EE 2092 – Laboratory Practice III
TEST ON DC MOTORS
Instructed by: Mr. Lathika Attanayaka
Group: 14Name: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’;
2
r=11.618cm=0.11618m
Armatur4e resistance (Ra) =4.1Ω, series field resistance (Rs) =3.3Ω
Sample Calculation
Considering first observation,
Weight (W) =0.4536 x 28lb=12.701kgWeight (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-2x0.11618m= 7.238 Nm
Mechanical Output Power = Nrad/s.T = 96.97 x 7.238= 701.87W
Efficiency==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 / CalculationsW(kg) / w (kg) / Speed(Nr) / Voltage (V) / Current (A) / Elec.input Power (kW) / Torque (Nm) / Mech.output Power (W) / Efficency / copper loss(W) / Mech.a loss(W)
rpm / rad/s
12.701 / 6.35 / 926 / 96.97 / 202 / 14.4 / 2.909 / 7.238 / 701.869 / 24.13% / 1534.46 / 672.667
13.608 / 6.35 / 940 / 98.437 / 202 / 14.2 / 2.868 / 8.272 / 814.271 / 28.39% / 1492.14 / 561.593
14.515 / 6.35 / 940 / 98.437 / 203 / 14.4 / 2.923 / 9.306 / 916.055 / 31.34% / 1534.46 / 472.481
15.422 / 6.8 / 930 / 97.389 / 203 / 14.8 / 3.004 / 9.822 / 956.555 / 31.84% / 1620.9 / 426.549
16.33 / 6.8 / 910 / 95.295 / 205 / 15 / 3.075 / 10.86 / 1034.618 / 33.65% / 1665 / 375.382
17.237 / 7.26 / 870 / 91.106 / 205 / 16 / 3.28 / 11.37 / 1036.149 / 31.59% / 1894.4 / 349.451
18.144 / 7.26 / 850 / 89.012 / 203 / 16.4 / 3.329 / 12.41 / 1104.372 / 33.17% / 1990.3 / 234.324
19.051 / 7.26 / 840 / 87.965 / 203 / 16.6 / 3.37 / 13.44 / 1182.338 / 35.08% / 2039.14 / 148.518
19.958 / 7.26 / 830 / 86.917 / 202 / 17 / 3.434 / 14.48 / 1258.124 / 36.64% / 2138.6 / 37.276
20.866 / 7.71 / 820 / 85.87 / 201 / 17 / 3.417 / 14.99 / 1287.449 / 37.68% / 2138.6 / -9.049
21.773 / 7.71 / 790 / 82.729 / 201 / 17.8 / 3.578 / 16.03 / 1325.898 / 37.06% / 2344.62 / -92.514
22.68 / 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 Calculation
Considering 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 x1=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)=== 0.643 Nm
Observation / CalculationsI2(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
1)Series DC motor
(i)Speed Vs Torque
speed(rad/s) / Torque(Nm)96.97 / 7.238
98.437 / 8.272
98.437 / 9.306
97.389 / 9.822
95.295 / 10.857
91.106 / 11.373
89.012 / 12.407
87.965 / 13.441
86.917 / 14.475
85.87 / 14.993
82.729 / 16.027
81.681 / 17.061
(ii)Torque Vs Armature current
Torque(Nm) / Armature current(A)7.238 / 14.4
8.272 / 14.2
9.306 / 14.4
9.822 / 14.8
10.857 / 15
11.373 / 16
12.407 / 16.4
13.441 / 16.6
14.475 / 17
14.993 / 17
16.027 / 17.8
17.061 / 18
(iii)Speed Vs Armature Current
speed (rad/s) / Armature current(A)96.97 / 14.4
98.437 / 14.2
98.437 / 14.4
97.389 / 14.8
95.295 / 15
91.106 / 16
89.012 / 16.4
87.965 / 16.6
86.917 / 17
85.87 / 17
82.729 / 17.8
81.681 / 18
(iv)Efficiency VsArmature current
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
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 / 16.6
2138.6 / 17
2138.6 / 17
2344.616 / 17.8
2397.6 / 18
(vi)Mechanical loss Vs Speed
Mechanical loss(W) / Speed(rad/s)672.667 / 96.97
561.593 / 98.437
472.481 / 98.437
426.549 / 97.389
375.382 / 95.295
349.451 / 91.106
234.324 / 89.012
148.518 / 87.965
37.276 / 86.917
-9.049 / 85.87
-92.514 / 82.729
-191.16 / 81.681
2)Separately excited DC motor
(vii)Speed Vs Torque
sep. ex. DC motor / series DC motorSpeed(rad/s) / Torque(Nm) / Speed(rad/s) / Torque(Nm)
156.221 / 0.643 / 96.97 / 7.238
155.195 / 1.884 / 98.437 / 8.272
154.985 / 3.038 / 98.437 / 9.306
154.388 / 4.171 / 97.389 / 9.822
153.561 / 5.195 / 95.295 / 10.857
153.037 / 6.207 / 91.106 / 11.373
152.451 / 7.169 / 89.012 / 12.407
151.927 / 7.862 / 87.965 / 13.441
151.121 / 8.698 / 86.917 / 14.475
150.367 / 9.345 / 85.87 / 14.993
82.729 / 16.027
81.681 / 17.061
(viii)Speed Vs Armature current
sep. ex. DC motor / series DC motorspeed(rad/s) / Armature current(A) / Speed(rad/s) / Armature current(A)
156.221 / 1 / 96.97 / 14.4
155.195 / 2 / 98.437 / 14.2
154.985 / 3 / 98.437 / 14.4
154.388 / 4 / 97.389 / 14.8
153.561 / 5 / 95.295 / 15
153.037 / 6 / 91.106 / 16
152.451 / 7 / 89.012 / 16.4
151.927 / 8 / 87.965 / 16.6
151.121 / 9 / 86.917 / 17
150.367 / 10 / 85.87 / 17
82.729 / 17.8
81.681 / 18
(ix)Pin Vs Pout
sep. ex. DC motor / series DC motorPin(W) / Pout(W) / Pin(W) / Pout(W)
210 / 100.475 / 2909 / 701.869
416 / 292.375 / 2868 / 814.271
618 / 470.875 / 2923 / 916.055
824 / 643.975 / 3004 / 956.555
1020 / 797.675 / 3075 / 1034.618
1224 / 949.975 / 3280 / 1036.149
1428 / 1092.88 / 3329 / 1104.372
1600 / 1194.38 / 3370 / 1182.338
1800 / 1314.48 / 3434 / 1258.124
1980 / 1405.18 / 3417 / 1287.449
3578 / 1325.898
3600 / 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 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 theseparately 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 thereis 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.