DC Motor Efficiency
Prepared By:
Engr. Renante D Junto REE,MTM
Technological University of the Philippines Taguig
Instructor
Power Losses in DC Motors
Two Classification of Power Losses in DC Machines
1. Cause by the rotation of the armature “ Rotational Losses” stray
power Losses
2. Those that result from a current flow in the parts of DC Machines
“electrical Losses”
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
Rotational Losses
1.Bearing friction
2.Brush friction
3.Wind friction “windage loss”
4.Hysteresis
5.Eddy Current loss
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
What is a Core Loss
Hysteresis loss take place in the revolving armature core because the
magnetic polarity in the iron changes in step with the changing positions of
the magnetic material under the various poles.
Eddy Current Loss are loops of electrical current induced
within conductors by changing magnetic field in the conductor according
to Faraday's law of induction. Eddy currents flow in closed loops within
conductors, in planes perpendicular to the magnetic field.
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
Electrical Losses
1. Armature Field Winding
2. Series Field Winding
3. Shunt Field Winding
4. Brush Contact
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
Losses in DC Machines
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
Power Flow Diagram of DC Motors
Power develop in Armature
PIN PD Pout
𝑃𝑖𝑛 = 𝑉𝐿 𝐼𝐿
2
𝑃𝑆𝐸 = 𝐼𝑠𝑒 𝑅𝑠𝑒 𝑃𝑓𝑤
𝑃𝐴 = 𝐼𝐴2 𝑅𝐴 2
𝑃𝑆𝐻 = 𝐼𝑠ℎ 𝑅𝑠ℎ
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
Sample Problem 1
A long shunt compound motor draws a line current of 42 Amp from a 230
volts DC source. The armature resistance is 0.1 ohms, while the series
shunt field resistances are 0.2 ohms and 50 ohms respectively. If the iron
and friction losses amount to 500 W, determine the copper loss of the
machine and overall efficiency
Ans; PL = 1977.63 W
𝜂= 79.5%
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
Sample Problem 2
A 220-V, d.c. shunt motor takes 4 A at no-load when running at 700
r.p.m. The field resistance is 100 Ω. The resistance of armature at
standstill gives a drop of 6 volts across armature terminals when 10 A
were passed through it. Calculate (a) speed on load (b) torque in N-m
and (c) efficiency. The normal input of the motor is 8 kW.
Ans:(a) 637.9 r.p.m. (b) 102 N-m (c) 80 %
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
Sample Problem 3
A 500-V, shunt motor takes a total current of 5 A when running unloaded.
The resistance of armature circuit is 0.25 Ω and the field resistance is 125 Ω.
Calculate the efficiency and output when the motor is loaded and taking a
current of 100 A
90.4% ; 45.2 kW
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
Sample Problem 4
A 50 kw generator has an efficiency of 91 % at full load. Calculate the input
in kilowatts and the power loss
Technological University of the
Engr. Renante D. Junto REE DC MOTOR Efficiency
Phils. Taguig
