Power Factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power, and is a number between 0 and 1 (frequently expressed as a percentage, e.g. 0.85 pf = 85% pf). Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can never be smaller than the real power.
In an electric power system, a load with low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor. For example, if smaller than 0,8.
Linear loads with low power factor (such as induction motors) can be corrected with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise power factor. The devices for correction of power factor may be at a central substation, or spread out over a distribution system, or built into power-consuming equipment.
How to Use
Each operator needs to set its own thresholds and compare same type of devices to determine if the power factor is the way how to determine whether investigation is needed. The Power Factor needs to take into account the historic values, the values compared to similar devices and the use case of the connected device:
- Compare the historic observed Power Factors: in case a significant change shows on the power factor of a device, it is worth looking at it. Set a value to be warned when the Power Factor lowers. When the threshold is reached, an alarm will be generated and you can check if something changed on the device's usage.
- Compare the Power Factor with similar devices: a server of the same brand with the same specifications and same usage should show less or more the same power factor. If significant difference is observed, it is worthwhile looking into it.
- Different use cases of a device influence the Power Factor: a low Power Factor does not need to be bad by definition, all depends on how the power is used. When a redundant power supply is connected, you might see one of the connectors has bad Power Factor, since it will have a bad efficiency on the power it is demanding versus the power is is consuming, for example 300W needed and consumption is only 100W, in that case you would have a lower power factor.
It is important to monitor the Power Factor, since it is thought that supplies may fail since a bad power factor needs higher current what makes that the supply is more sensible to failures (fuses that fall out is because current is too high).
The most collocation providers will charge the Ampère in kVAh, so if you have bad power supplies in your rack, you will consume a lot more current, since your kVAh is larger than if it would be charged in kWh!
