Electrical Machines
A commutator in DC machines can 1. provide half-wave rectification 2. Provide full-wave rectification 3. convert ac to dc 4. Convert dc to ac 5. provide controlled full-wave rectification From these, the correct answer is

 2, 3, 4
 2, 3, 4, 5
 2, 3 ,5
 1, 2, 3

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Electrical Machines
Short circuit test is performed on a transformer with a certain impressed voltage at rated frequency. If the short circuit test is now performed with the same magnitude of impressed voltage, but at frequency higher than the rated frequency then

 The magnitude of current will decrease but the power factor will increase.
 The magnitude of circuit as well as the power factor will decrease.
 The magnitude will both increase.
 The magnetude of current will increase, but power factor will decrease.

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