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Welcome to David City

Our mission is to enhance the vibrant community of David City by providing a tremendous quality of life defined by outstanding educational and employment opportunities for all citizens through provisions of quality, cost-effective governmental services that include infrastructure, utilities, affordable housing, physical environment, culture and recreation, public safety, land use planning, leadership and community participation.

August 2014 EnergyWiseSM Tip: Motor Myths
Electric motors account for 23 percent of all electricity consumed in the U.S. and almost 70 percent of manufacturing sector’s electrical consumption. In rural Nebraska during summer, the most common motor loads include air conditioning and irrigation systems. Consequently, you would think many of us would know how to optimize electric motor use. However, the following are a handful of “motor mistakes” many of us make:

MYTH: Temperature elevation has little influence on an electric motor.
FACT: Motors are designed with a specific insulation classification, which specifies the motor’s maximum operating ambient temperature. When fully loaded, a motor’s temperature increases. The ambient temperature rating accounts for the maximum amount of heat the motor can reasonably operate under. For each 10°C this threshold is exceeded, the motor’s life is cut in half. This can become a critical consideration when summer temperatures approach their peaks.

MYTH: Frequent startups do not impact a motor.
FACT: If not specifically designed for them, frequent starts will shorten a motor’s life. The brief inrush of starting current causes extra heat to buildup, which is usually dissipated while the motor is running. If the motor does not run long enough between starts to dissipate the extra heat, it can exceed the motor’s maximum operating temperature.

MYTH: High-efficiency motors always provide energy savings when compared to standard efficiency motors.
FACT: The actual operating speed of an induction motor is somewhat less than its synchronous speed. Synchronous speed would be produced if rotation of the motor’s shaft is at the same frequency of the supplied electrical alternating current. This difference between synchronous and actual speed is referred to as slip. Many energy-efficient motors tend to operate with less fullload slip or at a slightly higher speed than their standard efficiency counterparts.

For centrifugal fans and pumps, even a minor change in the motor’s operating speed translates into a significant change in the imposed load and annual energy consumption. Fan and pump “affinity” laws indicate that the horsepower loading placed on a motor by centrifugal loads varies as the third power or cube of its rotational speed. A seemingly minor 20 rotations-per-minute increase in a motor’s speed can result in a 3.5 percent increase in electrical load.

MYTH: Using soft-start equipment on big electrical motors can cut utility demand charges. FACT: The use of soft-start equipment can lead to savings, but it will not significantly reduce the demand charge on your electric bill. When a motor starts, it draws an “inrush” of electrical current often five to six times the motor's full-load running current. This creates a lot of heat, and heat is what kills motors over time. Soft-starters ramp up voltage applied to motor terminals over time, thereby limiting inrush, which significantly reduces heat buildup. By doing so, softstarters can extend motor lifetimes, in particular, for motors that are stopped and started frequently.

But why are demand charges from the utility not affected? If electrical kilo-watt (kW) demand is measured and billed on your electric utility account, the electric meter measures the average kW you consume over each 15 or 30-minute period. In contrast, a soft-starter affects a motor's power draw over the course of just a few seconds. The reduction of the motor's power draw over that short period is fairly insignificant when compared to the time over which the demand charge is calculated.

MYTH: Power-factor correction saves significant amounts of energy.
FACT: Power-factor correction can reduce energy use by only a small amount. But unless your utility requires power factor correction or charges a penalty for low power factor, improving it will not have a big impact on your electricity bill. The amount of energy saved is dependent on a number of site-specific factors, including the mix of electrical loads connected to your meter, the type and length of conductors, and where any power-factor-correction equipment may be placed (i.e. – closer to the meter versus closer to the motor loads). However, even in the most extreme cases, it is unusual for electrical consumption savings to exceed 2 percent.

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