DC Drive
Problem Symptoms:
-
Excessive (or
increasing) amplitudes at FLine and/or FSCR. These can indicate tuning
problems, grounding problems, winding problems, etc.
-
Peaks at other
FLine harmonics - 2x, 3x, 4x, 5x. With the exception of half-wave rectifiers
(where 3x FLine
is the SCR firing freq.), these peaks should never
be present. For example, consider the full-wave rectified signal below.
Imagine having a bad SCR. You would get an amplitude peak at 5 x FLine
(instead of 6x) plus an increased peak at 1 x FLine. A bad firing card,
which can control 1 SCR (half-wave) or 2 SCRs (full-wave), can cause the
loss of 1/3 of the power. This causes peaks at 1/3 x FSCR and 2/3 x FSCR.
The exact frequencies will depend on whether the drive is full-wave rectified
or half-wave rectified and the FLine supplied.
-
Sidebands around
FSCR. These sidebands typically indicate that the motor speed is fluctuating
or 'hunting'. This can be caused by comparitor card problems. A high resolution
spectrum (at least 1600 lines, probably 3200 and possibly even 6400 lines
- depends on motor speed) may be required to detect these sidebands. The
hunting may be most easily detected initially with a strobe light.
-
Whenever abnormal
symptoms develop, the drive itself must be thoroughly analyzed. The exact
symptoms, however, should provide important clues as to where to look first.
|
DC Drive Problems
typically show up on a vibration spectrum as amplitude peaks at multiples
of FLine between FLine and FSCR. With a full-wave rectifier, this
includes 2 x FLine, 3 x FLine, 4 x FLine and 5 x FLine. With a half-wave
rectifier, it means only 2 x FLine. In each case, however, it often
means high amplitudes at 1 x FLine and FSCR. It is important that the vibration
analyst either have a good understanding or work with someone who has a
good understanding of the electronic components in the drive (# of SCRs,
# of firing cards, control cards, how to tune the system, etc.). |