Excerpted from the book Motor and Drive Troubleshooting, written by Glen A. Mazur and published by American Technical Publishers
Use a good clamp meter or digital multimeter, such as this Fluke 87V multimeter, to take single-phase basic voltage or current measurements.
Troubleshooting is a step-by-step procedure with the purpose of quickly and easily identifying a problem in a system or process. Proper test instruments make the process smoother and make it possible to more easily identify secondary problems where they exist. To troubleshoot a system, a process, or equipment, start by collecting technical records from relevant sources. These include the original equipment manufacturer (OEM), suppliers, contractors, operators, and maintenance departments. Choose suitable personal protective equipment (PPE) and test instruments for the job, then isolate the portion of the system or equipment to be tested. Take readings on this portion to test for malfunction. Multiple readings may be necessary to either verify proper operation or to identify a problem.
Once a problem is located, repair and test the problem area to ensure proper function. The repair process may require technical service, parts replacement, or redemption of a warranty. Perform additional measurements after the initial repair is completed to determine whether any secondary problems still exist.
When all repairs are completed, document the processes performed. Include the original problem, all tests and measurements conducted, and the steps taken to repair the problem if one was found. Also include any secondary problems, if they exist, along with suggestions for solutions. Service and inventory all PPE and tools used while troubleshooting, and note anything that would have improved the process, such as additional tools or training.
When it comes to electrical problems, troubleshooting can range from as broad as a building or system to as specific as an individual component. Troubleshooting electrical equipment starts with basic tests first and moves toward more advanced testing as necessary. Voltage and current tests are the most common, and typically are first conducted at easily accessible points of measurement or access.
When the equipment is on, voltage testing is a logical first step in the troubleshooting process. For comparison, voltage should be measured both while the equipment is on and while it is off. When off, voltage delivered to the equipment in question should fall within -10% to +5% of the equipment nameplate rating.
When on, the operating voltage should not change by more than a maximum of ±3% of the voltage measured while the equipment is off. Ideally, in an appropriately sized power distribution system, the voltage measured should not change regardless of whether the equipment is on or off. A difference indicates a power distribution system that is overloaded, or conductors that are undersized or else delivering power for too long. High voltage drops typically require system testing.
When performing basic voltage tests, it is important to ensure correct grounding, as well as ensure that measurements are taken between each phase-to-hot conductor, phase-to-neutral (underground) conductor, and phase-to-ground (grounded) conductor. These phase-to-phase measurements should not vary more than 1% to 3%. If fuses are present where taking voltage measurements, test them to ensure that they are operating correctly.
If the voltage into and out of the fuse is equal, and the voltage across the fuse is 0 V, then the fuse is good (closed). If, on the other hand, no voltage is measured out of the fuse, then it is bad (open). This same measurement should also be taken at individual load points within the equipment, such as motors. The voltage delivered should fall within 110 % to +5 % of the motor’s nameplate rating when it is measured with the equipment on.
A handheld oscilloscope, such as this Fluke 190-204 ScopeMeter® test tool, is ideal for taking advanced, three-phase measurements.
Current tests are important because they can reveal things that voltage tests do not. A current test can detect whether a motor is delivering full power or half power, or is even inoperable, while the voltage measurement may be consistent at the motor and the load. A current test will accurately indicate just how loaded a motor actually is. When at full power, the motor should match the listed current rating on the motor nameplate. Typically, though, most motors should draw less. It is also important to measure current over time; unlike voltage, which generally stays the same, current changes as the required load changes.
Current and voltage tests can detect basic problems including open fuses, power loss, and overloaded motors. To understand how a motor or system functions over time, however, more complex tests and measurements are necessary. These can include minimum, maximum, relative, and peak temperature measurements, which can be taken using a digital multimeter (DMM) or portable oscilloscope. Portable oscilloscopes, in addition to power quality meters, can test for waveform distortion problems. Portable oscilloscopes are ideal for taking advanced measurements because of their advanced test functions.
Should a test indicate a possible problem, a portable oscilloscope can isolate the problem specifically. Common problems can be detected using these testing methods, and more serious and potentially damaging problems can be detected as well. Advanced measurements, taken over time and monitored closely, can provide crucial information about systems and equipment.