Introduction to Electric Drives
1
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)
• Advantages of Electrical Drives
• Enough overload capacity without loss of life of machine.
• Four quadrant operation.
• Modifiable torque-speed characteristics.
• No requirement of warming up period.
• Higher efficiency and Easy control.
• Clean operation, no pollution.
• Wide range of speed control.
• Electric braking can be employed
• Electric drives can be provided with automatic fault detection
systems.
• Electric motors have long life lower noise, lower maintenance
requirements and cleaner operation
• Adoptable to almost an operating conditions such as explosive and
radio active, submerged in liquids etc.
• They can be started instantly and immediately can be fully loaded.
Constant speed Variable Speed Drives
valve
Supply Supply
motor pump motor
PEC pump
Power Power out
In Power Power out
In
Power loss
Mainly in valve Power loss
Conventional electric drives (variable speed)
• Bulky
• Inefficient
• inflexible
Modern electric drives (With power electronic converters)
• Small
• Efficient
• Flexible
BLOCK DIAGRAM OF ELECTRIC DRIVE
Components in electric drives
Motors
• DC motors - permanent magnet – wound field
AC motors – induction, synchronous (IPMSM, SMPSM),
Brushless DC
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
AC-DC Converters or Rectifiers
AC-DC Converters or Rectifiers (Cont.)
AC Voltage Controller
VSI Controlled Inverter for IM Drive
CSI Controlled Drives for IM
DC – DC Converter (Chopper)
Overview of AC and DC drives
Extracted from Boldea & Nasar
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
Overview of AC and DC drives
Before semiconductor devices were introduced (<1950)
• AC motors for fixed speed applications
• DC motors for variable speed applications
After semiconductor devices were introduced (1950s)
• Variable frequency sources available – AC
motors in variable speed applications
• Coupling between flux and torque control
• Application limited to medium performance applications
– fans, blowers, compressors – scalar control
• High performance applications dominated by DC
motors – tractions, elevators, servos, etc
Overview of AC and DC drives
After vector control drives were introduced (1980s)
• 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
