28:- (d) the cohesive energy, which is the energy needed to take the solid apart into positive and negative ions, is relatively high for ionic solids. Therefore, they should be poor electrical conductors since there are no free elections available. They should be hard and stable with high vaporization temperatures and high melting points.
29:- (b) Peaked- flat topped.
30:- (a) When a slip becomes negative, the induction machine acts as a generator, therefore, the rotor speed should be more than the rated synchronous speed. Further, as the load increases, slip magnitude should increase as the power output is approximately proportional to slip magnitude for small slips. Therefore, speed will have to be increased.
31:- (b) It can be controlled by tapping the field at various points.
32:- (a) M =
M : Moment of inertia of the machine
: Second time derivative of the load or the torque angle which turns out to be same
as rotor acceleration.
: Shaft power
: Electrical power output
Since 0 on occurrence of a short circuit,
We have =
33:- (d) In a series motor, the torque is approximately proportional to current squared. The train resistance speed curve is such that there is very little increase in torque with speed; we can assume that torque remains constant. Therefore, the armature current remains constant with increase in applied voltage, the back emf reduces, which is achieved by a drop in speed.
34:- (d) Ball mill is used in power plants using pulverized fuel firing.
35:- (c) Let the current flow through resistance when – 12V Battery is removed. So,
Similarly current flow in resistance When + 12 V battery is removed. So,
Since the two voltages are in opposite directions, their currents will subtract. Hence,
= 10 x
12(1000x1000+) – 12(470x1000+)
10x (1000x1000+) x(470x1000+)
+ 1470x1000– 16.6x =0