Wireless speed control of AC motor using IR and ZCD (Zero cross Detector)

Here in this application, I will show you how we can control the speed of single phase 230V AC motor wirelessly using IC555. Along with IC555 I am using zero cross detector (ZCD) circuit to trigger the monostable and an optocoupler to trigger thyristor. Let us first see the block diagram of the AC Motor Speed Control System.

Block diagram: –

The major blocks of the circuits are rectifier, ZCD, IR modulator, monostable and AC motor driver. Let us understand all the blocks one by one

Rectifier: – It will step down the 230V AC to 12 VAC and then it produces rectified DC that is applied to ZCD as well as regulated power supply blocks

Regulated power supply: – This will generate 5 V regulated DC voltage that is applied as biasing voltage to all other blocks

Zero cross detector: – It generates very short duration positive pulses when AC cycle crosses zero line.

IR Modulator: –This will modulate the short duration positive pulses using 38 KHz frequency and generates IR light beam of 38 KHz

IR sensor: – When it receives 38 KHz IR light beam it generates short duration negative pulse that will trigger monostable multi-vibrator

Monostable multi-vibrator: – It generates variable duration pulse from 0-10 ms when it receives trigger input

AC motor driver: – It includes optocoupler and a thyristor. Optocoupler receives input form monostable and it triggers thyristor (TRIAC) that will rotate the motor.

Circuit diagram

Circuit diagram: –

Circuit Diagram is shown in the Circuit Diagram Tab.

Connections: –

· 230V @ 50 Hz AC is applied at the primary of transformer T1 (6-0-6, 2 A). It’s secondary is connected with AC input terminals of bridge rectifier BR1.

· The rectified output is connected with two diodes D1 and D3.

· Diode D3 couples rectified output to input of voltage regulator IC LM7805 through capacitive filter C1 (1000 µF)

· Output of LM7805 is regulated +5 VDC that is given to entire circuit. The power LED (RED) connected at the output through current limiting resistor R6 indicates power is on.

· Diode D1 couples rectified output to base of transistor Q1 through voltage divider formed by resistors R2 (1K) & R3 (1K).

· Q1 is connected in switch configuration as shown.

· The output of Q1 is applied as the biasing voltage to first IC555 chip. It is configured in astable mode. Its output drives IR LED directly.

· The output of IR sensor is applied at the trigger input of second IC555 that is configured in monostable mode.

· Output of U3 is applied to pin no 2 on U5 that is optocoupler MOC3021. Pin no.1 of MOC3021 is connected to Vcc through current limiting 220E register

· Pin no 6 is connected to MT2 terminal of TRIAC BT136 through resistor R12 and pin no. 4 is connected to gate of TRIAC

· AC Motor is connected between phase and terminal MT2 of TRIAC as shown. Terminal MT1 is connected to neutral.

· A snubber circuit consists of R13 (39?) and C3 (0.01 µF) is connected across TRIAC for dv/dt protection.

Operation

Operation: –

To understand the operation better let us divide the complete circuit into different sections

1. Power supply section

2. Zero cross detector (ZCD) section

3. IR Modulator

4. IR sensor & Pulse generator

5. AC motor driver

Power supply section: – A step down transformer T1 steps down 230 VAC into 6-0-6 VAC and give it to bridge rectifier. Bridge rectifier will produce rectified DC output. This output is filtered by C1 and pulsating DC is applied to voltage regulator chip LM7805. The output of LM7805 is regulated +5 VDC that is given to entire circuit. The red LED connected across this output indicates power is on.

Zero cross detector (ZCD) section: – This section consists of just two transistors Q1. As shown in figure the rectified output from bridge rectifier is coupled to input of this section through diode D1. The operation of this section will be better understood with the help of following waveforms.

As shown in above figure, the first waveform at point ‘A’ in the circuit is step down AC wave from -6 to +6 of 50 Hz. It is rectified by bridge rectifier and voltage divided by resistor R2 and R3 to half and given to base of transistor Q1. This is shown as second waveform at point ‘B’ in the circuit. Because transistor Q1 is connected in switch configuration, when the input at the base becomes lower than 0.7 V it comes into cutoff and produces very short duration positive pulse at point ‘C’. That is shown as 3^rd waveform in figure.

IR sensor & Pulse generator: – IR sensor receives short duration 38KHz IR beam and gives -Ve pulse (of same width) to the trigger input of monostable made up using IC555. When it receives –Ve pulse on its trigger input it will generate positive pulse. The time period of this pulse is determined by value of resistor R7 and capacitor C4. Actually it generates variable width pulse from 0 – 10 ms as anyone changes pot R7.

AC motor driver: – AC motor driver includes an optocoupler, TRIAC and a snubber circuit. the output available from pulse generator is applied at the cathode input of internal LED of optocoupler. When output from IC555 goes low it turns on internal LED. This will triggers internal DIAC. The DIAC will turn ON TRIAC till LED is ON. Let us again take the help of waveforms to understand the operation better. We shall discuss two different cases. If first case the time period of monostable is less and in second case the time period of output pulse is more

Case 1: –The figure given below shows the waveforms when output pulse time period from monostable is of 2 ms.