TEST 15

Problem 525:
A simplex lap wound armature has 530 conductors and carries a current of 125amperes per
armature current path. If the flux per pole is 20 mWb. Calculate the electromagnetic torque
developed by the armature?
A. 210.64 N-m
B. 252.72 N-m
C. 230.77 N-m
D. 207.63 N-m
Problem 526:
A 6-pole lap wound shunt motor takes 300 A when the speed is 500 rpm. The flux per pole is 0.6 Wb
and the armature has 80 turns. Neglecting the shunt field current, calculate the brake horsepower of
the motor. Assume a constant loss of 4%.

A. 175
B. 168
C. 172
D. 165
Problem 527:
A 220 V shunt motor driving its normal load draws an armature current of 50 A from a 220 V dc
source. The armature resistance of this motor including brushes is 0.25 ohm. How much armature
current (minimum) will this motor draw from a 200 V dc source when driving the same load with the
field adjusted to maintain the same speed.

A. 55.76A
B. 58.24A
C. 60.12A
D. 63.48A
Problem 528:

A DC shunt motor has a full load rating of 15 hp, 230 volts, 57.1 amperes, 1400115 ohms.
Neglecting the effect of armature reaction, determine the no-load line current.

A. 6.74A

B. 5.62A

C. 7.45A

D. 8.64A
Problem 529
At rated load, a shunt motor draws an armature current of 50 A from a 230 V DC mains while
running at 1000 rpm. At no load the armature current drops to 5 A. If the determine speed of the
motor at no-load. Assume the armature resistance to be 0.15 ohm.

A. 1202 rpm
B. 1221 rpm
C. 1122 rpm
D. 1010 rpm
Problem 530:
The resistance of the shunt field circuit is 240 ohms and the total armature circuit resistance is
0.15 ohm. Calculate the speed regulation of the motor.

A. 4.07%
B. 4.89%
C. 5.25%
D. 5.18%
Problem 531:
A DC shunt motor runs at 600 rpm on a 240-volt supply while drawing a line current of 30 A. Its
armature and field resistances are 0.5 Ω and 120 Ω respectively.What resistance shouid be placed
in series with the armature circuit in order to reduce the speed to 400 rpm. Assume no changes in
the armature or field current.

A. 2.69Ω
B. 2.41Ω
C. 2.07Ω
D. .2.83Ω

Problem 532:
A 120-V shunt motor has an armature equivalent of 0.5 Ω and a field resistance of 60 Ω. At full
load, the motor takes 10 A and the speed is 1000 rpm. At what speed must this motor maybe driven
as a generator to deliver 10 A to an external load at 120V?

A. 1158 rpm
B. 1086 rpm
C. 1262 rpm
D. 1045 rpm

Problem 533: EE Board April 1990

A belt-driven 150 kW shunt wound DC generator is running at 450 rpm and is supplying full load
to a bus bar at 240 V. At what speed will it run if the belt breaks and the machine continues to run
taking 8.5 kW from the bus bar? The armature and field resistances are 0.05 chm and 85 ohms
respectively. The brush contact drop is 1.5 volt per brush. Neglect armature reaction.

A. 395 rpm
B. 388 rpm
C. 402rpm
D. 358 rpm
Problem 534:
A shunt motor draws a current of 40 A from a 120 V source and runs at 1200rpm at rated load.
The armature and field circuit resistances are 0.1 Ω and 60Ω,respectively. Determine the speed of
the motor at half load. Assume a brush drop of 2 V at rated load and 1 V at half load.
A. 1180 rpm
B. 1238 rpm
C. 1230 rpm
D. 1026 rpm

Problem 535: EE Board October 1990

A DC shunt motor has a full load rating of 15 hp, 230 volts, 57.1 amperes, 1400rpm. The
armature circuit resistance is 0.13 ohm and the field circuit resistance is 115 ohms.Neglecting the
effect of armature reaction, determine the no-load speed.
A. 1392 rpm
B. 1264 rpm
C. 1441 rpm
D. 1380 rpm

Problem 536:
A 220-V shunt motor delivers 40 hp on full load at 950 rpm and has an efficiency of 88%. The
armature and field resistances are 0.2 ohm and 110 ohms,respectively.Determine the starting
resistance such that the starting armature current does not exceed 1.6 times the full load current.
A. 0.704 ohm
B. 0.732 ohrn
C. 0.712 ohm
D. 0.739 ohm
Problem 537:
A 120-V DC motor rated at 5-hp has a full loadefficiency of 86%. The field and resistance will be
required to limit the armature current at starting to 200% of its rated armature current?

A. 1.02 ohms
B. 1.12 ohms
C. 1.00 ohm
D. 1.22 ohms

Problem 538:
A 10-hp 230 V DC motor of 85% full load efficiency is located 450 ft from the the smallest cross-
sectional area of copper wire required when the allowable voltage drop in the feeder at starting is
limited to 24 volts?

A. 30 MCM
B. 28 MCM
C. 26 MCM
D. 27 MCM
Problem 539:
The nameplate rating of a shunt motor is 150 hp, 600 volts, 205 A, 1700 rpm.The resistance of
the shunt field circuit is 240 ohms and the total armature circuit resistance is 0.15 ohm. If the motor
is to be deliver full-load torque at 1,200 rpm,what value of resistance must be added to the armature
circuit?

A. 0.833 ohm
B. 0.802 ohm
C. 0.827 ohm
D. 0.862 ohm

Problem 540:
A 120 V shunt motor draws a current of 77 A at rated load. The armature and shunt field
resistances are 0.2 Ω2 and 60 ohins respectively. A multi-tap starter rheostat is inserted in the
armature circuit to limit the starting current. If the rheostat has a resistance, of 1.732 ohms,
determine the percentage tap used such that the starting current will be limited to 150% of its full
load value?

A. 60%
B. 50%
C. 55%
D. 45%

Problem 541: EE Board April 1985

A 10-hp power 220 V DC shunt motor has an armature and field resistance of 0.25 ohm and
100 ohms respectively. The full load efficiency is 83%. Determine the value of starting resistance in
order that the starting current will not exceed 200percent of full-load value.

A. 2.88Ω
B. 2.59Ω
C. 2.15Ω
D. 2.45Ω

Problem 542:
A 50 HP, 550 V shunt wound motor draws a line current of 4.5 A at no load.The shunt field
resistance is 275 ohms and the armature resistance exclusive of brushes,is 0.3 ohm, the brush
drop at full load is 2 V. At full load, the motor draws a line current of 84 A. Calculate the efficiency at
full load.

A. 92.0%
B. 88.5%
C. 91.2%
D. 89.9%

Problem 543:
A 100-volt shunt motor is developing 6 hp while operating at an overall efficiency of 86%. The
armature and shunt field resistances are 0.06 and 50 ohms respectively. Determine stray power
losses.

A. 378W
B. 367 W
C. 392W
 D. 364W

Problem 544: EE Board October 1998

The shaft power of a shunt motor is 7.8 hp. It draws 50 A from 120 V. The field winding draws-
1.2 A. What is the efficiency of the motor?

A. 94.7%
B. 93%
C. 96.9%
D. 98.2%

Problem 545:
A 500 V shunt motor running at.720 rpm takes an armature current of 50 A. The with the
armature to reduce the speed to 540 rpm while maintaining the same torque?

A. 2.2 ohms
B. 2.6 ohms
C. 2.4 ohms
D. 2.8 ohms

Problem 546: EE Board March 1998

A certain shunt motor has an armature resistance of 0.05 ohm. It draws 50 A at a terminal voltage
of 120 V. Assume other miscellaneous losses at 1%. Determine the output horsepower of the motor.
A. 8.5 hp
B. 7.1 hp
C. 6.8 hp
D. 7.8 hp

Problem 547:
The armature and field resistances of a shunt machine are 0.2 ohm and 110rpm. If the machine
is run as a shunt motor, it takes 4 A at 220 V. At a certain load the motor takes 30 A at 220 V.
However on load, the armature reaction weakens the field by 2 %. Find the motor speed at this load.
A. 1120 rpm
B. 1050 rpm
C. 1042 rpm
D. 1025 rpm

Problem 548: EE Board October 1990

A shunt motor, which has a field resistance of 220 ohms and an armature resistance of
0.8 ohm takes 26 A from a 260 V supply when running at 500 rpm on full load. In order to control
the speed of the motor a 1.2-ohm resistor is connected in series with the armature. Calculate the
speed at which the motor will run when supplying full load torque.
A. 438 rpm
B. 445 rpm
C. 424 rpm
D. 472 rpm
Problem 549: EE Board October 1992
The input and output powers of a 220 V, shunt motor are 30 and 28 kW respectively. The field
and armature circuit resistances are 40 ohms and 0.0125 ohm respectively. Determine the
efficiency of the motor if the output power is reduced to 10 kW.

A. 83.56%
B. 88.67%
C. 84.93%
D. 86.48%

Problem 550:

A 220 V series motor takes 15 A and runs at 700 rpm. The armature resistance including the
resistance of the series winding is 0.8 ohm. At what speed will it run if a 2-ohm resistance is
connected in series with supply voltage? Assume the motor is taking the same current.

A. 599 rpm
B. 601 rpm
C. 572rpm
D. 583 rpm

Problem 551:
The equivalent armature resistance of a series motor is 0.1 ohm. When connected across a 110-
V mains, the armature takes 20 A and its speed is 1000rpm. Determine its speed when the armature
takes 50 A from the same mains, with the field increased by 10%.

A. 809 rpm
B. 954 rpm
C. 856 rpm
D. 884 rpm

Problem 552:
A 400 V series motor has a field resistance of 0.2 ohm and an armature resistance of 0.1 ohm.
The motor takes 30 A of current at 1000 rpm while developing full load torque. What is the motor
speed when this motor develops 60% of full load torque.

A. 1302.4 rpm
B. 1256.2 rpm
C. 1297.6 rpm
D. 1135.5 rpm

Problem 553:
A 10 hp, 230-V, 1200 rpm series motor having rated load efficiency of 85.5%.The armature
resistance including brushes is 0.3 ohm. The field resistance is 0.25ohm. Assuming the flux varies
directly as the armature current, what value.of resistance should be placed in series with this motor
when starting, in order that the starting current maybe limited to a value that will exerț a starting
torque equal to 150% of its rated torque?
A. 4.7 ohms
B. 4.4 ohms
 C. 5.1 ohms
D. 3.8 ohms
Problem 554:
The field and an armature resistances of a 220-V series motor are 0.2Ωand 0.1Ω,respectively.
The motor takes 30 A of current while running at 700 rpm. If the total iron and friction losses are
350 W, determine the motor efficiency.

A. 90.6%
B. 91.5%
C. 89.4%
D. 92.2%
Problem 555:
A 100 – V series motor is used to drive a load through a pulley. This machine has an armature
resistance of 0.02 ohm and a series field resistance of 0.25 ohm. When a torque of 25 N-m is
applied to the pulley, the speed is 600 rpm. If stray power losses at this load is 300 W, calculate the
armature current.

A. 20.62 A
B. 21.89 A
C. 22.72A
D. 23.41A
Problem 556:
A 400 V series motor, having an equivalent armature circuit resistance of 0.5ohm, takes44 A of
current while running at 650 rpm. What is the mofor speed for a line current of 36
A. 803 rpm
B. 822 rpm
C. 812rpm
D. 806 rpm
Problem 557:
The resistance of each of the two coils of a series motor is 0.04 ohm, armature resistance, 0.04
ohm. The motor takes 50 A at 100 V while running at 800 rpm and the coils are in series. What will
it speed if the coils are re- connected in parallel while the load torque is doubled?

A. 800rpm
B. 400rpm
C. 200rpm
D. 650 rpm
Problem 558:
On full load a 500 V series motor takes 100 A and runs at 820 rpm. The armature and series
field resistances are 0.1 and 0.04 ohm respectively.What will be its speed when developing half-full
'load torque and with a 0.08- ohm diverter connected across the series field winding. Assume flux is
proportional to the field current.

A. 1563 rpm
B. 1634 rpm
C. 1359 rpm
 D. 1429 rpm

Problem 559:
A 400 V series motor working with unsaturated field is taking 60 A and running at 840 rpm. The
total resistance of the motor is 0.1 ohm. At what speed will the motor run when developing half-full
load torque.

A. 1193 rpm
B. 1202 rpm
C. 1167 rpm
D. 1352 rpm

Problem 560:
A long shunt compound motor takes a current of 42 A from a 230 V source. The generator
parameters are:nd Rsh = 50 Ω. If the friction and windage losses amount to 400 W, determine the
overall efficiency of the machine.

A. 82.24%
B. 80.56%
C. 81.22%
D. 79.53%.

Problem 561:
A 230 V, long shunt machine has the following parameters:460 Ω and Rse = 0.25 Ω. When the
machine is run at no-load at its normal speed and rated voltage, the armature draws 0.6 A.
Determine the armature current drawn if the machine delivers an output of 5 bhp.

A. 24.35A
B. 21.22 A
C. 20.36 A
D. 22.83A
Problem 562:
A 25 hp, 250 V long shunt compound motor takes 5 A when running light and runs at 1150 rpm.
Armature resistance, 0.15 Ω, Shunt field resistance, 200 Ω and series field resistance, 0.05 Ω. At
rated load the motor takes 86.5 A and runs at 720rpm. Determine the ratio of torque developed from
no-lóad to full load.

A. 1:26
B. 1:34
C. 1:22
D. 1:38

Problem 563:
A 200 V shunt motor has armature and shunt field resistances of 0.15 Ω and winding of 0.05
ohm is added to make it long shunt cumulatively compounded. This current of 100
A. Neglecting armature reaction, solve for the operating speed as a compound motor.

A. 887 rpm
B. 890 rpm
 C. 868 rpm
D. 865 rpm

Problem 564:
A 220 V DC motor has an armature resistance of 0.4 ohm, a shunt field resistance of 175
ohms and a series field resistance of 4 ohms. This machine is connected as a short shunt cmpound
motor to 220 V mains. Calculate the power developed by the armature if the armature current is 30
A. Neglect the brush drop.

A. 2254.6W
B. 2572.8W
C. 2652.6W
D. 2150.5W

Problem 565:
The input current to a 220 V, short shunt compound motor at no load is 6A.The shunt field
circuit resistance is 220 ohms; the armature resistance is 0.10 ohm and the series field resistance is
0.08 ohm. What is the stray power loss?

A. 1153.31 W
B. 1066.22 W
C. 1232.54W
D. 1073.25W

Problem 566: EE Board April 1988

The pony brake test of an elevator door drive shunt motor, the ammeter and voltmeter
measuring the input read 34 A and 20 V. The speed of the motor is found to be 910 rpm and the
balance on a 2 ft brake arm,read 27.2 pounds. The tare weight of the arm is found to be positive 2.3
pounds. Determine the efficiency of the motor at this load.

A. 87.03%
B. 86.05%
C. 84.45%
D. 88.32%

Problem 567: EE Board October 1986

In a brake test of an elevator door motor, the ammeter and voltmeter measuring the input read
34 A at 220 V. The speed of the motor is found to be 910 rpm and the balanced on a 0.61 m brake
arm reads 11.9 kg. The dead weight of the arm is found to be 1.09 kg. What is the output hp of the
door motor?
A. 9.40 hp
B. 10.38 hp
C. 8.26 hp
D. 9.58 hp

Problem 568:
A shunt motor with an armature and field resistance of 0.055 and 32 ohms,respectively, is to be
tested for its mechanical efficiency by means of a rope brake.When the motor is running at 1400
rpm, the longitudinal pull on the 6-inch diameter pulley is 57 pounds. The readings of the line
ammeter and voltmeter are 35 and 105,respectively. Calculate the efficiency.
 A. 77.10%
B. 75.32%
C. 78.28%
D. 79.12%

Problem 569:
A shunt motor was tested by means of a pony brake having a length arm of 3.5feet and a tare
weight of 5.7 lbs. The current drawn by the machine from a 240 V line was 50.9 A when the scale
reading was 24 Ibs and the speed of the motor was 1215rpm. Calculate the rotational losses of the
motor. The armature and shunt field resistances of the machine are 0.25 Ω and 120 Ω, respectively.

A. 674W
B. 692 W
C. 620W
D. 705W

Problem 570: EE Board October 1993

Calculate the force that will be exerted on the scale in a pony brake test when a 20-hp, 1,400
rpm motor is operating at full load. The length of the brake arm is 3 ft.and the tare weight of the
brake is 3.75 lb.

A. 30.45 lbs
B. 28.76 lbs
C. 21.54 lbs
D. 36.33 lbs