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A 230 V shunt direct-current machine on no-load has a speed of 900 rpm and a supply current of 2.5 A. The machine parameters are: • Field resistance: 230 Ω • Armature resistance: 0.3 Ω • Voltage drop/...

A 230 V shunt direct-current machine on no-load has a speed of 900 rpm and a supply current of 2.5 A. The machine parameters are:
• Field resistance: 230 Ω
• Armature resistance: 0.3 Ω
• Voltage drop/brush: 1.0 V (current independent)
a) Calculate the electromagnetic torque, speed, the output power and the overall efficiency when the supply current is 35 A, assuming that the friction and wind-age torque is constant. (16 points)
b) The machine operates now as generator and is driven to generate 35 A into the supply at 230 V and at 900 rpm. Determine the overall efficiency. (6 points)
c) Calculate the extra field resistance required to make to make the transition pos-sible from motor operation to generator operation. (3 points)

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