I get that when maximum power is used, there is a resistance of R = V^2/P = 230^2/80 = 661.25 Ω. This must therefore be a internal resistance that is active even when the varying resistance is inactive (i.e. when the pedal is not pushed).Grandma Greta has an electric sewing machine that she would like to donate to her granddaughter Stina. Stina think that the lowest speed of the machine, when the pedal is pressed only so that the machine starts, is good. However, the highest speed, when the pedal is at the bottom, is far too high.
The power of the sewing machine motor is 80 W at 230 V. The motor can be regarded as purely active, and its speed is controlled by a series resistor in the pedal. This resistance range from 550 Ω at the lowest speed to 0 Ω at the highest speed.
By fitting the two resistances at the pedal, the engine power can be halved at the highest speed without affecting the lowest speed. Help Stina to determine how these resistors should be connected as well as their resistance.
If the task was only to half the maximal power we could just connect another 661.25 Ω-resistance in series, but how are we supposed to do so that the lowest speed will not be affected?