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Single phase AC Motor speed controller


Table of Contents:

  1. Single phase AC Motor speed controller
  2. Operation
Ashutosh Bhatt

Here is a very simple example of AC motor speed control given by changing firing angle of TRIAC with the help of micro controller 89C2051. 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 1 AC motor used and its speed is varied using DIAC-TRIAC method. Here instead of using DIAC and other components for firing TRIAC here I am using micro controller 89C51 for this purpose.

A zero crossing detector circuit is used here to interrupt 89C2051 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


Zero crossing detector circuit: -


Connections: - as shown in Circuit Diagram Tab1 transformer T1 step downs 230 VAC in to 9 VAC and this is given to bridge rectifier D1. 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 Circuit Diagram tab2 micro controller 89C2051 along with opto coupler MCT2E (for triggering TRIAC) and common cathode 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. 6 that is external interrupt 0 (P3.2) pin. All port P1 pins are connected with anodes of bar graph display and common cathode of bar graph is grounded. One led LED1 is connected with P3.0 as shown. Pin P3.1 is connected with input of opto-coupler MCT2E. Output of MCT2E 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 S1 - S3 are connected with P3.4, P3.5 & P3.7 respectively through diodes. Pin P3.3 (external interrupt 1) is connected with all three push buttons through diodes D1, D2 & D4 as shown. A 12 MHz crystal along with 2 33pf capacitor is connected with crystal input pins. Capacitor C1 with push buttons RST forms power on reset circuit.

Circuit Diagram

Circuit Diagram 2

Source Code

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Comments (5)

Sir how to calculate values

Sir how to calculate values of RC snubber circuit for 1HP motor

In circuit diagram 2 what is

In circuit diagram 2 what is the value of capacitor and resistor in parallel with triac and AC motor?

 good project

 good project

Pleasae provide me link for

Pleasae provide me link for downloading the source code for this project as one of the two free downloads.





No link for schematic or

No link for schematic or source code

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