Servomotor Part 3-4 The torque-speed characteristics of a d.c. motor basically describes the static torque producing capability of the motor with respect to applied voltage and motor speed. The effect of armature inductance is negligible in the steady state and hence the torque equation for a d.c. servomotor can be written as T m = K m I a (flux is constant) where K m = Motor constant ω m = Speed in rad/sec V a = Applied armature voltage From the above equation (IV), it can be seen that the graph of T m against ω m is a straight line which is dropping in nature, for constant value of V a . This line represents torque-speed characteristics of a d.c. servomotor, for constant V a . Fig. 3 shows a typical set of torque-speed characteristics for various values of applied voltage. Fig. 3 Torque-speed characteristics of d.c. servomotor The slope of the characteristics can be obtained from the equation (IV), which is expressed as, The slope of such characteristics is negative. The torque-speed characteristics of d.c. servomotors are determined experimentally in practice, by conducting various tests on the servomotor. 1.1 Other Performance Characteristics The performance characteristics other than the torque-speed characteristics of a d.c. servomotor are shown in the Fig. 4. Fig. 4 Performance characteristics of a typical d.c. servomotor As torque developed increases, the current and power input increases while the efficiency increases upto certain limit and then starts decreasing.
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