Smooth start of a single phase induction motor

Smooth Start of a Single Phase Induction Motor

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Introduction


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.

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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.

SCR2N3668

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.


http://www.edgefxkits.com/induction-motor-protection-system






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.


http://www.edgefxkits.com/smooth-start-of-a-single-phase-induction-motor





Smooth start of a single phase induction motor

Engineering

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