The data acquisition system used to perform the dynamic speed
measurements consists mainly of a conventional dc tachometer, an
analog-to-digital (A/D) converter and a personal computer. The
A/D converter has 8 differential channels and can be operated at
avariable sampling rate up to 40 kHz. An external pulse generator
is used to trigger the A/D converter and set its sampling rate. If the
time required for the tested machine to reach its maximum speed
is larger than 0.2 second, a sampling rate of 5 kHz per channel is
quite sufficient for the acceleration tests.
All of the tests performed during the development of this method
were conducted at full rated voltage of the respective machine and
without any loading, except for the small tachometer. The machine
was rotated, by hand for small machines below 25-hp, in a direction
opposite to that of the expected operatingrotation, prior to energizing
the machine. In the case of large machines, plugging was used
to achieve starting from a reverse rotation. This was done in
accordance with IEEE standard test procedure [2]. It had the
advantages of deliminating the zero-crossing of the speed curve
representing the sampled data. It also allowed the switching transient
on the supply voltage waveform to disappear while the
machine was deccelerated from the imposed reversed rotation.
Therfore, the collected data in the speed range of zero to full speed
was not affected by the switching transients due to the supply bus.