The present invention relates to means for the measurement of torsional or rotational acceleration of a rotating body such as a shaft, and more particularly to the measurement of oscillatory starting and accelerating torques in rotating machines such as synchronous motors.
Synchronous motors are widely used for many types of drives because of their good efficiency and constant speed as well as the ability to provide power factor correction. The use of a synchronous motor, however, presents certain problems in the design of the driven system because of the nature of the starting torque of such motors. A synchronous motor, as such, has no starting torque and these motors are normally started as induction motors. The direct current field excitation is then applied when the speed approaches synchronism so that the motor pulls into step and runs at synchronous speed. Such motors are almost always of salient pole construction and the airgap torque during initial operation as an induction motor is, therefore, pulsating or oscillatory. When starting from standstill, the frequency of oscillation is initially twice the line frequency, and decreses to zero as the motor accelerates and finally pulls into synchronism, when the torque reaches a steady-state value. Thus, during the starting period, a range of frequencies extending from zero to twice the line frequency occurs and any natural resonant frequencies of the load or driven system in this range may be excited during this start-up period leading to undesirable and excessive vibration. The motor starting torque characteristics and the torsional response of the driven system can be analytically determined in many cases, but it is often desirable to make actual measurements of the magnitude and frequency of oscillation of the motor torque during the starting period, so that the design of the driven system can be properly coordinated with the motor characteristics and undesirable resonance and vibration problems avoided.