AC excitation usually refers to the use of a 50Hz (or 60HZ) sine wave power frequency city excitation sensor. The most important advantage of this excitation method is that it can reduce the polarization of the electrolyte liquid on the electrode. Therefore, the influence of drifting DC interference on the measurement is greatly reduced. Secondly, the AC excitation power supply is simple, directly using the mains power supply to generate the working magnetic field. Generally, the magnetic induction intensity of the sensor can be designed to be higher, so there is a larger signal electromotive force (such as 1mV per 1m/s), a higher signal-to-noise ratio, and higher measurement accuracy can be obtained.
At the same time, the excitation frequency is high and the measurement response is rapid, which is suitable for measuring slurry and pulsating flow. Therefore, in the 1950s and 1980s, commercial electromagnetic flowmeters were mainly based on this excitation method.
The biggest disadvantage of AC excitation is quadrature interference and in-phase interference caused by electromagnetic induction, as shown in the figure. They affect the measurement linearity and the stability of the zero point of the flowmeter. The reasons for these interferences will be discussed in detail later. Secondly, due to the electromagnetic induction of AC excitation, the magnetic circuit, the measuring tube, and the fluid will produce eddy current loss and hysteresis loss, which will increase the power consumption of the instrument.
Nevertheless, even though low-frequency rectangular wave excitation has gradually replaced AC excitation after the 1980s, it has become mainstream. When measuring solid and liquid two-phase slurry fluids, due to the high frequency of AC excitation, the solids rubbing against the surface of the electrode have become the mainstream. The slurry noise is small, and the swing amplitude of the output signal is low. Therefore, a new type of AC excitation electromagnetic flowmeter still appears on the market.