“DC VARIABLE SPEED DRIVES:

There are different types of DC drive setups with the designation depending upon the source of their DC current. For instance a bank of batteries could supply the DC current. This is often the case in emergency backup systems. For example, power plants have a bank of batteries that supply DC current to emergency oil pumps that supply oil to the bearings of a turbine during an emergency shutdown. A second source of DC current could be from a (Motor-Generator) MG set. In this case, the motor drives a DC generator that generates the DC current. These first two types of drives provide pure DC current, therefore they do not produce any significant electrically induced signals in regards to vibration. The last type of DC drive, and the one that will be discussed in this paper, uses silicon controlled rectifiers (SCRs) to rectify AC current into DC Current. These drives produce a DC signal with AC pulses that correspond to the firing of the SCRs. These pulses or lack thereof in the case of failed SCRs and or gating problems produce vibration signals that can be analyzed to determine the source of the defect. Improper tuning of variable speed drives can also generate vibration problems.

Rectification: The first concept that needs to be understood in regards to DC drives is rectification. Rectification is the process of converting AC current into DC current. Pictured below is a half wave rectifier and the resulting output. Only the positive part of the current Al passes through the rectifier.

Full Wave Rectification: The above plot shows a normal spectrum from a current probe on the lead to a DC drive. As can be seen, the only major component is at 360HZ. Note that there is some 2nd and 3d harmonic of 360 Hz present. This is normal due to the non-sinusoidal nature of the current waveform.

All of the above plots were taken by analyzing the current to the motor. In a normal situation, the vibration will be monitored and stored on a periodic basis. If the 360 Hz signal is present in the vibration spectrum, it is not normally of any concern. If, however, other unexplained frequencies appear in the vibration spectra, then it would be advisable to take a look at both the current waveform and the current spectrum. In the pages that follow, some examples of problems discovered in DC drives and motors are presented.”