Question

A dc shunt motor after running for several hours on constant voltage mains of 400 V takes a field current of 1.6 A. If the temperature rise is known to be 40°C, what value of extra circuit resistance is required to adjust the field current to 1.6 A when starting from cold at 20°C? Temperature coefficient = 0.0043/°C at 20°C.

Answer 1

resistance:

$R=R_{40}(1+\alpha(T-40))$

resistance for 40°C:

$R_{40}=400/1.6=250\ \Omega$

then resistance at 20°C:

$R=250(1+0.0043(20-40))=228.5\ \Omega$

so, value of extra circuit resistance:

$250-228.5=21.5\ \Omega$