Is Current Draw An Accurate Indicator Of Motor Failure
by Miles Budimir, Senior Editor
Testing electric motors doesn't have to be a mystery. Knowledge of the basics together with powerful new test equipment vastly simplifies the job.
Electric motors accept had a reputation for being a mix of science and magic. So when a motor fails to operate it may not exist obvious what the problem is. Knowing some bones methods and techniques along with having a few test instruments handy helps detect and diagnose problems with ease.
When an electrical motor fails to showtime, runs intermittently or hot, or continually trips its overcurrent device, in that location my be a diverseness of causes. Sometimes the problem lies inside the power supply, including branch circuit conductors or a motor controller. Some other possibility is that the driven load is jammed, bounden or mismatched. If the motor itself has adult a fault, the fault may be a burnt wire or connectedness, a winding failure including insulation deterioration, or a deteriorating begetting.
A number of diagnostic tools, such as clamp-on ammeters, temperature sensors, a Megger or oscilloscope, tin help illuminate the problem. Preliminary tests generally are done using the ubiquitous multimeter. This tester is capable of providing diagnostic information for all kinds of motors.
Electrical measurements
If the motor is completely unresponsive, no ac humming or fake starts, accept a voltage reading at the motor terminals. If at that place is no voltage or reduced voltage, piece of work back upstream. Take readings at accessible points including disconnects, the motor controller, any fuses or junction boxes, and so on, dorsum to the over-current device output at the archway console. What you're looking for is essentially the same voltage level as measured at the entrance panel main billow.
When there is no electrical load, the aforementioned voltage should appear at both ends of the branch circuit conductors. When the circuit electrical load is close to the circuit capacity, the voltage drop should non exceed three% for optimum motor efficiency. In a three-phase hookup, all legs should have substantially equal voltage readings, with no dropped phase. If these readings vary past a few volts, it may exist possible to equalize them by rolling the connections, taking care not to opposite rotation. The idea is to match supply voltages and load impedances so as to balance the 3 legs.
If the electrical supply checks out, examine the motor itself. If possible, disengage the load. This may restore motor performance. With power disconnected and locked out, endeavour to turn the motor by hand. In all only the largest motors the shaft should plow freely. If non, there is an obstruction inside or a seized bearing. Fairly new bearings are prone to seizure because the tolerances are tighter. This is particularly truthful if there is ambience moisture or the motor has been unused for a while. Frequently good functioning can be restored by oiling front and rear bearings without disassembling the motor.
If the shaft turns freely, set up the multimeter to its ohms office to check resistance. The windings (all iii in a three-phase motor) should read low but not zero ohms. The smaller the motor, the college this reading will be, only information technology should not exist open. It volition usually be depression plenty (under thirty Ω) for the audible continuity indicator to sound.
Small universal motors, such equally those used in portable electrical drills, tin contain extensive circuitry including a switch and brushes. In the ohmmeter mode, connect the meter to the plug and monitor the resistance as you wiggle the cord where it enters the enclosure. Move the switch from side to side and, with a trigger switch taped and so it remains on, press on the brushes and plow the commutator by hand. Any fluctuation in the digital readout may signal to a defect. Oft a new set up of brushes is what'southward needed to restore performance.
Amperage or electric current readings are useful in motor testing besides. With a voltage reading, y'all know the electrical energy available at the terminals, merely you don't know how much electric current flows. Multimeters always take a current function, but there are two problems with it. One is that the circuit under investigation must be cutting open (and later restored) to put the instrument in serial with the load. The other difficulty is that the typical multimeter is not capable of handling the corporeality of current present in fifty-fifty a small-scale motor. All the current would have to menses through the meter, burning the probe leads if non destroying the unabridged instrument.
An essential tool for motor electric current measurement is the clamp-on ammeter. It circumvents such difficulties by measuring the magnetic field associated with the current, displaying the issue in a digital or analog readout calibrated in amperes.
Clamp-on ammeters are user friendly. Just open the jump-loaded jaws, insert either the hot or neutral usher, so release the jaws. The wire demand not be centered in the opening and it'due south OK if information technology passes through at an bending. However, an entire cable containing hot and neutral conductors cannot be measured this style. That'due south because the current flowing through the 2 wires travels in opposite directions so the two magnetic fields cancel out. Consequently, it's non possible to mensurate the electric current in a power cord, every bit is often desired. The use of a splitter fixes the trouble. This is a brusque extension cord of adequate rating with almost 6 inches of jacket removed so that i of the conductors can be separated and measured.
Digital and legacy analog clamp-on ammeters work well and are capable of measuring upwardly to 200 A, which is acceptable for most motor work.
The basic procedure is to mensurate the start-upwards and running current for whatever motor while information technology's connected to a load. Compare the reading to documented or nameplate specifications. Equally motors historic period, the current drawn more often than not rises because winding insulation resistance drops. Excess current causes oestrus, which must be dissipated. Insulation degradation accelerates until there'south an avalanche effect, causing motor burn out.
The clamp-on ammeter reading volition tell you where you lot stand on this continuum. In an industrial facility, as office of routine motor maintenance, periodic current readings tin can exist taken and put into a log posted nearby so damaging trends tin be spotted in advance to avoid expensive downtime.
Insulation testing
The insulation resistance tester (or megohmmeter), generally known by its merchandise name Megger, tin can provide critical information regarding the condition of motor insulation. In an industrial facility, the recommended process is to perform periodic tests and record the results so damaging trends can be detected and corrected to foreclose an outage and all-encompassing downtime.
The insulation resistance tester resembles a conventional ohmmeter. But rather than the typical three-volt test voltage derived from an internal battery and nowadays at the probes, the Megger provides a much higher voltage practical for a proscribed length of time. The leakage current through insulation, expressed every bit resistance, is displayed so it can be graphed. This test may take place on installed or on-the-reel cable, tools, appliances, transformers, power distribution subsystems, capacitors, motors and any type of electrical equipment or wiring.
The test may exist non-destructive, for in-service equipment, or prolonged at elevated voltage to test prototypes to the point of destruction. A chip of a learning curve is involved in using the Megger. The right settings, connection procedures, test durations and safety precautions must be implemented to avoid damaging the equipment or electrocuting the operator or coworkers.
The motor under test must be powered downward and asunder from all equipment and wiring that's non to be included in the test. Besides invalidating the test, such extraneous equipment could exist damaged by the practical voltage. Additionally, unsuspecting individuals could be exposed to chancy loftier voltages.
All wiring and equipment has an inherent amount of capacitance, which is by and large significant in large motors. Considering the equipment is in upshot a storage capacitor, information technology's essential that lingering electrical free energy exist discharged before and after each examination. To do this, shunt the relevant conductor(s) to footing and to each other before reconnecting the power source. The unit should be discharged at least four times as long as the test voltage was applied.
The Megger is capable of applying different voltages, and the level should be coordinated with the type of equipment nether test and the scope of the inquiry. The test generally applies between 100 and 5,000 V or more. A protocol involving voltage level, time duration, intervals between tests and connectedness methods must exist composed, taking into account the type and size of the equipment, its value and role in the production process and other factors.
Motor testing equipment
Newer more than contemporary instruments make testing even easier. For case, exam equipment such as Fluke's 438-II Power Quality and Motor Analyzer uses algorithms to analyze not merely three-phase ability quality only also torque, efficiency and speed to make up one's mind organisation performance and detect overloaded conditions, eliminating the need for motor load sensors.
It provides analysis data for both the electrical and mechanical characteristics of the motor while in operation. Using proprietary algorithms, the 438-II measures the three-phase current and voltage waveforms and compares them against rated specifications to summate motor mechanical operation. The analysis is presented in elementary readouts, making it easy to gauge the operating performance and determine if adjustments are needed before failures crusade an operational shut downwards.
The analyzer too provides measurements to determine a motor's efficiency (for example, the conversion of electrical energy to mechanical torque) and mechanical ability under operating load weather. These measures allow for determining the motor's in-service operating power compared to its rated power to encounter if the motor is operating in overloaded condition or, conversely, if it'due south oversized for the application, energy may be wasted and operating toll increased.
Other developments include integrating multiple instrument functions into one unit. For example, a new thermal imaging clamp-on ammeter from FLIR has a born infrared photographic camera, which gives the user a visual indication of temperature differences and thermal anomalies.
Reprint info >>
FLIR
www.flir.com
Fluke
world wide web.fluke.com
Keithley/Tektronix
world wide web.tek.com/keithley
Source: https://www.testandmeasurementtips.com/basics-motor-testing/
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