VCAA 2011 Exam 2 question

[This-is-not-me]

Can someone please help me with this question?

[Stevensmay]

We have an DC generator. Has a commutator and is being turned by hand.

A DC generator will produce a sine wave output if it weren't for the commutator. But we have one.
A commutator reverses which wires the coil is connected to every half revolution.

An generator without a commutator would produce a full sine curve. But since the commutator reverses the wires every half revolution, the direction of the voltage every half turn is also flipped. This gives us a sine curve except any negatives bits have been reflected into the positive y-axis.

This gives us an answer of A.

Option B is wrong as that is the output of an AC generator, no commutator.
Option C is constant voltage which we know does not happen as the flux is not constant.
Option D is wrong as it implies that the voltage ceases to exist for half a revolution, which is of course untrue.

I will try and do a better answer in the morning if this is not clear.

[This-is-not-me]

Thanks a lot Stevensmay appreciate it.  So basically a DC generator always produces a full sine wave output if no commutator is present?

[Stevensmay]

Thanks a lot Stevensmay appreciate it.  So basically a DC generator always produces a full sine wave output if no commutator is present?

Well if a commutator isn't present it is an AC generator, and we know AC is a sine wave.
Commutator changes the connections every half revolution.

If it's missing and the wires stay the same, the voltage output will be positive and then go to negative as it reaches half way. This just repeats and we have our sine curve.

[This-is-not-me]

Oh ok thank you