1. AC Motor Winding
Prepared By:-
Muhammad Atta ul Ghaffar
(Sr.Electrical Engineer)
CENTURY PAPER & BOARD MILLS LTD.
2. Basic Rewind Terminology,
Formulas and Definitions
Synchronous Speed.
This term refers to the speed of the rotating
magnetic field in the stator. The motor rotor
will never turn at synchronous speed due to
various mechanical friction losses, windage,
and built-in slip.
The difference between synchronous speed
and actual rotor speed is called slip and is
expressed in a percent of synchronous
speed.
Synchronous speed must be known to
determine the number of poles and pole-
phase groups (PPGs) in the stator winding
and can be easily determined by use of the
following formula:
3. Basic Formula
N (speed) = 120 x F (frequency) / P
(pole)
Example: Synchronous speed of a 4
pole motor is:
N = 120 x 50 / 4
N = 6000 / 4
N = 1500 r/m
4. Phases
As the name implies, three-phase
motor stator windings will consist of
three separate groups of coils called
phases. Phases must be displaced
from each other by 120 electrical
degrees.
Phases must be electrically balanced
(contain the same number of coils)
and connected for the same number
of poles.
Phases will always be designated as
A, B, and C
5. Poles
Poles refer to a coil or group of coils wound
and connected to produce a unit of magnetic
polarity.
Poles are referred to as either north or south.
The number of poles a stator is wound for will
always be an even number and refers to the
total number of north and south poles. For
example, a four-pole motor will have two
north and two south poles alternately spaced
around the stator.
The number of poles in the stator must be
known to determine the number of PPGs in
the motor and can be easily determined by
the formula below.
6. Poles….
P = 120 x F / N
Example: P = 120 x 50 / 1500
P = 6000 / 1500
P = 4
8. Active Coil Per Phase
An active coil is a coil that is electrically
connected in the stator circuit.
Each phase must contain an equal
number of active coils to ensure
electrical balance (phase current).
In almost all cases, the total number of
coils in the stator will be equal to the
number of slots.
Active coils per phase can always be
determined by dividing the total number
of coils by three (number of phases).
9. Active Coil Per Phase
Example:-
Active coil per phase = total coil /
phases
= 36(total coils) / 3
phases
= 12 active coils per
phase
In this example the answer is a whole
number, meaning all coils in the
winding will be active. In this case the
winding is known as an integral slot
10. Active Coil Per Phase
Example:
62 (total coils) / 3 phases = 20 2/3
coils per phase
In this example, the winding will
require the use of two dummy coils to
compensate for the fractional number.
11. Dummy Coils
Dummy coils are used to fill slots but
are not electrically connected in the
stator circuit. If only one dummy coil is
required, it can be installed in any slot.
If two dummy coils are required, they
can be installed in any slots 180
mechanical degrees apart.
Dummy coils will always be left open
circuited.
12. Pole-Phase Groups (PPGs)
This term refers to the number of coils
connected in series to form the
magnetic pole of a phase. Each phase
must contain an equal number of
PPGs. Total PPGs can be determined
by multiplying the number of motor
poles by three (number of phases).
Example: 4 poles x 3 phases = 12
PPG
13. Coil Span or Pitch
Coil span or pitch is defined as the
number of slots separating the sides
of a coil including the slots in which
the coil sides lie.
The coil span or pitch is normally
referred to by the individual numbers
of the slots in which the coil sides are
laid during rewind, i.e., 1 and 6, 2 and
7, 3 and 8, etc.
15. Types of Winding
BASKET WINDING.
Defined as a single layer winding, the
basket winding is also called the half-
coil winding.
This winding will have one coil side
per slot, with the total number of active
coils equal to one-half of the total
number of slots.
17. Distributed Winding
This winding is defined as a double
layer winding and is the most common
type of winding.
It will have two coil sides per slot, with
the total number of active coils equal
to the total number of slots
18. Types of Winding Connection
WYE (STAR) CONNECTION
The wye or star connection is defined
as where the finishes of each phase
are connected together, and the starts
are connected to the line leads.
The diagram for this winding
resembles an inverted letter Y.
The wye can be connected in series or
parallel configuration.
20. Delta Connection
In the delta connection, the finish of
each phase is connected to the start
of the next phase.
The diagram for this winding
resembles the Greek letter delta.
The winding can be connected in
series or parallel configurations.
22. Winding Burning Reasons
Electrical Problems
Winding short between conductor & coils
Insulation to ground fault
Air gap fault
Rotor fault including casting voids and
broken rotor bars
Improper matching of motor to load
Loose Connections
Electrical vibrations /Mechanical
looseness
23. Burning Reasons…
Electrical Supply Problems
Over or Under Voltage
Voltage Unbalance
VFD waveform
Voltage Surge
24. Winding Burning Reasons
Insulation Failure
The reasons of Insulation failure are:.
Overload
Excessive Number of Starts
Contaminants/Ventilation failure
Voltage Spikes from VFDs
System Disturbances
Excessive Load Inertia
27. Mechanical Failure
Rotor Related
Bad Mechanical Fits
General Mechanical Unbalance
Shaft Deformation
Rotor Deflection
Mechanical Vibration
28. Mechanical Failure…
Shaft Related
Bad or Warn Shaft
Shaft deformation
Mechanical Vibration
Wrong Coupling Type
Mounting Breakage
Shaft alignment
29. Mechanical Failure…
Frame Related
Frame damage
Mounting Breakage
Base plate distortion
Foundation deterioration
Foundation size and design
