where T = time (in seconds) for one cycle of CAL indicator lamp
Example - if the CAL lamp period equals 2 seconds, the line frequency
is 60 c/s, and the RPM dial is set at 900; the SELF TEST error is:
-- . 7.5 rpm
2-8 SPEED MEASUREMENT.
2.8.1 FUNDAMENTAL SPEED.
When measuring the rotational speed of an object, it is recommended
that the RPM dial be set initially at a higher flashing rate than the speed
of the object. Then, slowly reduce the flashing rate until the first single
image is observed. At this point, the flashing rate of the stroboscope
will be equal to the rotational speed of the object, and the speed can be
read directly from the RPM dial.
when using the middle - or low-speed ranges, simply switch to the
next higher range without moving the RPM dial setting to check whether
the stroboscope is flashing at the fundamental speed of the object. Since
the ratio between ranges is exactly 6:1, six images will appear at the
next higher range, if the stroboscope has been set to the fundamental
speed. If only three images appear, for example, then the stroboscope
has been set to one-half the correct frequency. On the high-speed range,
it is necessary to double the speed setting to check for fundamental-speed
operation. A double image should occur when the frequency setting is
doubled. If the fundamental speed of the device being measured is above
12,500 rpm, it is not possible to check for correct speed setting by the
method outlined above. In this case, refer to paragraph 2.8.3.
(3) With practice, and especially when the approximate speed of the
object can be estimated, an operator can accurately measure the speed of
rotating objects quickly, and with confidence. It is necessary, however,
to thoroughly understand the following basic principles involved in making
speed measurements with a stroboscope:
The operator must distinguish between single and multiple images.
Odd shaped objects usually cause little difficult y, but objects which are
Symmetrical in shape (gear, disc, fan, etc) must be marked to provide
a visiblee reference (refer to PRINCIPLES OF OPERATION, paragraph
Multiple images will always be observed when the stroboscope
flashing rate is set to a multiple of the fundamental speed of the object.
When reducing the flashing rate from a rate higher than the funda -
mental speed of the object, the first single image will be seen when the
flashing rate is equal to the fundamental speed.
When the flashing rate is below the fundamental speed of the object,
single and multiple images will be observed. The single images will
always occur at integral submultipIes of the fundamental speed of the
object (refer to paragraph 2.8.2).