Download Direct Current Machines and more Exercises Electrical Engineering in PDF only on Docsity!
This Free Quality Manual is
Found only at POWERLINE
E-MAIL: powerlinereview@yahoo,com
M E Z Z A N IN E FLO O R , D O N A A M P A R O B UILD IN G
C O R N E R E S P A N A & G. M. TOLEN T1N O S T R E E T S
SAIVIPALOC, M A N IL A
TEL. N O S 7 3 5 -7 3 -0 2 & 7 3 3 -2 1 -1 8
TEL. NOS. ( 0 3 ) 2 6 1 - 2 2 4 4 & ( 0 3 2 )2 6 1 - 8 4 5 2
POWERLINE ENGINEERING REVIEW CENTER, INC,
DIRECT CURRENT MACHINERY
DYNAMO - is a rotating machine for converting mechanical energy into electrical energy, or the reverse process, electrical energy into mechanical energy.
Induced Voltage in Dy namo ZP(p N E = ------- L - * 1 0 ~ 8 Volts 60 a General Classification: [I]. Motor [II]. Generator
I - DC Motor - a continuous energy conversion device used to convert electrical energy to mechanical
energy.
General types of Self Excited Motors :
1. Series wound D.C. Motor- a motor where its winding called series field winding is connected in series with the armature. Speed Characteristics: Variable speed Torque Characteristics: high starting torque Uses: elevator, crane, converyor, hoist gear drive. *A tote: to reverse direction of rotation of this motor, interchange the brushes. Caution: Never operate the motor if unloaded because it will K'r<ice" or 'run away ■”
Schcrnafic Hiring Diagram fw/wvalcnl Circuit
VT - terminal voltage Eb - back emf or counter emf Vbc - brush contact voltage drop Vs - series field winding voltage drop h - line current or input current Ia - armature current
- series field winding resistance Rbc - brush contact resistance Ra - armature resistance R - equivalent resistance due to the power developed in the armature Pi - power point Pd ' - power developed in the armature P0 - power output
- useful power
DC MOTOR FORMULAS:
1. Induced EMF
60a
PZ
Let K =
60a
E = K<£N
Where:
P- number of poles
- flux per pole in weber
N- armature rotation in RPM
Z- total number of active conductor
Z= no. of slots x Conductor per slot
a- number of parallel paths
a = mP
m - winding multiplicity
2. Voltage Equations:
Series Motor:
Eb = Vt - IaRa - IaRs - Vbc
Pin
la = I, = Is = —
L Vt
Shunt M otor:
Eb = V t- IaRa - Vbc
la = IL - If
f Rf
3. Speed of DC Shunt Motors, N = -Vt' laRa b-C
F K0>
4. Armature Power Developed, Pdev = Eb •la
PQNZ
- ^ Torque Developed, J ^ T = (^) Pdev ------ (^) = -----------Eb-Ia (^) - o y a — -60a PZ ---- (^) O • la T
2*N' 2^1' l n V 2m
6. Motor Efficiency, 77
Po Vt • I, - Copper Losses - Rotational Losses
^OVERALL ~ ~ r '
Pin Vt • I,
=K<D-Ia
7? 'o v e r a ll ~
Po Po
'm e c h a n ic a l ~
Pin Po + Copper Losses + Rotational Losses
Pdev
Pdev + Copper Losses
________ Po _____
Po + Rotational Losses
Pdev
Pdev + Armature Copper Loss
7. Maximum Efficiency Condition
Variable Losses = Constant Losses
For Shunt Motor,
Ia2Ra = SPL + If2Rf
Ia’=
(Constant Losses
t Ra I L’= Ia M f
Vt • IL'- 2 • Constant Losses
^JxfAX ^ j i
8. Core Loss or Iron Loss, Wh+E
Wh+E = Hysteresis Loss + Eddy Current Loss
Hysteresis Loss, Wh = r]Bl bf • Vol
For constant flux density, Wh = A N
Eddy Current Loss - KhB 2f 2t2Vol
For constant flux density, Ws = B N 2
Where:
rj- Steinmetz Constant in Joule/(m’ cycle) or ergs/(cm3 cycle)
B - flux density in Tesla or Gauss
PN
f - frequency of magnetic reversal in cps f = y—
N - speed in rpm
Vol - volume of armature core in m3 or cm
t - thickness of armature laminations
Kh - hysteresis constant
DC GENERATOR FORMULAS:
1. Generated EMF
P$NZ
Eg =
60a
PZ
Let K =
60a
Eg = KN.
Where:
P- number of poles
- flux per pole in Weber
N- armature rotation in RPM
Z- total number of active conductor
Z= no. of slots x Conductor per slot
a- number of parallel paths
a,= mP
m - winding multiplicity
2. Armature Winding:
Conduc tor Current, / = —
Coil Pitch, Ys = —~ K
P
Back &.Front Pitch (Progressive Winding)
Back Pitch, Y„ = — +
p
FrontcPitch, YF = — -
P
Commutator Pitch, Yc
Lap Winding, Yc = ±m
Wave Winding, Yc = <:- ^ -
P! 2
C- number of commutator bars or number of coils
3. Voltage Equation:
Series Generator
Eg = Vt + IaRa + IaRs + Vbc
Shunt Generator
Eg = Vt + IaRa + Vbc
la = IL + If
Rf
Voltage/Speed Relation of DC Shunt Generators, N
Armature Power Generated, Pgen - Eg • la
Efficiency, 7
_ P ° _ V t I L V'o v e r a l l
Pin Vt • IL + Copper Losses + SPL
Maximum Efficiency Condition
Variable Losses = Constant Losses
For Shunt Motor,
Ia2Ra = SPL + If2Rf
Ia'=
Constant Losses
Ra
IL'= Ia '-If
Vt *1L
V m a x
Vt • Il '+2 •Constant Losses
POWERLIne r e v ie w c e n t e r Inc.
The Ultimate E l Review Center
DC MACHINES REVIEW PROBLEMS
- Calculate the voltage induced in the armature winding of a 4-pole, lap-wound, dc machine having 728 conductors running at 1800rpm. Flux per pole is 0.03Wb. A: 655.2 V C. 720.4 V B. 468.2 V D. 236.4 V
- A dc machine has a 4-pole, wave-wound armature with 46 slots and 16 conductors per slot. If the induced voltage in the armature is 480 V at 1200rpm, determine the flux per pole. A. 48 mWb C. 0.94 mWb B. 1.2 mWb D. 16.3 mWb
- A 115 volt shunt m otor has an armature whose resistance is 0.22 ohm. Assuming voltage across the brush contact is 2 volts, what armature current will flow when counter em f is 108 volts. A. 22.7A C. 34.6 A B. 31.8A D. 27.2 A
- The nameplate rating of a shunt m otor is 150 hp, 600 volts, 205 amp, 1700 rpm. The resistance of the shunt field is 240 ohms and the total armature resistance is 0.15 ohm. Calculate the counter emf. A. 569.63 volts C. 569.25 volts B. 586.52 volts D. 630.75 volts
- The term inal voltage of a motor is 240 volts and its armature current is 60A. The
armature resistance is 0.08 ohm. W hat is the hp developed by the motor. A. 18.92 C. 16. B. 17.26 D. 26.
- A certain load to be driven at 1750 rpm requires a torque of 60 Ib-ft. W hat hp will be required to drive the load? A. 20 C. 12 B. 15 D. 16
- A shunt motor is running at 1200 rpm for a load which requires an armature
current of 50 ampere from 230 volt source. At no load, the arm ature current is 5 amp. If the effect of armature reaction has reduced the air-gap flux by 2% from no load to full load, determ ine the no load speed. The armature resistance is 0.15Q A.. 1212 rpm C. 1200 rpm B. 1012 rpm D. 1182 rpm
- In a motor, the armature resistance is 0.1 ohm. W hen connected across 110 volts mains, the armature takes 20 amp and it speed is 1200 rpm. Determine its speed when the armature takes 50 A from the same mains, with field increased 10: : A. 1023 rpm C. 1171 rpm Bi 1061 rpm D. 1251 rpm
- A 120-V dc shunt m otor having an armature circuit resistance of 0.2 ohm and field circuit resistance of 60 ohms, draws a line current of 40A at full load. The brush drop is 3-V and full load speed is 1800 rpm. W hat is the speed at 125% full load? A 1765 rpm C. 3600 rpm B. 1863 rpm D. 720 rpm
- A Shunt generator while running light as a motor at 1000 rpm takes 12 ampere from 115-V mains. The field current is 7 A and the armature resistance is 0. ohm. Determine the stray power loss? A. 574W C. 942W B. 543W D. 862W
- A shunt motor runs at 1100 rpm, at 230 V, and draws a line current of 40 A. The various losses are: core loss, 200 W; friction and windage loss, 180 W; electrical loss due to brush contact, 37 W; stray-load loss, 37 W. The armature- and field- circuit resistances are 0.25 Q and 230 £2, resp. Solve for motor efficiency. A. 89.56% C. 87.89% B. 88.43% D. 92.48%
- A 50 hp 500 volt shunt m otor draws a line current of 4.5 amp at no load. The shunt field resistance is 250 ohms and the armature resistance is 0.3 ohm. The brush drop is 2 volts. The full load line current is 84 amp. W hat is the efficiency? A. 89.45% C. 86.25% B. 87.26% D. 76.64%
- Calculate the efficiency of a 230 Volt shunt motor from the following data : Input current is 19.8 Amp, balance reading is 14 lbs, tare weight is 2 lbs., brake arm is 2 ft long & Speed is 1100 rpm. A.' 82.4 % C. 84.2 % B. 8 1 .4 % D. 8 6 .5 %
- A 230 Volts, 15 hpd.c. shunt m otor has friction and iron loss of 200 W. The armature resistance is 0.3 ohm and shuntfield resistance is 115 ohms. W hat is the maximum efficiency of the motor? A. 86.9% C. 88.3% B. 87.4% D. 85.1%
- A 120 volts shunt m otor takes 75 A armature current at full load. Arm ature resistance is 0.2 ohms and brush contact drop is 2 V. Find the starting resistance to limit the starting torque to 150% of rated load torque at the instant of starting.
A. 0.85 Q C. 0.82Q
B. 0.82 Q D. 0.88 Q
16.The armature of a 230 volt shunt motor has a resistance of 0.18 ohm. A
resistance is inserted in series with the armature at the instant of starting so as not to exceed 76 amp at starting, calculate the value to which this resistance can be reduced when the armature accelerates until Ec is 168 volts. Brush drop is 2V
A. 0.61 Q C. 0.81 Q
B. 2.82 Q D. 0.28 Q
- A 10 kW 250 volts self-excited generator, when delivering rated load, has an armature-circuit voltage drop of 5% of the terminal voltage and a shunt field current equal to 5% of the rated load current. Calculate the resistance of the field circuit. A. 125 ohms C. 500 ohms B. 250 ohms D. 62.5 ohms
25.A self-excited s h u n t generator supplies a load of 12.5 kW at 125 V. the field
resistance is 25 £1 and the armature resistance is 0.1 Q. The total voltage drop
because of brush contact and armature reaction at this load is 3.5 V. calculate the induced armature voltage. A. 129 V C. 135 V B. 263 V D. 139 V
- A 2500 kW 600-V 16-pole generator has lap wound armature with 2360 conductors. If the pole face covers 65% of the entire circumference, calculate the number of pole face conductors in each pole of a compensating winding. A. 6 C. 5 B. 9 D. 4
- A certain 110-volt shunt generator has an armature and brush resistance of 0. ohm at full load of 85 amp. The resistance of the shunt field is 45 ohms, and the stray power losses are found to be 897 watts. Calculate the full load efficiency. A. 85.2% C. 86.5% B. 89.4% D. 84.9%
- A 250 kW 230 volt compound generator is delivering 800 amp at 230 V. The
shunt field current is 12 amp. The arm ature resistance is 0.007 ohm and the series field resistance is 0.002 ohm. The stray power at this load is 5500 watts. The generator is connected long shunt. Determine the generator efficiency at this load Stray load loss is 1% of output. / y 92% C. 95% B. 94% D. 90%
- A separately-excited dc generator has the following data: arm ature resistance, 0.04 Q ; field resistance, 110 Q ; total core and mechanical losses, 960 W; voltage across the field, 230 V. the generator supplies a load at a terminal voltage of 230 V. Calculate the maximum value of the generator efficiency. A. 93.8% C. 93.2% B. 92.7% D. 98.2%
- Two shunt generators A & B running in parallel share a load o f 100 kW equally at
a terminal voltage of 230 volts. On no-load their voltage rises to 240 and 245 volts respectively. How will they share the load when the total current is reduced to half of its original value. A. 20 kW & 30 kW C. 25kW each B. 18 kW & 32 kW D. 1 5 k W & 3 5 k W
DO Machines Supplementary Problems
- Dirt on the motor commutator may produce ___ A. isolation C. lower torque B. excessive sparking D. power loss
- If a motor frame is found excessively hot, which of the following is the LEAST likely cause A. very high ambient temperature C. inadequate internal ventilation B. faulty starter D. excessive loading
- The simplest form of a motor controller i s ...... A. drum switch C. magnetic switch B. relay D. toggle switch
- Rapid stopping of motor by momentarily reversing its connections to the supply is ... A. jogging C. inching B. plugging D. sequence operation
- A dc generator may loose its residual magnetism because o f ...... A. varying loads C. over-excitation B. vibrations D. heating or overload
- The simplest way to shift load from one dc shunt generator running in parallel with another is to .. A. adjust their fieldrheostatC. insert resistance in their armature circuits B. adjust speeds oftheirprime movers D. use equalizer connections
- For both lap and wave windings, there are as many commutator bars as the number of
A. poles % C. armature conductors
B. slots D. winding elements
- It is a DC motor that tends to over-speed when the field opened while running. A. shunt motor C. split phase motor B. series motor D. over-compounded motor,
- In DC shunt motors, how is the shunt field connected in relation with the armature? A. parallel C. parallel series B. series - parallel D. series
- DC shunt motor are commonly used in what application? A. high starting torque C. constant speed B low no-load speed D. high speed
- Which motor is used to start heavy loads? A. shunt C. series B. commulatively compound D. shaded pole
- When the load is removed, which motor will run at the highest speed? A. series C. shunt B. commulatively compound D. Differentially compound
- Which motor is suitable for elevator? A. series C, shunt B. commulatively compound D. Differentially compound
- A dc shunt motor runs at 900 rpm from a 460V supply when taking an armature current of 25 A. Calculate the speed at which it will run from a 230V supply when taking an armature current of 15A. The resistance of the armature circuit is 0.8 ohm. Assume flux per pole at 230V have to decrease to 75% of its value at 460V A. 495 C. 585 B. 485 p } 595
- A separately-excited motor runs at 1045 rpm, with a constant field current, while taking an armature current of 50 A at 120 V. the armature resistance is 0.1 Q. If the load on the motor changes such that it now takes 95 A at 120 V, determine the motor speed at this load. A. 1004 rpm C. 1014 rpm B. 1104 rpm D. 994 rpm
- A 20-hp, 250-V shunt motor has an armature-circuit resistance of 0.22 Q and a field
resistance of 170 Q. At no-load and rated voltage, the speed’is 1200rpm and the
armature current is 3.0 A. at full-load and rated voltage, the line current is 55 A, and the flux is reduced 6% (due to the effects of armature reaction) from its value at no-load. What is the full load speed? A. 1220 rpm C.)1200 rpm B. 1188 rpm D. 1260 rpm
- A shunt motor with an armature resistance of 0.15 ohm is running at 1200 rpm for a load which requires of armature current of 50 ampere from a 230 volt source. At no load, the armature current is 5 ampere. If the effect of armature reaction has reduced the air gap flux 2% from no load to full load, what is the no load speed? A. 1215 rpm C, 1,209 rpm B. 1268 rpm D. j,2 1 2 rp m
- The armature of a dc machine running at 1200 rpm carries 45 A in current. If the induced armature voltage is 130 V, what is the torque developed by the armature? A. 46.5 N-m C. 45.6 N-m B. 4.65 N-m D. 456 N-m
- Calculate the electromagnetic torque developed in the armature of a 4-pole, lap-wound dc machine having 728 active conductors and running at 1800 rpm. The flux per pole is 30 mWb. The armature is designed to carry a maximum current of 100A. A. 34.76 N-m C. 347.6 N-m B. 3.476 N-m D. 3476 N-m
- A 4-pole motor is lap wound with 728 conductors, and has a flux of 25 mWb per pole. The armature takes 50 A in current; the resultant demagnetizing effect due to armature reaction reduces the air-gap flux by 5%. Calculate the developed torque. A. 137.6 N-m C. 144.8 N-m B. 167.3N-m D. 176.3 N-m
- The armature of a motor has of 400 active conductors, the flux density in the field being 3000 gauss. The axial length of the armature is 20.32 cm. The current is 30 amperes. Determine the total force exerted in the armature.
A .> 31.5 N C. 0.07 N r - & \ l ^
B. 645.9 N D. 439.2 N
- A dc motor takes an armature current of 105 Amp at 480 volts when running at 720 rpm. The resistance of the armature circuit is 0.2 ohm and the field resistance is 96 ohms. What is the torque developed in the armature? A. 668 N-m C. 752 N-m B. 610 N-m D. 639 N-m
- An electric motor develops 20 metric HP at a speed of 600 rpm. What is the torque? A: 240 N-m C. 210 N-m B. 290 N-m D. 260 N-m
- A 4-pole lap-wound' armature has 144 slotswith two coil sides per slot, each coilhaving two turns. If the flux per pole is20mWb andthe armature rotates at 720rpm. What is the induced voitage? A. 138.24 V C. 69.12 V B. 34.96 V D. 276.48 V
- A d.c. shunt motor develops 7.8 hp at 120 Volt, the armature efficiency is 93%. What is the armature effective resistance? A. 0.1 ohm C. 0.16 ohm B. 0 06 ohm D. 0.05 ohm
- A certain shunt motor has an armature resistance of 0.05. It draws 50 amperes at a terminal voltage of 120! volts. Assume other miscellaneous losses of 1%. Determine the output horsepower of the motor. A. 8.5 hp C. 6.8 hp B. 7.1 hp D. 7.8 hp
- In a brake test the effective load on the branch pulley was 38.1 kg, the effective diameter of the pulley is 63.5 cm and the speed is 12 r.p.s. The motor took 49 ampere at 220 volts. What is the efficiency at this load? A. 85% C. 86% B 80% D) 83%
- A 240 volt d.c. motor on brake test took 52 Amp when running at 1500 rpm. The spring balance at the end of the 76 cm brake arm reads 93.5 N. What is the efficiency of the motor at this load? A. 0.85 C. 0. B. 0.95 , D. 0.
- The shaft power of a shunt motor is 7.8 hp. Its armature draws 50 Amperes from 120 Volts. The field winding draws 1.2 Amp. What is the efficiency of the motor? A. 94.7% C. 96.5% B: 93 % D. 98.2 %
- A 15 hp 220 volt, 1200 rpm shunt motor takes 50 Amperes. Its armature resistance is 0. ohm and field resistance is 220 ohms. Brush contact voltage drop is 2 volts, Friction and windage loss is 400 watts and stray load loss of 1% of the output. Calculate the efficiency. A. 91.8% C. 90.4% B. 92.4% D. 94.6%
49 A 20 Hp, 240 V shunt motor takes 72 Amperes when operating at full load. The shunt field resistance is 240 ohms and the armature resistance is 0.22 ohm. Calculate the starter resistance if the starting torque is limited to 1.5 times the full load torque (take a 3-V brush drop). A. 4Q C. 6 Q B. 2Q D. 1.8 Q
- A dc series motor develops 180 lbs.-ft of torque when the is current is 30 Amp. The load increases so that the motor current rises to 50 Amp. The torque now is... A 500 lbs.-ft C. 64.8 Ibs.-ft B. 300 Ibs.-ft D. 108 Ibs.-ft
51. A series motor, having an armature resistance of 0.1 Q and a field resistance of 0.15 Q ,
takes 48 A at 230 V and 720 rpm. The total core and friction losses are 650 W. neglecting stray-load and brush-contact losses, calculate output power at the shaft. A. 11.36 HP C. 13.16 HP B. 16.13 HP D. 113.6 HP
- For a 4-pole, 2 -layer, dc lap-winding with 20 slots and one conductor per layer, the number of commutator bars is. A. 80 C. 40 B. 20 D. 160
- A 4 pole, lap-wound dc armature, the back pitch is 17, if the winding is progressive what is the front pitch? A. 15 C. 17 B. 13 D. 16
- In a 4 pole, 54 slot d.c. armature, calculate the coil span. A. 18 C. 14 B. 9 D. 13
- If the armature of an eight-pole dc machine were wound with a simplex wave winding. ' How many parallel paths would there be? A. 16 paths C. 8 paths B. 2 paths D. 4 paths
- In a 4 pole, 35 slot d.c. armature, calculate the coil span. A. 8 slots C. 10 slots B. 7 slots D. 9 slots
- A 4 pole, 12 slot lap-wound dc armature has two coil side/slot. Assuming single turn coil, calculate the back pitch for progressive and retrogressive winding. A. 5 & 3 C. 7 & 5 B. 3 & 5 D. 6 & 4
- In a 10 pole, lap wound dc armature, the number of active armature conductor per pole is
- The number of compensating conductor per pole required is A. 5 C. 50 B 500 D. 10
- A 4 pole dc generator with simplex lap winding has 48 slots and four element per slot. The flux per pole is 2.5 x 106 maxwells and it runs at 1,500 rpm. What is the output voltage? A. 60 Volts C. 225 Volts B. 360 Volts D. 120 Volts
- A 4 pole dc generator with duplex lap winding has 48 slots and four element per slot. The flux per pole is 2.5 x 106 maxwells and it runs at 1,500 rpm. What is the output voltage? A. 60 Volts C 225 Volts
B. 480 Volts D. 120 Volts
- A separately excited generator generates 230 volts when driven at 1200 rpm, If the flux is decrease by 10%, What is the new emf with speed remains the same. A. 230 volts C. 207 volts B. 256 volts D. 253 volts
- The input current to a 220 volts, long shunt compound motor at no load is 6 amp. The shunt field circuit resistance is 220 ohms, the armature resistance is 0.1 ohm and series field resistance is 0.08 ohm. What is the stray power loss? A. 1095.5W —" C. 1086.5W B. 1097.5W D. 1189.6W
- A 20-kW 220-volt shunt generator has an armature resistance of 0.07 ohm and a shunt resistance of 200 ohms. Determine the power developed in the armature when it delivers its rated output. A. 27.9 kW C. 19.2 kW B. 20.83 kW D. 25.7 kW
- The pulley of an old gen-set has a diameter of 20 inches. The belt exerts a pull of 353 lbs on the pulley. the gen-set runs at 900 rpm. What is the approximate kW rating of the gen-set? , < y A. 75 C. 200 B. 250 D. 35
- A 288 volt shunt generator has constant losses of 2880 Watts. The armature circuit resistance is 0.20 ohm and field resistance of 144 ohms. If the generator is delivering power at maximum efficiency, then its armature current is. A. 10 A C. 130 A B. 120 A D. 64 A
- A 400- kW, 600 volt, d-c shunt generator has an armature resistance of 0.03 ohm. The field resistance is 48 ohms. The stray power loss for this machine at rated voltage and speed is 7200 watts, What is the maximum efficiency? A. 93.4% C. 95.3% B. 96.3% D. 98.5%
- A shunt generator delivers 195 Amp of terminal of voltage 250 Volts. The armature resistance and shunt field resistances are 0.02 ohm and 50 ohms respectively. The iron and friction losses equal to 950 watts. What is the overall efficiency? A 94 % C. 96 % B. 98% D. 95%
J
