What happens to the torque/speed curve of a brushless motor when I increase the voltage?

Question

Here is a an image of the torque/speed curve. My guess is that increasing voltage will simply translate the curve up because the slope is the torque constant, which is a physical motor property that will not change with added voltage. I know from this post that increasing voltage will increase stall torque:

Does increasing voltage increase the torque of a brushless motor?

So does the increasing voltage not only increase the stall torque, but also the overall range of torques the motor can achieve? In other words, the same RPM now correlates to a higher torque in the higher voltage curve?

Additionally, if the above is true, here is another question. If the original stall torque in the lower curve is 2.5 N-m, but the stall torque of the higher curve is 3.5 N-m, does this mean that with the added voltage I can now run the motor at 2.5 N-m more safely than I could before since it is no longer the stall torque? Or is the power dissipation the same in both curves so that 2.5 N-m will still be dissipating the same heat? So, if the motor was going to burn up at 2.5 N-m, it will still burn up at 2.5 N-m at the higher voltage input?

Answer 1

You characteristics is missing a straight horizontal line, which is called as nominal torque. Even if you increase the voltage, the nominal torque is still the same.

The torque is directly correlated with current. Therefore the nominal torque is related to the nominal current which is a continuous current that a motor can accept to maintain the temperature within working conditions. Small peaks are allowed as long the overall utilisation remains below the nominal curve. What you gain with higher voltage is the higher speed at nominal torque, see the double line.

Green, yellow and red rapresents load torque curve. The green one is acceptable for both voltages, the yellow would be good for low voltage but overloaded for hiher voltage, the red curve represents overload for both voltages.

The final speed is the point where torque curves (your red lines vs load curves) intersects.

Answer 2

The torque vs. speed curves presented are the curves for a permanent-magnet, brushed DC motor. The curves are valid for a brushless DC (BLDC) motor to the extent that the BLDC motor controller makes the motor perform like a brushed motor. That is generally the performance provided by many hobby motor-controller combinations. What is missing is an indication of the thermal limits and the control limits.

There is a safe operating torque for continuous operation. That corresponds to the safe operating current for continuous operation. Operation above the safe operating limits for continuous operation is safe for some time. For industrial DC brushed motors, the limit is often 150% of rated current for one minute. For hobby motors, the limits are not consistently published.

The voltages and currents that are often published for BLDC motors are the values for the input to the controller. The controller also likely has a minimum input voltage. That will set the limit of the downward movement of the curve.