How to Control AC Power?

Most of the electrical appliances used at home require AC power for their operation. This AC power or AC current is given to the appliances through the switching operation of some power electronic switches. For a smooth operation of the loads, it is necessary to control the AC power applied to them. This is achieved in turn by controlling the switching operation of the power electronic switches, like an SCR.

Two Methods to Control Switching Operation of SCR

  • Phase Control Method: This refers to controlling the switching of the SCR with a reference to the phase of the AC signal. Usually the Thyristor is triggered at 180 degrees from the beginning of the AC signal.  Or in other words at the zero crossing of the AC signal waveform, triggering pulses are applied to the gate terminal of the thyristor. Incase of controlling the AC power to the SCR, the application of these pulses are delayed by increasing the time between the pulses and this is called the control by firing angle delay. However these circuits cause higher order harmonics and generates radio frequency RFI and heavy inrush current and at larger power levels, it requires more filters to reduce RFI.
  • Integral cycle switching: Integral cycle control is another method used for direct conversion of AC to AC known as zero switching or cycle selection. Integral cycle triggering relates to alternating current switching circuits and particularly to integral cycle zero voltage alternating switching circuits. When a zero voltage switch is employed for switching a low power factor (inductive load) such as a motor or power transformer causes overheating of a power transformer on the utility lines. Hence the saturation of the current of the load is excessively high in rush currents. Another approach to integral cycle zero voltage switching involves the use of relatively complex arrangements of bi-stable storage elements and logic circuits which in effect count the number of half cycles of load current. Integral cycle switching consists of switching on supply to load for an integer number of cycles and then switching off supply for a further number of integral cycles. Due to zero voltage and zero current switching of thyristors, the generated harmonics will be reduced. Using integral cycle switching smooth voltage is not possible and frequency is variable. Integral cycle switching by bust triggering of thyristors as a method to remove whole cycle, cycles or portions of cycles of an AC signal, is a well-known and old method of controlling AC power, especially across AC heater loads. However the concept of achieving the cycle stealing of voltage waveform by use of microcontroller can be very precise as per the program written in Assembly/ C language. So that the average time of voltage or current experienced at the load is proportionally smaller than if the entire signal is to be connected to the load.

One side effect of utilizing this scheme is an imbalance in the input current or voltage waveform as the cycles are switched on and off across the load hence they are suitable for specific loads as against firing angle controlled method to minimize THD.

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Before going into examples for each type of control, let us brief up a little about zero crossing detection.

Zero Crossing Detection or Zero Voltage Crossing

By the term Zero Voltage Crossing we mean the point at the AC signal waveform where the signal crosses the zero reference of the waveform or in other words where the signal waveform intersects with the x-axis. It is basically used to measure the frequency or period of a periodic signal. It can also be used to generate synchronized pulses which can be used to trigger the gate terminal of the Silicon Controlled Rectifier so as to make it conduct at 180 degree firing angle.

A sine-wave by nature has nodes where the voltage crosses the zero-point, reverses direction and completes the sine-wave.

Zero Cross Sensing 1

By switching the AC load at the zero voltage point we virtually eliminate voltage induced losses and stresses.

Zero Cross Sensing or Zero Voltage Sensing ZVS or ZVR Circuit

ZCS Vs ZVS

Usually the OPAMP used in zero crossing detection works as a comparator comparing the pulsating DC signal (obtained by rectifying the AC signal), with a reference DC voltage (obtained by filtering the pulsating DC signal). The reference signal is given to the non inverting terminal whereas the pulsating voltage is given to the inverting terminal.

Incase of the pulsating DC voltage being lesser than the reference signal, a logic high signal is developed at the output of the comparator. Thus for every zero crossing point of the AC signal, pulses are generated from the output of the Zero Crossing Detector.

A Video on Zero Crossing Detectors

 

Integral Switching Cycle Control (ISCC):

To remove the disadvantages of integral cycle switching and phase control switching integral switching cycle control is used for the control of heating load. ISCC circuit is having 3 sections. First one consists of power supply to drive all internal amplifiers and feed the gate energy to the power semiconductor devices. Second section consists of zero voltage detection by sensing the instance of zero supply voltage and provides a phase delay. In the third section, an amplifier stage is needed which magnifies the control signal to provide the drive needed to turn on the power switch on. ISCC circuits consist of Firing circuit & Power Amplifier (FCPA) and power supply for controlling the load.

FCPA consists of gate driver for thyristor and triac is used as power devices in the proposed design. Triac can conduct current in either direction when it is turned on and it is formerly called as a bidirectional triode thyristor or bilateral triode thyristor. Triac is a convenient switch for AC circuits which allows the control of large power flows with milliamp scale control currents.

An Application of Integral Cycle Switching – Industrial Power Control by Integral Switching

This method can be used for controlling of AC power, especially across linear loads such as heaters used in electric furnace. In this the microcontroller delivers the output based on interrupt received as the reference for generation of triggering pulses.

Using these triggering pulses we can drive the opto- isolators for triggering the Triac to achieve integral cycle control as per switches which are interfaced to the microcontroller. In place of motor an electric lamp is provided for the observation of its functioning.

Block Diagram of Power Control by Integral Cycle Switching
Block Diagram of Power Control by Integral Cycle Switching by Edgefx Kits

Here a zero crossing detector is used to provide triggering pulses to the gate pulses of the Thyristor. The application of these pulses is controlled through a Microcontroller and an optoisolator. The Microcontroller is programmed so as to apply the pulses to the optoisolator for a fixed amount of time and then stop the application of pulses for another fixed amount of time. This result in complete elimination of few cycles of AC signal waveform applied to the load. The optoisolator accordingly drives the thyristor based on the input from the microcontroller. Thus the AC power given to the lamp is controlled.

An Application of Phase Controlled Switching – Programmable AC Power Control

Block Diagram of Power Control by Phase Control Method
Block Diagram of Power Control by Phase Control Method by Edgefx Kits

This method is used to control the intensity of the lamp by controlling the AC power to the lamp. This is done by delaying the application of triggering pulses to the TRIAC or using the firing angle delay method. The zero crossing detector supplies pulses at every zero crossing of the AC waveform which is applied to the Microcontroller. Initially the Microcontroller gives these pulses to the optoisolator which accordingly triggers the thyristor without any delay and thus the lamp glows with full intensity. Now using the keypad interfaced to the Microcontroller, the required intensity in percentage is applied to the Microcontroller and it is programmed to accordingly delay the application of pulses to the opto isolator. Thus the triggering of the thyristor is delayed and accordingly the intensity of the lamp is controlled.

 

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