Single Phase AC Motor speed controller using 8051



Single phase AC Motor speed controller
Abstract:
 Here is a very simple example of AC motor speed control given by changing firing angle of TRIAC with the help of micro controller AT89C51. Varying speed of AC motor by means of changing firing angle of any thyristor is very widely used method. One very nice example is fan regulator in which a fan motor is an AC motor used and its speed is varied using TRIAC method.
A zero crossing detector circuit is used here to interrupt AT89C51 after every 10 ms. After getting an interrupt 89C51 will fire TRIAC after some delay from 1 to 9 ms. This will cut the current supplied to motor and so the speed of motor will reduce. Thus by varying the delay after which the TRIAC is triggered one can change the speed of motor.
Main circuit is a combination of two sub circuits.
  1. Zero crossing detector circuit
  2. Firing angle control circuit


Resources:
    To download Code , Circuit : Click Here
 

Zero crossing detector circuit:
Connections:

As shown in above figure transformer T1 step downs 230 VAC in to 9 VAC and this is given to bridge rectifier. This rectified output is directly fed to base of Q1 through resistors R1 & R2. Same rectified output is filtered through C1 and given to voltage regulator IC 7805. Output of 7805 is regulated 5 VDC that is given as biasing voltage for both transistors Q1 & Q2 (same regulated 5 V supply is given to main control section also). Both transistors are connected in switch configuration. The final output ‘C’ is given to main control section.

Firing angle control circuit:
Connections:

As shown in above figure micro controller AT89C51 along with opto coupler MOC3011 (for triggering TRIAC) and common Anode type bar graph display (for indicating angle) are used for changing firing angle of TRIAC. Signal 'C' from zero crossing detector circuit is directly given to pin no 12 (INT0) that is external interrupt 0 (P3.2) pin. All port P1 pins are connected with cathode of bar graph display and common Anode of bar graph is connected to 5 volt through 220 ohm resistor. Two led’s of bar graph display is connected to P3.7 and other one is connected to P3.6. It is used to show the status of the motor power and zero cross circuit. If the led that is connected with P3.7 blinks it means zero cross circuit is working ok. The led that is connected with P3.6 shows the power status. If led is on it means motor power is enabled if led is off it means power is disabled.
P0.0 is connected with input of opto-coupler MOC3011. Output of MOC3011 is connected with gate of TRIAC. TRIAC is connected in loop with AC motor and 230 VAC supply as shown. RC snubber circuit is connected is connected in parallel with TRIAC. Three push buttons Sw1, Sw2 and Sw3 are connected with P1.0, P1.1 & P1.2 respectively. Pin P3.3 (external interrupt 1) is connected with all three push buttons through diodes D1, D2 & D3 as shown. A 12 MHz crystal along with two 33pf capacitor is connected with crystal input pins. Capacitor C2 with Resistor R6 performs power on reset.


Operation:

To understand operation let us take a help of waveform's.


  • As shown in figure the first wave form is full rectified wave that is fed to the base of Q1. Whenever this voltage falls below 0.7 V Q1 is switched off. So its output goes high.
  • This will produce one very short positive pulse at ‘B’ as shown in figure as second waveform
  • As this positive pulses are fed to Q2 which is again connected in switch configuration, it will produce one negative pulse at ‘C’ of same width of positive pulse. This is shown as third waveform

    • Now as this negative pulse output is given to interrupt pin of micro controller. It will generate interrupt every time.
    • After getting an interrupt micro controller will on the timer0. Timer 0 is used to generate a delay after some delay make P0.0 low. This is used to trigger (fire) TRIAC.
    • Depending upon the time delay in between interrupts and pulse on P0.0, the TRIAC is fired earlier or later.
    • As shown In figure I have given wave forms for two different cases one for 4 ms delay and second for 8 ms delay
    • In the first case for 4 ms delay the output positive cycle of AC wave is 60% of  input so only 60% current is delivered to load (the dotted line shows part of wave form that has been cut)
    • For second case 8 ms delay output cycle is 20% of input cycle so only 20% current is supplied to load

This change in delay is done through push buttons given. So let us see the functions of push button


Switch
                                                   Functions                                                 
 Sw1
 to switch on / off TRIAC
 Sw2
 S2 to increase delay by 1 ms
 Sw3
 to decrease delay by 1 ms

Variation in angle is displayed on bar graph display. If angle is more, less current supplied to motor and motor speed is reduced less bars appears on display. If angle is decreased, current supplied will be more, speed of motor is increased and more bars appear on display. 
The diodes D1, D2 and D3 are connected in such a manner that whenever any of three push buttons, is pressed it will generate external interrupt 1.

When Sw1 is pressed first time it will enable the switching of TRIAC. After every 10 ms external interrupt 0 is generated and that will start the entire operation. Pressing Sw1 again will disable switching of TRIAC. Now no more pulses are generated on P0.0 and complete operation is shut off.

On pressing Sw2 will increase delay by 1 ms (firing angle by 18o deg). So firing of TRIAC is delayed by 1 ms and amount of current supplied to load is decreased by 10%. Maximum delay is 9 ms (max angle is 162o deg). Bar is decreased by 1 step.

In same manner when S3 is pressed delay is decreased by 1ms (firing angle by 18o deg) and load current increases by 10%. Minimum delay is 0 means full positive cycle is applied. Bar is increased by 1 step.

The bar graph displays the variation in phase angle. More bars means angle is small and motor speed is high and vice versa.

Pc Software:

Hardware can be interfaced with the pc to control the speed of motor. It has no as such practical use it is just for learning point of view.
Hardware can be connected with pc through a serial port. Serial communication is used to communicate between hardware and software.
Visual basic 6 is used for making this GUI (Graphic user Interface).
Baud rate is 2400 bps.

Tested:

This hardware is tested with 120 watts Pedestal Fan, 200 watt bulb and with grander motor.

Motor specification is as

BLENDER MOTOR 7020
Voltage: AC 220V   
Frequency: 50/60 HZ  
Wattage: 250 WATT
Ampere: 1A
RPM: 13000 RPM


Software logic:

The complete operation is based on the software embedded in micro controller AT89C51. The software is written in C language. Compiler is keil 4. Different functions are used for different operations like incangle (), decangle (), init (), uart () and Interrupts functions etc. 


Resources:
    To download Code , Circuit : Click Here