Troubleshooting series: Power factor

It’s 3:37 pm — right between the lunchtime bulge and evening high-water-demand periods — when your EAM presents a notification from Motors@Work: “Motors@Work detects a declining power factor on High Service Pump #2.”

“Why is Motors@Work sending me a power-factor alert?” you wonder, “Low power factors are simply a part of operating motors, right? But why is it declining?”

It’s true that induction motors inherently generate reactive energy, lowering your power factor. However, low power factors — anything below 80% — incur utility fees; and, declining power factors may indicate a problem with your motor-driven equipment.

Here’s how Motors@Work can help you identify, troubleshoot, and resolve power factor issues that otherwise would have gone undetected without our timely condition-monitoring alerts.

POWER FACTOR, EXPLAINED                           

Power factor is the cosine of the phase angle, q (theta) — a unit related to both frequency and time — between when AC voltage and current waveforms cross zero.

When you run a purely resistive load, such as an incandescent lightbulb, voltage and current pass through zero at the same time; hence, q equals zero and power factor equals unity (cos q = cos 0 = 1), whether represented as one or 100% [FIGURE 1].

But, when you operate a capacitive or inductive load, current and voltage waveforms fall out of synchronization [FIGURE 1]. With inductive loads like motors, current lags voltage; with capacitive loads, current leads voltage. Both leading and lagging power factors cause issues for power utilities, which is why they often charge a fee to facilities whose power factor falls below 80%.

Power factor represents the percentage of the apparent energy supplied by the utility (measured in kilovolt-amps, kVA) that the inductive or capacitive load converts into active power (measured in kilowatts, kW). The difference between unity and the power factor represents the portion of apparent energy lost to reactive power (measured in kilovolt-amps reactive, kVAR). Reactive power can be thought of as friction, backwash in a piping system, or the foam on a beer: something you have to exert (or purchase) more energy to overcome; it’s unusable. The relationship between apparent, active, and reactive power is described by the power triangle, shown below [FIGURE 2].

Older induction motors and induction motors operating at low loads produce high reactive loads and low power factors. Newer, high-efficiency induction motors produce significantly less reactive power than older models; however, their power factor still declines sharply at low loads [FIGURE 3].

To fix low power factors caused by partially loaded motors, right-size motors to their application, use a VSD to control motor operation, or replace with a synchronous motor — which act like a capacitive load and help correct lagging power factors created by other induction motors.

 

You decide to send a maintenance technician out to see what’s going on. While waiting for the technician to report her findings, Motors@Work provides an update on this notification: “Low power factor on High Service Pump #2 worsening: was 72%, now 67%.”

The technician arrives onsite and begins troubleshooting following Motors@Work’s suggested actions:

1. Verify measurements & data transmitted to Motors@Work
2. Compare motor load to manufacturer’s performance data
3. Check motor controller, power-factor correction device, and/or VSD operation

The power sensor in the motor control center (MCC) reports that power factor is now 65% — having dropped an additional 2% since Motors@Work’s last update.

Next, the technician verifies the load. The motor is operating at 75% load. The technician looks up the manufacturer’s performance data on this motor in Motors@Work. At this load, the manufacturer expects the motor to operate with a power factor closer to 86.5%.

Finally, the technician checks the VSD. A fault in the rectifier circuit means the drive has disabled power-factor correction. The result: the rectifier bridge is injecting high harmonic content upstream, inducing a low power factor both on this motor and throughout the facility. As a result, all equipment at this facility is drawing more current than otherwise needed for the load.

The technician locates a similarly sized drive with a local vendor, picks it up, changes it out, and sends the faulty drive back to the manufacturer for an in-warranty repair.

Motors@Work’s continuous, near-real-time condition monitoring algorithms watch for energy-dollar-consuming low power factors and send out content-rich alerts that facilitate troubleshooting — so you can intervene before you get a surprise when your energy bill arrives.
How will condition monitoring benefit your organization? Email Nicole at info@motorsatwork.com to learn more.

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