Question

A PMDC machine is measured to va120Vdc, ia 5.5A at the electrical terminals and the shaft is measured to have Tmech 5.5Nm, ns1200RPM. State whether the machine acting as a motor or a generator and state the efficiency of the system. Q11. Motor, 89.4 %(a) Generator, 89.4%(b) Motor, 95.5%(c) Generator, 95.5 %(d) none of the abov

Answer 1

Answer:

(c) Generator, 95.5 %

Explanation:

given data

voltage va = 120V

current Ia = - 5.5 A

Tmech =  5.5Nm

ns = 1200 RPM

solution

first we get here electric input power that is express as

electric input power = va × Ia    ......1

put here value and we get

electric input power = 120 × -5-5

electric input power -660 W

here negative mean it generate power

and here

Pin ( mech) will be

Pin ( mech) = Tl × ω   .........2

Pin ( mech) = 5.5 × $\frac{2\pi N}{60}$    

Pin ( mech) =  5.5 × $\frac{2\pi 1200}{60}$  

Pin ( mech) = 691.150 W

and

efficiency will be here as

efficiency = $\frac{660}{691.150}$    

efficiency = 95.5 %

so correct option is (c) Generator, 95.5 %