Armature Reaction and Commutation Questions and Answers

Answer: A

Correct answer: providing a large number of segments

The voltage between adjacent segments is the total armature voltage divided among them, so more segments means less between any two. Keeping that figure low - conventionally below about 30 volts - is what prevents flashover across the commutator, and it is one of the constraints setting the size of a dc machine.

Answer: B

A surge of current distorts the flux badly and raises the voltage between segments, and an arc drawn at a brush can then track right round the commutator. Fast overcurrent protection and pole-face compensation both guard against it.

Answer: C

The interpoles look after the commutating zone and the pole-face winding looks after the distortion under the poles, and a mill or a hoist drive needs both. Together they nearly remove armature reaction as a practical concern.

Answer: D

A coil short-circuited by a brush while sitting in the main flux would carry a large circulating current, so it must be commutated where the flux is zero. That is why the neutral axis matters and why its shift with load is such a nuisance.

Answer: B

The most suitable material for a commutator is copper.

Commutator segments must carry the armature current with little loss and resist wear from the brushes, so they are made of hard drawn or drop-forged copper, insulated from each other with mica.

Cast steel and cast iron have far higher resistivity and would heat and spark badly. Carbon is the material of the brushes that slide on the commutator, not of the commutator itself.

Answer: A

Commutator segments are made of hard drawn copper.

The commutator must conduct the full armature current with little loss and also stand up to the continuous rubbing of the brushes and the centrifugal force at speed. Hard drawn (or drop-forged) copper has the conductivity of copper with much greater hardness and wear resistance, and the segments are insulated from each other by mica.

Ordinary annealed copper is too soft and wears and deforms quickly, which is why the plain 'copper' option is not the expected answer. Brass and aluminium have higher resistance, and aluminium is too soft for a sliding contact.

Answer: B

In a 230 V dc machine the brushes are made of electrographite.

Electrographite brushes are carbon baked at a very high temperature so that it turns to graphite. They are self-lubricating, wear slowly, and have a fairly high contact resistance, which helps commutation by limiting the current in the coil being short-circuited by the brush. That makes them the usual choice for ordinary industrial dc machines at a few hundred volts.

Copper and copper-graphite (or copper-carbon) brushes have a low contact drop and suit low-voltage, high-current machines such as electroplating and welding generators; on a 230 V machine they would cause sparking and rapid wear of the commutator.