In the below two diagrams, the Intermittent Torque Zones are different, figure 1 is triangle and figure 2 is rectangle. I don't know which one is correct, is there something related to motor's rated voltage? enter image description here enter image description here Here is my analyze:

  • τ = kt * I; I is phase current, τ is torque, kt is constant
  • Pin = U * I; Pin is input power, U is phase voltage
  • Pin = τ * ω + Ploss(I); ω is speed, assumming power loss is proportional to I(only heat loss)

Let's analyse the Intermittent Torque Zone in figure 2: Pin = τ * ω + Ploss(I), When "ω" increases toward rated speed, "τ" keeps constant and "I" keeps constant due to "τ", so Pin increases; Pin = U * I, because "Pin" increases and "I" keeps constant, so "U" increases. Now the question is changed to, if "I(phase current)" keeps constant and "U(phase voltage)" increases, will it burn the motor with a short time peak torque in figure 2 ? enter image description here

To answer this question, let's look at the motor coil's equivalent circuit, U = I * R + L(ω) + E(ω), "I" is constant so it can not burn "R", but "U" increases as "ω" increases toward rated speed in figure 2. Is there a limit for the voltages on "L(ω)" or "E(ω)(Back EMF)" in case it might degrade the motor?

  • Should they be under motor's rated voltage?
  • Which figure is correct for Intermittent Torque Zone?

Thank you.


Brushless motors are rated by attaching thermocouples to the stator windings and measuring maximum temperature while under load. The motor is attached to a dynamometer and the torque is increased until the steady-state temperature rise is equal to the maximum allowable temperature rise of the insulation system that is used in the motor. This is generally performed at rated voltage and rated speed. The intermittent torque zone just means that you can't run the motor in that area continuously without the motor getting too hot for the insulation system.

There are a number of reasons that the speed-torque curves in Figs. 1 and 2 look different. Figure 1 probably tested the motor at rated voltage and full speed and then let the torque pull the speed down. The intersection between rated speed and rated torque is the point where their motor's steady state temperature increase equaled the rated temperature rise of the insulation system. Then they probably just assumed that the rated torque holds for all speeds. This is a conservative assumption because in reality your rated torque should slightly increase as your speed decreases.

Figure 2 probably did maximum temperature tests at different speeds and that is why the line separating the continuous and intermittent torque zones isn't horizontal. The reason Fig. 2 has a rectangle shape is because the are limiting the torque in some way. In general if you continue to increase the torque on a brushless motor it will decrease the speed like in Fig. 1, forming a triangle shape. However, if you are current limited (say you have a current limit on your control) then that will prevent you from getting a speed-torque curve that takes the triangle shape and you will get the rectangle shape as in Fig. 2.

So my best guess is that Fig. 1 is showing you the speed-torque curve of a brushless motor with no current limit and Fig. 2 is showing you the speed-torque curve of a brushless motor with a current limit.

  • \$\begingroup\$ Yup, Figure 1 seems to show constant-voltage curve (no current limitation bar the winding resistance) while Figure 2 has a current limit. Reason? Perhaps a weak shaft not designed to handle as much torque as would be generated under constant voltage condition. \$\endgroup\$ – SunnyBoyNY Nov 5 '14 at 20:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.