DC Motor Control By Using A Chopper Circuit

EEE322 Course Project

DC Motor Speed Control By Using Chopper Circuit

©Birol İlker ARSLAN – Nurullah AÇIKGÖZ 2

Theory

• Dc motors are widely used in robotics for their small size and high energy output.

• The typical DC motors operate on as few as 1.5 volts on up to 100 volts.

• Roboticists often use motors that operate on 6 12 or 24 volts.

• A low voltage(e.g.12 volts or less ) DC motor may draw from 100MA to several amps at stall depending on its design.

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• Main characteristics of DC Motors include: High Speed, Low Torque, Reversibility and Continuos Motion.

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Principle Of Magnet DC Motors

If you were to place a loop of wire within the magnetic field of a horseshoe magnet in the manner (FİG 14) and apply current to it you would experience a motor action such that the loop of wire would try to turn.

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• This effect will only occur when the wire lays adjacent or in line with the magnetic field (FİG 15).

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• Let’s say the loop wire was arranged on a bearing so that it could freely rotate .What we need is some way of switching the current through the wire on each 180 degrees of rotation. We do this with a simple brush mechanism(FİG 14).In this case as the loop rotates to a vertical position you notice that at the end of wire leaves the switch contact. Under inertia,the loop continues to rotate until the other side of the loop strikes the right hand side contact and original loop contact makes contact with the left hand side brush.

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If you examine this concept carefully you can see that while current is applied to the brush contacts the loop of wire will rotate through 180 degrees where contact will be made again and the rotor (the revolving thing )will continue rotation

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• The direction of rotation depends on:The polarity of the battery (Power Source) • The stator is the stationary outside part of a

motor. The rotor is the inner part which rotates. The ends of each wire set for a connecting set is called a commutator. The torque of a motor is the rotary force produced on its output shaft.

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• When a motor is stalled it is producing the max. amount of torque that it can produce. Hence the torque rating is usually taken when the motor has stalled and is called the stall torque

• Motors that draw more current will deliver more power.

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Application Side Of the Project

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• The speed of the output shaft is directly proportional with the applied voltage.

• So the voltage drop on load (motor) was the thing we consider throgh all the design procedure.

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•Voltage drop is controlled by the variable resistance•Diac – triac compound: Purpose is to reduce loss. Diac simply triggers triac for frequencies to give output but applications without diac is also possible.•In fact, the same circuit could easily be done by using simple potentiometer and capacitor just adjusting the voltage drop on the load.

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•H-bridge is used to control direction manually with a digital logic. •Adapter is used to convert 230 AC into 12 V DC.•Dimmer (which is shown not by block diagram) is to adjust speed.

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• In fact there are ready integrated circuits for this purpose but we made it ourselves for design reasons. (To

reduce the power loss resistance values decreased. npn – pnp compund used instead of the same transistor couple which is much better.)

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• Flyback diodes– Small voltages– Upside npn downside

pnp

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• Specifications of the h-bridge we constructed– Can handle 2.2 to 9.6 V properly.– Won’t work with motors with resistance lower

than 5Ω. (Our motor has 20Ω resistance.)– Dangerous if R1-R2 or R3-R4 connected at the

same time since the battery or source is shorted.

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• 5,7 volt ouput is seen At the output of the H-bridge. • Efficiency of the h-bridge we use is about

%48.• 3V feed is used to handle control of the

bridge.

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Where to Use This Circuit & Suggestions

• This circuit can easily be used for fan applications and anywhere we need dc motor to rotate permanently.

• A switch can be used to switch between “turbo” mode in which we want more power and torque (overspeeding) from the motor where we need performance. (Which is directly connecting to 12V by a switch.)

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• For overspeeding there must be calculation done how much time it will be overspeeded and how much the motor can handle.

• For fan applications temperature sensors would be beneficial. (Closed loop control system)

• This circuit can also be used for lighting. Since there is a net voltage drop of about 6 volts a group of LED can be directly controlled. Small light bulbs can be directly handled by this circuit but for higher ranges need to increase voltage.

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• Can be used to drive small conveyor belts in 2 directions and with dimmer would be able to handle both directional control and speed control.

• It is also possible to use this circuit for any robotic application where directional control is needed with speed control such as RC helicopters’ or aircrafts’ propellers.

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RESOURCES• Bilgiç, Mustafa Murat. "Single Phase Induction Motor Speed Control Using PWM AC

Chopper Fan Applicaitons” M.Sc. Thesis Istanbul Technical University, 2007.

• Wikipedia Online Encyclopedia. 2012. Wikipedia. 28.5.2012 < http://en.wikipedia.org/wiki/H_bridge>.

• Which.net 2012 “Speed Controllers” Online Source. 25 May 2012. < http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html>

• Cook, David “h-bridge Motor Driver Using Bipolar Transistors” May 2012 <http://www.robotroom.com/BipolarHBridge.html>

• Datasheets for parts used throughout the project are available at;• BC307 from Motorola Semiconductor™ • <http://www.datasheetcatalog.org/datasheet/motorola/BC308C.pdf>• 2N3904 from Fairchild Semiconductor™• <http://www.datasheetcatalog.org/datasheet/fairchild/2N3904.pdf>

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Thanks...

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