EE 2092 – Laboratory Practice III

TEST ON DC MOTORS

Instructed by: Mr. Lathika Attanayaka

Group: 14 Name: V.I.P. Dasanayake

Group Members: Dasanayake V.I.P. Index No: 090075M

Dayarathne H.K.C.O. Field: EE

De Silva J.G.D.S. Date of Experiment: 01/12/2010

De Silva O.S.D. Date of Submission: 15/12/2010

CALCULATIONS

Absorption Dynamometer

Considering radius of pulley as ‘r’;2πr=73 cm∴r=11.618cm=0.11618m

Armatur4e resistance (Ra) =4.1Ω, series field resistance (Rs) =3.3ΩSample CalculationConsidering first observation,

Weight (W) = 0.4536 x 28lb=12.701kg & Weight (w)= 0.4536 x 14lb=6.350kg Speed (rad s-1) = 2 x л x Nr/60= 2 x л x 926/60=96.97rads -1 Electrical Input Power = V x I = 202 x 14.4=2.909k W Torque produced (T) = (W-w).g.r = (12.701-6.350) kg x 9.81ms-2 x 0.11618m = 7.238 Nm Mechanical Output Power = Nrad/s .T = 96.97 x 7.238= 701.87W

Efficiency= Mech.outputElec . input

×100=701.872908.8

×100=24.13%

Copper loss= I2R=14.4A2 x (3.3+4.1)Ω=1534.464W Mech. loss= Elec. Input – Mech. output – Copper loss=(2908.8-701.87-1534.464)W=672.667W

Observations Calculations

W(kg) w (kg)

Speed(Nr)

Voltage (V)

Current (A)

Elec.input Powe

r (kW)

Torque (Nm)

Mech.output Power

(W)

Efficency

copper loss(W)

Mech.a loss(W)rp

m rad/s

12.701 6.35 926 96.97 202 14.4 2.909 7.238 701.869 24.13% 1534.46 672.66713.608 6.35 940 98.437 202 14.2 2.868 8.272 814.271 28.39% 1492.14 561.59314.515 6.35 940 98.437 203 14.4 2.923 9.306 916.055 31.34% 1534.46 472.48115.422 6.8 930 97.389 203 14.8 3.004 9.822 956.555 31.84% 1620.9 426.54916.33 6.8 910 95.295 205 15 3.075 10.86 1034.618 33.65% 1665 375.38217.237 7.26 870 91.106 205 16 3.28 11.37 1036.149 31.59% 1894.4 349.45118.144 7.26 850 89.012 203 16.4 3.329 12.41 1104.372 33.17% 1990.3 234.32419.051 7.26 840 87.965 203 16.6 3.37 13.44 1182.338 35.08% 2039.14 148.51819.958 7.26 830 86.917 202 17 3.434 14.48 1258.124 36.64% 2138.6 37.27620.866 7.71 820 85.87 201 17 3.417 14.99 1287.449 37.68% 2138.6 -9.04921.773 7.71 790 82.729 201 17.8 3.578 16.03 1325.898 37.06% 2344.62 -92.51422.68 7.71 780 81.681 200 18 3.6 17.06 1393.56 38.71% 2397.6 -191.16

Separately Excited DC Motor

Armature Resistance (Ra) = 4.7 Ω

Sample CalculationConsidering first observation,

Speed (rad s-1) = 2 x л x Nr/60= 2 x л x 1491.8/60=156.22rads -1

Electrical Input Power(Pin) = V2 x I2 = 210 x 1=210W

Copper loss= I22 .Ra = 12*(4.7) = 20.1W

Mechanical loss= P2(0) = V2(0).I2(0) - I22(0).Ra =212x0.5-0.52*4.7=104.825

Mechanical Output Power(Pout) = P2 – P2(0) = Mech. Power developed – Mech. Loss = (Elec. input power– Armature copper loss) – Mech. Loss

= (Pin - I22.Ra) - P2(0) =[(210-12*4.7)-104.825]=100.475

Torque produced (T)= Pout

N r =100.475

156.22 = 0.643 Nm

1) Series DC motor

Observation Calculations

I2(A) V2

(V)

Speed(Nr)

I2(0)(A) V2(0)(V) Pin(W)Copper

loss (W)

Mech. Loss (W)

Pout (W) Torque(Nm)rpm rad/s

1 210 1491.8 156.22 0.5 212 210 4.7 104.825 100.475 0.643

2 208 1482 155.2 0.5 212 416 18.8 104.825 292.375 1.884

3 206 1480 154.99 0.5 212 618 42.3 104.825 470.875 3.038

4 206 1474.3 154.39 0.5 212 824 75.2 104.825 643.975 4.171

5 204 1466.4 153.56 0.5 212 1020 117.5 104.825 797.675 5.195

6 204 1461.4 153.04 0.5 212 1224 169.2 104.825 949.975 6.207

7 204 1455.8 152.45 0.5 212 1428 230.3 104.825 1092.88 7.169

8 200 1450.8 151.93 0.5 212 1600 300.8 104.825 1194.38 7.862

9 200 1443.1 151.12 0.5 212 1800 380.7 104.825 1314.48 8.698

10 198 1435.9 150.37 0.5 212 1980 470 104.825 1405.18 9.345

(i) Speed Vs Torque

5 7 9 11 13 15 17 1975

80

85

90

95

100

Torque(Nm)

spee

d(ra

d/s)

speed(rad/s)Torque(Nm

)96.97 7.238

98.437 8.27298.437 9.30697.389 9.82295.295 10.85791.106 11.37389.012 12.40787.965 13.44186.917 14.47585.87 14.993

82.729 16.02781.681 17.061

(ii) Torque Vs Armature current

12 13 14 15 16 17 18 19 208

9

10

11

12

13

14

15

16

17

18

Armature current(A)

Torq

ue(N

m)

Torque(Nm) Armature current(A)7.238 14.48.272 14.29.306 14.49.822 14.8

10.857 1511.373 1612.407 16.413.441 16.614.475 1714.993 1716.027 17.817.061 18

(iii) Speed Vs Armature Current

(iv) Efficiency Vs Armature current

14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 1980

85

90

95

100

Armature current(A)

Spee

d(ra

d/s)

speed (rad/s) Armature current(A)96.97 14.4

98.437 14.298.437 14.497.389 14.895.295 1591.106 1689.012 16.487.965 16.686.917 1785.87 17

82.729 17.881.681 18

Efficiency Armature current(A)

0.241 14.4

0.284 14.2

0.313 14.4

0.318 14.8

0.336 15

0.316 16

0.332 16.4

0.351 16.6

0.366 17

0.377 17

0.371 17.8

0.387 18

(v) Copper loss Vs Armature current

14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 1923.0%

25.0%

27.0%

29.0%

31.0%

33.0%

35.0%

37.0%

39.0%

Armature current(A)

Effice

ncy

Copper loss(W) Armature current(A)

1534.464 14.4

1492.136 14.2

1534.464 14.4

1620.896 14.8

1665 15

1894.4 16

1990.304 16.4

2039.144 16.62138.6 172138.6 17

2344.616 17.82397.6 18

(vi) Mechanical loss Vs Speed

14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 191450

1550

1650

1750

1850

1950

2050

2150

2250

2350

2450

Armature current(A)

copp

er lo

ss

Mechanical loss(W) Speed(rad/s)672.667 96.97561.593 98.437472.481 98.437426.549 97.389375.382 95.295349.451 91.106234.324 89.012148.518 87.96537.276 86.917-9.049 85.87

-92.514 82.729-191.16 81.681

2) Separately excited DC motor(vii) Speed Vs Torque

80 82 84 86 88 90 92 94 96 98

-220

-120

-20

80

180

280

380

480

580

680

Speed(rad/s)

Mec

hani

cal l

oss(

W)

sep. ex. DC motor series DC motorSpeed(rad/s) Torque(Nm) Speed(rad/s) Torque(Nm)

156.221 0.643 96.97 7.238155.195 1.884 98.437 8.272154.985 3.038 98.437 9.306154.388 4.171 97.389 9.822153.561 5.195 95.295 10.857153.037 6.207 91.106 11.373152.451 7.169 89.012 12.407151.927 7.862 87.965 13.441151.121 8.698 86.917 14.475150.367 9.345 85.87 14.993

82.729 16.02781.681 17.061

(viii) Speed Vs Armature current

0.5 2.5 4.5 6.5 8.5 10.5 12.5 14.5 16.575

95

115

135

155

175

Sep. ex Dc motor Linear (Sep. ex Dc motor)Series DC motor Exponential (Series DC motor)

Torque(Nm)

Spee

d(ra

d/s)

sep. ex. DC motor series DC motorspeed(rad/s) Armature current(A) Speed(rad/s) Armature current(A)

156.221 1 96.97 14.4155.195 2 98.437 14.2154.985 3 98.437 14.4154.388 4 97.389 14.8153.561 5 95.295 15153.037 6 91.106 16152.451 7 89.012 16.4151.927 8 87.965 16.6151.121 9 86.917 17150.367 10 85.87 17

82.729 17.881.681 18

(ix) Pin Vs Pout

0.5 2.5 4.5 6.5 8.5 10.5 12.5 14.5 16.5 18.575

95

115

135

155

175

sep. ex DC motor Linear (sep. ex DC motor)Series DC motor Linear (Series DC motor)

Amature current(A)

Spee

d(ra

d/s)

sep. ex. DC motor series DC motorPin(W) Pout(W) Pin(W) Pout(W)

210 100.475 2909 701.869416 292.375 2868 814.271618 470.875 2923 916.055824 643.975 3004 956.5551020 797.675 3075 1034.6181224 949.975 3280 1036.1491428 1092.88 3329 1104.3721600 1194.38 3370 1182.3381800 1314.48 3434 1258.1241980 1405.18 3417 1287.449

3578 1325.8983600 1393.56

DISCUSSION

(1) Types of materials employed in construction

High grade steel: -Mainly there two advantages of using high graded steel. One is to keep hysteresis loss low, which is due to cyclic change of magnetization caused by rotation of the core in the magnetic field and the other one is to reduce the eddy currents in the core which are induced by the rotation of the core in the magnetic field

Cupper (Cu): -Cu is used to make Field windings and Armature windings

Carbon/Carbon graphite/ Graphite/Metal graphite: -Those are used to make brushes due to its reluctance for deterioration

Insulating Material: -Insulating materials are used to provide electrical insulation between parts at different potentials. An insulating material should have high resistivity, high dielectric strength, low dielectric loss, good heat conductivity, sufficient mechanical strength to withstand vibrations etc. These materials begin to deteriorate at relatively

0.5 200.5 400.5 600.5 800.5 1000.5 1200.5150

650

1150

1650

2150

2650

3150

3650

sep. ex DC motor Linear (sep. ex DC motor)Series DC motor Linear (Series DC motor)

Pout(W)

Pin(

W)

small temperatures. For reliable operation, it is essential that the temperature rise in electrical machines and equipment do not exceed the permissible temperature of the insulating materials used therein.Some of the most important insulating materials used for insulation in electrical machines and apparatus are mica, cotton, asbestos, paper and glass

Cast iron/Cast steel/Fabricated steel: -Cast iron yokes are preferred in smaller machines; because of its cheapness but yoke fabricated steel yokes are preferred in larger machines due to its high permeability. Because weights of large machines are the main considerable fact. As the permeability of cast steel is nearly twice of cast iron, the weight of cast steel required will be only half of the cast iron if used for the same reluctance. Pole cores are usually not laminated and made of cast steel.

(2) Part of the DC machine

Armature: -This is the rotating part of a DC motor and is built up in a cylindrical shape. The purpose of the armature is to rotate the conductor in the uniform magnetic field. It consists of coils of insulated wires wound around an iron and so arranged that electric currents are induced in these wires when the armature is rotated in a magnetic field. It provide a path of very low reluctance to the magnetic flux. The armature core is made from high permeability silicon-steel stampings, each stamping, being separated from its neighbouring one by thin paper or thin coating of varnish as insulation. Due to this the eddy currents in the core induced by the rotation of the core in the magnetic field, is cut into several. The laminations should be perpendicular to the paths of eddy currents and parallel to the flux.

Stator: -The stator is the stationary part of a rotor system. It mainly consists with stator poles pole shoes field windings (winding that produces main magnetic flux.), etc.

Shaft: -The shaft is made of mild steel with a maximum breaking strength. The shaft is used to transfer mechanical power from or to the machine. The rotating parts such as armature core, commutator, cooling fan etc. are keyed to the shaft.

Brushes: -The brushes are rectangular in shape and rest on the commutator.The function of brushes is to collect current from the commutator and supply it to the external load circuit (the armature of the machine being connected to the external load circuit via the commutator and brushes). The brushes are rectangular in shape and rest on the commutator. Brushes are manufacture in a variety of compositions and degrees of hardness to suit the commutation requirements.

Commutator: -The commutator is a cylindrical structure and is built up of wedge shaped segments of high conductivity hard drawn copper and the segments are insulated from each other. Commutator provides the electrical connections between the rotating armature coils and the stationary external circuit, keeps the rotor or the armature mmf stationary in space, when the rotor rotates perform switching action reversing the electrical connections between the external circuit and each armature coil in turn so that the armature coil voltage add together and result in a DC output voltage. So this is a main part of motor.

(3) Types of armature windings and their applications

There are several types of armature windings called Lap winding, wave winding, Non lap winding. The difference between lap winding and wave winding is different arrangement of the end connections at the front or commutator end of armature. Each winding can be arranged progressively or retrogressively and connected in simplex, duplex and triplex.

Commonly for windings these things should be considered The number of commutator segments is equal to the number of slots or coils because the front

ends of conductors are joined to the segments in pairs. The winding must close upon itself Both pitches should be odd, otherwise it would be difficult to place the coils properly on the

armature. As windings should be full-pitched the front and back pitch are each approximately equal to the

pole-pitch. This results in increased e.m.f round the coils

Lap Winding

In the case of lap winding, the end of a wire conductor is connected to the commutator, and then the other wire end is connected to the beginning of the next coil segment. This winding configuration refers to the fact that the wire "laps over" each segment as the winding structure reaches its terminus.

Wave Winding

With wave winding, one wire conductor is wrapped under one pole, and then connected to the back of the next pole. In this case, the series of wire conductors do not directly overlap, but when it's completed, the structure looks like a series of copper "waves" wrapped around the commutator.

Non-Lapped Winding

Non-lapped winding refers to a wire process that does not employ overlapping at any point across the commutator but employs a linear side-by-side configuration from the front to the rear of the structure.

(4) Performance characteristics of the DC Series Motor

EFFICIENCY IN PERCENTAGE

ARMATURE CURRENT IN A

SPEED IN rpm

TORQUE IN Nm

In the above figure, four important characteristics of a DC series motor, namely torque,

speed, current and efficiency, each plotted against useful output power, are shown.

Components of a series motor include the armature and the field. The same current is

impressed upon the armature and the series field. The coils in the series field are made of a few

turns of large gauge wire, to facilitate large current flow. This provides high starting torque,

approximately 2 ¼ times the rated load torque. Series motor armatures are usually lap wound. Lap

windings are good for high current, low voltage applications because they have additional parallel

paths for current flow. Series motors have very poor speed control, running slowly with heavy

loads and quickly with light loads.

A series motor should never drive machines with a belt. If the belt breaks, the load

would be removed and cause the motor to over speed and destroy itself in a matter of seconds.

Common uses of the series motor include crane hoists, where large heavy loads will be raised and

lowered and bridge and trolley drives on large overhead cranes. The series motor provides the

starting torque required for moving large loads. Traction motors used to drive trains are series

motors that provide the required torque and horsepower to get massive amounts of weight moving.

On the coldest days of winter the series motor that starts a car overcomes the extreme cold

temperatures and thick lubricant to get the car going.

(5) Performance characteristics of the separately excited DC motor

Rated load

Т

ω

Armature control Field Control

Torque

Speed (ω)

Series DC motor Separately excited DC motor

Mainly there are two methods to control the speed in safe operate region which are

called armature control and field control. In armature control there is a constant torque while

constant power in the field control

The separately excited DC motor is probably the most common dc motor used in industry

today. Components of the separately excited DC motors are the armature and the field. The coils in

the shunt field are composed of many turns of small wire, resulting in low shunt field current and

moderate armature current. This motor provides starting torque that varies with the load applied

and good speed regulation by controlling the shunt field voltage. If the separately excited DC

motor loses its field it will accelerate slightly until EMF rises to a value sufficient to shut off the

torque producing current. In other words, the shunt motor will not destroy itself if it loses its field,

but it won’t have the torque required to do the job it was designed for. Some of the common uses

of the shunt motor are machine shop lathes, and industry process lines where speed and tension

control are critical.

When comparing the advantages of the series and separately excited DC motor, the

series motor has greater torque capabilities while the separately excited DC motor has more

constant and controllable speed over various loads.

(6) Difference between performance characteristics of series DC motor and separately excited DC motor.

When you increase the load, Speed of Separately excited DC Motors will nearly remain constant where as speed of series DC Motors will drastically decrease. Therefore shunt DC Motors is more suitable for traction applications. Separately excited DC meter has good speed controllability, safe no load speed and good speed controllability.

In series DC motor it can give high torque at starting without demanding similar high power. Series DC motor has high torque capability and reasonable good power cushioning ability. But Unlike Separately excited DC motors, series DC motors can produce high starting torques. Therefore series DC motors are more suitable for starter applications.

(7) Applications of motors with limitations

1. Shunt excited dc motors

These have fairly constant speeds against a varying load or torque. Therefore applications include situations where a constant speed is required. (E.g. Lathes, Conveyors, Fans, Machine tool drives )

2. Compound excited dc motors

These have Combine characteristics of both shunt and series wound motors. The series winding gives good starting torque and shunt winding ensures a comparatively constant speed. (E.g. Planers, Shears, Guillotines, Printer machines, Power presses which needs peak loads at certain instances)

3. Permanent magnet motors

These are used for low power applications. (E.g. Automobiles, Starter motors, Wiper motors, Lowering windows, Toys, Electric tooth brushes)

4. Adjustable speed DC shunt motor

Starting torque should be medium. Usually limited to 250% by a starting resistance but may be increased. Maximum momentary operating torque-usually limited to about 200% by commutation. Speed regulation-10-15%. Speed control-6:1 range by field control, lowered below normal speed by armature voltage control.

Used for constant speed applications which require medium starting torque & which require adjustable speed control, either constant torque or constant output.

5. Differential compound wound DC motor with relatively weak series field

It has almost constant torque, constant speed and tendency towards speed instability with a possibility of motor running away and strong possibility of motor starting in wrong direction. Applications are mainly for experimental and research work

REFERENCES

Electrical Machines and Drive Systems, by C.B.Gray

Electrical Machines, by Draper

Machine Elements in Mechanical Design, by Robert L. Mott.

http://electricalandelectronics.org/2009/04/29/types-of-armature-windings/,