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When a D.C. series motor is connected to A.C. supply, the power factor will be low because of
The fine copper wire winding
The induced current in rotor due to variations of flux
High inductance of field and armature circuits
None of the above
High inductance of field and armature circuits
Quick Trick: In AC circuits, reactance XL equals 2 * ╧А * f * L. Since DC series motors have large field and armature windings, their high inductance causes massive AC reactance, driving the power factor (cos phi = R / Z) to a very low value.
In AC circuits, reactance XL equals 2 * ╧А * f * L. Since DC series motors have large field and armature windings, their high inductance causes massive AC reactance, driving the power factor (cos phi = R / Z) to a very low value.
Analyze AC Reactance Effect
When an AC voltage is applied to a DC series motor, the alternating current encounters inductive reactance XL = 2 * ╧А * f * L in both the field winding and the armature winding, which are connected in series.
Relate Inductance to Power Factor
Because both circuits have high inductance L, total inductive reactance XL becomes very high compared to resistance R. The power factor is given by cos phi = R / Z = R / SQRT(R┬▓ + XL┬▓). As XL dominates, the power factor drops significantly.
A: Fine copper wire alters resistance but does not cause the severe inductive voltage drop or low power factor. B: Rotor induced currents cause eddy losses and heating, not primarily the overall circuit low power factor. D: Incorrect because Option C accurately states the primary reason.
C is correct because high inductance in the series field and armature windings creates massive inductive reactance under AC supply, causing a severe phase lag and low power factor.
Always link AC supply drawbacks in DC machines to frequency-dependent parameters: inductance creates high reactance (XL = 2*╧АfL), reducing current and power factor, while changing flux causes eddy current and hysteresis losses.