Working of AC Regulation using Electromagnetic Relay

In this section, the working of whole technique is explained. This technique can be divided into these major parts.

1.          Multi-tap transformer
2.          Micro-controller
3.          Relays
4.          Relays triggering circuit
5.          Voltage sensing circuit
6.          Rectification for dc voltage

All these parts are explained in previous sections. Now, we have to combine these parts to implement this technique. The flow chart of the technique is shown below .

Microcontroller is the heart of the control circuit. It senses the output voltage and generate input pulses for the triggering of the relays. Measuring circuit converts the high voltage ac into measurable dc voltage. 

Measuring circuit

For the selection of appropriate tap of multi-tap transformer, we need output measuring circuit. We must constantly measure the output voltage because when voltage goes below certain range or goes above certain range, we must change the tap of transformer that can restore the voltage in the desired range. To measure the output voltage, it is reduced to a certain value, that can be fed to micro-controller. Potential transformer is used to step down the voltage, that will be exact replica of input voltage. As we are concern about the output RMS value. So, the output of potential transformer is converted to dc using bridge and smoothing capacitor. By dividing this dc voltage using voltage divider circuit into value that can be fed to micro-controller, we can measure the output voltage RMS value. The constant factors of PT ratio, peak value (Underwood 2) and voltage divider ratio is accommodated in the software. The measuring circuit used in this technique is shown in Figure 1.

Figure 1: Measuring circuit for micro-controller

Complete diagram for regulatory circuit

Figure 2, shows the complete regulatory circuit, compatible with our transformer. Common terminal of all the relays are connected with the neutral of the transformer, while each tap of transformer is connected with the NO terminal of the relay. This part is shown at the right side of the circuit diagram. While at the left side, opto-couplers are present for the isolation of circuit with the microcontroller. The micro-controller, measuring circuitry is not shown in this circuit diagram.

Figure 2: circuit diagram for regulatory circuit

Input after passing through the relays connects with the multi-tap transformer. Rectifier circuit converts the input ac into dc voltage that is used to energize the coil of relay. Cycle starts by sensing the output voltage by micro-controller via measuring circuit. If the output is less than the desired value, a tap of higher turn ratio is selected automatically by micro-controller, which gives signal to the relay triggering circuit. Only one relay is triggered at a time giving the selection of desired tap. If the output is greater than the desired value, a tap of higher turns ratio is selected automatically. If the voltage is in the desired range, the same tap remains selected and pulses generated by controller remain same. So, the whole mechanism is working in closed loop giving the desired output voltage. The speed of response of this technique depends on the speed of relays. This technique has lot of switching transients as the output can be changed at any instant without detecting zero crossing. The problem of transients has been solved in a new similar technique which is discussed in chapter 6. In the testing phase of this technique, power quality analysis has been performed which gives the limitations of this technique. Moreover, this technique has certain pros and cons, which is discussed when comparative analysis of different techniques has been performed.