Auto Control of 3-Phase Induction Motor

7/30/2019 Auto Control of 3-Phase Induction Motor

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Under Guidance Of-:

Ms. Nikita Kharkhatte

(Lect. Elect. And Elex Department)

Presented By-:

Varun Bhaseen,

Amit Bawane,

Navneet Behra,

Hemant Kumar Dewangan


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Necessity For Controlling

Induction motors are the most widely used motors for

appliances, industrial control, and automation; hence, they

are often called the workhorse of the motion industry.

They are robust, reliable, and durable. When power is

supplied to an induction motor at the recommendedspecifications, it runs at its rated speed. However, many

applications need variable speed operations.

For example, a washing machine may use different speeds

for each wash cycle.

Historically, mechanical gear systems were used to obtainvariable speed. Which resulted in many losses so the recent

developments have overtaken mechanical systems by

electrical and electronics.


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Comparison Of Electrical and

Mechanical controlling Electrical -Electronics

controlling requires less

maintenance.

They are more efficient

as losses are reduced.

Less complicated and

easier to handle withless overall cost.

Mechanical controlling

requires more

maintenance.

They are less efficientdue to more losses

(frictional, mechanical

etc.)

Complicated in

handling and

increased overall cost.


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Principle of operation-:

Variable Voltage Variable Frequency (VVVF) or V/fis

the method of speed control in open loop.

This method is most suitable for applications without

position control requirements or the need for highaccuracy of speed control.

Thus the whole motor is therefore controlled by the V/f

characteristics of the induction motor used. The speed-torque characteristics along with the V/f characteristics

are further explained in detail of the three phase

induction motor.


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Speed-Torque Characteristics

Of Induction Motors-:

The characteristics are drawnwith rated voltage and

frequency supplied to the

stator.

During start-up, the motor

typically draws up to seventimes the rated current.

This high current is a result of

stator and rotor flux, the losses

in the stator and rotor

windings, and losses in the

bearings due to friction.

This high starting current

overcomes these components

and produces the momentum

to rotate the rotor.


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Contd.. At start-up, the motor delivers 1.5 times the rated torque of

the motor. This starting torque is also called locked rotor

torque (LRT).

As the speed increases, the current drawn by the motor

reduces slightly.

At base speed, the motor draws the rated current and

delivers the rated torque.

When the motor is running at approximately 80% of the

synchronous speed, the load can increase up to 2.5 timesthe rated torque. This torque is called breakdown torque.

As seen in the speed-torque characteristics, torque is highly

nonlinear as the speed varies. In many applications, the

speed needs to be varied, which makes the torque vary.


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V/f Control Theory-: As we can see in the

speed-torquecharacteristics, the

induction motor draws the

rated current and delivers

the rated torque at the

base speed. When the load is

increased while running at

base speed, the speed

drops and the slip

increases. the voltage applied to the

stator is directly

proportional to the product

of stator flux and angular

velocity.


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Contd.. This makes the flux produced by the stator proportional to

the ratio of applied voltage and frequency of supply. By

varying the frequency, the speed of the motor can be varied.

Therefore, by varying the voltage and frequency by the same

ratio, flux and hence, the torque can be kept constant

throughout the speed range This makes constant V/fthe

most common speed control of an induction motor

At base speed, the voltage and frequency reach the rated

values as listed in the nameplate. We can drive the motor

beyond base speed by increasing the frequency.

However, the voltage applied cannot be increased beyond

the rated voltage. Therefore, only the frequency can be

increased, which results in the field weakening and the

torque available being reduced.


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Implementation-:

Standard AC supply is converted to a DC voltage by usinga 3-phase diode center-tap rectifier. A capacitor filters theripple in the DC bus. This DC bus is used to generate avariable voltage and variable frequency power supply.

A voltage source power inverter is used to convert the DC

bus to the required AC voltage and frequency.

The power inverter has 4 switches that are controlled inorder to generate an AC output from the DC input.

PWM signals generated from the microcontroller controlthese 4 switches. The phase voltage is determined by theduty cycle of the PWM signals.


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Rectified input-:


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Contd.. A full-wave rectifier converts the whole of the input waveform to

one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveformto DC (direct current), and is more efficient.

In the positive half cycle diode (D1) will be forward biased andhence will conduct; while diode (D2will be reverse biased and willact as a open circuit and will not conduct. And diode D1 supplies

the load current & this current is flowing through upper half of thesecondary winding.

In the next half cycle of ac voltage polarity reverse and diode(D2) conducts, being forward biased; while diode (D1) doesntconduct, being reverse biased. And diode D2 supplies the load

current .now the lower half of the secondary winding carries thecurrent.

The op-amp lm393 is a analog voltage comparator which givesout a 9v dc output for the microcontroller with ripples absent.


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Main Circuit Diagram-:


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Contd.. The Circuit Diagram is used in controlling the motor. If LED 1

is on then it is Auto On. If LED 2 is set then it is Auto Off.

If LED 3 is on or set then it is active for Motor.

The Switch is used as a regulator and LCD is for display. Therelay is used for providing power to the Motor.

Then crystal oscillator is provided. There is an Auto On &Auto Off switch provided to work there. The three phaseinput is provided at Pin 10,11 & 12.

When the power is supplied to the circuit it checks the On &Off switches then if it is On then the Motor is switched on bythe PWM signals being sent by the Controller.

If the switch is Off then no connection is made to the motorand it is stop. The start and stop of motor depends uponrelay input signals.


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Advantages -:

More Efficient and more effective.

Easy to use & applicability.

Portable Circuitry consumes less area.

Reduced errors and losses as no mechanical parts.

Less initial cost.

Automatic Control.


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Applications and uses-:

In Industries and Factories to auto Control

Induction Motors.

In Plants to auto Control Induction Motors.

In laboratories and other places where Induction

motor control is required.

Auto Control of 3-Phase Induction Motor

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Transcript of Auto Control of 3-Phase Induction Motor