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Select the problems caused during if we provide AC supply to a DC series motor?
Its efficiency will poor due to hysteresis and eddy current losses
The power factor of the motor is low due to inductive reactance of the field and armature windings
The sparking at the brushes will be excessive due to the effect of armature reactance
All of these
All of these
Quick Trick: When an AC signal is applied to a DC machine, multiple issues arise simultaneously (magnetic core losses, inductive reactance drop, and commutation/sparking issues). Seeing multiple valid core physical problems instantly points to 'Al...
When an AC signal is applied to a DC machine, multiple issues arise simultaneously (magnetic core losses, inductive reactance drop, and commutation/sparking issues). Seeing multiple valid core physical problems instantly points to 'All of these'.
Analyze Core Loss Effects
Alternating current (AC) causes continuous reversals of magnetic flux in the unlaminated or thicker iron parts of a DC series motor. This leads to heavy hysteresis losses and large eddy currents, significantly reducing efficiency.
Analyze Reactance and Power Factor
Both the armature and field windings have significant inductance. On AC supply, the inductive reactance (XLтАЛ = 2*╧АfL) causes a large voltage drop and introduces a large phase lag, resulting in a very low overall power factor.
Analyze Commutation and Sparking
The alternating magnetic field induces a transformer EMF in the armature coils undergoing commutation. Combined with reactance voltage, this causes severe, destructive sparking at the brushes.
A: True statement, but not the only problem. B: True statement, but not the only problem. C: True statement, but not the only problem.
D is correct because operating a standard DC series motor on AC supply simultaneously causes high core losses, poor power factor, and heavy brush sparking.
Questions listing multiple valid physical consequences almost always point to 'All of the above'. Remember that AC motors modified from DC series motors are called AC Series / Universal motors, designed with laminated yokes and compensating windings to overcome these exact three issues.