Question

A dc motor running at 2000 rpm has a hysteresis loss of 500 W and eddy-current loss of 200 W. The flux is maintained constant, but the speed is reduced to 1000 rpm. At the reduced speed the total iron-loss would be
1. 500 W
2. 400 W
3. 300 W
4. 200 W

Answer 1

Correct Answer - Option 3 : 300 W

N₁ = 2000 rpm

Hysteresis loss \({p_{h{L_1}}} = 500\;W\)

Eddy current loss \({p_{e{L_2}}} = 200\;W\)

Now, flux is constant, and the speed is N₂ = 1000 rpm

Hysteresis loss, \({p_{hL}} \propto f \propto N\)

\( \Rightarrow \frac{{{p_{h{L_1}\;}}}}{{{p_{h{L_2}}}}} = \frac{{{N_1}}}{{{N_2}}}\)

\(\Rightarrow {p_{h{L_2}}} = 250\;W\)

Eddy current loss \({p_{eL}} \propto {f^2} \propto {N^2}\)

\(\frac{{{p_{e{L_1}}}}}{{{p_{e{L_2}}}}} = \frac{{N_1^2}}{{N_2^2}}\)

⇒ p_eL = 50 W

Total iron loss \(= {p_{h{L_1}}} + {p_{e{L_2}}} = 300\;W\)