12-1
EE 442-642
(AC) Motor Drives
Fall 2012
12-2
Control Structure of Drives
12-3
Servo Drives
The basic structure is the same regardless of the
drive that is selected
12-4
An Example of Adjustable Speed Drives
12-5
Two Commonly-Used Coupling Mechanisms
12-6
Simplified Circuit of a Drive
12-7
Control of Servo Drives
The structure is application dependent
12-8
Limiters in the Control Structure
By providing ramp limiters, for example, a drive can be
prevented from “tripping” under sudden changes.
12-9
AC Motor Drives: Pump Application - Adjustable
Flow rate
Fixed versus adjustable speed drive
12-10
AC Motor Per-Phase Representation
Assuming sinusoidal steady state
12-11
Important Relationships in an Induction Machine
12-12
Torque-Speed Characteristics
The linear part of the characteristic is utilized in
adjustable speed drives
12-13
Plot of Normalized Rotor Current
It increases with slip and slip frequency
12-14
Acceleration Torque at Startup
Intersection represents the equilibrium point
12-15
Torque Speed Characteristics at various Frequencies of
Applied Voltage
The air gap flux is kept constant
12-16
Adjusting Speed of a Centrifugal Load
The load torque is proportional to speed squared
12-17
Phasor Diagram at Small Value of Slip Frequency
The rotor branch is assumed to be purely resistive
12-18
Voltage Boost to Keep Air Gap Flux at its Rated Value
Depends on the torque loading of the machine
12-19
Induction Motor Drive Capability Curves
12-20
Generator Mode of Operation
Rotor speeds exceed the synchronous speed
12-21
Regenerative Braking Mode to Slow Down
Machine is made to go into the generator mode
12-22
Per-Phase Equivalent Circuit at Harmonic Frequencies
The magnetizing branch is ignored
12-23
Torque Pulsations due to Harmonics
Rotations of fields due to the fifth and the seventh
harmonics are in opposite directions
12-24
Classification of Converter Systems
PWM-VSI is now most commonly use
12-25
PWM-VSI System
Diode rectifier for unidirectional power flow
12-26
PWM-VSI System
Options for recovered energy during regenerative
braking
12-27
General-Purpose Speed Controller
12-28
Change in Switching Frequency based on the required
Fundamental Frequency
Can be significant in large power ratings
12-29
Static Slip Recovery (Type 3 Wind Gen.)
Applications in very large power ratings where the speed
is to be adjusted over a very limited range