When driven at 1500 rev/min, the open-circuit armature voltage of a d.c. machine

Question

When driven at 1500 rev/min, the open-circuit armature voltage of a d.c. machine is 110 V a) Calculate the e.m.f. constant. b) Suppose the machine was at rest and a block of mass 5 kg was suspended [5 marks] from a horizontal bar (neglect its mass) of length 0.80 m attached to the shaft (see Figure 1 below). What current must be applied to the armature to keep the arm horizontal? Will the equilibrium be a stable one? Justify your answer [10 marks] c) When the machine runs as a motor drawing 25 A from a l10 V supply, the speed is 1430 rev/min. Calculate the armature resistance Hence find the voltage needed to keep the same bar horizontal. [10 marks] Figure 1 Diagram for Q2(b and c) Turn over 3 of 7

Explanation / Answer

Answer:

We know that relation between emf constant and emf is given by following relation:

E = ke

[where, E = 110 V; ke = emf constant; = angular speed in rad/s = 2N/60 = 2*3.14*1500/60 = 157 rad/s]

Thus, 110 V = ke*157 rad/s

=> ke =110/157 V/rad/s

=> ke = 0.70 V/rad/s (emf constant) Answer(a)

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