Q4. a) What design factor limits the maximum torque of a d.c. motor? (6)
(b) A shunt motor runs on no load at 700 r/min off a 440 V supply. The resistance of the shunt circuit is 240Ω. The following table gives the relationship between the flux and the shunt current:
Shunt current (A): 0.5 0.75 1.0 1.25 1.5 1.75 2.0
Flux per pole (mWb): 6.0 8.0 9.4 10.2 10.8 11.2 11.5
Calculate the additional resistance required in the shunt circuit to raise the no-load speed to 1000r/min (10)
Q5. (a) On what factors does the capacitance of a parallel-plate capacitor depend? (6)
(b) A tuned circuit consisting of a coil having an inductance of 200µH and a resistance of 20Ω in parallel with a variable capacitor is connected in series with a resistor of 8000Ω across a 60 V supply having a frequency of 1 MHz. Calculate: (10)
(a) The value of C to give resonance.
(b) The dynamic impedance and the Q factor of the tuned circuit.
(c) The current in each branch
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