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ELECTRIC DRIVES INTRODUCTION TO ELECTRIC DRIVES Electrical Drives Drives are systems employed for motion control Require prime movers Drives that employ electric motors as prime movers are known as Electrical Drives Electrical Drives • About 50% of electrical energy used for drives • Can be either used for fixed speed or variable speed • 75% - constant speed, 25% variable speed (expanding) CONTENT • Power electronic system • Modern electric drives • Power electronic converter in electric drives Modern electric drives (With power electronic converters) • Small • Efficient • Flexible Components in electric drives Motors • DC motors - permanent magnet – wound field • AC motors – induction, synchronous (IPMSM, SMPSM), brushless DC • Applications, cost, environment • Natural speed-torque characteristic is not compatible with load requirements Power sources • DC – batteries, fuel cell, photovoltaic - unregulated • AC – Single- three- phase utility, wind generator - unregulated Power processor • To provide a regulated power supply • Combination of power electronic converters • More efficient • Flexible • Compact • AC-DC DC-DC DC-AC AC-AC 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 Components in electric drives 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 DC motors: Regular maintenance, heavy, expensive, speed limit Easy control, decouple control of torque and flux AC motors: Less maintenance, light, less expensive, high speed Coupling between torque and flux – variable spatial angle between rotor and stator flux Torque-speed quadrant of operation T -ve +ve Pm -ve 2 1 T +ve +ve Pm +ve T 3 T -ve -ve Pm +ve 4 T +ve -ve Pm -ve • Quadrant of operation is defined by the speed and torque of the motor • Most rotating electrical machines can operate in 4 quadrants • Not all converters can operate in 4 quadrants Torque-speed quadrant of operation Te m Te Quadrant 2 Forward braking m Quadrant 1 Forward motoring T Te Te m Quadrant 3 Reverse motoring m Quadrant 4 Reverse braking • Quadrant of operation is defined by the speed and torque of the motor • Most rotating electrical machines can operate in 4 quadrants • Not all converters can operate in 4 quadrants Load steady state torque-speed characteristic Hoist drive Speed Torque Gravitational torque
