By John Olobri
Until the recent past when electricians, service technicians and contractors needed to test, calibrate or troubleshoot an electrical network, machine control circuit or any other electrical system, they would turn to either their clamp-on meter or a multimeter for assistance. These devices were small enough to fit into their tool bag, low cost enough to buy one when needed and easy enough to use such that reading the manual wasn’t necessary. Every tool bag had one, if not both of these meters. As time went on and system designs got increasingly complex, signal levels got smaller and smaller and transmission speeds became lightning fast; such that these meters could no longer stand up to the task. Detecting a signal that is a few microvolts or microamps and is only present for a few milliseconds requires different instruments. Compound this with the fact that these signals may be plagued with intermittent anomalies that need to be detected, and you are left with only a few choices of instruments to do the job.
High speed data acquisition systems, logic analyzers and oscilloscopes are the likely choices; all of which are big, hard to use, and too expensive for most electricians and therefore, hard to justify purchasing. More recently however, the oscilloscope has come onto the electrician’s radar screen. Why? Because a new generation of oscilloscopes, called the hand-held scope, has come onto the market and has addressed the needs of the electrical tradesman. They are smaller in size, about the size of an average textbook, more affordable, and easier to operate. The added advantage of a color screen makes it much easier to analyze multiple parameters, a task not possible or practical with a multimeter. Add to this the fact that scopes now have direct access buttons with icons that visually show the operator what function will be executed, and touch screen displays with “Windows® like” menus. They are practically as easy to use as a multimeter. The input probes are self-calibrating, eliminating guess work and tedious set up. Locking onto the incoming signal with the right time base and sensitivity to capture these very small high speed signals is as easy as pushing one button.
One significant advantage the oscilloscope brings to the data analysis arena is triggering. The electrician or technician is now able to set up conditions to capture these problematic intermittent signals that have eluded him in the past. With this new generation of hand held oscilloscope, it is now easy to capture a signal based on its level, duration or even based on the number of occurrences or time between occurrences. This makes capturing high-speed intermittent signals faster and more efficient than can practically be accomplished by other instruments. Testing can also be performed using a much simplified set up with easy to understand prompts on screen (see figure 1).
Several oscilloscopes now offer the ability to function as multiple instruments that are well known to the electrician. At the push of a button the hand held scope can be a multi-channel multimeter with both text and graphic display of the data (see figure 2), a data logger displaying and storing trend data for short or long periods of time, as well as an analyzer capable of measuring and displaying the harmonic content as shown in figure 3. Of course, the oscilloscope function is there with the ability to graphically display waveforms and perform Fast Fourier Transform (FFT) analysis which provides the user the ability to identify specific frequencies associated with the troubleshooting process.
Let’s look at a practical application of these hand held oscilloscopes. First, a very common application is motor start up analysis. This is most commonly achieved by looking at the in-rush current over the first few cycles once voltage is applied to the motor, typically one half to 20 hertz. On a 60 hertz power line, that is between 8 and 320 milliseconds.
When switching power supplies are first turned on, they present high initial currents resulting from filter capacitor impedance. These large filter capacitors act like a short circuit, producing an immediate inrush surge current with a fast rise time.
When switching on solid state equipment such as computers, copiers, and electronic ballasts, found in lighting systems, along with magnetic devices like motors, drives, and core & coil ballasts, peak inrush currents several orders of magnitude greater than the circuit's steady state current could be produced. It can be as high as 20 times normal operating current.
One practical reason to measure these in-rush currents is that in high-efficiency motors these currents are much more likely to trip their circuit protectors when started. Initial start-up current (in-rush current) is usually the cause. In a 3-phase motor, for example, in-rush current generally lasts between 60 milliseconds and 180 milliseconds, with a current between 5 and 10 times that of steady state operation. Additionally, it is often necessary to analyze equipment operation, particularly of medium voltage motor starting, in order to determine the motor actual locked-rotor-amps (LRA), full-load-amps ( FLA), acceleration time, and power factor. This data is often required to verify proper protective relay settings, for inclusion in commissioning reports, and is very helpful in establishing an operating baseline for future system troubleshooting.
The oscilloscope is a good choice for this application for several reasons. First, the operator can see the actual in-rush current on a cycle by cycle basis as shown in the example in figure 4. This can be accomplished on all phases of a three phase motor if the oscilloscope has three or more channels. Secondly, the user can set up triggering to be sure that all of the vital data associated with the motor start up is captured. Letting the oscilloscope decide when to capture the data frees up the operator to observe other conditions of the test with confidence that the desired information will be captured.
Once the waveforms are captured, it is then a simple and straightforward task to analyze the data on screen to determine such important parameters as peak value, min/max values and duration of the in-rush by simply placing a cursor at the desired points on the waveform and reading the displayed values presented by the oscilloscope for those points. This information can then be downloaded to a computer for report generation and to archive the data as a baseline for future tests comparisons.
Now that oscilloscopes incorporate industrialized functionality and straightforward on-screen prompts as well as direct access button functions, and are built into rugged cases, they are now far more practical and useful to electricians, service technicians and contractors than their predecessors found in engineering labs. An added bonus is the fact that they are now available at an affordable cost. The ability to see real time waveforms also provides the operator with the opportunity to make on the spot decisions with a higher degree of confidence saving both time and money.
About the Author:
John Olobri holds degrees in both Electrical and Industrial Engineering and has worked in the design and marketing of instrumentation for over 35 years. For the past 10 years, John has been the director of sales and Marketing for AEMC Instruments where he has been actively involved in the areas of Insulation Resistance, Ground Resistance and Power Quality testing.
