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Today, many processes require continuous operation of motors. Failure — because of the expense of production loss — is not an option. Luckily, proper maintenance, monitoring, and testing can extend motor life and eliminate unplanned downtime.

Current

One reason for measuring a motor’s operating current is to determine how much power it’s producing. Suppose a motor’s full-load current is 100 A and it is drawing 75; the motor is producing threequarters of its nameplate horsepower, right? This is a false assumption because as the load on a motor increases, its power factor improves. This phenomenon is very common in modern motors. As a result, measuring only current will not give an accurate indication of the motor load. Approximate load may be determined by comparing measured current versus the motor performance data.

No-load currents vary widely from motor to motor because of differences in efficiency, power factors, and design; uncoupled, these motor currents can range from 30% to 50% of full-load current. As long as measured values of all three current phases are near the motor’s typical no-load current value, within 1% of each other, and with balanced voltage, current consumption is satisfactory. No-load current can also be determined from the motor’s performance data sheets.

Measuring current is done with a portable clamp-on current transformer with an appropriate voltage insulation level. If the equipment has a panel-mounted amp meter or ammeter, all the better. The current signature can be used to identify motor problems such as broken rotor bars or loose laminations. These faults can be detected by analyzing current with a spectrum analyzer or through a computer program. These faults are very rare, and the procedures followed to perform the tests depend on the equipment being used.

Voltage

Voltage measurements on operating equipment should be made as close to the motor as possible, so that the voltage drop of the motor’s feeder doesn’t influence results. For most industrial installations the motor starter is close enough to the motor so that measurements are satisfactory. With appropriately rated equipment, the phase-tophase voltage of all three phases should be measured. Each measured value should be 6 10% of the motor’s nameplate voltage, and all measured values should be within 1% of each other. Next, the voltages in the three phases to ground should be measured; ideally these voltages should be equal to each other and equal to the phase to phase voltage divided by = 3.

If phase-to-ground voltages are not equal, it implies that ground fault problems exist that must be immediately corrected. If this unbalance is significant and not corrected, the motor’s insulation system will be severely overstressed, resulting in significant reduction of motor life.

Watts

The measurement of a running motor’s wattage is the most accurate measurement of a motor’s produced work.

Approximate values of efficiency and power factor can be obtained from charts. Today’s high quality, easy-to-use instruments make taking power readings relatively simple.

Surface temperature

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Non-contact infrared pyrometers make abnormal hot spots, as well as bearing, air flow, and cooling problems, stick out like a sore thumb.

Standard motors are designed to operate with a maximum total winding temperature rise of 80°C (by resistance) for NEMA class B or 105°C (by resistance) for NEMA class F above 40°C ambient. (The motor’s insulation class can be found on the motor’s nameplate.) If the surface of a motor approaches these temperatures, it may indicate a winding or cooling problem; the motor winding temperatures should be measured or the motor should be taken out of service as soon as possible so that the problem can be fixed.

Anti-friction or ball bearings should not be operated above 130°C. Elevated temperatures degrade their greases and may begin to anneal rolling elements. If exact bearing temperature measurements can’t be made, approximate measurements will suffice. If the bearing box or shaft next to a bearing approaches 100°C, the bearing is probably near 130°C and is in danger of sustaining thermal damage. The motor should be shut down immediately and the problem rectified. Sleeve or babbitted bearings have a temperature limitation of about 110°C. The bearing material begins to get soft at about 130 to 170°C, depending on whether the bearing is made from tin or leadbased materials. If there are no thermometers or temperature detectors embedded in the bearing, an indication of the bearing’s temperature can be obtained by scanning the bearing box and shafting next to the bearing, and removing sight plugs to inspect oil rings and bearing metal.

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