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7/1/2015 Lesson 7: Construction of Elementary Dc Generators 1 ET 332A DC MOTORS, GENERATORS AND ENERGY CONVERSION DEVICES Learning Objectives 2 After this presentation you will be able to: List the parts and explain the function of a dc machine Identify the commutator of a dc machine and explain how it operates on a dc machine Explain armature reaction and how interpoles improve dc machine performance 1 7/1/2015 Construction of Elementary Dc Generator output leads Brushes Field Poles Field Poles Armature Commutator Field poles - produce the main magnetic field of the machine (permanent magnet or electromagnet) Armature - rotating part of the machine. Holds coils where induced voltage is developed. Commutator - connect armature coils to the brushes. Commutator and brushes act as mechanical rectifier. Brushes - pick up current and voltage from armature. Remain fixed while armature spins. 3 Construction of Elementary Dc Generator Pictorial Diagram of small dc machine 4 2 7/1/2015 Commutation in Elementary Dc Machines 5 Induced voltage is sinusoidal. Commutator and brushes act as mechanical rectifier to always connect positive voltage to the same terminal. e Effects of Adding More Armature Coils 6 Two coils require four commutator segments Adding more coils and commutator segments gives smoother dc output. Each coil needs 2 commutator segments. eout eout Communtation point 3 7/1/2015 Commutator Details 7 Commutator combined with Brushes forms a mechanical rectifier Insulation Copper Segments Segments connect the armature coils to the load through the brushes. Communtation Process 8 Commutation - switching or reversing of current in a circuit Commutator Detail Copper Segments Insulation Brush Segments rotate past fixed brushes during commutation Rotation 4 7/1/2015 Commutation Process Resistance commutation - uses contact resistance of brush commutator to switch the current in the motor or generator coils. R2 R1 IT I2 I1 Circuit Model Rotation R2 Generator Load R1 I2 I1 R2 increasing due to lack of contact area R1 decreasing due to increasing contact area IT 9 Brush Position Affects Commutation 10 Good commutation takes place with zero voltage across bushes Communtation above 2-3 V causes sparking and reduced terminal V 5 7/1/2015 Armature Reaction and Commutation 11 Armature reaction - distortion of the field flux distribution due to load current in armature brush Neutral Zone E=0 under brush Low sparking Amount of field distortion determined by load current For mechanically fixed brushes, armature reaction causes poor commutation, (sparking) Point of zero induced voltage shifts Move brushes to neutral zone to improve commutation under load Add interpoles (modern solution) Interpoles (Commutating Poles) 12 Interpole constructed on a few turns of heavy gage wire. Carries load current. Produces cancelling mmf so flux distortion is minimized. Interpole Poles Field Pole Field Pole 6 7/1/2015 Machine Schematic Diagram 13 Interpole Winding Field winding Armature and Brushes End Lesson 7: Construction of Elementary Dc Generators 14 ET 332A DC MOTORS, GENERATORS AND ENERGY CONVERSION DEVICES 7