The ratio of the Peak value to the root mean square(RMS) value of AC quantity is called the Crest Factor or Peak Factor. Any periodic waveform has its varying magnitude with time. The waveform attains maximum or peak value at a point in time. The direct current has its maximum amplitude at all the times i,e. it has a constant magnitude, and the magnitude does not change with time. In the case of the alternating current, the instantaneous value varies with time and attains maximum or peak value, and again, the magnitude decreases.
What is Crest Factor?
The crest factor of an AC current or voltage waveform is the ratio of its peak value to the root mean square(RMS) value. The crest factor of an AC voltage or current can be found by measuring its peak value and root mean square value. The root mean square value of an AC current is equal to the heating effect produced by the AC current, which is equal to the heating effect produced by the DC current. The root mean square value of a perfect sinusoidal AC voltage or current has a certain relationship with its peak or maximum value.
RMS Value of AC Waveform
The RMS value of an AC voltage is given below.
Crest Factor Formula Derivation
The crest factor of an AC current or voltage waveform can be mathematically expressed as;
Importance of Crest Factor
The crest factor 1 indicates no peaks. A higher crest factor indicates peaks. The crest factor of DC current or voltage is equal to unity because the RMS value of the DC is equal to the peak value.
The crest factor of the perfect sinusoidal voltage or current is 1.414. The crest factor of AC other than 1.414 shows that the waveform is not sinusoidal.
When the sinusoidal voltage is fed to semiconductor devices, the current through the devices is not linear and non-sinusoidal. The crest factor of the current drawn by the semiconductor devices is much higher than 1.414.
A crest factor different than 1.414 indicates distortion in the waveform. Typically, distorted current waveforms have crest factors higher than 1.414 or may have lower than 1.414. Distorted voltage waveforms with crest factor lower than 1.414 are called flat top voltage waveforms.
The Computer and Business Equipment Manufacturing Association (CBEMA) recommends a method for de-rating transformers based on the current crest factor. The de-rated KVA of the transformer would be the nominal KVA of the transformer multiplied by the total harmonic distortion factor(THDF).
Crest Factor of sinusoidal and non-sinusoidal waveform
From the above figure,
Crest Factor of a sinusoidal waveform
Crest Factor= I peak/ Irms= 8.20/5.8=1.414
Crest Factor of non-sinusoidal waveform
Crest Factor= I peak/ Irms= 24.2/5.8=4.17
Significance of Crest Factor
Power Supply Capacity Selection
The rating of the power supply is decided based on the peak current demanded by the load. Higher crest factor loads demand a much higher capacity power supply, or the supply needs to be de-rated.
Current Transformer(CT) Selection
If the peak current is too high, the metering CT inputs can clip, causing inaccurate readings. This means that when measuring loads with high current crest factors, the CT current rating needs to be selected considering the crest factor of the load. For example, if your load draws 15 amps RMS but has a crest factor 4.0, then the peak current is 60 amps. If you use a 20 amp CT, the meter will not be able to measure the 30 amp peak current accurately.
The following graph shows the maximum RMS current for accurate measurements as a function of the current waveform crest factor. The current is shown as a percentage of CT-rated current. For example, if you have a 10 amp load with a crest factor 3.0, the maximum CT current is approximately 58%. Fifty-eight percent of 20 amps is 11.60, which is higher than 10 amps, so your measurements should be accurate.
On the other hand, if you have a 100-amp load with a crest factor of 4.0, the maximum CT current is 42%. Forty-two percent of a 250-amp CT is 105 amps, so you would need a 250-amp CT to measure this 100-amp load accurately.
If the load’s crest factor is not known, it can be assumed to be in the range of 1.4 to 1.5, and a CT current rating of 150% of the expected RMS current can be selected for measurement. So, if the measuring current is up to 50 amps, select a 75 amp CT.
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