DC Drive Problems

Figure 1 - Full-Wave Rectified Velocity Spectrum w/ Drive Problems

Figure 2 - Half-Wave Rectified Velocity Spectrum w/ Drive Problems

Figure 3 - Spectrum on DC Motor w/ Speed Fluctuations
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.).

Figure 4 - Half-Wave Rectified

Figure 5 - Full-Wave Rectified