Electrical motors make up a major portion of a building’s mechanical systems. If these motors are maintained properly, operations personnel can protect an owner’s investment and help ensure long-term, efficient, and cost-effective operation of the equipment.
The most important element of motor testing is safety. Always use lockout/tagout procedures for any equipment you test. Remember that motors and the equipment they drive pose mechanical hazards, such as turning shafts or fan blades and the danger of electric shock.
After performing mechanical tests, such as visual inspection, as well as inspections for free rotation and proper alignment, the following electrical tests should be performed. Because most electrical tests require a hot circuit, it may be necessary to remove the lockout/tagout devices. Exercise caution when performing electrical tests, and reinstall lockout/tagout devices as soon as possible.
Testing Supply Voltage
Voltage is electrical pressure that provides the potential for current to flow. If motors are connected to an improper voltage, they will not operate correctly, can be damaged, and may require repair or replacement.
To test the supply voltage of a motor, first determine the proper voltage required to operate the motor. The most accurate source for motor voltage information is generally the nameplate. Once the proper voltage has been determined, use a voltmeter to check the voltage potential between all three legs. Make sure to use all safety procedures. If the proper voltage is being supplied, proceed with the electrical checks described in the following paragraphs. If there is no voltage or incorrect voltage, check the supply circuit.
Testing Voltage between Phases
Unbalanced voltage in three-phase induction motors can cause a loss in torque and can overheat the motor. Therefore, the voltage should be checked periodically for imbalances at the motor terminals. Most electrical systems are designed so the load is balanced between each phase. Over time, however, additional equipment is often added to the electrical system without concern for the phase balance. This imbalance often leads to a loss of mechanical power and an overheated or damaged motor. When disassembled, a motor that has an unbalanced system will have a noticeably darker stator winding on one or two phases.
To test for a voltage imbalance, check the voltage between all legs while the motor is running. When the voltage is unbalanced by more than 2 percent:
- check the surrounding power system for excessive loads connected to one phase
- if excessive loads are not found, check the voltage balance at the main disconnect and contact the power company if an imbalance exists at this point
Testing Current Draw
To test the current, or amperage, draw, the motor must be operated. The most convenient way to check the amperage is to use a clamp-on ammeter. With the motor running under a normal load, check the amperage draw of each power line, or leg. Since the current draw may vary slightly over time, you may want to take several readings and then average the data. To obtain an average, record several amp readings at one-minute intervals. Next, add the amp readings together and divide that number by the number of readings taken.
After testing each leg, compare the amperage draw data to the amperage information on the nameplate or in the operating maintenance logs. A higher-than-expected amperage draw could indicate one of two problems: a bearing or alignment problem has occurred in either the motor or the driven equipment, or there are winding problems in the motor. Large motors often draw an amperage slightly higher than the nameplate rating because of high power factors. In these cases, the operating log is generally a more accurate measure of amperage.
When checking a motor for unbalanced currents, use the loaded amp readings; the currents may vary more when the motor is not loaded. If the motor is driving the load, and the highest amperage of the three legs is below the nameplate full-load rating, then it is probably safe to operate the motor. If the highest leg is above the nameplate rating but within the normal service factor amps, it is probably safe to operate the motor. In general, if the highest leg is not more than 10 percent above the average of the three legs, it is usually safe to operate the motor.
A motor that does not run properly should be checked for open winding circuits by testing for continuity.
Testing Continuity Using Meters: Using a multimeter set for resistance checking is a convenient way to check continuity. Remember that any time a multimeter is used for continuity testing the device being tested must be deenergized. To test a motor for continuity, first isolate, or disconnect, it from the circuit. Then use the multimeter to check the resistance between phases.
Testing Continuity Using Test Lamps: To test continuity in the windings of a motor using a test lamp, touch one test lamp prod to a coil terminal and the other lamp prod to the opposite coil terminal. If the test lamp does not light, the winding is open. If it does light, an open circuit does not exist and the winding is serviceable.
Testing for Shorts and Grounds
A short in a motor occurs when two wires make contact, causing the circuit to bypass the normal load and creating a path with low resistance. Damaged insulation on a motor winding can also cause a short or ground. In these cases, contact is established between the winding and the motor housing and a path with low resistance is created.
Testing for Shorts and Grounds Using Meters: Use a multimeter to test for a grounded motor. Set the multimeter for resistance testing, and connect one lead to the motor housing, or frame. Next connect the other lead to a winding. If the winding is grounded, the meter will indicate a low resistance.
Although both shorts and grounds can create similar electrical situations, testing for a short is more complicated than testing for a ground. In fact, a motor with a short in one winding may continue to operate for some time. If the motor is operable, the winding with the short will have a lower resistance than the other windings and will operate at a higher amperage and temperature. To test for a short, check the amperage draw between each phase using a clamp-on ammeter. If any resistance variation between phases cannot be explained by system conditions, remove the motor for bench testing. To test for shorts in the field winding of a motor, measure the voltage drop in each field winding section with a voltmeter. A voltage drop decrease in a section indicates a short circuit.
Once a motor is disassembled, you can test for a short or a ground using an ohmmeter. In this case, an infinite resistance indicates an open, and a resistance of zero indicates a short or a ground.
Testing for Shorts and Grounds Using Test Lamps: After a motor has been disassembled, you can perform tests to determine which components are faulty. To locate a grounded field winding, disconnect and separate the internal connections between the windings. Position one lamp prod of a test lamp to the housing, and touch each winding lead individually with the other lamp prod. If the test lamp lights, that particular winding is grounded.
Testing Phase Rotation
Before the initial start-up of a large motor, make sure the motor is turning in the proper direction using a phase rotation meter. If the meter indicates that the motor is turning in the wrong direction, check to make sure that all connected elements, such as transformers and branch circuits, are connected correctly. If all connections are correct, check the individual phases of the motor. The rotation direction of a three-phase motor can be changed by switching any two of the three supply wires. When a direction change occurs, the source of the phase exchange should be located. It can take place anywhere from the motor connecting taps back through the system to the power distribution grid. It is important to locate the source of the problem so that all three-phase motors within the affected circuit will operate properly.
The phases are purposely rotated when the application calls for reversing motor rotation. For safety reasons, it is common to interchange phases one and three when reversing a three-phase motor.
This article is adapted from BOMI International’s course Electrical Systems and Illumination, part of the SMA and SMT designation programs. More information regarding this course or the new High-Performance certificate courses is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.