A $$dc$$ motor with armature resistance $${R_a}$$
is fed from a step down chopper in the
continuous mode, and operates at some known speed and known excitation current. The motor current rises from $${I_{\min }}\,\,$$ to $${I_{\max }}\,\,$$ in the $${T_{on}}$$ period Ton of the chopper; and drops from $${I_{\max }}\,\,$$ to $${I_{\min }}\,\,$$ in the $${T_{off}}$$ period Toff of the same circuit. Both the rise and fall of the current may be assumed to be approximately linear.
What is the average power loss in the machine armature?
Answer
Average power loss $$ = {\left( {{{{{\rm I}_{\max }} + {{\rm I}_{\min }}} \over 2}} \right)^2}\,\,{R_a}$$
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