“Run ’til it fails” No Longer Applies
Electrical motors are a critical element of any manufacturing or commercial plant. Their failure can produce sizeable losses, not only in replacement but also in lost production time and materials. Unfortunately, much of motor “maintenance” consists of little more than a policy of “run ‘til it fails, then change out.” But even if the motor itself is deemed replaceable, this practice may still lead to unacceptable losses. The cause of the failure may not lie with the motor itself but in some other part of the electrical system, producing excess current draw and voltage drop that will “burn up” one replacement after another.
“…Electric motors do not ‘just burn out.’ They burn out for given, and usually preventable, reasons.” [Lynn Lundquist, “Industrial Electrical Troubleshooting”, Delmar, p. 81] Fortunately, effective motor maintenance can be accomplished easily and readily with only a few practical test instruments. Electrical problems include shorts, opens, “grounds,” leakage, and system problems. (Mechanical problems, which include vibration, heat, wear, and overloading, will not be discussed here.) Opens typically involve conductor damage, while shorts, grounds, and leakagea are insulation faults.
“… Almost all motor losses are a result of some form of insulation failure.” [Lundquist, p. 122] System problems include voltage unbalance and surges, phase unbalance, improper phase sequencing, ‘single phasing,’ and voltage and frequency variations. These tend to originate with the utility or building wiring, and are the types of problems that lead to too-frequent motor replacements. To begin with, start with the contactor or motor starter. After checking for loose connection, this becomes a voltmeter test. First, check the voltage upstream from the starter. This should be within 10 percent of the rated voltage of the motor. Voltage drops can draw more current to make up required horsepower, thereby producing more heat in the windings and leading to burnout. Voltage rises similarly will pull more current, producing the same effect.
Next, check voltage on the overload relay, and replace if zero. If voltage is present, the problem is downstream from the starter. If contacts are burned or welded, use a clamp-on ammeter to test inrush current, and resize the relay if necessary.
On shaded-pole motors, the stator winding should be checked for opens and shorts. This requires only a multimeter or the continuity range of an insulation tester. However, a simple test will reveal only present failures. Use of a megohmmeter will expose possibly impending failures, and may be necessary with large windings having inductive impedance. Many motors, such as split-phase motors, have a thermal switch, which could be the cause of the problem. The switch should be checked for continuity across the contacts and heater element. Also, check resistance across the starting and running windings, looking for shorts (zero) and opens (infinity). A similar continuity check should be made of the centrifugal switch. Manually operating the switch should cause resistance to drop as it switches from starting running winding. A problem exists if no change occurs.
Capacitor-start motors should have the capacitor checked. On a good capacitor, resistance increases from zero to infinity. At the midpoint, one lead of the ohmmeter should be disconnected for 30 seconds; when reconnected, the same reading should be observed, then rising to infinity. This demonstrates the capacitor can hold a charge (is not “leaky”). A shored capacitor reads zero and an open reads infinity, with no change over the time of test.
Three-phase motors do not require these additional elements, and are relatively trouble-free. Basic winding maintenance with a megohmmeter can prolong useful life indefinitely. “A motor that has failed will most frequently have an insulation-related problem. Whatever condition outside of the motor initially caused the problem, the resultant motor winding heat will destroy the insulation, which is the immediate cause of motor failure.” [Lundquist, p. 121] An insulation test phase-to-phase, phase to ground, and stator to rotor will reveal developing deterioration and head off total breakdown.
Maintenance of DC motors is primarily a function of the brushes and their holders. Brushes should ride smoothly on the commutator without sparking. They should not gouge or wear the commutator surface. Proper maintenance is a mechanical, not electrical, function. However, the windings need to be tested for opens, shorts, and grounds as in an AC motor, and the commutator must be checked for opens and shorts by means of a bar-to-bar test. All of the checks are frequently done with no more than a simple test light, but should be performed more rigorously with a meter. A test light tells only if the motor is fully faulted at time of test, and has no predictive/preventive capabilities. A measuring instrument tells the operator the actual condition, and can intercept failures. With a megohmmeter, shorts from winding to case, or winding to winding, and opens within a winding, are not only detected but can be anticipated. Similarly, a high-current, low-resistance ohmmeter can perform bar-to-bar testing for shorts between turns of the commutator or grounds to the shaft.
All of the electrical tests thus far described can be easily performed with the various selections on a single tester - a full-function insulation tester. System problems can be addressed with a variety of other instruments. Phase and voltage unbalance can be detected with a 3-phase power meter. If the meter has recording capabilities, it can monitor voltage, current, and frequency trends and disturbances. ‘Single phasing,’ the case of one phase of a 3-phase system being lost, can be detected with a phase sequence indicator, as well as establishing phase rotation. Similarly, a combination phase sequence indicator/motor rotation tester is indispensable for proper hookup of 3-phase motors.
Megger offers a variety of insulation testers that meet the requirements of a full-function insulation tester. The megohmmeter chosen will depend largely on the rated voltage of the motor winding. The IEEE Std 43-2000 “IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery” provides guidelines for the DC voltages to be applied during an insulation resistance test. Note that voltages up to 10kV are recommended for winding voltages rated at greater than 12kV.
| *Winding Rated | Insulation Resistance Test |
| Voltage (V) | Direct Voltage (V) |
| <1000 | 500 |
| 1000-25000 | 500-1000 |
| 2501-5000 | 1000-2500 |
| 5001-12,000 | 2500-5000 |
| >12,000 | 5000-10,000 |
* Rated line-to-line voltage for three-phase AC machines, line-to-ground voltage for single-phase machines, and rated direct voltage for DC machines or field windings.
Below is a list of Megger products that are suitable for the type of tests discussed in this article:
- BMM2000 - 1kV Insulation Tester with multimeter functions
- BM25 - 5kV Insulation Tester with automated tests
- MEG10-01 - 10kV Insulation Tester with internal storage
- DCM2000P - Power Clampmeter
- PA9+ - Power Quality Analyzer
- PSI-700 - Phase Sequence Indicator
- 560060 - Phase Sequence/Motor Rotation Tester
Other application guides are available on the Megger website at www.megger.com.
