91Ó°ÊÓ

a. In Fig. 38 determine the polarity of \(E_{34}\) between commutator segments 3 and 4 , knowing that the armature is turning clockwise. b. At the same instant, what is the polarity of segment 35 with respect to segment 34 ?

The armature shown in the following figure has 81 slots, and the commutator has 243 segments. It will be wound to give a 6-pole lap winding having 1 turn per coil. If the flux per field pole is \(30 \mathrm{mWb}\), calculate the following: a. The induced voltage at a speed of \(1200 \mathrm{r} / \mathrm{min}\) b. The average flux density per pole c. The time needed to reverse the current in each armature coil, knowing that the brushes are \(15 \mathrm{~mm}\) wide and that the diameter of the commutator is \(450 \mathrm{~mm}\)

A \(200 \mathrm{~W}, 120 \mathrm{~V}, 1800 \mathrm{r} / \mathrm{min}\) dc generator has 75 commutator bars. The brush width is such as to cover 3 commutator segments. Show that the duration of the commutation process is equal to \(1.33 \mathrm{~ms}\).

A \(240 \mathrm{~kW}, 500 \mathrm{~V} 1750 \mathrm{r} / \mathrm{min}\) separately excited dc generator has an overall efficiency of \(94 \%\). The shunt field resistance is 60 ohms and the rated current is \(5 \mathrm{~A}\). The \(I^{2} R\) loss in the armature is \(0.023 \mathrm{pu}\). a. the rated armature current b. the total losses in the machine c. the \(I^{2} R\) losses in the armature

The generator in Problem 24 weighs \(2600 \mathrm{lb}\). Calculate the output in watts per kilogram.

A 4-pole dc generator delivers a current of 218 A. The average brush voltage drop on each of the four brush sets is found to be \(0.6 \mathrm{~V}\). Calculate the total brush loss in the machine, neglecting friction loss.