about dc machines and transformers

1. Good Morning
Week 6: DC Motors
Ibrahim AL-Zughaibi
202333100

2. Video
Question 1
The field coils can be connected to the rotor winding in the
two different ways, mention them?
1. Series connection
2. Parallel connection
Question 2
The result of connection between the field coils and the
rotor winding is two different kinds of DC Motors
construction, mention them?
1. A Shunt Motor
2. A Series Motor
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3. Methods of Starting
Introduction
Thus, to avoid the above dangers
while starting a DC motor, it is
necessary to limit the starting
current. For that purpose, starters
are used to start a DC motor.
There are various starters like, 3
point starter, 4 point starter and
DC series motor starter.
The main concept behind
every DC motor starter is, adding
external resistance to the
armature winding at starting.
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4. Methods of Starting
1- 3 Point Starter
When lever touches point 1, the field
winding gets directly connected
across the supply, and the armature
winding gets connected with
resistances R1 to R5 in series.
When the motor is overloaded
beyond a predefined value,
overcurrent release electromagnet D
gets activated, which short circuits
electromagnet E , and hence releases
the lever and motor is turned off.
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5. Methods of Starting
2- 4 Point Starter
The main difference between a 3
point starter and a 4 point starter is
that the no voltage coil is not
connected in series with field coil.
The electromagnet pull of the hold-on
coil will always be sufficient so that
the spring does not unnecessarily
restore the lever to the off position.
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6. Partner Work
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7. Search by using google website to find
information about methods of starting on DC
Motor.
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Googling

8. Methods of Starting
3- DC series motor starter
A start arm is simply moved towards
right to start the motor.
Thus at first maximum resistance is
connected in series with the armature
and then gradually decreased as the
start arm moves towards right.
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9. (1) All of three types of starting play a very significant role
in limiting starting current of DC motor for proper
starting and running of the DC moto ( ) 10 points
(2) These three types couldn’t use in industry ( ) 10
points
(3) The 3 point starter and 4 point starter Used for the
starting of shunt wound DC motor and compound
Wound DC motor ( ) 10 points
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Team Work

11. There are two types of losses occur in DC motors. These
losses are,
• Constant or fixed losses,
• Variable losses.
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Losses and Efficiency

12. Constant or Fixed Losses
Constant losses are those losses which are considered to
remain constant over normal working range of DC motor.
The fixed losses can be easily obtained by performing no-load
test on DC motor. These losses are further classified as:
1. Iron or core losses.
2. Mechanical losses.
3. Brush friction losses.
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Losses and Efficiency

13. Losses and Efficiency
1. Iron or Core Losses
Iron or core losses are further divided into hysteresis
and eddy current losses. Eddy current losses are
minimized by using lamination.
The core losses depend upon frequency.
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14. Losses and Efficiency
2. Mechanical and Brush Friction Losses
Mechanical losses occur at the bearing and brush friction
loss occurs in wound rotor DC motor.
These losses occurs with the change in speed. In DC
Motor the speed usually remains constant. hence these
losses almost remains constant.
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15. Googling
Use Google website to find more information about the
third type of losses which is Brush friction losses.
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16. Partner Work
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17. Losses and Efficiency
Variable Losses
These losses are also
called copper losses.
These losses occur due
to current flowing in
stator and rotor windings.
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18. Losses and Efficiency
Efficiency is defined as the
ratio of the output to that of
input
Efficiency =
𝑂𝑢𝑡𝑝𝑢𝑡
𝐼𝑛𝑝𝑢𝑡
DC Motor efficiency Equal
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19. Losses and Efficiency
Electric motor has input power
consumption of 50 watts.
The motor was activated for 60
seconds and produced work of
2970 joules.
Find the efficiency of the motor.
Solution:
Pin = 50W
E = 2970J
t = 60s
Pout = E / t = 2970J / 60s = 49.5W
η = 100% * Pout / Pin = 100 * 49.5W /
50W = 99%
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20. Alone Work
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