A 120 V DC shunt motor takes 2 A at no load and 7 A at full load, where it runs at 1200 rpm. Armature resistance is 0.8 Ω and field resistance is 240 Ω. Its no-load speed in rpm is ________.
The field current is 120/240=0.5 A in both conditions. Armature currents are therefore 1.5 A at no load and 6.5 A at full load.
The corresponding back emfs are 120−1.5(0.8)=118.8 V and 120−6.5(0.8)=114.8 V. With unchanged field flux, speed is proportional to back emf.
Thus $n_0=1200(118.8/114.8)=$ **1241.81 rpm**, within 1235–1250.
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