copy of rem lab record

8/13/2019 Copy of REM LAB Record

1/17

EXPERIMENT NO- 01

AIM: To perform no load test on 3ø induction motor. Apparatus required:

S.NO. EQUIPMENTS USED RATINGS TYPE QUANTITY

1. 3Ø INDUCTION MOTOR 1No.

2. 3Ø ALTERNATOR 1No.

3. 3Ø VARIAC - 1 No.

4. WATTMETER 2 Nos.

5. VOLTMETER 1 No.6. AMMETER 1 No.

Theory: No load test is also called Open Circuit Test and is similar to the O.C. test on a

transformer. The motor is uncoupled from its load and the rated voltage at the

rated frequency is applied to the stator to run the motor without load. The inputis measured by the two wattmeter method.

The ammeter measures the no load current and voltmeter gives the normal

rated voltage. Since the no-load current is 20-30% of full load current, I²R

losses in the 1˚ may be neglected as they vary with the square of current. Since

the motor is running at no-load, total input power is equal to the constant i.e.

iron loss, friction and windage loss of the motor.

Pconstant (PI)= P1+P2 = Sum of two wattmeter readings.

Since the power factor of the induction motor under No-load condition is

generally less than 0.5, 1 wattmeter will show negative reading. It is therefore

necessary to reverse the direction of the current coil, terminal to take reading.

As in the case of transformer, the constant R0 & X0 can be calculated from the

reading obtained in No-load test.

If ViNL = Input line voltage

PiNL = Total 3Ø

I0

= Input line currentVip = Input phase voltage

PiNL = 3 ViNL∙I0∙cosØ0

Circuit diagram:

3Ø supplyat ratedvoltage

&frequency

3Ø Indⁿ Motor

P2

P1


2/17

Connection diagram:

Result: Parameters of No-Load test i.e. R0 & X0 are observed.

Precaution:

Connection should be as per circuit diagram. Supply should be as per the circuit diagram. No mechanical load should be connected on o/p terminal of an induction motor.

VRotor

R

Y B

R

YB3-Ø AC

SUPPLY

3-Ø AC

SUPPLY

Fuse

W₁

W₂

M

LM

C

C

A

V V

3-Ø AutoXmer

Stator

3-Ø alternator

L

Fig.-No-Load Test


3/17

EXPERIMENT NO- 02

AIM: To perform blocked rotor test on 3ø induction motor.

Apparatus required:


1. 3Ø INDUCTION MOTOR 1 No.

2. 3Ø ALTERNATOR 1 No.

3. 3Ø VARIAC 1 No.4. WATTMETER 2 Nos.5. VOLTMETER 1 No.6. AMMETER 1 No.

Theory: Blocked rotor test is performed on induction motor to calculate it’s leakage

impedance for performing this test the rotor shaft is blocked by external meant i.e. held

stationary by best pulley arrangement or by hand. Now a balanced poly-phase voltage

at the rated frequency are applied to the stator terminal through a poly-phase variac per

phase value of applied voltage VVR, input current IBR, input power are recoded.

The current drawn by the motor in blocked rotor test should be closed to its

rated voltage Rated current value is obtained by applying reduced voltage to the stator,

as block rotor represent short circuited conditions at the same stator terminal (low

impedance ZBR). The core loss at this reduced voltage can be ignored. This justifies theblocked rotor circuit modes.

The following readings are recorded during the test.

VBR = Stator voltage(Line to line)

IBR = Stator current (Avg. of 3 ammeter readings)

ZBR = Low impedance

PBR = Power fed into the Stator

From the test we can compare-

ZBR =√

RBR=√

RBR=


4/17


5/17

EXPERIMENT NO- 03

AIM: To conduct OC & SC tests on three-phase alternator.Name Plate details:

S.NO. DESCRIPTION D.C MOTOR 3-ALTERNATOR

1. CAPACITY 5 H.P 3 KVA

2. VOLTAGE 220V 415V

3. CURRENT 19A 4.2A

4. SPEED 1500rpm 1500rpm

5. Excitation 220V, 1.5A 220V, 1.4A

Apparatus required:

S.NO. EQUIPMENTS USED RANGE TYPE QUANTITY

1. Ammeter 0-5A , 0-2A M.I 1 No

2. Voltmeters 0-300V , 0-50V M.I 1 No

3. Rheostat 250/1.5A Wire wound 2 No’s 4. Tachometer 0-50000rpm Digital 1 No

5. Fuses 10A , 2A ------ 2 No’s

Theory: Alternator is a machine, which converts mechanical energy to Electrical

energy. Regulation of an Alternator can be calculated by Synchronous impedance

method. In this test, the alternator is run with the help of a prime mover. In OC test the

terminals of the alternator Are kept opened and a voltmeter is connected i.e. the

alternator is run on no load , therefore it is known as no load test Keeping speed

constant a relation between field current & open circuit voltage are obtained.

The curve is known as open circuit characteristics (O.C.C)

The general shape of this characteristic has been depicted by the Graph 1.

Circuit diagram:


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Graph:

1) A graph is drawn b/w If and V which is known as OC Curve , by taking If on X-

axis and V on Y-axis.

2) A graph is drawn b/w If and Isc which is known as SC curve, by Taking If on X-

axis and Isc V on Y-axis.

Observations: Synchronous speed of alternator =

Measured value of armature resistance = Ω Effective value of armature resistance = Ω

Calculation:

S.NO. Field current -If (amp.) OC voltage(volts)

Field current If (AMP.) SC current (amp.)

1.2.

3.

4.

5.

Fig.- Arrangement for Open circuit test on 3-phase alternator

Fig.- MODEL GRAPH 1 Fig.- PHASOR DIAGRAM


7/17

Synchronous Impedance (Z S ) =

Synchronous reactance (X S ) = (ZS)2- (Ra)2

From Phasor diagram, Eo =√

EO =

Result: The open circuit & short circuit test has been performed .

The regulation of the alternator is found to be ______ %

Precaution:

Operate the 3-point starter slowly & uniformly. . Keep the speed of the prime mover to its rated value throughout the experiment. In OC test, there should not be any load on Alternator. In SC test, the SC current should not exceed its rated value.

% Regulation =

= %


8/17

EXPERIMENT NO- 04

AIM: Determination of direct axis synchronous reactance (Xd) andQuadrature axis synchronous reactance (Xq) by performing

slip

test on synchronous machine.

Apparatus required:


1. 3Ø ALTERNATOR 415V,4.2A,1500rpm - 1No.2. 3Ø VARIAC 400V,15A - 1 No.

3. DC REGULATOR 0-220V,1.5A - 1 No.4. VOLTMETER 0-300V M.I 1 No.5. AMMETER 0-15A M.I 1 No.

Theory: This method is used to determine Xq and Xd , the direct and quadrature axis

reactance is called slip test. In an alternator we apply excitation to the field winding and

voltage gets induced in the armature but in the slip test, a three phase supply is applied

to the armature, having Voltage must less than the rated voltage while the field winding

circuit is kept open.The alternator runs at a speed close to synchronous speed but little less than

synchronous value. The three phase current drawn by the armature from a three phase

supply produce a rotating flux. Thus the armature mmf wave is rotating at synchronous

speed as shown above figure. The rotor is made to rotate at as speed little less than the

synchronous speed. Thus armature mmf having synchronous speed, moves slowly past

the field poles at a slip speed ‘(Ns-N)’, Where ‘N’ is actual speed of rotor .This causes an

emf to be induced in the field circuit. When the stator mmf is aligned with the d-axis of

field poles then flux Ød per pole is setup and the effective reactance offered by the

alternator is Xd. When the stator mmf is aligned with the q-axis of field poles then fluxØq per pole is setup and the effective reactance offered by the alternator is Xq.


9/17

As the air gap is non-uniform, the reactance offered also varies and hence current

drawn by the armature also varies cyclically at twice the slip frequency.

Hence, the r.m.s current is minimum when machine reactance is Xd and it is maximum

when machine reactance is Xq.

As the reactance offered varies due to the non-uniform air gap, the voltage drops also

varies cyclically .Hence the impedance of the alternator also varies cyclically. And

therefore the terminal voltage (VT ) and armature current (Ia=Iload) also varies cyclically.

Hence, voltage at terminal is maximum when all the reactive drops are minimum and

vice-versa.

The waveforms of voltage induced in rotor, terminal voltage and current drawn by

armature as shown in oscillogram below.

Fig.- OSCILLOGRAM OF LINE CURRENT, TERMINAL VOLTAGE, VOLTAGE INDUCED IN ROTOR

DIRECT AXIS AND QUADRATURE AXIS COMPONENTS OF ARMATURE MMF

NS=RMF

Circuit Diagram:


10/17

Observations:

Calculation: The minimum to maximum variations in terminal voltage and current will

record by the analogues voltmeter and ammeter as connected in the

circuit. Hence, direct and quadrature axis reactance can be calculated as:-

Xd=

= Ω/phase

Xd=

= Ω/phase

Result: The accurate value of direct axis reactance Xd= …………Ω.

The accurate value of quadrature axis reactance Xq=…………..Ω.

Precaution:

All connection should be made tight and according to circuit diagram. The zero settings of all the meters should be checked before connecting them in the

circuit.

The slip of the synchronous machine should be kept below 5%. At all the times during this experiment, the voltage induced in the rotor circuit

should remain below rating of the voltmeter connected there.

3-ø AC voltage applied to the stator of the synchronous machine should be 25-30%of the rated value of synchronous machine.

S.NO. FIELD EXCITATIONVOLTAGE

IL (max) Il(min) VT(max) VT(min) Xd Xq

1.

2.3.

4.

5.

VTL(max)

√3*IL(min)

VTL(min)

√3*IL(max)


11/17

EXPERIMENT NO- 05

AIM: To Study starting methods of single phase induction motors. Apparatus required:


1. 3Ø INDUCTION MOTOR 1500 rpm Delta 1No.

2. AMMETER 0-100 A MI 1 No.

3. VOLTMETER 0-500 V MI 1 No.

4. TACHOMETER - - 2 No.

5. PATCH CHORDS - - 1-15 Nos.

Theory: Before studying starting methods of single phase induction motors, we will

explain why single phase induction motors are not self starting.

Constructionally, this motor is, more or less, similar to poly-phase induction motor,

except that its stator is provided with a single phase winding and a centrifugal switch is

used in some types of motors, in order to cut out a winding, used only for starting

purposes. It has distributed stator winding and a squirrel cage rotor. When fed from

single phase supply, its stator winding produces a flux ( or field ) which is onlyalternating i.e. one which alternates along one space axis only. It is not a synchronously

revolving ( or rotating ) flux, as in the case of two or three phase stator winding, fed

from 2 or 3 phase supply. Now, alternating flux acting on a stationary squirrel cage rotor

cannot produce rotation (only a revolving flux can). That is why a single phase induction

motor is not self starting.

Double Field Revolving Theory-

This theory makes use of the idea that an alternating uni-axial quantity can be

represented by two oppositely rotating vectors of half magnitude. Accordingly, analternating sinusoidal flux can be represented by two revolving fluxes, each equal to the


12/17

half value of the alternating flux and each rotating synchronously (N s = 120f/P) in

opposite direction.

A shown in fig., let the alternating flux have a maximum value of Xm . Its component A

and B will each be equal to Xm /2 revolving in anticlockwise and clockwise direction

respectively.After some time, when A and B would have rotated through angle +Y and –Y, as shown

in fig, the resultant flux would be

= 2 x (Xm/2) cos (2Y/2) = XmcosY

After a quarter cycle of rotation, fluxes A and B will be oppositely directed so that the

resultant flux will be zero.

After half a cycle, fluxes A and B will have resultant of -2 x Xm/2 = -Xm. After three

quarters of cycle, again the resultant is zero and so on. If we plot the values of resultant

flux against Y between limits Y =00

and Y = 3600

, then a similar curve is obtained. That iswhy an alternating flux can be looked upon as composed of two revolving fluxes, each of

half the values and revolving synchronously in opposite directions.

It may be noted that if the slip of the rotor is s w.r.t the forward rotating flux (i.e. one

which rotates in the same direction as rotor) then its slip w.r.t the backward rotating

flux is (2-s).

Each of the two component fluxes, while revolving round the stator, cuts the rotor,

induces an e.m.f and this produces its own torque. Obviously, the two torque (called

forward and backward torque) are oppositely directed, so that the net or resultanttorques is equal to their difference.

Now, power developed by a rotor is Pg = (1-s/s)I22R2

If N is the rotor r.p.s then torque is given by Tg =

x

I22 R2

But N=Ns(1-s)

Tg = (1/2x3.14xNs) x {I22R2/s} = k. I22R2/s

Hence, the forward and backward torques are given by

Tf = K (I22R2/s) and Tb = - K (I22R2/2-s)

Tf = I22R2/s synch. Watt and Tb = - I22R2/(2-s)

T=Tf + Tb

The fig. Shows both torques and resultant torques for slips between 0 and +2. At

standstill, s=1 and (2-s) = 1. Hence Tf and Tb are numerically equal but, being

oppositely directed, produce no resultant torque. That explains why there is no startingtorque in single phase induction motor.


13/17

However, if the rotor is started somehow, say, in the clockwise direction, the clockwise

torque starts increasing and, at the same time, the anticlockwise torque starts

decreasing. Hence, there is a certain amount of net torque in the clockwise direction

which accelerates the motor to full speed.

Classification Of Single Phase Induction Motors

1. Split Phase Motor(i) Resistance start induction run (iii) Capacitor start capacitor run(ii) Capcitor start induction run (iv) Capacitor start & run

2. Shaded Pole Motor

3. Synchronous Motor(i) Reluctance motor (ii) Hysterisis motor4. Commutator Motor

(i) Universal motor5. Stepper Motor(i) Variable reluctance motor (ii) Permanent magnet motor(iii) Hybrid type motor

Capacitor start & run motor

This motor is similar to capacitor start motor except that the starting winding and

capacitor are connected in the circuit at all times. The advantages of leaving capacitor

permanently in the circuit are(i) improvement of overload capacity of the motor

(ii) a higher power factor(iii) higher efficiency(iv) quieter running of the motor which is so much desirable for small power drives inoffices and laboratories.

Some of these motors which start and run with one value of capacitance in the cicuit are

called single value capacitor run motors. Other which start with high value of

capacitance but run with low value of capacitance are known as two value capacitor run

motors.

Single Value Capacitor Run Motor

It has one running winding and one staring winding in series with a capacitor. Since

capacitor remains in circuit permanently, this motor is referred to as permanent split

capacitor run motor and behaves practically like an inbalanced 2 phase motor. There is

no need to use a centrifugal switch which was necessary in the case of capacitor start

motors. Since the same capacitor is used for starting and running, it is obvious that

neither optimum starting nor optimum running performance can be obtained because

value of capacitance used must be a compromise between the best value for starting and

that for running. Generally, capacitors of 2 to 20 microfarad capacitance are employed

and are more expensive oil capacitors because of continuous duty rating. The low valueof the capacitors result in small starting torque which is about 50% to 100% of rated


14/17

torque. Consequently, these motors are used where the required starting torque is low

such as air moving equipments fans, blowers and voltage regulators and also oil burners

where quiet operation is particularly desirable.

One unique feature of this type of motor is that it can be easily reversed by an external

switch provided its running and external windings are identical. One serves as therunning winding and other as starting winding for one direction of rotation. For reverse

rotation, the one that previously served as running winding becomes that the starting

winding while the former starting winding serves as the running winding. When the

switch is in forward position, winding B serves as running winding and A as starting

winding. When switch is in “reverse” position, winding A becomes the running winding

and B the starting winding,

Such reversible motors are often used for operating devices that must be moved back

and forth vary frequently such as rheostats, induction regulations, furnace controls,valves, and arc welding controls.

Result: Hence we studied the staring methods of single phase induction motors.


15/17

EXPERIMENT NO- 06

AIM: Speed Control of 3 Phase Induction Motor.

Apparatus required:


1. 3Ø INDUCTION MOTOR 1500 rpm Delta 1No.

2. AMMETER 0-100 A MI 1 No.

3. VOLTMETER 0-500 V MI 1 No.

4. TACHOMETER - - 2 No.5. PATCH CHORDS - - 1-15 Nos.

Theory: The rotor speed of an induction motor is given by-

Nr = ( 1-s ) Ns

Since, Ns= 120f/p

so, Nr =(1-s)120f/p --- (1)

From Eq. no (1) we can conclude that the

rotor speed can be changed by change in

frequency f, No. of poles p and slip s.

The main method employed for speed control

of induction motor is as follows-

Pole changing Method Supply Frequency Method Tandom method

Pole Changing Method

In this method, the single stator winding is divided into few coil groups. The terminal of

all these group are brought out the no. of poles can changed in only single change in

connection.

In process the stator winding is divided into two pole groups. The no. of poles can be

changed in the ratio 1:2.

Suppose the motor winding for groups 6 poles and 4 poles, for 50Hz supply

synchronous speed will be 1000rpm and 1500rpm respectively.

NN N NS S S S

Fig.(b)-8 –POLE MACHINE

SN S N

Fig.a)-4 –POLE MACHINE


16/17

From the fig (a) shows one phase of stator winding consisting of 4coil divided into two

group a-b and c-d. Group a-b consists of Odd numbered coil (1,3) and connected in

series. Group c-d have even numbered coils (2,4) connected in series. The terminals a, b,

c and d are taken out. The coils can be made to carry current in the given direction by

connecting coil group either in series or parallel with this connection, there will be atotal of 4 pole giving a synchronous speed of 1500 rpm for a 50Hz system.

If the current through the coils of group a-b is reversed, shoen in fig (b) then all coils

will produce North pole. In order to complete magnetic path, flux of the pole group must

pass through the space between the group, thus indicating magnetic poles of opposite

polarity (s poles) in the interpole spaces. These poles are called consequent poles. Thus,

the machine has twice as many poles as before (8 poles) and the synchronous speed is

half of the previous speed (i.e 750rpm)

Supply frequency Method

From Eq. no (1), we can conclude that rotor speed is directly proportional to the supplyfrequency.A shown in fig., let the alternating flux have a maximum value of X m . Its

component A and B will each be equal to Xm /2 revolving in anticlockwise and clockwise

direction respectively.

Hence, as the supply frequency increases rotor speed increases and vice versa. The

variable frequency supply is generally obtained by the following converter-

1. Voltage source inverter2. Current source inverter

3. CycloconverterTandem Method

Whenever multiple speeds are desired, motors are sometimes operated in tandem or

cascade connection. In tandem connection, two motors are rigidly coupled to the same

shaft or otherwise mechanically linked by means of gears. The stator winding of the first

is connected to the mains in the usual way, while the second stator winding is fed from

the rotor winding of the first.

Result: Precaution:1. The supply of the induction motor during the pole changing method should

be done carefully.

2. Connection should be as per circuit diagram.Result: Hence Speed of the three phase Induction Motor is controlled

Precaution:


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The supply of the induction motor during the pole changing method should be donecarefully.

Connection should be as per circuit diagram.

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