Any fluctuation in the digital readout may point to a defect. Move the switch from side to side and, with a trigger switch taped so it remains on, press on the brushes and turn the commutator by hand. In the ohmmeter mode, connect the meter to the plug and monitor the resistance as you wiggle the cord where it enters the enclosure.
Small universal motors, such as those used in portable electric drills, can contain extensive circuitry including a switch and brushes. A wide range of DMMs are available to measure voltage, current, and resistance, depending on the motor power ratings. A DMM (digital multimeter), such as this Keithley DMM7510 from Tektronix, is a must-have instrument for motor testing. It will usually be low enough (under 30 Ω) for the audible continuity indicator to sound. The smaller the motor, the higher this reading will be, but it should not be open. The windings (all three in a three-phase motor) should read low but not zero ohms. If the shaft turns freely, set the multimeter to its ohms function to check resistance. Often good operation can be restored by oiling front and rear bearings without disassembling the motor. This is especially true if there is ambient moisture or the motor has been unused for a while. Fairly new bearings are prone to seizure because the tolerances are tighter. If not, there is an obstruction inside or a seized bearing. In all but the largest motors the shaft should turn freely. With power disconnected and locked out, attempt to turn the motor by hand. If the electrical supply checks out, examine the motor itself. The idea is to match supply voltages and load impedances so as to balance the three legs. If these readings vary by a few volts, it may be possible to equalize them by rolling the connections, taking care not to reverse rotation. In a three-phase hookup, all legs should have substantially equal voltage readings, with no dropped phase. When the circuit electrical load is close to the circuit capacity, the voltage drop should not exceed 3% for optimum motor efficiency.
When there is no electrical load, the same voltage should appear at both ends of the branch circuit conductors. What you’re looking for is essentially the same voltage level as measured at the entrance panel main breaker. Take readings at accessible points including disconnects, the motor controller, any fuses or junction boxes, and so on, back to the over-current device output at the entrance panel. If there is no voltage or reduced voltage, work back upstream. If the motor is completely unresponsive, no ac humming or false starts, take a voltage reading at the motor terminals. This tester is capable of providing diagnostic information for all kinds of motors. Preliminary tests generally are done using the ubiquitous multimeter. If the motor itself has developed a fault, the fault may be a burnt wire or connection, a winding failure including insulation deterioration, or a deteriorating bearing.Ī number of diagnostic tools, such as clamp-on ammeters, temperature sensors, a Megger or oscilloscope, can help illuminate the problem. Another possibility is that the driven load is jammed, binding or mismatched. Sometimes the trouble lies within the power supply, including branch circuit conductors or a motor controller. When an electric motor fails to start, runs intermittently or hot, or continually trips its overcurrent device, there my be a variety of causes. Knowing some basic methods and techniques along with having a few test instruments handy helps detect and diagnose problems with ease. So when a motor fails to operate it may not be obvious what the problem is. Knowledge of the basics together with powerful new test equipment vastly simplifies the job.Įlectric motors have had a reputation for being a mix of science and magic. Testing electric motors doesn’t have to be a mystery.