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INDUCTION MOTOR

 Basic theory & construction

 Phasor diagram & equivalent circuit

 Torque speed characteristics

 Testing & circle diagram

 Starting & sped control

 Other modes of operation

 Single-phase induction motor

 

Induction Motors-Demonstration file

 

Internet site: Induction Motor Simulation

Internet site: induction motor program -equivalent circuit

 Objective questions

HOME takes you to the start page after you have read these Topics. Start page has links to other topics.

 

Cherry Tree Scientific Software - Graham J. Rogers - Induction Motor Simulation

 

MENU-DRIVEN GRAPHICAL USER INTERFACE (GUI) PROGRAMS- by Prof. H.Sadat. -The transformer program obtains the transformer equivalent circuit from open-circuit and short-circuit tests. It also finds the transformer performance characteristics using the transformer parameters. In the induction motor program the parameters of the equivalent circuit can be entered on the circuit diagram. The program draws the Thevenin's equivalent circuit. Performance characteristics are then obtained for the specified slip and the torque slip curve is plotted on the screen. The analysis can be repeated for different slips. A GUI program deals with the design of a first-order controller in the forward path of a closed-loop control system.  This GUI program designs the following controllers: Proportional, phase-lag, phase-lead, PD, PI, and PID controllers.  To run the controller design program at the MATLABprompt type>> rldesigngui

 

Basic theory & construction

 

Introduction to the concept of rotating magnetic field and production of rotating magnetic flux by two-phase and three-phase currents flowing through concentrated windings (1slot/pole/phase), magnitude & speed of resultant rotating flux, Constructional features of induction motors, three-phase windings

Expression for the stator induced emf, principle behind the development of torque by the rotor, rotor frequency & slip.

 

Phasor diagram & equivalent circuit

 

Expressions for rotor current, establish the relationship-rotor input: copper loss: output-1: s: (1-s)

 

Torque speed characteristics

 

Effect of rotor resistance variation, maximum torque and the slip at which it occurs

 

Testing & circle diagram

 

No load and blocked rotor tests equivalent circuit parameters, circle diagram

 

Starting & sped control

 

Requirements to develop high starting Torque and reduced starting current, Deep bar and double -cage rotors, Starters (star-delta, auto transformer & rotor resistance starters)

 

Other modes of operation

 

Braking of induction motor, induction generator

 

Single-phase induction motor

 

Double revolving field theory, Torque sped characteristics, equivalent circuit, Starting methods, auxiliary winding parameters for maximum starting torque, Characteristics of split-phase motor, capacitor start motor, two value capacitor motor, shaded pole motor

 

Induction Motors-Demonstration file- Moduleg.xls

 

This module demonstrates the torque versus speed characteristics of a three-phase induction motor. Draw the exact equivalent circuit of the motor and observe the effects of changing various parameters on the torque versus speed curve. In particular, vary the rotor resistance, applied voltage, and the supply frequency and observe the effects on the torque-sped curves. Also verify whether the motor can supply a particular load. Draw two types of plots: torque versus slip and torque versus speed. The type of plot can be chosen by selecting the desired plot type from the plot pull-down menu under commands. There is a list of parameters of the induction motor that can be changed by clicking on the values and editing them to the desired values. The curve is updated by clicking on the calculate button. The curve is drawn for the displayed set of parameters.

                                                                                                                                      

 

 

OBJECTIVE QUESTIONS

 

  1. The speed at which the rotating magnetic field produced by stator currents rotates in an induction motor is:

 

(A)    Synchronous speed

(B) Rotor speed

(C) Greater than synchronous speed

 

Ans.  (A)

 

2.       A 3-phasde 50 Hz induction motor runs at speed of 940 RPM; the speed of the rotating magnetic field will be:

 

(A) 940 RPM

(B) 1000RPM

(C) 1050 RPM

(D) 1100 RPM

 

Ans.  (B)

 

  1. The no-load speed of an induction motor is of the order of

 

(A) 8%

(B) 4%

        (C) 2%

(D) 6%

 

Ans.  (C)

                                                                                                                                      

 

  1. Iron losses in the rotor of a 3-phase induction motor are negligible, because

 

(A)    Frequency of rotor emf is too low

(B)    Flux linking the rotor is of constant magnitude

(C)    Flux density in the rotor parts is too low

 

Ans: (A)

 

  1. Full-load copper losses in a 3-phase 50 Hz 4-pole induction motor running at 1455 rpm are 300 W. The rotor input is

 

(A)    5 kW

(B)    10 kW

(C)    20 kW

(D)    50 kW

 

Ans.  (B)

 

  1. The no-load current of a 3-phase induction motor in terms of its full-load current is of the order of

 

(A)    5%

(B)    10%

(C)    25%

(D)    50%

 

Ans.  (C)

 

  1. The power factor of a 3-phase induction motor at no load is approximately

 

(A)   0.2

(B)    0.7

(C)    0.85

(D)    1

 

Ans: (A)

 

8.The maximum torque in a 3-phase induction motor occurs at a slip

 

(A)       R2/ X2

(B)       X2/ R2

(C)      R22/ X2

(D)      R22/ X22

 

Ans: (A)

 

9. For high starting torque, the most 3-phase induction motor is

 

(A)    Squirrel-cage type

(B)    Slip-ring type

(C)    Deep bar squirrel-cage type

(D)    Double-cage induction motor

 

Ans.  (B)

 

10.     The starting torque in a single-phase induction motor is

 

(A)   High

(B)    Low

(C)    Zero

(D)    Very low

 

Ans.  (C)

 

11.     The efficiency of a single-phase induction motor as compared to a 3-phase induction motor rating is

 

(A)   Lower

(B)    Higher

(C)    Equal

 

Ans: (A)

 

12.     The starting torque of a 3-phase induction motor can be increased by increasing

 

a)        the rotor reactance

b)       the rotor resistance

                                               c ) the stator resistance

 

Ans. B

13.  The maximum starting torque in the induction motor is developed when the rotor reactance is

 

a)      half of  the rotor resistance

b)       twice of the rotor resistance

c)       equal to the  rotor resistance

 

Ans. C

 

14.  A change of 55 in the supply voltage of an induction motor will produce  a change of approximately

 

a)      5%

b)       7%

c)       10%

d)       25%

in the rotor torque.

Ans. D

 

15.     The transformation ratio of an induction motor can be found by

 

a)       blocked rotor test

b)       open circuit test

c)       slip test

d)       stator resistance test

 

16.     The crawling in the induction motor is caused by

 

a)       improper design of the machine

b)       low supply voltage

c)       high loads

d)       harmonics developed in the motor.

 

Ans. D

 

17.     Which one of the following motors is most suitable for signalling devices and timers?

 

a)       dc series motor

b)       ac series motor

c)       induction motor

d)       reluctance motor

 

Ans. D