Wednesday, November 12, 2008

Inductive loads and power correction

The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power, and is a number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 pf = 50% 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 be greater 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.

Thinking about power-factors all came from various discussions with the wiremen and engineers whilst working on site over the last couple of weeks - In the case of the kind of builds we do we always ask customers to provide a power supply with inductive rated breakers. This is from our standard Scope Of Works document;

The equipment cabinets will be wired with 14-way IEC mains distribution units. Every circuit is fuse-protected and the mains input is terminated in a 16A C-Form connector (AKA a ‘Commando’ plug) – Root6 requirement is that the client’s electrician provides a separate spur connection for each bay and all feeds are provided via a D-rated 16A MCB. We recommend the area is protected by an Earth Leakage Breaker. We will test all mains installs in accordance with edition 4 of the 16th ed. IEE regulations using a standard suite of PAT tests.

We've fallen foul of a customer who didn't pass this on to their electrician and we've been battling with B-Rated (i.e. resistive-load) breakers not being able to hold an 16A circuit to a bay full of switch-mode power-supplies (i.e. all modern broadcast equipment!). When a strongly inductive load such as a switch-mode/resonant-mode power supply is switched on input surge current which may be several times larger than the steady current flows. It's also why the mains in Soho is nearly a square-wave - all those odd-order harmonics being dumped back out onto the incoming supply.

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