Question

A d.c. 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 36.69 Q

Answer 1

To adjust the field current to 1.6 A when starting from cold at 20°C, an extra circuit resistance of 1.23 Ω is required.

Step-by-step explanation:

To adjust the field current to 1.6 A when starting from cold at 20°C, we need to find the value of extra circuit resistance. The formula to calculate the extra resistance is:

Rextra = Rt × (1 + α(ΔT)) - Rt

Where,

Rextra is the extra circuit resistance

Rt is the resistance at the temperature at which you want to adjust the field current

α is the temperature coefficient of resistance

ΔT is the difference in temperature

In this case, Rt = 1.6 A, α = 0.0043/°C, ΔT = 40°C - 20°C = 20°C

Substituting these values into the formula, the extra circuit resistance is:

Rextra = 1.6 × (1 + 0.0043 × 20) - 1.6

Rextra = 1.23 Ω