07-12-2012, 02:05 PM
INTRODUCTION TO ELECTRIC DRIVES
ELECTRIC DRIVES.ppt (Size: 1.25 MB / Downloads: 207)
Electrical Drives
About 50% of electrical energy used for drives
Can be either used for fixed speed or variable speed
MEP 1523 will be covering variable speed drives
Components in electric drives
Control unit
Complexity depends on performance requirement
analog- noisy, inflexible, ideally has infinite bandwidth.
digital – immune to noise, configurable, bandwidth is smaller than the analog controller’s
DSP/microprocessor – flexible, lower bandwidth - DSPs perform faster operation than microprocessors (multiplication in single cycle), can perform complex estimations
Electrical isolation between control circuit and power circuit is needed:
Malfuction in power circuit may damage control circuit
Safety for the operator
Avoid conduction of harmonic to control circuit
Sensors
Sensors (voltage, current, speed or torque) is normally required for closed-loop operation or protection
Electrical isolation between sensors and control circuit is needed for the reasons previously explained
The term ‘sensorless drives’ is normally referred to the drive system where the speed is estimated rather than measured.
Overview of AC and DC drives
AC motors used in high performance applications – elevators, tractions, servos
AC motors favorable than DC motors – however control is complex hence expensive
Cost of microprocessor/semiconductors decreasing –predicted 30 years ago AC motors would take over DC motors
Elementary principles of mechanics
As the motor speed increases, the kinetic energy also increases. During deceleration, the dynamic torque changes its sign and thus helps motor to maintain the speed. This energy is extracted from the stored kinetic energy:
Thermal considerations
Electrical machines can be overloaded as long their temperature does not exceed the temperature limit
Accurate prediction of temperature distribution in machines is complex – hetrogeneous materials, complex geometrical shapes