Crest factor

The crest factor or crest factor (English crest factor) describes the electrical measurement technique, the ratio of peak value to the RMS value of an AC size and is always greater than or equal to 1, he finds application in areas of communications engineering, sound engineering and acoustics.

He serves as the form factor or the total harmonic distortion as a characteristic value for a rough description of the waveform of an alternating size as an AC voltage or AC current.

Definition

The crest factor of the variable X is defined as:

For example, if a sinusoidal alternating voltage having an effective value of 230 V, the peak value is about 325 V. The peak factor is in this case.

A measuring instrument must therefore be able to process of accuracy in RMS measurement increased to the peak value of voltage (peak value) without much loss. High crest factors represent a high proportion of harmonics, in particular higher order. The following is a high distortion power. High crest factors are therefore undesirable as a rule.

Electronic energy and power measuring instruments as well as sound level meters show at high crest factors of distorted values ​​. High accuracy means special effort. Power transformers are far more contaminated by connected rectifier and filter capacitors than for a resistive load, because the current to charge the capacitor to flow only during the current flow angle.

Many power supplies, thyristor dimmer and have a highly distorted, pulsed power consumption particularly at part load operation. Has the power absorbed by them partly a very high crest factor of up to about 10 affected not only power supplies with power transformer and rectifier downstream, but especially switching power supplies, converters and inverters without power factor correction ( PFC). To consider is the crest factor even in uninterruptible power supplies ( UPS) for personal computers. When a computer power supply without PFC has a crest factor of about 3, then you have a UPS for a short time current of 3 times the RMS current can deliver, which is 2.1 times of the peak current in sinus. ( Inexpensive UPS devices provide in backup mode also only roughly approximated by a sine wave from a square wave. Effort is made to achieve as sine a crest factor of about 1.4. , See also duty cycle ).

Crest factors

The following table shows the peak factors for a variety of simple waveforms.