07-03-2011, 11:30 AM
PRESENTED BY:
D .Sunil
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ABSTRACT
The main objective of the thesis is to study the operation of six pulse inverter connected to an induction motor.
• In this thesis the induction motor is fed with a three phase six step voltages of variable frequency and magnitude are applied.
o For purposes of comparison the transient behavior of the motor operated with fixed supply voltage and no feedback control. For comparison purposes 20HP, 220-V, 60HZ, Four-pole, squirrel cage induction motor is used.
o In this thesis the harmonics in the line current and voltage of the motor are studied in addition to dynamic response of the system. For the above operations startup and loading transients, effect of changing the limits on slip, response to changes in speed reference are observed and compared and related plots are drawn.
Introduction
The three-phase squirrel-cage induction motor fed from a six-step voltage-source inverter is analyzed.
Particular attention is paid to the influence of the non sinusoidal voltage source on the steady and dynamic performance of the machine.
The stator quantities such as currents and flux linkages respond to the switching of the voltage by the inverter.
The presence of the sixth harmonic torque is confirmed both by simulation and the fast Fourier transform of the electromagnetic torque
Three Phase Induction Machines
Three-phase induction machines are asynchronous speed machines, operating below synchronous speed when motoring and above synchronous speed when generating.
They are comparatively less expensive to equivalent size synchronous or dc machines and range in size from a few watts to 10,000 hp.
They draw large starting currents, typically about six to eight times their full load values, and operate with a poor lagging power factor when lightly loaded.
Construction of Three-phase Induction Machines:
Most induction motors are of the rotary type with basically a stationary stator and a rotating rotor.
The stator has a cylindrical annulus magnetic core that is housed inside a metal frame onto which are mounted the bed-plates, end shields, and a terminal box. At the center of the end shields are bearings to support the rotor’s shaft.
The stator windings are formed by connecting coils of copper or aluminum conductors. The coils are insulated from the slot walls.
The rotor consists of a cylindrical laminated iron core with uniformly spaced peripheral slots to accommodate the rotor windings.
Slip
When the rotor is rotating at a steady speed of ωrm mechanical radians per sec, the relative or slip speed between the rotor and synchronous rotating stator field, F, is
Six-step inverter induction motor Drive
The main objective of this is to study the operation of six pulse inverter connected to an induction motor and its closed loop speed control operation.
In this, the induction motor is fed with a three phase six step voltages of variable frequency and magnitude are applied.
Each of the voltages are displaced by 1200 each. Start up transients of the motor with a preset ramp rate of the speed reference, the dynamic response of the drive to cyclic changes in speed command and load torque are studied.
For purposes of comparison the transient behavior of the motor operated with fixed supply voltage and no feedback control.
The harmonics in the line current and voltage of the motor are studied in addition to dynamic response of the system.
startup and loading transients, effect of changing the limits on slip, response to changes in speed reference are compared.
SIX STEP DESIGN (6-STEP)
The 6-Step type of inverter uses a two part conversion process to produce the variable frequency and voltage needed to control the AC motor.
It takes the 3-phase input line voltage and rectifies it into a variable DC value which is later converted into six discrete steps per output cycle.
Because there is no high frequency carrier, a 6-Step type of inverter produces no audio noise in the AC motor.
Six step wave form:
Closed-loop volts/hertz speed control with Six-step inverter
APPLICATIONS:
An alternative to the analog techniques and devices used in detecting the speed of induction motors in adjustable-speed drives.
The method involves digital processing of one stator phase current in order to retrieve the speed information contained in the slot-ripple harmonics, using FFT-based spectral analysis.
A noninvasive digital speed detector, suitable for steady-state operation, was developed.
APPLICATIONS:
Compressors.
Crushers.
Punching Presses.
Shears.
Hoists.
Elevators.
Reciprocating Pumps.
Mining & Metals:
Slurry pumps
Ventilation fans
De-scaling pumps
Conveyors
Bag house fans
Cyclone feed pumps
Electric Power
Forest Products:
Fan pumps Induced draft fans
Boiler feed water pumps Pulpers
Refiners Kiln drives Line shafts
Raw sewage pumps Bio-roughing tower pumps Treatment pumps Freshwater pumps
Petrochemical:
Pipeline pumps Gas compressors Brine pumps Mixers / extruders
Electrical submersible pumps Induced Draft Fans
Boiler feed water pumps
Cement
Kiln induced draft fans Forced draft fans Cooler baghouse fans Preheat tower fans
Raw mill induced draft fans Kiln gas fans
Cooler exhaust fans Seperator fans Baghouse
Miscellaneous
Test stands
Wind tunnels
Agitators
Rubber mixers
