INTRODUCIION
A new measurement technique for the determination of the
torque-speed characteristics of induction motors was developed
recently in the Power Research Laboratory at McMaster University
in response to a request from Westinghouse Canada Inc.,
Motors Division [l]. The need for this development became necessary
when the accuracy of the torque-speed profiles for two, three
phases, 2@hp, induction motors was in question. Specifically, the
value of the minimum torque at an approximate speed of one third
rated speed, obtained from the graph shown in figure 1 was
doubted. The torque-speed charactesistics illustrated in figure 1 for
one of the motors was obtained using conventional equipment and
an analog differentiator. The oscillations in the curve, particularly
in the region of minimum torque, obfuscated the actual torque. A line drawn along the middle of the oscillations would indicate tha
the minimum torque of this motor is at least 160% of the rated
torque. This is in accordance with the specifications for this motor.
However, the user experienced difficulty bringing the machine to
full load speed. The motor stalled before it reached 600 rpm. The
user was led to believe that the lower envelope of the oscillation
should be considered the representative characteristic of the motor.
Clearly, a new measurement technique was required. It must be
capable of producing a smooth torque-speed profile without distorting
the actual characteristics of the machine. The problem was
to eliminate the need for interpolation of the digitally differentiated
analog signal. The analog signal is itself very noisy due to
coupling noise, interference, etc. Hence differentiation is highly
suspect.