Troubleshooting series: Voltage unbalance

It’s 2:35 pm, you’ve just gotten off the phone with me, and minutes ago we went live with Motors@Work’s condition-monitoring service. You’ve already received your first notification from Motors@Work on your cellphone via your EAM application: “Motors@Work detects a 1.73% voltage unbalance on Blowers #1, #2, #3 & #4.”

Thus begins nearly every go-live on our near-real-time condition monitoring service.

Unfortunately, voltage unbalance is a common power-quality problem. A voltage unbalance greater than one percent causes increased losses and heating in motors, drives, and other powered electronics — reducing the life of your equipment. A voltage unbalance greater than 5% can kill certain motors within days or weeks.

Here’s how Motors@Work helps our clients identify, troubleshoot, and resolve a voltage unbalance that otherwise would have gone undetected without Motors@Work’s timely condition-monitoring alerts.

Since your voltage unbalance is less than five percent, you assign a maintenance technician to troubleshoot the voltage unbalance as part of the next scheduled electrical preventive maintenance at the site.

A week later, an electrical technician gets assigned the PM. To help with troubleshooting, the message from Motors@Work contains help content as well as that asset’s most recent measurements. From this message, the technician quickly sees that the voltage unbalance remains constant and varies slightly over time, ranging between 1.67% and 1.82%.

Had the voltage unbalance been intermittent, Motors@Work would’ve recommended verifying that spikes in unbalance coincide with the operation of highly reactive and discontinuous single-phase loads, such as a welder. But, because this unbalanced voltage is continuous, Motors@Work suggests identifying whether the voltage unbalance originates from the utility or within your facility by checking the voltages at your utility meter.

ANSI C84.1 requires utilities operate with a maximum voltage unbalance of three percent; yet, many utilities hold themselves to a tighter standard. I’ve seen many utilities commit [in their tariff or customer service agreement] to supplying power at less than one percent voltage unbalance.

If your utility regularly supplies you unbalanced voltages in excess of ANSI C84.1 or the level stated in your tariff or service agreement, you may be eligible either to receive a rebate on your electricity bills or to be reimbursed costs related to failed motors and motor-driven equipment.

The technician checks the incoming line voltages at the main facility breaker and finds an unbalance of 0.47% [e.g., 462.1 Vab, 458.8 Vbc, 462.0 Vca]. This means the majority of the unbalance originates within your facility.

Next, the electrical maintenance technician begins working through Motors@Work’s suggested actions for investigating the intra-facility cause of unbalanced voltages. These blowers don’t use power-factor-correction devices, starters, or VFDs, so the issue can’t be faulty devices. All ground-fault indicators are clear. Transformer tap settings are equal.

Single-line diagrams show that loads should be balanced; however, the technician knows that some electrical work was recently done at the facility and so decides to trace the circuits to confirm they follow the single-line diagram. He finds a 277-Volt lighting circuit upstream from the motor that’s supplied by Phase B to Neutral. The single-line assigns this lighting circuit to Phase C. While a relatively small load compared to these blowers, it’s enough to drop Phase B by 8.5 Volts. Moving this load to Phase C corrects the unbalance.

A deviation of 8.5 Volts on a 460-Volt system may not sound significant, which is why it often goes unnoticed by operators and maintenance technicians. However, as shown in the table below, the excess heating a voltage unbalance creates increases exponentially. Furthermore, motor life goes down by 50% for each 10°C increase in winding temperature, on average.[1] That means even a small voltage unbalance reduces your motors’ expected life significantly.

Motors@Work’s continuous, near-real-time condition monitoring algorithms watch for motor-damaging voltage unbalances and send out content-rich alerts that facilitate troubleshooting — so you can intervene before a voltage unbalance damages your motor-driven asset.

How will condition monitoring benefit your organization? Email Nicole at info@motorsatwork.com to learn more.

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