The iron losses in a d.c. motor depends upon _____________

Question

Examoogle · Accepted Answer

Both flux and speed

Explanation: Iron losses (also known as core losses) in a DC motor consist of hysteresis and eddy current losses occurring in the armature core. Since these losses are functions of magnetic flux density and the frequency of magnetic reversals, they directly depend on both the operating magnetic flux and the motor speed.

Working Principle: Hysteresis loss ($P_h = k_h B_{max}^{1.6} f$) and Eddy current loss ($P_e = k_e B_{max}^2 f^2 t^2$) are both dependent on the frequency of flux reversals ($f$), which is proportional to speed ($N$). Additionally, they depend on the flux density ($B$), which is proportional to the flux per pole ($Phi$). Thus, the total iron loss $P_i$ varies with both $\Phi$ and $N$.

Key Formulas: $P_h = k_h B_{max}^{1.6} f$ — Hysteresis loss formula; $P_e = k_e B_{max}^2 f^2$ — Eddy current loss formula; $f = \frac{PN}{120}$ — Frequency of flux reversals

Key Points: Iron losses are constant for a specific speed and excitation.; Eddy current losses are proportional to the square of the speed.; Hysteresis losses are proportional to the speed (first power).; Increasing flux increases $B_{max}$, which significantly increases iron losses.

Advantages: Predictable in constant-speed applications; Independent of load variations

Disadvantages: Results in heating of the armature core; Reduces overall machine efficiency

Applications: Calculating machine efficiency; Thermal rating analysis of DC motors

Additional Info: Iron losses are typically grouped with mechanical losses under 'Constant Losses' or 'Stray Losses'.. Option A and B are incomplete because iron loss is a multifaceted phenomenon involving magnetic domain flipping and induced currents, both of which rely on frequency (speed) and magnetic field strength (flux).

Answer: C is correct — Iron losses depend on both flux and speed because hysteresis and eddy current losses are functions of magnetic flux density and the frequency of magnetic field variation.

Exam Tip: In DC machine questions, always remember that iron losses are 'constant losses' at a fixed voltage and speed, but vary if the operating point (Speed or Flux) changes.

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