Archive for the ‘Capacitor’ Category

Electrical Systems: Power Factor Correction (2)

November 29, 2009

Power Factor Charges
Many utilities charge for low power. To measure power factor, the most common type of utility meter measures the total kVAr-hours and kVA-hours over the billing period and calculates the average power factor as:

PF = Cos [ ArcSin (kVArh / kVAh) ]

The most common methods of charging for low power factor are:

1. Adding a demand penalty when the power factor dips below a set amount (usually 90%)

2. Basing the demand charge on the supplied power Ps (kVA), rather than the actual power used Pa (kW).

3. Basing part of the overall charge on the reactive power kVAr, which increases as power factor decreases.

Electrical Systems: Power Factor Correction (1)

November 29, 2009
Resistive devices, like electric resistance heaters and incandescent lights transform all the power supplied to the device into heat or useful energy. Inductive devices, like motors, use some of the power supplied to the device to energize the inductive windings and create a magnetic field. This power, called reactive power, is alternately stored and given up by the windings, but is not used to do actual work. When this happens, the line supplying power to the device now carries the actual power used by the device and the reactive power created by the device.

Actual power used by the device is measured in kW, reactive power created by induction devices is measured in kVAr, and the apparent power in the supply lines is measured in kVA. The mathematical relationships between these types of power are described by the “power triangle” shown below. For example,

The ratio of the actual power consumed by equipment (Pa) to the power supplied to equipment (Ps) is called the power factor.

PF = Pa / Ps = kW / kVA = cos Phi

Devices which generate/require large amounts of reactive power in relation to actual power consumed have low power factors. Such devices include:

• Motors
• HID and fluorescent lights with low PF ballasts
• Devices which convert AC power to DC power such as:
• DC drives
• Welding machines
• VFDs
• Induction furnaces

Fully loaded motors generally have a power factor of about 80%. However, if the motor is under loaded, the fraction of reactive power (for the coil) to actual power (for mechanical work) increases and the power factor decreases.

Two potential problems are associated with low power factor. First many utilities have explicit or implicit charges for low power factor. Second, low power factor increases the current, and hence losses, in transformers and the electrical distribution system. These losses cost money and generate excess heat in the electrical distribution system, which may shorten equipment lifetime or cause production shut downs. These potential problems are discussed in the sections that follow.