University of Tehran
Electrical Machines I
Homework 6 (DC Motors)
Due: Sunday, 31 May
1- A DC motor has the following characteristics
Prated = 15 hp
IL,rated = 55 A
VT = 240 V
NF = 2700 turns per pole
nrated = 1200 r/min
NSE = 27 turns per pole
RA = 0.40Ω
RF = 100Ω
RS = 0.04Ω
Radj = 100 to 400Ω
Rotational losses = 1800 W at full load. Magnetization curve as shown below:
(a) Assume that the motor is operating at full load and that the variable resistor Radj is again
175Ω . If the armature reaction is 1200 A.turns at full load, what is the speed of the
motor?
(b) If the motor is connected cumulatively compounded as shown in following figure and if
Radj = 175Ω , what is its no-load speed? What is its full-load speed? What is its speed
regulation? (Neglect armature effects in this problem.)
(c) The motor is now connected differentially compounded. If Radj =175Ω , what is the noload speed of the motor? What is the motor’s speed when IA = 20 A? 40 A?
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University of Tehran
Electrical Machines I
Homework 6 (DC Motors)
Due: Sunday, 31 May
2- An automatic starter circuit is to be designed for a shunt motor rated at 15 hp, 240 V,
and 60 A. The armature resistance of the motor is 0.15Ω, and the shunt field
resistance is 40Ω. The motor is to start with no more than 250 percent of its rated
armature current, and as soon as the current falls to rated value, a starting resistor
stage is to be cut out. How many stages of starting resistance are needed, and how big
should each one be?
3- A 300-hp 440-V 560-A, 863 r/min shunt dc motor has been tested, and the following
data were taken:
Blocked-rotor test:
VA = 16.3V
(exclusive of brushes)
VF = 440 V
IF = 8.86 A
I A = 500 A
No-load operation:
VA = 16.3V
(including brushes)
IF = 8.76 A
IA = 23.1 A
n = 863 r/min
What is this motor’s efficiency at the rated conditions? [Note: Assume that (1) the
brush voltage drop is 2V; (2) the core loss is to be determined at an armature voltage
equal to the armature voltage under full load; and (3) stray load losses are 1 percent of
full load.]
4- When operated from a 230-V dc supply, a dc series motor operates at 975 r/min with a
line current of 90 A. Its armature-circuit resistance is 0.11Ω and its series-field
resistance is 0.08Ω. Due to saturation effects, the flux produced by an armature
current of 30 A is 48 percent of that at an armature current of 90 A. Find the motor
speed when the armature voltage is 230 V and the armature current is 30A.
5- A 15 hp, 120V, 1800 r/min shunt dc motor has a full-load armature current of 60 A
when operating at rated conditions. The armature resistance of the motor is RA =
0.15Ω, and the field resistance RF is 80Ω. The adjustable resistance in the field circuit
Radj may be varied over the range from 0 to 200Ω and is currently set to 90Ω.
Armature reaction may be ignored in this machine. The magnetization curve for this
motor, taken at a speed of 1800 r/min, is given in tabular form below:
EA
5
78
95
112
118
126
IF
0
0.8
1
1.28
1.44
2.88
(a) What is the speed of this motor when it is running at the rated conditions specified
above?
(b) The output power from the motor is 7.5 hp at rated conditions. What are the
copper losses and rotational losses in the motor at full load (ignore stray losses)?
(c) What is the efficiency of the motor at full load?
(d) If the motor is now unloaded with no changes in terminal voltage or Radj , what is
the no-load speed of the motor?
6- Magnetization curve of a series DC motor is as the following table. Assume RA= 0.1
and Rseries = 0.05Ω. When this motor is connected to a 230V source, 60A current
flows through its terminals and rotates at 750rpm. Find the diverter resistor which
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University of Tehran
Electrical Machines I
Homework 6 (DC Motors)
Due: Sunday, 31 May
should be added in parallel to series winding to speed up the motor to 900rpm under
constant armature current.
IF (A)
Magnetic Flux per pole (mWb)
10 20 30 40 50 60 70
80
90
2.9 5.5 7.4 8.5 9.2 9.7 10.2 10.6 10.8
Project 6 (Bonus)
Remember the previous notes!
1- Analyze the performance of different DC motors at “under-rated load starting”
and “under-over load starting” modes and find the most stable one.
2- Analyze the performance of different DC motors at suddenly disconnecting
load from the motor and find the most stable one.
Good luck!
H. Ahmadi
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