Armature Reaction and Commutation Questions and Answers

Answer: A

Correct answer: the current density is high and the voltage low

Metal-graphite brushes contain copper or another metal, so their resistivity and their contact voltage drop are much lower than plain carbon. That suits a machine carrying a large current at a low voltage, such as a plating or traction machine, where the loss in a carbon brush would be a large fraction of the output.

Answer: B

With a fixed drop of about a volt per brush, the power lost is that drop times the current, which is a linear rather than a square law. On a low-voltage machine those two volts are a significant fraction of the output.

Answer: C

Correct answer: rapid wear of the brush and the commutator

Excessive pressure increases the friction and the mechanical work at the contact, so both the brush and the copper wear quickly and the machine runs hot. Too little pressure gives the opposite fault - the brush bounces, contact becomes intermittent and sparking follows - so the pressure is specified within a range.

Answer: D

Correct answer: a thin film of copper oxide and carbon

The patina is a thin oxide film with carbon from the brushes embedded in it, and it forms in service on a commutator that is running correctly. It lubricates the contact and provides the resistance that assists commutation, so it should be left alone - polishing a commutator back to bare copper does real harm.

Answer: A

Correct answer: a fault in the armature winding at that spacing

Marking that repeats at the pole pitch corresponds to particular coils rather than to a general condition, since those are the coils commutating at the same points. It therefore points at the winding - a high-resistance joint at the riser, or a shorted turn - rather than at the brushes or the interpoles, which would mark uniformly.

Answer: B

Correct answer: abrasive contamination or an unsuitable brush grade

Grooves cut into the copper mean something abrasive is passing between brush and commutator: dust drawn in with the cooling air, or a brush grade harder than the application calls for. The remedy is to clean the air and to change the grade, since resurfacing without doing so simply repeats the damage.

Answer: C

Correct answer: its visible degree, from none to heavy with burning

Sparking is graded by inspection, from none through pin-point sparking at part of the brush edge, to sparking at the whole edge, to heavy sparking with visible burning and blackening. The first two grades are acceptable in continuous service; the last two are not and call for investigation.

Answer: D

Interpoles in series with the armature track the load almost instantly, but a compensating winding that is absent or wrongly proportioned leaves the flux distorted during a surge. A machine with rapid load changes therefore needs pole-face compensation.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: with the brushes on the geometric axis the armature field is entirely across the main field, and shifting them converts part of it into opposition. Statement 3 attributes the wrong remedy - interpoles sit between the main poles and correct the commutating zone, while it is the compensating winding, embedded in the pole faces, that cancels the distortion under them.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

Three remedies for three distinct problems. The interpole induces an emf cancelling the reactance voltage in the coil being commutated. The compensating winding, in slots in the pole faces, cancels the armature’s distortion of the flux under the poles. And the brush grade supplies the contact resistance that assists the reversal.