Speed Control of 3-Phase Induction Motor Using PIC18 Microcontrollers
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Speed Control of 3-Phase Induction Motor Using PIC18 Microcontrollers
INTRODUCTION
Induction motors are the most widely used motors forappliances, industrial control, and automation; hence,they are often called the workhorse of the motion industry.They are robust, reliable, and durable. When poweris supplied to an induction motor at the recommendedspecifications, it runs at its rated speed. However,many applications need variable speed operations. Forexample, a washing machine may use different speedsfor each wash cycle. Historically, mechanical gear systemswere used to obtain variable speed. Recently,electronic power and control systems have matured toallow these components to be used for motor control inplace of mechanical gears. These electronics not onlycontrol the motor’s speed, but can improve the motor’sdynamic and steady state characteristics. In addition,electronics can reduce the system’s average powerconsumption and noise generation of the motor.Induction motor control is complex due to its nonlinearcharacteristics. While there are different methods forcontrol, Variable Voltage Variable Frequency (VVVF) orV/f is the most common method of speed control inopen loop. This method is most suitable for applicationswithout position control requirements or the needfor high accuracy of speed control. Examples of theseapplications include heating, air conditioning, fans andblowers. V/f control can be implemented by using lowcost PICmicromicrocontrollers, rather than usingcostly digital signal processors (DSPs).Many PICmicro microcontrollers have two hardwarePWMs, one less than the three required to control a3-phase induction motor. In this application note, wewill generate a third PWM in software, using a generalpurpose timer and an I/O pin resource that are readilyavailable on the PICmicro microcontroller. This applicationnote also covers the basics of induction motors anddifferent types of induction motors.
Induction Motor Basics
NAMEPLATE PARAMETERS
A typical nameplate of an induction motor lists thefollowing parameters:• Rated terminal supply voltage in Volts• Rated frequency of the supply in Hz• Rated current in Amps• Base speed in RPM• Power rating in Watts or Horsepower (HP)• Rated torque in Newton Meters or Pound-Inches• Slip speed in RPM, or slip frequency in Hz• Winding insulation type - Class A, B, F or H• Type of stator connection (for 3-phase only), star(Y) or delta ()When the rated voltage and frequency are applied tothe terminals of an induction motor, it draws the ratedcurrent (or corresponding power) and runs at basespeed and can deliver the rated torque.MOTOR ROTATIONWhen the rated AC supply is applied to the stator windings,it generates a magnetic flux of constant magnitude,rotating at synchronous speed. The flux passesthrough the air gap, sweeps past the rotor surface andthrough the stationary rotor conductors. An electromotiveforce (EMF) is induced in the rotor conductorsdue to the relative speed differences between the rotatingflux and stationary conductors.The frequency of the induced EMF is the same as thesupply frequency. Its magnitude is proportional to therelative velocity between the flux and the conductors.Since the rotor bars are shorted at the ends, the EMFinduced produces a current in the rotor conductors.The direction of the rotor current opposes the relativevelocity between rotating flux produced by stator andstationary rotor conductors (per Lenz's law).To reduce the relative speed, the rotor starts rotating inthe same direction as that of flux and tries to catch upwith the rotating flux. But in practice, the rotor neversucceeds in 'catching up' to the stator field. So, therotor runs slower than the speed of the stator field. Thisdifference in speed is called slip speed. This slip speeddepends upon the mechanical load on the motor shaft.The frequency and speed of the motor, with respect tothe input supply, is called the synchronous frequencyand synchronous speed. Synchronous speed isdirectly proportional to the ratio of supply frequencyand number of poles in the motor. Synchronous speedof an induction motor is shown in Equation 1
INDUCTION MOTOR TYPES
Based on the construction of the rotor, induction motorsare broadly classified in two categories: squirrel cagemotors and slip ring motors. The stator construction isthe same in both motors.Squirrel Cage MotorAlmost 90% of induction motors are squirrel cagemotors. This is because the squirrel cage motor has asimple and rugged construction. The rotor consists of acylindrical laminated core with axially placed parallelslots for carrying the conductors. Each slot carries acopper, aluminum, or alloy bar. If the slots are semiclosed,then these bars are inserted from the ends.These rotor bars are permanently short-circuited atboth ends by means of the end rings, as shown inFigure 1. This total assembly resembles the look of asquirrel cage, which gives the motor its name. The rotorslots are not exactly parallel to the shaft. Instead, theyare given a skew for two main reasons:a) To make the motor run quietly by reducing themagnetic hum.b) To help reduce the locking tendency of the rotor.Rotor teeth tend to remain locked under the statorteeth due to direct magnetic attractionbetween the two. This happens if the number ofstator teeth are equal to the number of rotorteeth.


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to get information about the topic Voltage frequency control of 3phase induction motor. full report ,ppt and related topic refer the page link bellow

http://studentbank.in/report-speed-contr...ique-refer

http://studentbank.in/report-speed-contr...pic30f4011

http://studentbank.in/report-spedd-contr...ull-report

http://studentbank.in/report-speed-contr...ontrollers

http://studentbank.in/report-three-phase...tion-motor

http://studentbank.in/report-linear-indu...9#pid61329
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