Test on Dc Motors

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Tests on DC Motors

Instructed By: M.G.H Wickramasingha

Name : S.R Dahanayake

Index No : 100073B

Field : EE

Group : 2

Date of Per : 02/01/2011


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Apparatus:

Part 1

DC series motor 1 No of ammeter 020A 1 No of voltmeter 0-300V 1 No of rheostat 20A/6 ohms Tachometer Absorption meter

Part 2

DC shunt motor 3 point starter 2 Nos 490, 2A rheostats Tachometer

Procedure (part1)1. Examine the machine and identify the terminals2. Connect the circuit according to the given figure. Start the motor with sufficiently load on

the pan, note down V,I,w,W and speed Nr. Decrease the load on the pan in suitable steps,

and note down the above quantities.

3. Measure the armature and field winding resistance4. Measure the circumference length of the pulley

Procedure (part2)Speed control using field resistance control

1. Connect the circuit as shown in the lab sheet, and keep Rf=0 and Ra=0, move the starter armslowly to its range.

2. Note down the speed. Then increase Rfand note down the speed at different settings of Rf.Speed control using armature resistance control

3. Keep again Rf=0 and Ra=0. Move the starter to its full range. Note down the speed. Thenincrease Raand note down speed at different settings of Ra.

Calculations:

Plots the following graphs for the both motors

1. Plot the speed vs. Torque2. Plot the speed vs. Ameture current3. Plot the P-in vs P-out


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Calculations

Part 1

1. Let radius of the pulley= rC=2r

r=

r = 11.4591cm

Torque can be calculated using,

T=(W-w)grNm

Calculated values are,

W-kg 12.7 13.6 14.5 15.4 16.3 17.2 18.1 19.0 19.9 20.8 21.7 22.6 22.6

w-kg 4 4.5 5 5 5 5.5 5.5 5.5 5.5 6 6 6 6.5

T-Nm 9.78 10.2 10.6 11.7 12.7 13.1 14.2 15.2 16.2 16.7 17.7 18.7 18.

2. Mechanical output power of motor =

N = speed of the dynamometer

Input to the motor = VI

Thereforeefficiency of the motor=


Speed(Rad/s) Torque(Nm) Input power-W Efficiency Current-A

203.053333 9.7806501 2720 73.0144706 13.6

196.773333 10.2383877 2800 71.95148849 14

193.633333 10.6961141 2880 71.91403568 14.4

186.306667 11.7159206 3000 72.75847042 15

177.933333 12.7357158 3151.2 71.91255306 15.6

173.746667 13.1934535 3272.4 70.05007221 16.2

171.653333 14.2132487 3312.8 73.64620625 16.4

169.56 15.2330552 3393.6 76.11141094 16.8

167.466667 16.2528505 3434 79.26064911 17

165.373333 16.7105881 3474.4 79.53850032 17.2161.186667 17.7303833 3555.2 80.38651523 17.6

159 093333 18 7501898 3595 6 82 96334967 17 8


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0

50

100

150

200

250

0 2 4 6 8 10 12 14 16 18 20

Speed(rpm)

Torque (Nm)

Speed vs Torque


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0

50

100

150

200

250

0 2 4 6 8 10 12 14 16 18 20

Speed(rpm)

Ameture current

Speed vs. Ameture current


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0

500

1000

1500

2000

2500

3000

3500

0 500 1000 1500 2000 2500 3000 3500 4000

p-out

P-in

P-out vs P-in


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Part 2

1. Input power to the motorInput Power = V2*I2

2. Mechanical loss of the motor is given by,Mechanical loss = mechanical output copper loss

mechanical output = V*ICopper loss = I*I*Ra

Ra = 2.6

3. Output powerOutput power = electrical input Mechanical loss copper loss

4. Torque

Torque =


Observations Calculations

I2(A) V2(V) N(rpm) I2(O)(A) V2(O)(A) Pin(W)

Copper

loss

(W)

Mechanical

loss (W) Pout(W)

1 212 1487.6 0.4 210 212 2.6 83.584 125.816

2 212 1474.6 0.4 210 424 10.4 83.584 330.016

3 212 1466.2 0.4 210 636 23.4 83.584 529.016

4 210 1456.7 0.4 210 840 41.6 83.584 714.816

5 208 1449.5 0.4 210 1040 65 83.584 891.416

6 206 1443.4 0.4 210 1236 93.6 83.584 1058.82

7 206 1435.9 0.45 210 1442 127.4 93.9735 1220.63

8 206 1431.3 0.45 210 1648 166.4 93.9735 1387.63

9 204 1428.5 0.45 210 1836 210.6 93.9735 1531.43

Speed(Rad/s) Torque(Nm) Input power-W Current-A

155.7021333 0.808055723 212 1

154.3414667 2.13821993 424 2

153.4622667 3.447205697 636 3

152.4679333 4.688303858 840 4

151.7143333 5.875621508 1040 5

151.0758667 7.008505219 1236 6150.2908667 8.121761003 1442 7

149 8094 9 262613027 1648 8


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0

1

2

3

4

5

6

7

8

9

10

150 151 152 153 154 155 156 157

Speed(rpm)

Torque (Nm)

Speed vs Torque


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0

1

2

3

4

5

6

7

8

9

10

150 151 152 153 154 155 156 157

Speed(rpm)

Ameture current

Speed vs. Ameture current


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0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5 6 7 8 9 10

p-out

P-in

P-out vs P-in


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DISCUSSION

Parts of the DC machine (motor) & types of materials employed in construction.

Armature

The armature takes the place of the nail in an electric motor. The armature is an electromagnet mad

by coiling thin wire around two or more poles of a metal core. The armature has an axle, and th

commutator is attached to the axle. The armature constructed by using copper wires.

CommutatorThe commutator is simply a pair of plates attached to the axle. These plates provide the tw

connections for the coil of the electromagnet. As it rotates, it changes the direction of the electricity i

electric motors and generators. By reversing the electricity flow in an electric motor armature (movin

coil) a rotating force is produced which in turn rotates the armature and converts the electrical force t

mechanical force. In a generator, the commutator does exactly the reverse; it converts the mechanic

force to electrical force. The commutator constructed with a pre-designed number of copper strip

separated by a small insulation layer.

Brushes

The function of brushes is to collect current from the commutator and supply it to the external loacircuit (the armature of the machine being connected to the external load circuit via the commutato

and brushes). The brushes are rectangular in shape and rest on the commutator. Brushes ar

manufacture in a variety of compositions and degrees of hardness to suit the commutatio

requirements. . The brushes constructed by using carbon.

Field systemIn a field system of a magnet type D.C. motor constructed of a cylindrical yoke and a plurality of po

pieces, each of which is made of a permanent magnet member bonded to the inner peripheral surfac

of the yoke, an improved field system wherein each pole piece is formed with a recess along its centra

part. The yoke is provided with bent portions protruding inwards of the yoke, in correspondence wit

each recess, and the bent portions are held in engagement with the recess through an elastic member

Types of armature windingAn armature is a rotating, copper-wrapped assembly, induced by a magnetic field to create electric

energy. This component is central to the manufacture of electric motors. An armature's "windings

refer to the network of metal conductors that enclose the structure's central commutator. Depending o

the motor type, there a number of winding configurations.

Lap Winding

In the case of lap winding, the end of a wire conductor is connected to the commutator, then the othe

wire end is connected to the beginning of the next coil segment. This winding configuration refers t

the fact that the wire "laps over" each segment as the winding structure reaches its terminus.

Wave WindingWith wave winding, one wire conductor is wrapped under one pole, then connected to the back of th

next pole. In this case, the series of wire conductors do not directly overlap, but when it's complete

the structure looks like a series of copper "waves" wrapped around the commutator.

Non-Lapped WindingNon-lapped winding refers to a wire process that does not employ overlapping at any point across th

commutator but employs a linear side-by-side configuration from the front to the rear of the structure.


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Performance characteristics of the DC series motors

DC series motors are used in applications where a high load is used. Also it can provide a very hig

starting torque, when it is first energized. But they don't have a precise speed regulation. In serie

motors, the field winding and the rotor is connected in series, so that the armature current and the fiel

winding current will be the same. The amount of torque that can be produced by the shaft depends o

the current passing through it. Series motor armatures are usually lap wound. Lap windings are goo

for high current, low voltage applications because they have additional parallel paths for current flow

The field winding can carry large amounts of current because it made of large conductors. Since thfield winding can carry large amounts of current motor can produce large torques.

Removal of mechanical load from series motors results in an indefinite speed increase which ca

destroy the motor or bearings. Small series motors usually have enough internal friction to preven

high-speed breakdown, but larger motors require to be controlled.

Some applications are,

Driving cranes

Steel rolling mils

Electric locomotives

Power tools ( hand drills, saws, power screwdrivers)

Performance characteristics of the DC series motors

Separately excited DC motor has its field winding separated from the armature in series DC motor w

have it in series with the armature.seperate field resistance is normally large for a example a 200V D

motor Rf can be about 250.seperate field usually carries a small current, so we need a large no o

turns of tine mires for the separate field. This support a large Rf . No load speed of the separate

excited DC motor is approximately proportional to the armature voltage and inversely propotional t

the field flux.It is important to note that motor will tend to reason dangerous high speed if the fir

circuit is disconnected while running on no load. All motors, therefore should carry loss of fielprotection.

Differences between performance characteristics of two motorsIn many traction applications we use separately excited DC motor and in simple brush DC moto

applications we use series DC motor. In series motor armature and field windings are series and

separate excited DC motor these two windings are parallel.


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Constant speed DC shunt motor

ApplicationsEssentially for constant speed applications requiring medium starting torque. May be used for

adjustable speed not greater than 2:1 range. For lathes, centrifugal pumps, reciprocating pumps, fans,

blowers conveyors , wood working machines, machine tools, printing presses, spinning and weaving

machines etc.

LimitationsStarting torque-medium, Usually limited to 250% by a starting resistor but may be increase

Maximum operating torque usually limited to about 200% by commutation. Speed regulation is 5-10%Speed control increase upto speed 200% by field control, decrease by armature voltage control.

Cumulative compound wound DC motor

ApplicationsFor drives requiring high starting torque and only fairly constant speed, pulsating loads

with fly wheel action. For shears, conveyors, crushers, bending rolls, punch presses, hoists, elevator

heavy planers, ice making machines, air compressors, rolling mills, printing presses

LimitationsStarting torque-high, upto 450% depending upon the degree of compounding.Maximum momentar

operating torque-higher than shunt, upto 350%. Speed regulation-varying depending upon degree o

compounding upto 25 to 20%.

Differential compound wound DC motor with relatively weak series field

Applications

For experimental and research work

LimitationsAlmost constant torque and constant speed. Tendancy towards speed instability with

possibility of motor running away and strong possibility of motor starting in wrong direction

References

http://ayyarao.blog.co.in/files/2008/07/types-of-armature-winding.pdf

http://www.answers.com/DC+machines+and+their+applications
http://ayyarao.blog.co.in/files/2008/07/types-of-armature-winding.pdfhttp://ayyarao.blog.co.in/files/2008/07/types-of-armature-winding.pdfhttp://www.answers.com/DC+machines+and+their+applicationshttp://www.answers.com/DC+machines+and+their+applicationshttp://www.answers.com/DC+machines+and+their+applicationshttp://ayyarao.blog.co.in/files/2008/07/types-of-armature-winding.pdf


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Documents

Test on Dc Motors