This document discusses the design of a closed-loop speed controller for a single-phase AC induction motor using pulse-width modulation (PWM) of the TRIAC firing angle. It describes how varying the TRIAC firing delay can control motor speed, and how adding a tacho generator and error amplifier in a closed control loop enables set-point speed regulation. Key components include the zero-crossing detector, PWM generator, TRIAC driver, power supply, and control loop circuitry around the motor and tacho generator.

1. PWM based single phase ac induction
motor with close loop controller.
Introduction :-
Closed-loop control allows single-phase ac motors to close the
gap in performance and efficiency that exists between these
motors and other higher-performance, higher-cost motors. These
control techniques enable design solutions that are competitive for
a wide range of applications while maintaining their low-cost
appeal. This paper explores several key aspects of these speed
controls utilizing a traditional low-cost phase-control (TRIAC-drive)
technique.

2. Introduction
Here is a very simple example of AC motor speed control given
by changing firing angle of TRIAC with the help of 555 TIMER Ics.
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. A zero crossing detector circuit
is used here to interrupt 555 timer after every 10 ms. After getting an
interrupt timer 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.

3. What is closed loop control ?
An automatic control system is a preset closed-loop
control system that requires no operator action. This
assumes the process remains in the normal range for
the control system. A controlled variable is the
process variable that is maintained at a specified
value or within a specified range.

4. Design principle
Motor control
In this project the mains and the sample voltage is isolated from the
main through an opto isolator. The output of the opto isolator as a
square wave and used as a ZCD output. The ZCD output pulses are
inverted and feed to the mono-short based PWM genarator configured
on a 555 Timer IC. and the output is a TTL compatible pulse of 1ms
to 10ms pulse width. These pulses are used as the synchronization
pulses for determining the firing angle. The monoshot is used for
varying the firing pulse by varying its resistor and this o/p is given to
the triac driver to drive the TRIAC through the driving circuit. Which
can be incremented or decremented by an external variable resistor.

5. Design principle
Closed-loop control
A techo generator is coupled with that motor for closed loop control.
Its o/p voltage is directly propertional with the main motor. This o/p
voltage is rectified and sampled and then given to a subtractor. By
using subtractor we are calculate the error voltage by subtracting set
voltage and techo voltage. Then that error voltage added with the set
voltage and goes to the PWM generator. To roate the motor in
different speed you have to vary the set voltage.

6. Block diagram
ZCD PWM
Firing
circuit
TRIAC
LOAD
Techo
generato
r
Subtract
or
Adder
V set
230v ac
+12v -12v
Power
supply
230v ac
230v ac
Inverter

7. Circuit description
Power supply :-
Here 230V, 50 Hz ac signal is given as input to the primary of
the transformer and the secondary of the transformer is
given to the bridge rectification diode. The o/p of the diode
is given as i/p to the IC regulator (7912 &7812) through
capacitor (1000mf/35v). The o/p of the IC regulator is
given to the LED through resistors.

8. +12v
2.2k
7912
LED
TRANSFORMER CT
T1
1 5
6
4 8
- +
IN 4007
BRIDGE
1
4
3
2
1000uf LED
Power supply
2.2k
7812
1000uf
230v
ac
-12v

9. Advantages
Energy savings and process control are the two primary reasons for
using adjustable speed drives.
1. Energy savings is gained by using speed control of fans or
pumps as a means of flow control.
2. Speed control as a means of process control can:
3. Provide smoother operation
4. Provide acceleration control

10. Advantages
5. Accommodate the requirements of different processes with
one machine
6. Compensate for changing process variables
7. Allow slow operation for setup purposes
8. Adjust the rate of production
9. Allow accurate positioning
10.Control torque or tension

11. Future expansion
This project is designed with limitation of time but this can be
developed in the following directions as follows,
• With slight modification a PID controller can be developed with
low cost and simple design.
• A multi stage amplifier and filter can be provided to improve the
output signal condition.