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

12

3 4

T +ve +vePm +ve

T -ve +vePm -ve

T -ve -vePm +ve

T +ve -vePm -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

T

Te

mTe

Te

Te

m

m m

• 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

Quadrant 1Forward motoring

Quadrant 2Forward braking

Quadrant 3Reverse motoring

Quadrant 4Reverse braking

Load steady state torque-speed characteristic

Hoist drive

Speed

Torque

Gravitational torque