SINGLE PHASE POWER CONTROL
Abstract
POWER control can be achieved by
changing firing angle of THYRISTOR with the help of micro controller
PIC16F877A.
Varying the POWER of an induction 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 THYRISTOR method.
A zero crossing detector circuit is
used here to interrupt PIC16F877A after every 10 ms. After getting an interrupt
pic16f877a will fire THYRISTOR 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 THYRISTOR 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:
From
the circuit diagram transformer TR1 step downs 240 VAC in
to 12 VAC and this is given to bridge rectifier. This rectified output is
directly fed to base of Q1 through resistors R3& R2. Same rectified output
is filtered through C5 and given to voltage regulator IC 7805 (U2). 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 PIC16F877A along with opto-coupler MOC3022(for triggering THYRISTOR)
and LCD display (for indicating percentage) are used for changing firing angle
of THYRISTOR.
Signal 'C' from zero crossing detector
circuit is directly given to pin no 6 (RA4) that is use as an external
interrupt RB0 pin of PORTB. All portD pins are connected to an LCD display.
Twoled’sareconnected to RC0 and other
one is connected to RC1.It isused to show the status of the motor power and
zero cross circuit. If the led that is connected with RC0blinks it means zero
cross circuit is working ok. The led that is connected with RC1shows the power
status. If led is ON it means motor power is enabled if led is OFF it means
power is disabled.
RB6is connected with input of
opto-coupler MOC3022. Output of MOC3022 is connected with gate of THYRISTOR.
The THYRISTOR is connected in loop with
AC motor and 240 VAC supply as shown.
Three push buttons Sw1,Sw2 and Sw3 are
connected with RA0, RA1& RA2respectively. PinRB1 (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,C1 with Resistor R1 performs power on reset.
Operation:
To
understand operation let us take a help of waveforms.
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