events unless there is a corresponding voltage event. Most voltage and current recorders can easily
capture the high steady state voltage that leads to a drive trip, but they may not capture the capacitor
switching waveshape disturbance. Many voltage recorders can capture voltage imbalance, current
imbalance, and some of them will trigger a capture of voltage and current during a current event,
such as the drive tripping off.
To select the best monitor for the job, it is necessary to understand the characteristics of the available
monitors. The following sections will discuss the various types of data that may be needed for a power
quality investigation, and the characteristics of some commercially available monitors.
33.3.1 Voltage
The most commonly recorded parameter in power quality investigations is the RMS voltage delivered to
the equipment. Manufacturers of recording equipment use a variety of techniques to reduce the volume
of the data recorded. The most common method of data reduction is to record Min=Max=Average data
over some interval. Figure 33.1 shows a strip chart of rms voltages recorded on a cycle-by-cycle basis.
Figure 33.2 shows a Min=Max=Average chart for the same time period. A common recording period is 1
week. Typical recorders will use a recording interval of 2–5 minutes. Each recording interval will produce
121120.5120119119.5118.5FIGURE 33.1 RMS voltage stripchart, taken cycle by cycle.
Maximum 1 Cycle Voltage121120.5120119.5119118.5Minimum 1 Cycle VoltageAverage Of Every Cycle In Recording IntervalFIGURE 33.2 Min=Max=Average stripchart, showing the minimum single cycle voltage, the maximum single cycle
voltage, and the average of every cycle in a recording interval. Compare to the Fig. 33.1 stripchart data.