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? 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 ?
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?