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 ECE 441 1

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 2

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

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

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

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

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 7

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

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 9

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

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

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

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 13

Compensating Windings -- Generator ECE 441 14

Compensating Windings -- Motor ECE 441 15

DC Machine with Interpoles and Compensating Windings ECE 441 16