13.5 Electrical Systems, Motors & Standby Generators

Three-Phase Fundamentals

Most water and wastewater equipment runs on three-phase power at 480 V (commonly 460 V nameplate), with 4,160 V and higher for very large motors and 120/240 V single phase for controls and small loads.

$\text{kW} = \frac{\sqrt{3} \times V \times I \times \text{PF}}{1{,}000} \qquad \text{HP} = \frac{\sqrt{3} \times V \times I \times \text{PF} \times \text{Eff}}{746}$

Worked example. A motor draws 62 A at 460 V with a power factor of 0.88 and an efficiency of 0.93. kW = (1.732 x 460 x 62 x 0.88) / 1,000 = 43.5 kW HP = (1.732 x 460 x 62 x 0.88 x 0.93) / 746 = 54.2 HP delivered to the shaft

Power Factor

Power factor is the ratio of real power (kW) to apparent power (kVA). Induction motors are inductive loads that draw magnetizing current, which lowers power factor - especially when lightly loaded.

• Low power factor means higher current for the same real work, which means larger conductors, more heat, and more losses.
• Most utilities apply a power factor penalty or a kVA demand charge, so correcting power factor with capacitor banks directly reduces the bill.
• A motor running at 40 percent load has a much worse power factor than the same motor at 85 percent load. Oversizing motors is expensive twice: in capital and in every month's demand charge.

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Motor Nameplate

Everything you need to size protection and troubleshoot is on the plate:

$\text{Percent Load} \approx \frac{\text{Measured Amps}}{\text{Nameplate FLA}} \times 100$

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Starting Methods

VFDs

A VFD changes motor speed by changing the frequency (and voltage) supplied. Because centrifugal pump behavior follows the affinity laws:

$Q \propto N \qquad H \propto N^{2} \qquad P \propto N^{3}$

power varies with the cube of speed. Running a pump at 80 percent speed uses roughly 0.8³ = 51 percent of the power. That is why VFDs are the single largest energy savings opportunity at most water and wastewater plants.

VFD cautions operators must know:

• Harmonics distort the supply and can overheat transformers and neutral conductors; line reactors and filters mitigate this.
• Shaft currents can pit bearings; shaft grounding rings or insulated bearings are used on larger motors.
• Reduced cooling at low speed on TEFC motors whose fans are shaft-mounted - hence minimum speed limits.
• Do not run a centrifugal pump below the minimum speed that produces enough head to open the check valve, or it will churn and heat.
• Cable length limits between drive and motor to control reflected wave voltage.

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Motor Protection and Troubleshooting

[!WARNING] Single phasing is the classic motor killer. If one leg of a three-phase supply opens - a blown fuse, a burned contactor pole, a broken conductor - the motor keeps turning but the remaining two legs carry roughly 1.7 to 2 times normal current. Standard thermal overloads may not act fast enough. Phase loss and phase imbalance relays are inexpensive insurance on any critical motor. A rule of thumb: voltage imbalance above about 1 percent should be investigated; above 2 percent, derate or shut down.

Voltage imbalance is calculated as the maximum deviation from the average, divided by the average, times 100. A 1 percent voltage imbalance can cause a 6 to 10 percent current imbalance and a large increase in winding temperature.

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Standby Power

Water and wastewater plants are life-safety facilities. Loss of power means loss of pressure, loss of disinfection, or a sanitary sewer overflow.

[!WARNING] A transfer switch must make backfeed physically impossible. Backfeeding the utility through an improper connection energizes what a line worker believes is a dead line and can kill. Never use a "suicide cord," never bypass an interlock, and never connect a portable generator except at a proper transfer means.

Generator Testing

Also verify fuel supply duration (typically 24 to 72 hours on site with a refueling contract), air permit compliance for the engine, and spill containment for the day tank.