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The Citadel ELEC 316 - DC Machines

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2/28/20131DC MachinesIntroduction to Rotating Machines• The rotor and stator each consist of a magnetic core, some electrical insulation, and the windings necessaryto establish a magnetic flux (unless it is created by a permanent magnet).• The rotor is mounted on a bearing supported shaft, which can be connected to mechanical loads (motor) or to a primer mover (generator), by means of belts, pulleys or other mechanical couplings. • The basic electrical machine creates two magnetic fields. One on the rotor and one on the stator. The field of the rotor will seek to align with the field of the stator, thereby creating torque and rotation.Introduction to Rotating MachinesThe windings carry the electrical currents that generate the magnetic fields and that flow to the electrical loads.If the current serves the sole purpose of providing a magnetic field and is independent of the load, it is called a magnetizing, or exciting current. The winding in which it flows is called a field winding.If the windings carry the load current, it is called the armature winding or armature.Introduction to Rotating MachinesMachine TypeWinding PurposeWinding TypeLocation Current TypeDC MachineIn/OutExcitationArmatureFieldRotorStatorAC->DCDCACSynchronousIn/OutExcitationArmatureFieldStatorRotorACDCACInductionInputOutputPrimarySecondaryStatorRotorACACIntroduction to Rotating MachinesGenerator : A machine that converts mechanical energy from a prime mover to electrical energy.Prime MoverArmature Current CounterTorqueF = ilBRot-ationArm-atureEMFE = dλ/dtElec.PowerOutIntroduction to Rotating MachinesGenerator: A machine that converts mechanical energy from a prime mover to electrical energy.Power flow and losses in DC generator2/28/20132Introduction to Rotating MachinesMotor : A machine that converts electrical energy from a source to mechanical energy.Input VoltageArmature Current TorqueF = ilBRot-ationArmature EMFE = dλ/dtMech.PowerOutIntroduction to Rotating MachinesMotor: A machine that converts electrical energy from a source to mechanical energy.Power flow and losses in DC motorIn a generator the prime mover causes the rotation and the commutator serves to rectify voltage at the brushes.Rectification occurs when the rotor winding experienceminimum flux.Introduction to Rotating Machines Introduction to Rotating MachinesHow to keep the rotor turning.• In order to prevent the rotor from stopping for ϒ = 0°which implies alignment of Bstatorand Brotortwo schemes are possible.1. Supply the stator windings with an alternating currentcausing it’s Bstatorfield to change direction periodically. Thus preventing the rotor field from aligning with that of the stator. This is how an AC synchronous machine works.Introduction to Rotating MachinesHow to keep the rotor turning.2. Supply the rotor winding with an alternating current causing Brotorto change direction periodically. The reversal is accomplished by the comutator assembly attached to the rotor.• Since the torque is a maximum for ϒ = 90°, thecommutator is configured so that the current distribution in the rotor windings remain constant and the rotor poles are always at 90° with respect to the fixed stator poles.Introduction to Rotating Machines2/28/20133As the commutator rotates CCW with the rotor, the magnetic field Br also rotates CCW . After θ = 30°, a new segment pair will be connected to the brushes. The current through winding coils L3 and L6 will reverse direction and the rotor magnetic field will shift by 60°in the direction opposite to rotation (CW).BrotorBstator+30°-30°BstatorBrotorIntroduction to Rotating MachinesL3L6As the commutator continues to rotate, the magnetic field Brotorwill change from -30 to +30°. Then a new segment pair will be connected to the brushes and the rotor magnetic field will again shift by 60°in the CW direction.BrotorBstator+30°-30°BstatorBrotorL6L3In this machine, the torque angle (between Bstatorand Brotor) is not always the ideal value of 90° , but actually varies from 60 ° to 120°as the machine rotates. The resulting torque will vary by up to 14% {sin(90° -30°) = 0.86} from the maximum value. As the number of segments are increased, the torque fluctuations produced by commutation will be reduced. For example, with 60 segments the torque angle will vary from 90° by +/- 3 °. The resulting torque will vary by only 0.5% from the maximum value. In this way a DC motor can produce nearly constant torques and DC generators can produce nearly constant voltages.Introduction to Rotating Machines• ARMATURE REACTION (AR)The mmf produced in the armature windings effects the spatial distribution of the air gap flux and the magnitude of the net flux per pole.The effect of armature reaction is to create flux crossing the main field flux. This is called cross magnetizing armature reaction, or just armature reaction.Armature reaction causes a decrease in the air gap flux under part of the pole face and an increase under the other part of the pole. Because of magnetic saturation of the machine iron, the net flux is decreased by a greater amount. Thus the net flux per pole is less than that produced by the field windings alone.Introduction to Rotating MachinesArmature Reaction Effects• The effect of the magnetic flux generated by the armature currenton the main magnetic field of the machine is called armature reaction.• Shift in the magnetic neutral plane: arching and sparking at the brushes when machine is loaded.17Figure 7-23The development of armature reaction in a dc machine.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.o The flux-weakening caused by the armature reaction.18Armature Reaction EffectsCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.2/28/20134• ARMATURE REACTIONQuantitatively, the effect of AR is complex due to the non-linear behavior of machine saturation. Typically the effects of AR are determined experimentally, and are displayed via the machines magetization curve. The effect on the net field is loosely proportional to the armature current Ia. Therefore, a family of mag. curves are drawn for a series of values of Ia. So for analysis purposes, the effect of AR can be accounted for simply by using the appropriate curve for the Ia in question. Introduction to Rotating Machines• ARMATURE REACTIONIn general the amount of AR increases proportional to Ia. Therefore, the result of


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