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Effect of Armature Inductance on Commutation – No Load

Effect of Armature Inductance on Commutation – No Load. Consider Coil #2 Induced Voltage changes from clockwise to counter-clockwise as the coil passes from under the South pole, through the neutral plane, to under the North pole. Coil #2 Under the South Pole.

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Effect of Armature Inductance on Commutation – No Load

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  1. Effect of Armature Inductance on Commutation – No Load Consider Coil #2 Induced Voltage changes from clockwise to counter-clockwise as the coil passes from under the South pole, through the neutral plane, to under the North pole ECE 441

  2. Coil #2 Under the South Pole Clockwise voltage induced in coils under the South pole Counter-clockwise voltage induced in coils under the North pole ECE 441

  3. Coil #2 in the Neutral Plane Coil #2 is shorted out by the brush No voltage is induced ECE 441

  4. Coil #2 Under the North Pole Counter-clockwise voltage induced in the coil ECE 441

  5. Under “Loaded” Conditions Coils #2 and #3 supply current to the load through the commutator bar #3 and the brush to LOAD ECE 441

  6. Coil #2 in the Neutral Plane Current in Coil #2 wants to go to zero, but cannot change instantaneously! ECE 441

  7. Coil #2 Under the North Pole As soon as commutator bar #3 slides off the brush, the current in coil #2 is forced to zero A large emf is induced between commutator bar #3 and the brush, resulting in an “arc” ECE 441

  8. Ideal Commutation For Ideal Commutation, need the coil current in phase with the coil voltage (no delay) ECE 441

  9. Interpoles – Generator Action • Install narrow poles in the Neutral plane • Only the coil undergoing commutation is affected • A neutralizing voltage forces the reversal of current in each coil as it moves through the region ECE 441

  10. Interpoles – Motor Action Connected in series with the armature winding Incorrect polarity makes the sparking problem worse ECE 441

  11. Armature Reaction -- Generator Neutral Plane shifts in the direction of rotation ECE 441

  12. Armature Reaction -- Motor Neutral Plane shifts opposite to the direction of rotation ECE 441

  13. Compensating Windings • Eliminate armature reaction by setting up an mmf that is always equal and opposite to the armature mmf • Connect in series with the armature • Use in connection with interpoles ECE 441

  14. Compensating Windings -- Generator ECE 441

  15. Compensating Windings -- Motor ECE 441

  16. DC Machine with Interpoles and Compensating Windings ECE 441

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