A variable d.c. drive system is used to drive a fan which has the load characteristic T₁ x n², where T₁ is the load torque and n is the mecha- nical speed in rpm. When the motor runs at 600 rpm the back emf generated is 120 V and the current drawn by the armature is 28 A. Calculate the external resistance that must be added to the armature circuit to reduce the speed of the motor to 400 rpm. Neglect all electrical and mechanical losses of the motor including the armature resistance loss. [Ans: 3.21 22]

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A variable d.c. drive system is used to drive a fan which has the load
characteristic T₁ x n², where T₁ is the load torque and n is the mecha-
nical speed in rpm. When the motor runs at 600 rpm the back emf
generated is 120 V and the current drawn by the armature is 28 A.
Calculate the external resistance that must be added to the armature circuit
to reduce the speed of the motor to 400 rpm. Neglect all electrical and
mechanical losses of the motor including the armature resistance loss.
[Ans: 3.21 22]
Transcribed Image Text:A variable d.c. drive system is used to drive a fan which has the load characteristic T₁ x n², where T₁ is the load torque and n is the mecha- nical speed in rpm. When the motor runs at 600 rpm the back emf generated is 120 V and the current drawn by the armature is 28 A. Calculate the external resistance that must be added to the armature circuit to reduce the speed of the motor to 400 rpm. Neglect all electrical and mechanical losses of the motor including the armature resistance loss. [Ans: 3.21 22]
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