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# Polyphase Induction Motors

About two-thirds of the electric power in the U.S. is consumed by motors, with industrial three-phase motors above 5 HP (7 kW) being by far the bulk of that load. They are linear loads and therefore don’t contribute to harmonics. They are, however, the major contributor to reduced Displacement Power Factor, which is a measurement of the effective use of system capacity.

Measurements

1. Voltage unbalance

Voltage unbalance should not exceed 1-2% (unless the motor is lightly loaded). Why such a small number? Voltage unbalance has a very large effect on current unbalance, in the neighborhood of 8:1. In other words, a voltage unbalance of 1% can cause current unbalance of 8%. Current unbalance will cause the motor to draw more current than it otherwise would. This in turn causes more heat and heat is the enemy of motor life, since it deteriorates the winding insulation.

2. Voltage %THD and harmonic spectrum

Voltage THD should not exceed 5% on any phase. If the voltage distortion on any phase is excessive, it can cause current unbalance. The usual culprit is the 5th harmonic and therefore the harmonic spectrum should be examined for the 5th in particular. The 5th is a negative sequence harmonic which creates counter-torque in the motor. A motor fed by a voltage with high 5th harmonic content will tend to draw more current than otherwise. This is a major problem when across-the-line or soft-start motors share the same bus with ASDs.

3. Current unbalance

To find current unbalance, measure amps in all three phases. Do the same calculation as for voltage unbalance. In general, current unbalance should not exceed 10%. However, unbalance can usually be tolerated if the high leg reading doesn’t exceed the nameplate FLA (Full Load Amps) and SF (Service Factor). The FLA and Service Factor are available on the motor nameplate. If the voltage unbalance and the voltage THD are within limits, high currentunbalance can be an indication of motor problems, such as damaged winding insulation or uneven air gaps.

Current measurement will also find single-phasing. If a three-phase motor loses a phase (perhaps caused by a blown fuse or loose connection), it may still try to run single phase off the remaining two phases. Since the motor acts like a constant power device, it will simply draw additional current in an attempt to provide sufficient torque. A voltage measurement alone will not necessarily find this condition, since voltage is induced by the two powered windings into the non-powered winding.

Measure current draw of the motor. If the motor is at or near its FLA rating (times the Service Factor multiplier), it will be more sensitive to the additional heating from harmonics, as well as current unbalance. A motor that is only lightly loaded is usually safe from overheating. On the other hand, its efficiency and DPF are both less than optimal. Most motors reach maximum efficiency at 60%-80% of full load rating. Displacement Power Factor is maximum at rated load (including S.F.) and drops off, especially at less than 80% of rated load. This leads to the conclusion that, to the degree a motor load is constant and predictable, 80% of rated load is the most efficient operating range.

5. Inrush

Motors which are started across-the-line (as opposed to those using soft-starts or drives) draw a current inrush, also called locked rotor current. This inrush tapers off to normal running current as the motor comes up to speed.

• Older motors draw an inrush of typically 500-600% of the running current. Newer energy efficient designs draw brief inrushes as high as 1200% of running current, a direct result of the lower impedances which help make them more energy efficient in the first place.

• High torque, high HP motor loads require proportionally higher inrush.

• Motor loads started at the same time will have a cumulative inrush.

Another source of inrush is UPS and ASD systems with diode converters. They draw inrush current as their cap banks first charge.

Effects of inrush current

1. Inrush causes voltage sags if the source voltage is not stiff enough:

• Relays and contactor coils might drop out (typically, the sag would have to get as bad as about 70% of normal line voltage); or, if they hold in, their contacts might chatter (especially if the additional load causes a long-term undervoltage).

• Control circuits might reset or lock up (at 90% and below).

• Drives might trip off-line (undervoltage trip).

2. High peak demand periods, which may cause higher utility bills.

3. Cycling loads can cause periodic sags, which might show up as flickering lights.

4. If the motor is required to start up a high torque load, the inrush can be relatively prolonged (e.g., 10 to 20 seconds or more) and this can cause nuisance tripping as the overload heaters trip the motor starter.

6. Power Factor

To size PF correction capacitors, it is necessary to measure the DPF (Displacement PF) and Active Power consumption (kW) of the motor load. These measurements assume that the motor voltage and current is balanced. Therefore, before undertaking PF correction, first make sure that voltage and current unbalance are within limits. Either problem can shorten motor life and should take priority over DPF correction.

Fluke Corporation offers an extensive range of power quality test tools for troubleshooting, preventive maintenance, and long-term recording and analysis in industrial applications and utilities. For more information on Fluke Predictive Maintenance Products and Services go to http://www.fluke.com/pdm

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