The power output decreases with speed because of the constant-torque loss due to detent torque and other losses. The same effect causes a slight decrease in torque with speed in the constant torque region as well. Finally, there is a rounding of the torque curve at the corner speed because the drive gradually transitions from being a current source to being a voltage source. The drive limits current to the motor below the corner speed and thus is a current source. Above the corner speed, the motor’s inductive reactance limits the current and the drive becomes a voltage source as it applies all of the power supply voltage to the motor.
At this point it is important to introduce the concept of overdrive ration. This is the ration between the power supply voltage and the motor’s rated voltage. An empirically derived maximum is 25:1, meaning the power supply voltage should never exceed 25 times the motor’s rated voltage or 32 times the square root of motor inductance. Below is a graph of measured iron losses for a 4A, 3V motor. Notice in Figure 16 how the iron losses range from insignificant to being the major cause of heating in the motor compared to a constant 12W copper loss (4A times 3V).
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