Precise replication of load torque for process control
7) Converter losses reduced to a minimum
1) Generally not suitable for harsh explosive, or corrosive environments
2) Controls relatively complex, particularly for high performance drives
3) Motor requires maintenance
4) May create line harmonics
1) Log carriages,
2) Metals industry primary reduction mills, cold mills& processing lines
2) A.C DRIVE SYSTEM:
The four important considerations that enter into motor & inverter selection are:
1) Actual motor capabilities of ampere Vs torque
2) Thermal (low speed) limits
3) High speed limitations &
4) Starting & peak running torque
MICROCOMPUTER CONTROL OF D.C DRIVES:
A conventional analog control scheme implemented in d.c drives consists of an
outer speed control loop and an inner current control loop
A synchronizing circuit interface is required so that the microprocessor
can synchronize the generation of the firing pulse data with the supply
Although the microcomputer can perform the functions of gate pulse
amplifier provides the necessary isolation and produces gate pulse of
required magnitude and duration.
A set of instructions is stored in the memory, and those instructions are
executed by the microprocessor for proper functioning of drive.
After receiving the start signal it begins executing the program in a
continuous cyclic manner.
MICROPROCESSOR CONTROLED A.C DRIVES:
A block diagram of the microprocessor-controlled synchronous motor drive is shown
Better solutions to process engineering problems
More adaptable dynamic characteristics
High operational reliability
High product consistency
Low maintenance & low wear &tear
Fast communications with a higher order automation systems
Quick fault diagnosis
Accurate monitoring of protective functions such as phase loss, over
current & high& low voltages with out the use of discrete relays