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Electrical Machines Dcmotor Jun2015

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dc motor, chapter 2 for det3043 electrical machines

Text of Electrical Machines Dcmotor Jun2015

Electrical machines

Electrical machines-chapter 2-Dc motor1Learning Objectives:

Understand the construction and principle operation of a DC motorApply the principle operation of DC motor.22Electric MotorThe input is electrical energy (from the supply source), and the output is mechanical energy (to the load).3

ConstructionDC motors consist of one set of coils, called armature winding, inside another set of coils or a set ofpermanent magnets, called the stator. Applying a voltage to the coils produces a torque in the armature, resulting in motion.4NSStator with with polesBrushRotorFieldStator: Stationary part of the machine.The stator carries a field winding that is used to produce the required magneticfield by DC excitation. Often know as the field.Rotor: The rotor is the rotating part of the machine. The rotor carries a distributed winding, and is the winding where the emf is induced. Also known as the armature.ConstructionStator The stator is the stationary outside part of a motor. The stator of a permanent magnet dc motor is composed of two or more permanent magnet pole pieces. The magnetic field can alternatively be created by an electromagnet. In this case, a DC coil (fieldwinding) is wound around a magnetic material that forms part of the stator.5ConstructionRotor The rotor is the inner part which rotates. The rotor is composed of windings (called armature windings) which are connected to the externalcircuit through a mechanical commutator. Both stator and rotor are made of ferromagnetic materials. The two are separated by air-gap.6ConstructionA winding is made up of series or parallel connection of coils. Armature winding - The winding through which the voltage is applied or induced. Field winding - The winding through which a current is passed to produce flux (for the electromagnet) Windings are usually made of copper.7Construction8The rotor iron core is mounted on the shaft.

Coils are placed in the slots.

The end of the coils are bent and tiedtogether to assure mechanical strength.

Note the commutator mounted on theshaft. It consists of several coppersegments, separated by insulation.CommutatorPoles BrushesRotor windingFanBearingDC Motor Basic PrinciplesEnergy ConversionIf electrical energy is supplied to a conductor lying perpendicular to a magnetic field, the interaction ofcurrent flowing in the conductor and the magnetic field will produce mechanical force (and therefore,mechanical energy).9DC Motor Basic PrinciplesValue of Mechanical Force

There are two conditions which are necessary to produce a force on the conductor. The conductor mustbe carrying current, and must be within a magnetic field. When these two conditions exist, a force will beapplied to the conductor, which will attempt to move the conductor in a direction perpendicular to the magneticfield. This is the basic theory by which all DC motors operate.10DC Motor Basic PrinciplesThe force exerted upon the conductor can be expressed as follows : F = B x i x l (unit Newton)where B is the density of the magnetic field, l is the length of conductor, and i the value of current flowing in the conductor. The direction of motion can be found using Flemings Left Hand Rule.11

Flemings Left Hand RuleThe first finger points in the direction of the magnetic field (first - field), which goes from the North pole to the South pole. The second finger points in the direction of the current in the wire (second - current). The thumb then points in the direction the wire is thrust or pushed while in the magnetic field (thumb - torque or thrust).DC Motor Basic Principles12

DC Motor Basic PrinciplesPrinciple of operationConsider a coil in a magnetic field of flux density B. When the two ends of the coil are connected across a DC voltage source, current I flows through it. A force is exerted on the coil as a result of the interaction of magnetic field and electric current. The force on the two sides of the coil is such that the coil starts to move in the direction of force.13

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Induced voltage in the armature winding of DC motorDC Motor Basic PrinciplesThe value of current flowing through the armature is dependent upon the difference between the applied voltage and this counter-voltage. The current due to this counter-voltage tends to oppose the very cause for its production according to Lenzs law. It results in the rotor slowing down. Eventually, the rotor slows just enough so that the force created by the magnetic field (F = Bil) equals the load force applied on the shaft. Then the system moves at constant velocity.16DC Motor Basic Principles17DC Motor Basic PrinciplesNote that equation (induced voltage) gives the emf induced in one coil. As there are several coils wound all around the rotor, each with a different emf depending on the amount of flux change through it, the total emf can be obtained by summing up the individual emfs.The total emf induced in the motor by several such coils wound on the rotor can be obtained by integrating equation, and expressed as:where K is an armature constant, and is related to the geometry and magnetic properties of the motor, and m is the speed of rotation. The electrical power generated by the machine is given by:18

DC Motor Basic Principles19Generated EMF in a Real DC MachineWhereZ = total number of conductors, P = total number of polesa = P for lap winding, a = 2 for wave winding, = flux, = speed in rad/s and n = speed in rpm.

DC Motor Basic Principles20Counter EMF:When the motor is running, internally generated emf, (EG = EC) opposes the applied voltage, thus:I=V ERA T C AWhere: VT = terminal voltage, Ec = counter EMF, RA is the armature resistance and IA is the armature currentDC Motor Basic PrinciplesThe Relationship Between The Induced Emf And Torque21

DC Motor Basic PrinciplesExample 122

DC Motor Basic PrinciplesExample 223

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DC Motor Basic PrinciplesEquivalent circuitThe equivalent circuit of DC Motors (and Generators) has two components:Armature circuit: It can be represented by a voltage source and a resistance connected in series (the armature resistance). The armature winding has a resistance, Ra.

The field circuit: It is represented by a winding that generates the magnetic field and a resistance connected in series. The field winding has resistance Rf.

25DC Motor Basic PrinciplesShunt Motors-Field and armature windings are connected in parallel.

Series Motors-Field and armature windings are connected in series.

Compound Motors-Has both shunt and series field so it combines features of series and shunt motors.26DC Motor Basic Principles27

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DC Motor Basic PrinciplesExample 1


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DC Motor Basic PrinciplesExample 234

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DC Motor Basic PrinciplesShunt Motor: Speed Control44

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46DC Motor Basic PrinciplesShunt Motor : The effect of an open field circuit47

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70Compound Motor: Speed ControlDC Motor Basic PrinciplesTorque-Speed Characteristics:In order to effectively use a D.C. motor for an application, it is necessary to understand its characteristic curves. For every motor, there is a specific Torque/Speed curve and Power curve. The relation between torque and speed is important in choosing a DC motor for a particular application.71DC Motor Basic Principles72

DC Motor Basic PrinciplesDC Motor Speed Control


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DC Motor Basic PrinciplesDC Motor Starting


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DC Motor Basic PrinciplesDC Motor Efficiency83

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86DC Motor Basic PrinciplesIt is important to mention that the total input and output power can be calculated in many different ways using the power flow diagram, depending on the information given. Also note that the torque developed inside the rotor is different from the final (output) torque supplied to the load due to rotational losses.87

DC Motor Basic PrinciplesExample 188

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DC Motor Basic PrinciplesSelf Test 192

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