LIMIT | Everything can be measured 1
ABOUT RMS It is fairly difficult to measure curre
nts accurately on today’s industrial and office premises, where personal computers, motor controllers and equipment that draws current in brief pulses instead of using evenly sinusoidal current is the norm. This type of equipment means that standard instruments detecting rectified average values display errors to varying degrees. If you have ever hunted for the cause of a tripped fuse but without success, your multimeter may be the source of the problem. Rectified average value detection When referring to alternating current measurements, we normally mean the effective value, i.e. the thermal output, referred to in the data sheets as RMS (Root Mean Square). This value is equivalent to a direct current that gives the same thermal output in a resistor as the measured alternating current. The most common way of measuring this thermal output with a multimeter is to rectify the alternating current, find out the average value of the rectified signal and multiply the result by a factor of 1.11. This factor represents a relationship between the average value and the effective value on a perfect sine curve. However, this relationship no longer applies unless the curve is perfectly sinusoidal. This is why instruments that detect the rectified average value mostly report incorrect values when measuring current in modern electrical systems. Linear and non-linear loads Linear loads mainly consist of resistors, coils and capacitors, which always draw a sinusoidal current. This is why there are no problems with these loads (see Figure 1). Non-linear loads, such as frequency converters and the switched power units in office electronics, draw a distorted current (see Figure 2). Measuring the effective value with instruments that detect the rectified average value may result in measurements that are up to 50% too low. As a result, you may end up wondering why a 14A fuse is constantly tripping, even though your multimeter is displaying a value of 10A. True effective value To measure such distorted waveform shapes, you can check the waveform shape with a graphic instrument first and only use an instrument that detects the rectified average value if the curve is perfectly sinusoidal. If you do not want to take any chances, you should always use an instrument that measures the true effective value directly. A modern multimeter that measures the effective value uses electronic measuring technology to always show the correct effective value of an alternating current, regardless of whether the waveform shape is sinusoidal or distorted. Fig. 1. Current curve of a linear load. Fig. 2. Current curve of a non-linear load. | 91