Electromechanical Energy conversion Principles.

1. EE8301 – ELECTRICAL MACHINES - I
Unit – III – ELECTROMECHANICAL ENERGY
CONVERSION AND CONCEPTS
IN ROTATING MACHINES
By
Mr. D. Karthik Prabhu,
Assistant Professor,
Department of Electrical and Electronics Engineering.
Email: karthikprabhu@ritrjpm.ac.in
1

2. UNIT – III
ELECTROMECHANICAL ENERGY
CONVERSION AND CONCEPTS
IN ROTATING MACHINES

3. Winding Terminologies
• Conductor: The part of the wire, which is under
the influence of the magnetic field and
responsible for the induced e.m.f. is called active
length of the conductor. The conductors are
placed in the slots.
• Turn: A conductor in one slot, when connected to
a conductor in another slot forms a turn. So two
conductors constitute a turn.

4. Winding Terminologies
• Coil: As there are number of turns, for simplicity
the number of turns are grouped together to form
a coil. Such a coil is called multiturn coil. A coil
may consist of single turn called single turn coil.

5. Winding Terminologies
• Coil side: coil consist of many turns. Part of the
coil in each slot is called coil side of a coil.
• Pole pitch: It is centre to centre distance between
the two adjacent poles. We have seen that for
one rotation of the conductors, 2 poles are
responsible for 360o electrical of e.m.f., 4 poles
are responsible for 720o electrical of e.m.f. and so
on. So 1 pole is responsible for 180o electrical of
induced e.m.f. So 180o electrical is slots are under
one pole which are responsible for 180o electrical,
are measured to specify the pole pitch.

6. Winding Terminologies
• Consider 2 pole, 18 slots armature of an
alternator. Then under 1 pole there are 18/2 i.e. 9
slots. So pole pitch is 9 slots or 180o electrical.
This means 9 slots are responsible to produce a
phase difference of 180o between the e.m.f.s
induced in different conductors.
• This number of slots/pole is denoted as “n”.
• Pole pitch = 180o electrical
= slots per pole (no. of slots/P) = n

7. Winding Terminologies
• Slot angle (β): The phase difference contributed
by one slot in degrees electrical is called slot angle
β.
• As slots per pole contributes 180o electrical which
is denoted as “n”, we can write,
• 1 slot angle = 180o/n
• β = 180o/n
• In the above example n = 18/2= 9 while
β= 180o/n=20o

8. Winding Terminologies

9. Winding Terminologies
• Types of three phase windings
• In general, the types of three phase windings used
in a.c. machines are,
1. Single layer and double layer
2. Full pitch and short pitch
3. Concentrated and distributed

10. Winding Terminologies
• Single layer and double layer: if a slot consists of only one
coil side, winding is said to be single layer. While there are
two coil sides per slot, one at bottom and one at the top
the winding is called double layer.
• As lot of space gets wasted in single layer hence in
practice generally double layer windings is preferred.

11. Winding Terminologies
• Full pitch and short pitch winding: If coil side in one slot is
connected to a coil side in another slot which is one pole pitch
distance away from first slot, the windings is said to be full pitch
winding and coil is called full pitch coil.
• If coils are used in such a way that coil span is slightly less than a
pole pitch i.e. less than 180o electrical, the coils are called, short
pitched coils or fractional pitched coils. Generally coils are shorted
by one or two slots.

12. Winding Terminologies
• Advantages of short pitch coils:
• The length required for the end connections of
coils is less i.e. inactive length of windings is less.
So less copper is required. Hence economical.
• Short pitching eliminates high frequency
harmonics which distort the sinusoidal nature of
e.m.f. Hence waveform of an induced e.m.f. is
more sinusoidal due to short pitching.
• As high frequency harmonics get eliminated, eddy
current and hysteresis losses which depend on
frequency also get minimized. This increases the
efficiency.

13. Winding Terminologies
• Short pitching angle is α then β = 180 – α
• Pitch factor = phasor addition of emf induced in
two sides of coil
-----------------------------------------------
Arithmetic addition of emf induced
in two sides of coil
• Pitch factor Kp = cos (α/2)

14. Winding Terminologies
• Concentrated and distributed winding:
• In this type of winding the whole of the armature
is mechanically divided into three sectors for a 3
phase connection and each phase is housed in one
120o sector
• If one phase winding is started at a slot, first that
phase is completed and then the other phase is
wound. But here the main disadvantage is the
vectorial addition of the e.m.f. induced in the
conductors is not maximum and hence not
followed in 3 phase ac machines mostly.

15. Winding Terminologies
• Concentrated and distributed winding:
• The flux per pole and hence flux density are
limited to certain dimensions and cannot exceed
certain limits from design point of view. Hence for
inducing emf of required value, many number of
turns or coils are required that cannot be
accommodated in a single slot pair. So the
windings of a single phase are distributed in more
than one slot pair distributed in space with an
angular separation of ϒ electrical degrees. This
angle between adjacent slots is given by
ϒ = (π P/S) elect rads

16. Winding Terminologies
• The total phase angle spread covering one
phase winding is called phase spread. (σ)
• σ = m ϒ where m = no. of slots/pole/phase
• Distribution factor Kd = sin (m ϒ/2)
-----------------
m sin (ϒ/2)

17. Relation between electrical and mechanical angle
• Let there be ‘P’ no. of poles in the stator of a dc
machine. The physical angle between any two
consecutive poles is 360/P and this mechanical
degrees as the poles are mechanically spaced
apart by θm = 360/P.
• But any conductor on the armature as rotation
experience a half cycle of emf induced in it when
it moves from one pole centre to the other. This is
the angle spanned by the electrical parameter
and hence θe = 180 for one pair of poles.
• θe = 180 × (2/P) × (P/2) = θm (P/2)