TI-MotorControlCompendium-2010.ppt

1、“Absolutely amazing!I wish I had a tool like this when I was learning about motors.Five stars!”,Navigating the complexities of motor control,-Nikola TeslaInventor of the AC Induction Motor,2010-2011,Motor Control Compendium,By Dave WilsonTI MCU Application Manager for Motor Control,Copyright Texas I

2、nstruments 2011,Motor Glossary,Flux,is flux,in Webers,In the mid 1800s Michael Faraday speculated that magnetic fields exist as lines of force,which we today refer to as“magnetic flux”,which is measured in units of“Webers”.Essentially,the more flux there is,the stronger the magnet will be.We convent

3、ionally think of flux leaving the north pole,and re-entering the south pole of a magnet,as shown by the arrows in the diagram.If we measure how much flux cuts through a surface area which is perpendicular to the flux path,then we have a measure of the flux density at that particular spot in space.On

4、e Weber of flux cutting through one square meter of area constitutes a flux density of one“TESLA”,named after the Serbian engineer,Nikola Tesla,who is ALSO the inventor of the AC induction motor.,Motor Glossary,Right-hand Rule,current,field,force,In 1820,Hans Christian Oersted discovered that curren

5、t flowing in a wire creates its own magnetic field,and when this field interacts with a second magnetic field,the result is a force acting on the conductor.This force is proportional to the amount of current flowing in the wire,the strength of the second magnetic field,and the length of wire that is

6、 affected by the second magnetic field.The direction of the force can be determined by a technique know as the Right-hand Rule.If your right hand is configured as shown below,where your thumb points in the direction of positive current flow,and your index finger points in the direction of the second

7、 magnetic fields flux(i.e.,flowing from the North pole to the South pole),then your middle finger will be pointing in the direction of the force acting on the wire.,Motor Glossary,Left-hand Rule,S,N,Direction of Motion,A wire moving in a magnetic field will generate a voltage across it whose magnitu

8、de is proportional to how many flux lines are cut by the wire per second.The polarity of this back-EMF voltage can be determined by the Left-hand Rule.If you hand is configured as shown below where your index finger is pointing in the direction of the magnetic flux(pointing towards the South pole),a

9、nd your middle finger is pointing in the direction of motion,then your thumb will be pointing towards the positive end of the wire.,Motor Glossary,Torque,Clockwise torque=r1 F1+r2 F2-r3 F3,Torque has units of distance times force,i.e.,foot-pounds or newton-meters.,The sum of all of the perpendicular

10、 magnetic forces acting on a rigid body(such as the rotor of a motor),times the radiuses at which these forces are applied,results in torque.Pancake motors achieve high torque by maximizing the radius at which the forces are applied to the rotor.On the other hand,longer motors with smaller radiuses

11、can achieve the same torque by maximizing the force that is applied.This trade-off allows a motor customer to select from a wide range of motor form factors to meet their space constraints,and still have good torque performance.Using the Right-hand Rule,we see that a force is exerted on a wire which

12、is carrying current in a magnetic field.This force is proportionalto the amount of current flowing in the wire,the strength of themagnetic field,and the length of wire that is affected bythe magnetic field.In most motor topologies,the strengthof the magnetic field and the wire lengths exposed to the

13、magnetic field are fixed as a function of the motor design.The current however is adjustable.So we can combine theeffects of the magnetic field strength and the wire lengthsinto one term called the torque constant.This term,whenmultiplied by the motor current results in motor torque.So we see that i

14、n order to control motor torque,we must control motor current.,Motor Glossary,Torque Components,The torque produced by a motor can be divided into two different categories.The first one is called reaction torque or alignment torque.This accounts for the torque created by the reaction between the mag

15、nets on the rotor and stator.All motors with the exception of the switched reluctance motor exhibit reaction torque.A lesser known torque component is called reluctance torque.This torque is generated as a result of a magnetic field trying to minimize the reluctance of its flux path.For example,if y

16、ou place a kitchen knife on a countertop,and approach one end of the knife with a magnet,the knife will move toward the magnet in order to minimize the path of the magnetic field,even though the knife itself is not magnetized.Since the switched reluctance motor does not use a magnet or an electromagnet on the rotor,reluctance torque is its only torque component.Both of these torque components are a function of the alignment between the rotor and stator.Consider for example Figure A,where the rot