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V/f Speed Control for Engineers

This document describes an experiment to control the speed of a three-phase induction motor using the V/F method by plotting the variation of speed versus frequency while keeping the V/F ratio constant. The apparatus includes a three-phase induction motor, DC shunt motor, voltmeter, ammeter, rheostats, tachometer, and three-phase alternator. The procedure involves connecting the circuit, running the motor at rated speed, adjusting the alternator voltage, varying the alternator voltage and frequency while measuring speed, and recording multiple readings in a table. The results will show that the calculated and observed speeds are nearly the same, demonstrating that speed can be controlled by varying frequency while maintaining a constant V/F

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Krushna Pisal
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73 views4 pages

V/f Speed Control for Engineers

This document describes an experiment to control the speed of a three-phase induction motor using the V/F method by plotting the variation of speed versus frequency while keeping the V/F ratio constant. The apparatus includes a three-phase induction motor, DC shunt motor, voltmeter, ammeter, rheostats, tachometer, and three-phase alternator. The procedure involves connecting the circuit, running the motor at rated speed, adjusting the alternator voltage, varying the alternator voltage and frequency while measuring speed, and recording multiple readings in a table. The results will show that the calculated and observed speeds are nearly the same, demonstrating that speed can be controlled by varying frequency while maintaining a constant V/F

Uploaded by

Krushna Pisal
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOCX, PDF, TXT or read online on Scribd
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EXPERIMENT NO DATE:

To control speed of a three phase induction motor by V/f method.

AIM: To plot variation of speed verses frequency keeping V/f constant.

APPARATUS:

Sr. Name of the Apparatus Rating Quantity


No
415 V, 1500 rpm, 1 HP
1 3Ø Induction Motor 1

2 DC Shunt Motor 5 Hp 220 Volt, 19 Amp, 1


1500 rpm
3 VOLTMETER (0-600V).A.C. 1

4 AMMETER (0-10A) A.C 1


5 RHEOSTATS (750Ω, 1.2A), 2

6 TACHOMETER (0-9999 RPM) 1

7 3 Phase Alternator 415 Volt, 50 Hz, 3.5 KVA, 5 -


Amp, 1500 rpm,

CIRCUIT
DIAGRAM
THEORY :

Control of speed by changing supply frequency .The speed of an induction motor is directly
proportional to supply frequency by gradually changing the supply frequency., speed can be
increase and decreased smoothly. There is however, electrical drawback in this system. Electrically
supply authorities supply power at a fixed frequency of 50Hz .Provision of supply at various
frequency can be made by the consumer by having separate arrangement .Frequency conversion
equipment are ,therefore to be installed by the industries at additional costs .variation frequency
supply can be obtained from solid-state equipment or rotary converter i.e. motor generator set.

If speed control is achieved by changing frequency ,the supply voltage should also be
simultaneously changed .this is because if the supply frequency is reduced keeping the applied
voltage constant ,the flux is increased (E=4.44 ΦmfT) if flux is increased core-losses will reduced
the efficiency .On the other hand if the frequency is increased ,flux will decreased .thereby
reducing the torque developed .It is important ,therefore that the frequency changing device should
change frequency voltage simultaneously as a direct ratio ,that is if frequency in increased &if the
frequency is decreased the supply voltage must also decreased proportional.

PROCEDURE :

1) Connect the circuit as per circuit diagram.


2) Keep the Prime mover rheostat at minimum position , switch on the supply .
3) Run the motor at its rated speed.
4) Check Frequency and voltage,
5) adjust the field rheostat of the alternator field winding such that approximate 250 v
is available at the alternator terminal.
6) Connect the induction motor to alternator by closing switch.
7) Slowly increase the voltage to its rated value of the induction motor and measure the
speed, frequency & voltage
8) Measure the alternator voltage, frequency and speed of induction motor .
9) Obtain the V/f ratio.
10) Decrease voltage, frequency of alternator keeping constant.
11) Measure the speed of induction Motor.
12) Take Sr Alternator Alternator V/f Speed (N) 4-5 reading.
No. Supply Frequency Ratio
Vtg
OBSERVATION TABLE :
1
2

6
CALCULATION : N = ---------RPM , V = -------- V

Ns = 120f/p

Therefore f = N x p / 120

RESULT TABLE:

Sr Frequency (Hz) Speed


No. By calculation By observation N (rpm)

CONCLUSION: From this experiment we can conclude that calculated value by observation and
by calculations are nearly same.

Question and Answers


V/F Method of 3 Phase Induction Motor

1. 3 phase Induction motor is self starting or not ?


2. Difference between 3 phase Induction motor and 3 Phase synchronous motor
3. Different methods of speed control of 3 phase Induction motor
4. How to achieve variable speed and variable frequency
5. If you change the speed of prime mover which quantity will get affected.
6. How you keep V/F ratio constant
7.  Mention different types of speed control of slip ring induction motor?
e) By changing supply frequency     
f) By changing the number of stator poles
g) By rotor rheostat control   
h) By operating two motors in cascade

How can varying supply frequency control speed?


 We know that Ns = 120f / P
From the equation it is clear that by varying frequency speed can be varied

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