MCH Rest Experiments

8/12/2019 MCH Rest Experiments

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(a) Stator(b) Rotor

(a) Stator:

The stator is the outer body of the motor which houses the drivenwindings on an iron core. In a single speed three phase motor design, thestandard stator has three windings, while a single phase motor typically hastwo windings. The stator core is made up of a stack of round pre-punchedlaminations pressed into a frame which may be made of aluminum or castiron.

The laminations are basically round with a round hole inside throughwhich the rotor is positioned. The inner surface of the stator is made up of a

number of deep slots or grooves right around the stator.

It is into these slots that the windings are positioned. Thearrangement of the windings or coils within the stator determines thenumber of poles that the motor has.A standard bar magnet has two poles, generally known as North andSouth. Likewise, an electromagnet also has a North and a South pole.

As the induction motor Stator is essentially like one or moreelectromagnets depending on the stator windings, it also has poles in

multiples of two. i.e. 2 pole, 4 pole, 6 pole etc. Greater the no of poles,greater the speed and vice versa. The stator winding ,when supplied with 3phase currents ,produces a magnetic flux which is for constant magnitudebut revolves at synchronous speed (given by Ns=120f/p).This revolvingmagnetic flux induces an emf in the rotor by mutual induction.

The winding configuration, slot configuration and lamination steel allhave an effect on the performance of the motor. The voltage rating of themotor is determined by the number of turns on the stator and the power

rating of the motor is determined by the losses which comprise copper lossand iron loss, and the ability of the motor to dissipate the heat generated bythese losses. The stator design determines the rated speed of the motorand most of the full load, full speed characteristics.


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(b) Rotor:

The Rotor comprises a cylinder made up of round laminationspressed onto the motor shaft, and a number of short-circuited windings.The rotor windings are made up of rotor bars passed through the rotor,from one end to the other, around the surface of the rotor. The barsprotrude beyond the rotor and are connected together by a shorting ring ateach end. The bars are usually made of aluminum or copper, butsometimes made of brass.

The position relative to the surface of the rotor, shape, cross sectionalarea and material of the bars determine the rotor characteristics.Essentially, the rotor windings exhibit inductance and resistance, and thesecharacteristics can effectively be dependent on the frequency of the current

flowing in the rotor. A bar with a large cross sectional area will exhibit a lowresistance, while a bar of a small cross sectional area will exhibit a highresistance.

Likewise a copper bar will have a low resistance compared to a brassbar of equal proportions. Positioning the bar deeper into the rotor,increases the amount of iron around the bar, and consequently increasesthe inductance exhibited by the rotor. The impedance of the bar is made upof both resistance and inductance, and so two bars of equal dimensions willexhibit different A.C. impedance depending on their position relative to thesurface of the rotor. A thin bar which is inserted radialy into the rotor, withone edge near the surface of the rotor and the other edge towards theshaft, will effectively change in resistance as the frequency of the currentchanges. This is because the A.C. impedance of the outer portion of thebar is lower than the inner impedance at high frequencies lifting theeffective impedance of the bar relative to the impedance of the bar at lowfrequencies where the impedance of both edges of the bar will be lowerand almost equal. The rotor design determines the starting characteristics.

Types of Rotor:

(a) Squirrel cage rotor(b) 2-phase Wound or Wound rotor


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(a) Squirrel Cage Rotor:

Most common AC motors use the squirrel cage rotor, which will befound in virtually all domestic and light industrial alternating current motors.The squirrel cage takes its name from its shape - a ring at either end of therotor, with bars connecting the rings running the length of the rotor. It istypically cast aluminum or copper poured between the iron laminates of therotor, and usually only the end rings will be visible. The vast majority of therotor currents will flow through the bars rather than the higher-resistanceand usually varnished laminates. Very low voltages at very high currentsare typical in the bars and end rings; high efficiency motors will often usecast copper in order to reduce the resistance in the rotor.

In operation, the squirrel cage motor may be viewed as a transformer

with a rotating secondary - when the rotor is not rotating in sync with themagnetic field, large rotor currents are induced; the large rotor currentsmagnetize the rotor and interact with the stator's magnetic fields to bringthe rotor into synchronization with the stator's field. An unloaded squirrelcage motor at synchronous speed will consume electrical power only tomaintain rotor speed against friction and resistance losses; as themechanical load increases, so will the electrical load - the electrical load isinherently related to the mechanical load. This is similar to a transformer,where the primary's electrical load is related to the secondary's electricalload.

This is why, as an example, a squirrel cage blower motor may causethe lights in a home to dim as it starts, but doesn't dim the lights when itsfan-belt (and therefore mechanical load) is removed. Furthermore, a stalledsquirrel cage motor (overloaded or with a jammed shaft) will consumecurrent limited only by circuit resistance as it attempts to start. Unlesssomething else limits the current (or cuts it off completely) overheating anddestruction of the winding insulation is the likely outcome.Virtually every washing machine, dishwasher, standalone fan, record

player, etc. uses some variant of a squirrel cage motor.

(b) Wound Rotor:

An alternate design, called the wound rotor, is used when variablespeed is required. In this case, the rotor has the same number of poles asthe stator and the windings are made of wire, connected to slip rings on the


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shaft. Carbon brushes connect the slip rings to an external controller suchas a variable resistor that allows changing the motor's slip rate. In certainhigh-power variable speed wound-rotor drives, the slip-frequency energy iscaptured, rectified and returned to the power supply through an inverter.

Compared to squirrel cage rotors, wound rotor motors are expensive andrequire maintenance of the slip rings and brushes, but they were thestandard form for variable speed control before the advent of compactpower electronic devices. Transistorized inverters with variable frequencydrive can now be used for speed control, and wound rotor motors arebecoming less common. (Transistorized inverter drives also allow themore-efficient three-phase motors to be used when only single-phasemains current is available, but this is never used in household appliances,because it can cause electrical interference and because of high power

requirements.)

This type of motor is becoming more common in traction applications suchas locomotives, where it is known as the asynchronous traction motor.The speed of the AC motor is determined primarily by the frequency of theAC supply and the number of poles in the stator winding, according to therelation:

Ns= 120f / p

Where

Ns = Synchronous speed, in revolutions per minuteF = AC power frequencyp = Number of poles per phase winding

Slip:Actual RPM for an induction motor will be less than this calculated

synchronous speed by an amount known as slip, that increases with the

torque produced. With no load, the speed will be very close tosynchronous. When loaded, standard motors have between 2-3% slip,special motors may have up to 7% slip, and a class of motors known astorque motors are rated to operate at 100% slip (0 rpm/full stall).

The slip of the AC motor is calculated by:


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s = (Ns Nr) / Ns

Where

Nr = Rotational speed, in revolutions per minute.

s = Normalized Slip, 0 to 1.

Of all AC machines the poly phase induction motor is the one which isextensively used for various kinds of industrial drives. It has the followingsmain advantages and disadvantages

Production of Rotating Magnetic Field:

The fundamental principle of operation of AC machine operation isthat if a 3-phase set of currents, each of equal magnitude and difference by120 (a balanced 3-phase system) flows in a 3-phase winding, then it willproduce a rotating magnetic field of constant magnitude.

Conditions for Rotating Magnetic Field:

To produce a rotating magnetic field, the following conditions must be met:

1. The supply must be poly phase2. There must be angular displacement between the axis of the coilsfor the a, b, and c phases

Production of Torque:

The 3-phase stator windings set up a rotating magnetic field. The fluxproduced passes through the air-gap, sweeps past the rotor surface and socuts the rotor conductors which are yet (at startup) stationary. Due to therelative speed between the rotating flux and the stationary conductors, anemf is induced in the latter according to Faradays law of electromagneticinduction and this is dynamically induced emf. The frequency of theinduced emf is the same as the supply frequency. Its magnitude isproportional to the relative velocity between the flux and the conductorsand its direction is given by Flemings Right-hand rule. Since the rotorconductors form a closed circuit, rotor current is produced whose direction


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as given by Lenzs law is such as to oppose the cause producing it. In thiscase, the cause which produces the rotor current is the relative velocitybetween the rotating flux and the stationary conductors. Hence to reducethe relative speed, the rotor starts to rotate in the same direction as that ofthe flux and tries to catch up with it.

Percentage Slip:

In practice, the rotor never succeeds in catching up with the statorfield. If it really did so, then there would be no relative speed between thetwo, hence no rotor emf, no rotor current and therefore no torque tomaintain rotation. That is why the rotor runs at a speed which is always lessthan the speed of the stator field. The difference in speed depends on the

load on the motor.

The difference between the synchronous speed NS and the actualspeed Nr of the rotor divided by the synchronous speed is called thepercentage slip. Mathematically,

100%

=

S

rS

N

NNSlip

Losses in an Induction Motor:

The power input to an induction motor, Pin, is in the form of three-phase electric voltages and currents. The first loss in the machine is I2Rlosses in the stator windings (the stator copper loss). Then some amount ofpower is lost as hysteresis and eddy currents in the stator (stator core

losses). The power remaining at this point is transferred to the rotor of themachine across the air gap between the stator and the rotor. This power iscalled the air-gap power of the machine. After the power is transferred tothe rotor, some of it is lost as I2R losses (the rotor copper loss), and the restis converted from electrical to mechanical form. Finally, friction andwindage losses and stray losses are subtracted. The remaining power isthe output of the motor, Pout.


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The core losses in the induction motor come partially from the stator

circuit and partially from the rotor circuit. Since an induction motor normallyoperates at a speed near synchronous speed, the relative motion of themagnetic fields over the rotor surface is quite slow, and the rotor corelosses are very tiny compared to the stator core losses.

The higher the speed of an induction motor, the higher its friction,windage and stray losses. On the other hand, the higher the speed of themotor, the lower will be its core losses. Therefore, these three categories oflosses are sometimes lumped together and called rotational losses. Thetotal rotational losses of a motor are often considered to be constant withchanging speed, since the component losses change in opposite directionswith change in speed

Advantage:

It has very simple and extremely rugged, almost unbreakableconstruction (especially squirrel cage type).

Its cost is low and is very reliable.

It has sufficiently high efficiency. It requires minimum of maintenance.

It starts up from rest and needs no extra starting motor and has notto be synchronized .its starting arrangement is simple especially forsquirrel cage type motor.

Disadvantages:

Its speed cannot be varied without sacrificing some of its efficiency. Just like a dc shunt motor its speed decreases with increase in load. Its starting torque is somewhat inferior to that of the dc shunt motor.

Block Diagram:


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Procedure:

Set the apparatus as shown in the block diagram.

Set the rated line voltage, VL, and for each different value note thevalues of line current, IL, the total true 3-phase power, WT, and thetotal reactive power, Q, using measuring unit when you are applyingload on the DC Generator using Brake Control Unit. For P and Qmeasurement refer to Appendix of Manual.

Measure the total power using the concept of 2-wattmeter method(Use appendix for 2-wattmeter method)for 3-phase measurement.

Note the values of torque, T, the output power, Pout, and the motorspeed, Nr, are measured from the brake control unit (BCU) for eachvalue.

Calculate the values of power factor, cos , % slip and the efficiency,from the formulas given.

Plot the graphs as mentioned


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Observations: Synchronous Speed = Ns= _______ rpm

S.No.

Line toLine

Voltage

VL

(V)

LineCurrent

IL

(A)

TotalPower

WT

(W)

ReactivePower

Q

(VAR)

PowerFactor

cos

RotorSpeed

Nr

(rpm)

%Slip

Torque

(N.m)

OutputPower

Pout

(W)

Efficiency,

(%)

1

2

3

4

5

6

7

8

9

Formulas

100%

=

S

rS

N

NNSlip

LL

T

IV

W

3cos = 100=

in

out

P

P


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Graphs:

1. Plot output versus efficiency2. Plot reactive power versus efficiency3. Plot speed versus efficiency4. Plot power factor versus efficiency5. Plot torque versus efficiency6. Plot torque slip curve

Scale__________________

________________


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Scale_______

___________________________

Scale__________________________________


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Scale

__________________________________

Scale__________________________________


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Scale__________________________________

Scale__________________________________


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Comments:

What is the purpose of load test of induction motor?____________________________________________________________________________________________________________________________________________

____________________________________________________________________________________________________________________________________________________________________________________

Why one of the watt meters gives negative, zero and then positive readingas the load is increased gradually.

________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Is it possible for both watt meters to give same reading, if possible thenwhy?

________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________What would you do with W1if its reading becomes negative?

____________________________________________________________________________________________________________________________________________

____________________________________________________________________________________________________________________________________________________________________________________What is power factor of induction motor at no load and why?

________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Why does power factor increases with load?________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________


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What type of motor was used in experiment, slip ring or squirrel?_____________________________________________________________________________________________________________________________________________________________________________

What type of motor was used in experiment, slip ring or squirrel?________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Is induction motor a variable speed motor?

_____________________________________________________________________________________________________________________________________________________________________________Why does motor damage due to over loading.

________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Can power factor of induction motor be leading?

________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Marks Obtained = _________Date = __________________

Signature = ______________


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EXPERIMENT NO 8

TO RUN AN INDUCTION MOTOR AS INDUCTION GENERATOR

BLOCK DIAGRAM:

APPARATUS:

Induction motor

DC Machine Break control unit Measuring unit Power supply

PROCEDURE:

First run DC motor (separately excited) and note the direction of rotation of the motor.

Connect ac motor separately and run it and note its direction of rotation.

If it is same as that of the DC motor then procedure further otherwise first make thedirection of rotation of both same by changing the phase sequence.

Start both of them together. Measuring unit will show the values of voltage, current, realpower and reactive power.

At this point the speed is less than 1500 rpm. Now increase the speed of the motor so that

it becomes equal to the rated speed that is 1500 rpm. At this speed the motor is in floating condition i.e when slip becomes zero the machines

would be in floating condition. Again note the values of current, voltage, active and

reactive power for the floating condition.

Now increase the speed further from 1500 rpm. At this point the motor becomesgenerator and the values of voltage, current active and reactive power become negative.

Note down these readings in the table.

Ind

Motor

DC

M/CM.U BCUPower

supply


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OBSERVATIONS

State of

machineV I W1 W2 WT p.f Q

Ind. Motor

Floating

Ind.

Generator


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EXPERIMENT # 9

CONSTRUCTION OF V-CURVES OF A SYNCHRONOUS MOTOR

THEORETICAL BACKGROUND

SYNCHRONOUS MOTOR

Synchronous motors are AC motors that have a field circuit

supplied by an external DC source. They convert AC electrical power to mechanical power. It is

electrically identical to an alternator or AC generator. Some characteristics of the synchronous

motor are:

It runs either at synchronous speed or not at all i.e. while running it maintains a constantspeed. The only way to change its speed is to vary the supply frequency.

It is not self-starting. It has to be run up to synchronous or near synchronous speed bysome means before it can be synchronized to the supply.

It is capable of being operated under a wide range of power factors both lagging andleading.

PRODUCTION OF TORQUE:

In a synchronous motor, a three-phase set of stator currents

produces a rotating magnetic field, BS. The field current, IFof the motor produces a steady-state

magnetic field, BR. Therefore, there are two magnetic fields present in the machine, and the rotor

field will tend to line up with the stator field, just as two bar magnets will tend to line up if

placed near each other. Since the stator magnetic field is rotating, the rotor magnetic field willconstantly try o catch up. Larger the angle between the two magnetic fields, the greater the

torque on the rotor of the motor. The basic principle of a synchronous motor operation is that the

rotor chases the rotating stator magnetic field around in a circle, never catching up with it.

SPEED OF SYNCHRONOUS MOTOR:

The rotor (which is initially unexcited) is speeded up to

synchronous or near synchronous speed by some arrangements and then excited by the DC

source. The moment this synchronously rotating rotor is excited, it is magnetically locked into

position with the stator i.e. the rotor poles are engaged with the stator poles and both run

synchronously in the same direction. It is because of this interlocking of stator and rotor polesthat the motor has either to run synchronously or not at all. The synchronous speed is given by

the usual relation:

P

fNS

120=


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However, this engagement is not very rigid. As the load on the motor is increased, the rotor

progressively tends to fall back in phase by some angle but it still continues to run

synchronously.

V-CURVES:

The V-curves of a synchronous motor show how armature current varies with its

field current when motor input is kept constant. These are obtained by plotting armaturecurrent while motor input is kept constant and are so called because of their shape. There is a

family of such curves, each corresponding to a definite power intake.

To draw these curves experimentally, the motor is run from constant voltage and constant

frequency bus bars. Power input to motor is kept constant at a definite value. Next, the field

current is increased in small steps and corresponding armature currents are noted. When plotted,

we get a V-curve for a particular constant motor input. Similar curves are drawn by keeping

motor input constant at different values.

EFFECT OF FIELD CURRENT CHANGES:

Considering a synchronous motor in which the

mechanical load is constant. When the field current is increased, the magnitude of the back emf,

EAin the motor increases, but does not affect the real power supplied by the motor. The power

supplied by the motor changes only when the shaft load torque changes. Since a change in

armature current, IA, does not affect the shaft speed and since the load attached to the shaft is

unchanged, the real power supplied is unchanged. The terminal voltage is also kept constant by

the power source supplying the motor.

Therefore, as the value of EA increases, the magnitude of IA first decreases and then increases

again. At low EA, the armature current is lagging and the motor is an inductive load. It is

therefore consuming reactive power Q. As the field current is increased, EA increases and thearmature current eventually lines up with the voltage and the motor is purely resistive. As the

field current is further increased, the armature current becomes leading, and the motor becomes a

capacitive load. So now it consumes negative reactive power Q or alternatively supplying

reactive power to the system.

A plot of IA versus IF for a synchronous motor is as

shown in Figure-1 above. For each curve, the minimum armature current occurs at unity power

factor, when only real power is being supplied to the motor. At any other point on the curve,

some reactive power is being supplied to or by the motor as well. For field currents less than the

value giving minimum IA, the armature current is lagging, consuming Q. In this situation, the

motor is said to be underexcited. For field currents greater than the value giving minimum IA,the armature current is leading, supplying Q to the power system as a capacitor would. This case

is for an overexcitedmotor.Therefore, by controlling the field current of a synchronous motor,the reactive power supplied to or consumed by the power system can be controlled.

Also, as explained above that an overexcited motor

can be run with leading power factor, this property renders it extremely important in phase

advancing purposes in industrial loads driven by induction motors and lighting and heating loads


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supplied through transformers. Both transformers and induction motors draw lagging currents

from the line. Especially on light loads, the power drawn by them has a large reactive component

and the power factor has a very low value. This reactive power results in losses in many ways.

By using synchronous motors in conjunction with induction motors or transformers, the lagging

reactive power required by the latter is supplied locally by the leading reactive component taken

by the former, thereby relieving the line and generators of much of the reactive component.When used in this way, the synchronous motor is called a synchronous capacitor because it

draws leading current from the line.

BLOCK DIAGRAM

APPARATUS:

Terminal Board

Measuring unit

Synchronous motor

variable DC supply

DC generator

Break control unit

PROCEDURE

Set up the apparatus as shown in Figure.

Give a constant voltage and constant frequency supply to the motor (using infinite bus-

bars). Vary the field current and note the corresponding value of armature current for each

value.

Calculate the power factor using equation (1) below.

Plot a graph of field current against armature current which is the V-curve.

Repeat the above steps for different loads.


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OBSERVATIONS

V = 220V

At no load With medium load With full Load

Ia If Ia If Ia If

DRAW THE V-CURVES HERE

MCH Rest Experiments

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