Smooth start of a single phase induction motor

Smooth Start of a Single Phase
Induction Motor

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Introduction
Smooth Start of a Single
Phase Induction Motor
 The main aim of Smooth Start of a Induction Motor is to protect the
induction motor with a supply of low voltage at start and then
gradually increasing the voltage so that the motor can be operated
smoothly. Due to supply of abrupt voltage at start, the motor may gets
damaged and sometimes the windings can also get burnt. Hence the
project aims at giving a smooth start to the motor by using the concept
of SCR firing angle control.

Block Diagram

Hardware Components
• Op-amp
• Opto-isolators
• SCRs
• Voltage Regulator
• Transformer
• Diodes
• LED
• Resistors
• Capacitors
• Lamp

Circuit Diagram

Operational Amplifier
 An operational amplifier is a DC-coupled high-gain electronic voltage
amplifier.
 It produces an output potential typically hundreds of thousands of
times larger than the potential difference between its input
terminals.

Opto-oscillators
 Opto-isolator transfers electrical signals between two isolated
circuits by using light.
 It prevents high voltages from affecting the system receiving the
signal.

Silicon Controlled Rectifier
 SCR Controls the conduction under forward bias by applying a
current into the Gate terminal.
 Under reverse bias, SCR looks like conventional pn junction
diode.
SCR
2N3668
ANODE
CATHODE
GATE

Voltage Regulator
 It is used to regulate input voltage from a power source.
 A simple voltage regulator can be made from a resistor in series
with a diode.
 Voltage across the diode changes only slightly due to changes in
current drawn or changes in the input.

Transformer
 Transformer is an electrical device, which transfers electrical energy
between two or more circuits.
 These are used to increase or decrease the alternating voltages in
electric power applications.
 A varying current in the transformer's primary winding creates a
varying magnetic flux in the transformer core and a varying field
impinging on the transformer's secondary winding.

Diode
 A Diode is a semiconductor device with two terminals allowing the
flow of current in one direction only.
 It is an electronic component with two electrodes.
 It is often used in power supplies and sometimes to decode
amplitude modulation radio signals also.

Power Supply
 The 230V AC supply is first stepped down to 12V AC using a step
down transformer.
 This is then converted to DC using bridge rectifier.
 The AC ripples is filtered out by using a capacitor and given to the
input pin of voltage regulator 7812.
 At output pin of this regulator, we get a constant 12V DC which is
used for different ICs in this project.

Soft
Starter
 A soft starter is a device which controls the acceleration of an electric
motor by means of controlling the applied voltage.
 It is another form of reduced voltage starter for AC Induction motors.
 The soft starter employs solid state devices to control the current flow
and therefore the voltage applied to the motor.
 These are connected in series with the line voltage applied to the
motor, controlling the voltage applied to each winding.

Description about Project
 Basically, the main purpose of this project is to make the system
which can easily start the single phase induction motor.
 It is used to give protection to the induction motor, by applying
voltage gradually from low to high, by which the motor can start
easily.
 During initial conditions, it takes up larger current than its capacity.
 This leads the motor capture the speed resulting into mechanical
stress and also causing some damage to the motor windings.
Sometimes the windings may also get burnt.

Description
 Smooth start to the induction motor is based on the SCR firing angle
control principle.
 This project consists of a two SCR’s which are anti-parallel and are
further connected in series with an induction motor to the supply.
 So from that starting point of time, the angle which is called the firing
angle is heavily delayed by received delayed pulses from an operational
amplifier.
 The op-amp operates in comparator mode which compares zero voltage
pulses to develop a saw-tooth voltage.

Description
 This waveform is further compared against a varying voltage by
charging and discharging of the capacitor to develop delayed
pulses for triggering the SCRs.
 A lamp is provided in this project in place of an induction motor
for demonstration use.

Working Principle
 To start the motor with low current, two SCR's are connected back to base
in each phase.
 They are triggered slowly at initial states and gradually the triggering
pulse is increased.
 So that motor gets high current after a time delay when windings are in
the position of withstanding the high current.
 To trigger the gates of SCR's the op-amps are used.

Working
Principle
 Op-amp is configured to get a level voltage that will initially be high
and gradually fall to zero.
 Smooth Start of an induction motor have certain time constant and
back to back SCR.
 SCR being triggered by opto-oscillators from the rc time delay
arrangement where the induction motor start slowly.
 It starts from slow to full speed.

Working
Principle
 Switch ON the circuit.
 And then the circuit will slowly pick up the speed.
 This is how the smooth start of induction motor works which is
important as per the industrial applications.
 It starts drawing less current in the start and that is how a smooth
starter works.

Applications
 These are main applications of smooth starter:
• Steel industries (Rolling mills and processing lines)
• Cement industries
• Textile industries
• Machine tool applications
• Power sector

Advantages
 Vast improvement in efficiency relative to the primary resistance starter.
 Durability.
 Ease of operation.
 Lightweight, Compact Unit.
 Speed Variation.

Disadvantages
 High Insulation Costs.
 Not Uniform Torque.
 Cannot Handle Overload.

Future Scope
 Further the project can be enhanced by using six SCRs, two connected in
back to back for each phase.

Conclusion
 In Smooth Start of a Induction Motor, protection is provided to
induction motor. By using SCR firing angle control it provides a
smooth start to the motor to stop from the windings getting burnt.

Smooth start of a single phase induction motor

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