electric machines

1. 1
Electric Machines
Electric Machines,
Mechanical Students
Prof. Dr. Mohsen Fadl Morsi
Semester: 2

2. 2
Aims: The aim of this course is to introduce: the theory,
operation and performance of the electrical
machines and electrical motors.
Course Contents:
Chapter 1: Electrical Machines
Chapter 2: DC motors
Chapter 3: Power Transformer
Chapter 4: AC Induction Motor
References
1) Electrical Machines and Power Electronic Applications, P. C. Sen.
2) Handouts.

3. 3
Course Evaluation
1. Final Term Exam: Written exam, 70% , (week-15),
2. Class work: 30%
a) Mid Term Exam: written exam, 15% ,
b) Reports, Activities and Attendance: 15%

4. 4
Chapter-1: Electrical Machines and
Electrical Drive System
 Electric Machines: Definition, Function, Classification,
Power Flow Diagram, Rotating Machine Construction,
 Theory of Operation and Name Plate Data,
 Voltage (induced emf) and Torque Equations: in DC and
AC Machines. Primitive Machine.

5. 5
1. Definition and Function of EM
• Electric machine is an Energy Converter Device used to
convert Energy from one form to another form of Energy:
Mechanical to Electrical:
or, Electrical to Mechanical
Rotating M/c:
Non Static machine
Electrical to Electrical:
Transformer:
Static machine

6. 6
1. Definition and Function of EM
1) Electric motor: converts electrical energy to
mechanical energy,
2) Electric generator: converts mechanical energy
to electrical energy,
3) Transformer: convert Electric to Electric with
different level of Voltages and currents.
Mechanical energy used to drive:
• Pumps, fan, blower, compressors and lifting devices.
Industrial Motors is around 70% of electrical load.

7. 7
2. Power Flow Diagram of EM
• Power flow diagram for electric motor is shown:
Ploss/e = ∑ I2 R Ploss/Field   Ploss/m  
• Power flow diagram for transformer is shown:

8. 8
3. Electric Machines Classification

9. 9
4. Rotating Machine Structure
Any rotating machine contains three different parts,
 Electrical part: electrical wires especially prepared with forms
in suitable windings, and connected to electric power supply.
To decrease electrical loss: electrical winding resistance must
be very small,
 Mechanical part: includes outer frame and steel shaft, which
coupled to the mechanical load. To decrease mechanical loss:
regular maintenance must be made,
 Magnetic part: laminated sheet from good magnetic material, it
carry the magnetic flux. To decrease the magnetic losses:
Thickness of the Laminated sheets must be very small (<0.5
mm) ,.
All these above consideration will be increase the rotating
electric machine efficiency and performance.

10. The Basic structure of a rotating electric machine is a
cylindrical form and has 3 parts:
1.Stator and stator windings, (electric),
2. Rotor windings in the Rotor,
which stands on bearings
3. Air gap separate them,
4. Both internal stator and external rotor
surfaces are slotted,
5. The magnetic flux link both stator and rotor. (Magnetic part)
{The magnetic parts of both stator and rotor are manufactured of the
same ferromagnetic laminated sheets, as shown}, while, {The non
magnetic parts of the rotor is solid steel, and it is called, the “Mechanical
Shaft“. For stator, it is called “Yoke”}.
3/25/2021
10
4. Rotating Machine Structure

11. 3/25/2021
4. Rotating Machine Structure

12. 12

13. 3/25/2021
There are many configuration for both the stator and rotor
as follows:
13
4. Rotating Machine Structure

14. 3/25/2021
14
The power transformer contains only two parts:
i) 2 Electrical coils, and,
ii) Magnetic core (which is of iron or air type)
5. Power Transformer General Construction

15. 15
Chapter-1: Introduction to EM
5. Power Transformer General Construction
The transformer core (magnetic part) is made from
laminated sheets has different configuration as shown.

16. If any parameter exceeds the listed value, it is abnormal condition
and overloaded. Overload is permissible within (+10%).
5. Name Plate: It lists the predesigned/nominal values of
machine including: Power (P or S), V, I, f, , n, t, as shown.
Operation within these limits is the normal operation.
3/25/2021

17. This is a basic rule for the motor operation:
– IS produce stator magnetic field, BS
– Ir produce rotor magnetic field, Br
and given by: Td = k x Br x BS
 In other words: both BS and Br
will be attracted (CW) or repulsed,
 This make a torque, that causes
the rotor to rotate at a speed, .
 Both fields are rotating and
the motor continuously runs.
3/25/2021 17
Both BS, Br
make Torque
6. Theory of Operation “Motor”

18. 18
Theory of Operation “Generator”
As the generator’s conductors will be driven (rotate) by a
prime mover, (in a magnetic field), this will result an induced
emf (E), in these conductors, (induced emf by motion).

19. 7. Induced emf by induction (change in  & i)
Magnet can be created by electric current and electricity can
be generated from magnet.
Faraday’s Law State that: When the flux linking a coil is changing, an
induced emf is produced and it is proportional to the rate of change of
the flux linkages. The flux linking is:
For AC:
It can be applied to the 2-windings of the transformer .
)
(
)
( i
L
t
t
N
t
t
e
i
L
N





















)
cos(
)
( max wt
t 


)
sin(
)
sin(
))
cos(
(
)
( max
max
max
t
E
w
wt
N
t
wt
N
t
e 













max
max
max
44
.
4
2
2


 fN
f
N
E
Erms 






L= coil inductance,
N = № of coil turns,
φ, i = flux and current,
f = Supply frequency

20. Induced emf by motion and induction
For the previous single excited magnetic system, the internal
induced emf (e) computed as follows:
This induced emf is dependent on: i) the excitation current,
ii) the speed of the movable part (v), and,
iii) the derivative of (L(x)).
For DC excitation:
For AC excitation:
)
(
)
( i
L
t
t
t
e 









i
L
N 


 

v
x
L
i
t
i
L
t
x
x
L
i
t
i
L
t
e 















)
(
v
x
A
N
I
v
x
A
N
x
I
x
v
e o
dc
o
dc 







 2
2
2
)
(
)
,
(


v
x
A
N
t
i
t
t
i
x
A
N
x
v
t
e o
o







 2
2
2
)
(
)
(
)
,
,
(


)
(
)
(
t
L
i
t
i
L
t
t
e













21. How is the Electric Machine wounded
• Assume the total No of the conductors is = Z, which is wounded in series
parallel based on the require Vt (Ea) and PN ,
• Assume the induced emf (The voltage can a conductor withstand) is = ec ,
• Assume the current can a conductor carries is = ic (based on its CSA),
• Based on the required Vt , the No of conductors must be connected in
series is (Zp) and its given by, Zp = Vt / ec, or, Zp = Ea / ec. Also: Zp = Z /a
• Based on the required P, the machine current must be I (Ia) = PN / Vt ,
• The No of parallel paths, a = Ia / ic , or, a = Z / Zp
21

22. Induced emf by motion (DC Machines) Faraday’s Law
If a conductor (of length-L) moving at a speed (v) in a field
(of density-B), there is an induced emf (e) in it given by:
For one conductor:
For Zp-series conductors:
The total emf is:
Where: a, p = № of parallel paths and poles,
Z, Zp = Total and per path № of conductors,
φ, B = Flux per pole and flux density,
L, r = Length & mean radius, L = N2/, = l /A
Ap = area per pole,
, l,  and A = The magnetic reluctance, length, permeability and area.
22
a
Z
wr
BL
e
E total
Z
a P



 )
(
60



a
Z
p
Ke





n
K
n
Z
a
P
n
a
Z
Lr
p
Lr
E e
a 





60
60
2
)
/
2
(
a
Z
n
Lr
A
E
p
a 


60
2

BLV
ec 
p
Z Z
BLV
e p


a
Z
Z p 

23. Torque in DC Machines (Lenz’s Law)
If a conductor (of length-L) carries a current (i) and lies in a field
(of density-B), there is force (f) affects it given by:
and the torque is:
For Z-conductors:
The conductor current is:
The developed torque is:
Where: Td = Developed torque, a, p = № of parallel paths and poles,
Z, Zp = Total and per path № of conductors,
φ, B = Flux and flux density per pole,
L, r = Length & mean radius, Ap = area per pole
Notice that, for DC machine: , 23
c
c BLi
f 
r
BLi
r
f
T c
c
c 



a
I
i a
c /

Z
r
a
I
BL
T a
d 









a
T
a
a
a
p
d I
K
I
Z
a
p
Z
r
a
I
L
p
Lr
Z
r
a
I
L
A
T 









2
2

2
Z
a
p
KT 
 60
2

T
e
K
K
Z
r
BLi
Z
T
T c
c
d 





24. 8. The Primitive Machine (Generator)
The primitive machine is consisting of a hollow thin disc which is
internally isolated and coated with a metallic material at different
radii of r1 and r2 and brushes are put on the metallic surface. This
disc is rested vertically on a fixed stand. A magnetic pole is allocated
around this disc.
If the disc is running, the metallic surface will cut the magnetic field
lines and an emf is induced at the brushes, as shown.
Based on the Faraday’s law ( ), the incremental emf is:
The same applied for motor.
24
2
2
2
1
2
2
2
2
1
2
1
r
r
B
r
B
rdr
B
de
e
r
r
r
r
r
r
r












r
r dr
r
B
dl
B
de 
 





BLV
e 

25. 8. The Primitive Machine(Motor)
If a thin layer from the disc is fed (at the brushes) with DC or AC
current, the metallic surface carrying current and cut the magnetic
field lines, an electromagnetic torque is generated and make the
disc to rotate.
Based on the Len’s law ( ), the incremental force is:
25
2
2
2
1
2
2
2
2
1
2
1
r
r
I
B
r
B
rdr
I
B
df
F
r
r
r
r
r










I
dr
r
B
I
dl
B
df 





c
c BLi
f 