**Imagine an ideal brushed DC motor whose +/- coil wires are directly connected to an adjustable DC voltage power supply, and whose output shaft is connected to a dynamometer capable of maintaining a constant torque.
The power supply is adjusted to 12 volts applied to the motor, and the dynamometer is set to maintain a constant load of To ounce inches on the motor’s output shaft, and as a result the motor spins at Wo RPM and the motor is drawing Io Amps. Assume this operating point is well within the motor’s safe continuous-duty operating range.
Without changing anything else, the applied voltage is adjusted down to 10 volts. Assume the motor is still spinning at this lower voltage. Ignoring secondary effects such as windage and bearing friction etc in the motor, to a first approximation what happens to the motor speed and current due to the decrease in applied voltage?
a) speed decreases and current decreases
b) speed decreases and current stays roughly the same
c) speed decreases and current increases
d) speed increases and current decreases
e) speed increases and current stays roughly the same
f) speed increases and current increases**
*** assume free speed current is zero Amps**