Many electricians and technicians are facing a couple different challenges, at the same time:
There has to be some learning involved, to meet these challenges. Learning new methods to be more efficient, to fix problems faster, to troubleshoot complicated systems.
But who has time for training?
The good news is that you may already have a great opportunity in your toolbag.
A lot of technicians buy a really good multimeter but never fully explore all of its functions. Well, that day may have come. Use this guide to get familiar with the more advanced features of your meter, as they apply to electronic equipment such as VFDs, and then start using them to improve your accuracy, lower your troubleshooting time, and do more - with less.
Picking meters for VFD work
Everywhere you look today, you'll find Variable Frequency Drives (VFDs) and other types of electronically-controlled equipment. Selecting the right multimeter for VFD maintenance is not as simple as it used to be. Drive output is not a sinusoidal waveform of 60 Hz. Yet, many multimeters only give accurate readings on "clean" waveforms. The distorted waveforms and harmonic currents in VFD outputs can create gross inaccuracies on these meters.
To work with a VFD, first select a meter that is "True RMS." Average responding meters give accurate RMS readings only if the ac voltage signal is a pure sine wave. A True RMS meter will read the RMS values of the various waveforms produced by the VFD.
Second, look for labeling indicating the meter meets recognized testing laboratory requirements and has appropriate Category Ratings. (CAT III should be the minimum for VFD work inside of a plant). Always perform a safety inspection of your meter including test leads and any accessories you have available.
Third, consider a meter with a built-in "low-pass" filter, such as the Fluke 87V, 1587, or 289. These meters filter out distortion for the highest accuracy VFD readings.
Measuring electronic equipment
Some may argue that a DMM is not required for modern VFDs, since they have a digital display tracking many different drive parameters and diagnostic codes. Such keypad control interfaces are invaluable for programming the VFD and identifying specific problems. However, maintenance and troubleshooting often finds the technician not in a position to read the VFD display, or, in many cases, the technician is interested in values elsewhere in the drive system.
For example, on larger drives, there may be separate cabinet doors for accessing various sections of the VFD. Some drive systems may have a disconnect between the drive and the motor. Certain types of motors, such as IEC motors may have open terminals within the terminal box allowing voltage readings directly at the motor. Controls and analog and digital inputs may be located anywhere in the plant. The right DMM, properly used, is a must.
Using the low-pass filter
VFDs vary the voltage and frequency applied to the motor to vary motor speed. Since the waveform output of VFDs is not a standard sinewave, the meter must be able to provide accurate readings of the fundamental values applied to the motor.
The "low pass filter" mentioned earlier filters out the higher unwanted frequencies. When reading ac voltage and frequency, technicians must manually activate this filter. Push the "Low Pass" button and verify the low pass filter "On" indication appears on the meter face.
What the bar graph is for
Depending on the drive application, there may be ramp up times programmed into the drive. Also, the drive output voltage and frequency will vary due to changes in load. Output values can change rapidly and be difficult, if not impossible to read on the digital display.
The bar graph on the modern DMM display provides the much needed analog indication to note trends and observe rapidly changing values. Happily, it does not overshoot like older analog meters do. And, the bar graph responds up to 10 times faster than the digital display. The bar steadily expands horizontally from left to right over its scale, providing a stable, usable indication of drive output.
To determine the approximate value displayed by the bar, first observe the bar display scale and count the divisions along the graph. Divide the number of divisions into the displayed scale. Then determine the value at the end of the bar. For example, if the meter is on the 600 volt scale and there are five division marks on the scale, then each of the division marks represents 150 volts. (The first mark represents zero, then divide 600 volts by the four remaining marks for 150 volts each.) See Figure 1.
If the end of the bar reaches the second division along the scale, then the voltage reading is 150 volts. If the end of the bar appears between two divisions of the scale, interpolate to determine the value. Remember that the bar graph provides an approximate value; but, is invaluable for estimating rapidly changing readings that cannot be observed digitally.
Tracing voltage issues with Min/Max
When loads do not operate properly, fingers often point at erratic drive output voltage and frequency. As it happens, mechanical issues such as a faulty chain drive or worn gears are usually the real culprit. However, as part of the troubleshooting process the electrical professional should verify proper VFD operation.
Tip: Min/Max can also be helpful when measuring separate analog inputs, such as 0 - 10 V dc from a remote control potentiometer.
Measuring output changes with Auto Hold
As a drive ramps up to speed, the voltage output gradually builds. Also, small changes in motor speed are created by small changes in voltage and frequency. Once again, attempting to observe the readings as they change on the digital display may be too difficult.
Press the "Auto Hold" function key and the DMM will lock in and hold the reading on the display. Then, when the changing voltage steadies out at the next value, the meter will beep and the new reading is locked in on the meter display.
This process continues, providing a set of incremental voltage readings as the drive adjusts itself to the next desired level. The "Auto Hold" mode is especially useful when verifying drive output voltages during longer ramp up and deceleration periods.
Your Auto Range habit
Many electricians and technicians develop the habit of using their meter in "Auto Range." With this function, the meter will determine the best scale for the reading and go to that scale automatically. However, reading a variable voltage may cause the meter to continually cycle between scales. Determining exact values then becomes difficult.
Depress the "Range" function on the meter to manually select the desired scale. Keep pressing this button, changing scales as needed until the desired scale appears and the appropriate number of significant digits is indicated.
Using "Rel" to measure terminal voltage
Use the "Relative" mode to determine how much values change based on another value. For example, measure voltage at the terminals on the VFD that provide input for speed control.
Depress the "Rel" mode key. The measured value now becomes the reference value to which all other values are compared. The meter also resets to zero at the same time.
Now, as the measured input values change, a reading appears on the display indicating the amount of change. If the reference value in the meter is 7 V DC, for example, and the meter reads -1 V DC, then, the value has decreased by one volt to 6 V DC.
Troubleshooting inverters with the Diode function
If the main fuse supplying the drive blows, drive diagnostics may later reveal a problem with the inverter section. Place the DMM function switch to the diode symbol to read resistance of the drive output transistors.
Technicians can also make capacitor checks by selecting the appropriate function on the DMM and verifying measured capacitor values fall within the rating of the capacitor.
Trace overheating with temperature measurements
Overtemperature trips may be due to a dirty air filter. Or, they could indicate more severe problems. To find out, select temperature on the DMM and use a thermocouple or other temperature probe to check actual temperatures of components such as heat sinks. Compare temperature variations from other similar components to determine the possible source of the overheating. If applicable, check thermistors with the resistance function on the DMM.
Checking input power with voltage and low-pass filtering
You can also make a few quick checks on the input power to the VFD. Certain problems here could cause drive malfunctions.
Checking crest factor with Peak mode
Use the "Peak" mode to approximate the "crest factor" of the incoming power supply.
Input sine waves that are flattened across the top may cause operational problems with the VFD.
In these high-pressure times, knowing how to troubleshoot complex problems requires using every trick in your DMM. The modern DMM is the 'right hand" of the field technician. Learn to get the most out of your meter!
(This is the first installment of a series of articles on test equipment usage. In the next article we will examine the use and application of multimeters that log data and then display it on screen).