Torque calculation for a sample data:
Speed N= 850 rpm
For speed 850rpm- Eb = 0.96V
Therefore power P = Eb*Ia = 0.96*0.17 = 0.1632 watts.
T= P*1.019 * 10^4*60/2piN
T = 0.1632*1.019*10^4 *60/2*3.142*850
T = 18.68 Gm-cm
Compared with induction motor, a ac servo motor does not conform to the characteristic, namely high inductance to resistance ratio. Instead, it is featured with:
1. Linear torque-speed characteristics.
2. Negative slope of the torque-speed characteristics.
3. Be parallel to one another for various values of the control voltage applies.
It is obvious that the torque-speed is becoming more linear with the increasing of rotor’s resistance. When in a low inductance to resistance ratio, the torque-speed characteristics are almost linear.
Actually, motor with almost linear torque-speed characteristics is well designed. Only when encountered with a detectable error at a low voltage, can it smoothly rotates. And It rarely works at high speed. Hence for a given value of control voltage, torque-speed characteristics are almost linear. The following fig illustrates the torque-speed characteristics for various control voltages.
All the characteristics are equally spaced for equal increments of control voltage as shown.