Question

(a) By how many percent is the torque of a motor decreased if its permanent magnets lose 3.9% of their strength? %(b) How many percent would the current need to be increased to return the torque to original values? %

Answer 1

a) The torque of a DC motor is directly proportional to the flux of the magnetic field, thus, when we reduce the flux (strength of permanent magnet) by 3.9%, we also reduce the torque by 3.9%

b) The torque of the DC motor can also be written as T=k*phi*I, where I is the current on the static part of the assembly. Thus, in order to maintain this relation, if the new phi is 0.961 times the old phi, the new current will need to be 1/0.961 times the old current, and thus 4% higher