Gates Belts & Applications - Tips and guidelines written by Gates Industrial Engineers

Understanding Power: Power Factor, Apparent/Reactive/Real Power

Tuesday, August 31, 2010

posted by Brent at 9:05 AM

In order to understand energy and the efficiency gains made possible by synchronous belt drives, it is important that four terms be explained and their relationship to each other defined. Those four terms are apparent power, reactive power, real power, and power factor.


Apparent power is the total power supplied by the utility company to the end user’s facility. Apparent power is measured in kiloVolt Amperes (kVA). This power is comprised of 2 components - reactive power and real power.

Reactive power is measured in kilovolt Amperes Reactive (kVAR). For power transmission purposes, we are dealing primarily with electric motors. Reactive power is the non-working power necessary to create the magnetic field in an electric motor. Some facilities utilize capacitors in their electrical system to reduce the amount of reactive power required. Depending on the size of the end user, his power demands, and the limitations of the local utility company, capacitance correction can be “encouraged” by the utility company. This “encouragement” typically takes the form of a penalty if the utility company feels that the facility is requiring too much reactive power (and total apparent power as a result). Remember that since it is not included in their billing, reactive power is of no interest to the end user. Only the utility company is concerned with reactive power. Since reactive power is part of the power supplied by the utility company, it must also be accounted for in the utility company’s power generation capacity. By forcing users to control reactive power, utility companies can stretch their resources.

Real power is the power paid for by the end user, and is measured in kiloWatts (kW). Real power is also the power that can be reduced by converting V-belt drives to synchronous belt drives and utilizing the mechanical efficiency advantages of a positive drive. Real power is the component of the power supplied by the utility company that actually does useful work. Utility customers are actually billed for kiloWatt-Hour
(kW-Hr) usage, a typical energy value.

Power factor is simply the ratio of real power to apparent power. Utility companies prefer high power factors (over .90) since that results in a lower reactive power and thus a lower total apparent power that they must supply. However, this tends to run opposite of the effects of energy saving devices at the output of electrical motors. Anytime load is reduced on the motor (by increasing the efficiency of the power transmission system, or increasing the driveN equipment efficiency), the power factor will drop. In order for electric motors to operate at a high power factor, they must be loaded to their full capacity. Motors operating at loads less than 100% will see corresponding drops in power factors.

The real power being used for work and paid for by the end user is basically calculated by multiplying the power factor, voltage (volts), and current (amperes) together. In order to reduce the real power (kW) being consumed, the end user must therefore reduce either the power factor, the voltage, or the current (or any combination of the three). Energy savings measurements performed using a computer data logger system have shown that conversions to synchronous belt drives from V-belt drives rarely result in a voltage reduction. Small savings in amperage have been noted, but are typically small enough to be undetectable by ordinary clamp-on ammeters. The primary savings in real power (kW) usage is in power factor reduction. This has been demonstrated on every successful conversion that has been measured, and is an expected occurrence. The characteristics of electric motors demands that the power factor decline as the power demands decline.

The final result is that when looking at the bottom line - energy being paid for by the end user (kW-Hrs) - synchronous belt drives do result in energy savings. Field testing has proven that the 5% energy savings claim is valid, and in many cases conservative. For a thorough energy savings evaluation, it is necessary to use equipment which measures all of the components of energy usage: time, current, voltage, and power factor. Without measuring all of the components, an incomplete picture of the energy usage is provided. Common hand held instruments (ammeter, voltmeter) are insufficient by themselves to document energy savings. If an installation must be surveyed for energy usage, equipment which has the ability to measure all four components of energy usage (such as a data logger) must be used.

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