Tuesday, 23 May 2017

Flux density RMS versus peak.

Why quote flux density as an RMS value rather than the peak value?

Historically the reason for quoting the RMS value of an alternating voltage comes from a desire to know the heating effectiveness of that voltage.
The instantaneous voltage v(t) is constantly changing, from zero to a positive peak value, back through zero to a negative peak value, and returning to zero to complete each cycle (Fig. 1).

The first commercial electrical lighting system, from Thomas Edison, used a constant (DC) voltage. How can you tell how hot a light-bulb filament will become if you put an AC voltage across it?
The RMS value of an alternating voltage is the value of the DC voltage which would have the same heating effect. The UK mains voltage of 240V AC (RMS) has a peak voltage of 340V, but produces the same heat in an electric bar-fire (or the same light in a filament bulb) as 240V DC would.

The RMS value, or heating effectiveness, varies for different waveforms (Fig. 2).

We can see that the heating effectiveness is not defined by the peak but is always defined by the RMS value. RMS values can be directly compared, irrespective of waveform.

So why quote a peak value in a device specification?
............It's a bigger number!

There is another reason why it is important to compare the RMS value of the magnetic flux.
Some RF magnetic field generators are adapted from industrial-heating equipment originally intended for metal treatment. When using a magnetic field to induce eddy currents in the metal work-piece, the waveform is not important, so long as the desired temperature is achieved. The waveform will be nominally sinusoidal, but it may be distorted, depending on how energy is fed into the resonant circuit.

It is easy to calculate the RMS value of a simple wave shape but not so easy for a complex waveform, or a waveform which has significant distortion.

The heating effectiveness of a clean sine-wave is the peak value divided by √2. If you assume the RMS value of a distorted sine-wave is Vpeak2, this is an approximation and will introduce errors into your calculations; not ideal for scientific measurement!

Note that the NanoHeat device is specifically designed for laboratory work. The output is a high quality sinusoidal waveform. The field measurements are displayed as RMS values for accuracy, consistency and reproducibility.