FIELD OF DISCLOSURE
The present disclosure relates to electrical machines.
Particularly, the present disclosure relates to a method of winding magnetic
wires through winding slots in the stator of a low voltage electrical machine for
facilitating use of such machine in applications requiring Variable Frequency
Drive (VFD) operations.
BACKGROUND
Squirrel cage induction motors are extensively used in industrial drives such as
fans and pumps due to its lower cost, efficiency, ruggedness and ease of
maintenance. One of the main disadvantages of squirrel cage induction motors
is lack of speed control. In case of prior art motors, flow regulation is achieved
by devices like valves, dampers, vanes and the like. However, Variable
Frequency Drive (VFD) application of motors is preferred because it facilitates
regulating the output in an efficient manner. It also facilitates soft starting of
motors, thereby protecting motors from electro-mechanical stress generated due
to heavy starting currents.
VFDs generally deploy semiconductor devices such as SCR, Triac, MOSFET,
IGBT's to convert a fixed frequency input to variable frequency output to
motors. VFDs pose a challenging environment for motor operation in the form
of increased heating due to reduced motor cooling and harmonic current
heating. Further, motors with VFD applications experience additional voltage
stress due to reflected wave phenomenon and is particularly subjected to
dielectric stresses. Such motors may also be subjected to currents due to nonuniform
distribution of magnetic field. Winding and overhang portions of
motors used in VFD applications are subjected to increased double frequency
2
mechanical forces that result in higher vibrations due to switching current
harmonics. Further, the motors used in VFD applications are subjected to partial
discharge due to higher rate of voltage change. Again, VFD application of
induction motors result in higher thermal stresses due to higher operating
temperature of the motors. The operating temperature of an induction motor is
the resultant of various heat sources, for instance, copper losses including
harmonic heating, iron loss due to eddy and hysteresis and windage and friction
losses. In order to remove heat during operation of induction motors, fans are
provided for facilitating forced cooling of motors. However, it has been
observed that the heat transfer coefficient reduces linearly with fan speed.
There is thus felt a need for eliminating the problems associated with motors
used in VFD applications and subjected to a higher rate of voltage change,
higher dielectric stresses, higher thermal stresses and higher voltage stresses.
Further, there is a need to increase reliability of motors by enabling motors to
sustain VFD environments in terms of voltage spikes at motor terminals, higher
dielectric stress at first tum, higher vibration due to fast changing magnetic
field, increased heating due to harmonics and decreased cooling.
OBJECTS
Some of the objects of the present disclosure aimed to ameliorate one or more
problems of the prior art or to at least provide a useful alternative are described
herein below:
An object of the present disclosure is to provide a method that eliminates the
disadvantages associated with the low voltage electrical machines used in VFD
applications and subjected to a higher rate of voltage change, higher dielectric
stresses, higher thermal stresses, and higher voltage stresses.
3
Another object of the present disclosure is to provide a method for winding
magnetic wires in low voltage electrical machines that enhances dielectric
capabilities ofthe machines.
Yet another object of the present disclosure is to provide a method for winding
magnetic wires in low voltage electrical machines that enhances thermal
capabilities ofthe machines.
Still another object of the present disclosure is to provide a method for winding
magnetic wires in low voltage electrical machines that enhances physical
capabilities of the machines.
Still another object of the present disclosure is to provide a method for winding
magnetic wires in low voltage electrical machines that enables low voltage
electrical machines to sustain VFD environment.
Another object of the present disclosure is to provide a method for winding
magnetic wires in low voltage electrical machines that enables low voltage
electrical machines to withstand increased voltage at motor terminals.
Still another object of the present disclosure is to provide a method for winding
magnetic wires in low voltage electrical machines that enables low voltage
electrical machines to withstand higher dielectric stress on the first tum.
Yet another object of the present disclosure is to provide a method for winding
magnetic wires in low voltage electrical machines that enables low voltage
electrical machines to bear higher vibration due to fast changing magnetic field.
4
Another object of the present disclosure is to provide a method for winding
magnetic wires in low voltage electrical machines that increases reliability of
the motor.
Still another object of the present disclosure is to provide a method for winding
magnetic wires in low voltage electrical machines that provides better insulation
system thereto for sustaining VFD environment.
Yet another object of the present disclosure is to provide a method for
e refurbishing low voltage induction motors and making them compatible for
VFD applications.
Other objects and advantages of the present disclosure will be more apparent
from the following description when read in conjunction with the accompanying
figures, which are not intended to limit the scope of the present disclosure.
SUMMARY
In accordance with the present disclosure, there is provided a method of
winding magnetic wires through a plurality of winding slots in the stator of a
low voltage electrical machine. The method includes the following steps:
• selecting spike resistant magnetic wires having a dual coating of
polyester imide and polyamide imide insulation;
• providing a slot liner along the inner periphery of each of the
winding slots for insulation and providing mechanical strength;
• winding the spike resistant magnetic wires through the plurality of
winding slots until each phase is completely wound;
• providing a top stick for preventing wires near the top of the slot
from contacting the stator which is grounded;
5
• providing silicon sleeves on the first tum of the winding;
• providing support to the end turns of the winding using at least
one of a supporting member, thermal insulation and electrical
insulation for mitigating vibrations exerted on the windings;
• increasing voltage and thermal stress withstand capabilities by
covering the mouth of the slots with a formulation comprising mica
powder blended with epoxy resin;
• impregnating the stator with Elmotherm H-71 varnish by multiple
dip-bake process at a predetermined controlled temperature; and
• engaging a holder with each of the winding slots for holding the
magnetic wires in respective winding slots, the holder being in the
form of wedges provided over the top stick to reduce relative
motion between the winding and the stator;
the slot liner and the top stick being insulating paper comprising an
aramid and a polyester in predetermined proportions.
Typically, the step of selecting spike resistant magnetic wires is preceded by at
least one step selected from the group consisting of cleaning the stator and
ascertaining core healthiness.
Additionally, in case of low voltage polyphase machines, the step of winding
the spike resistant magnetic wires further includes the step of disposing phase
separators between phases to provide a dielectric barrier therebetween.
Further, the step of disposing phase separators between phases is carried out by
providing insulating paper comprising an aramid and polyester in predetermined
proportions.
6
Alternatively, the step of disposing phase separators between phases is carried
out by using insulating paper wrapped in a glass cloth.
Further, the step of winding includes a step of providing insulation between the
turns of the winding using an insulating paper comprising an aramid and a
polyester in predetermined proportions as a center stick.
In accordance with the present disclosure, the step of providing silicon sleeves
further comprises the step of reducing a winding tum to create space for
accommodating the sleeve.
Typically, the step of providing support is carried out by bracing the overhang
portion of the winding using Polytetrafluoroethylene tape having a thickness in
the range of 2 to 3 mm.
Further, the step of increasing voltage and thermal stress withstand capabilities
includes the step of preparing a formulation comprising mica powder blended
with Dobeckot 52 0F.
In accordance with the present disclosure, the step of impregnating the stator
with varnish includes a triple dip and bake process.
Typically, the step of engaging a holder is carried out by using a wedge made of
hot pressed cotton cloth impregnated with a thermosetting phenol-formaldehyde
based binder.
7
In accordance with the present disclosure, the step of impregnating the stator
further includes the step of curing the varnish at a temperature of 120 °C for 4
hours and subsequently at a temperature of 150 °C for 8 hours.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
The method of winding magnetic wires through the slots in the stators of low
voltage electrical machines of the present disclosure will now be described with
the help of accompanying drawings, in which:
Figure 1 illustrates a graphical representation depicting variation of heat
transfer coefficient as a function of speed of fan used for cooling of winding
coils of a prior art induction motor;
Figure 2 illustrates a graphical representation depicting variation of iron losses,
hysteresis losses and eddy current losses as a function of frequency of a prior art
induction motor;
Figure 3 illustrates a graphical representation depicting variation of mechanical
losses, windage losses and friction losses as a function of frequency of a prior
art induction motor;
Figure 4 illustrates a graphical representation depicting variation of insulation
life with respect to the operating temperature in a prior art induction motor; and
Figure Sa - Figure Sh illustrates the steps involved in the method of winding
magnetic wires through the slots in the stator of a low voltage electrical machine
in accordance with the present disclosure.
8
DETAILED DESCRIPTION
A method of winding magnetic wires through the slots in the stator of an
electrical machine of the present disclosure will now be described with
reference to the embodiments which do not limit the scope and ambit of the
disclosure. The description provided is purely by way of example and
illustration.
The embodiment herein and the vanous features and advantageous details
thereof are explained with reference to the non-limiting embodiment in the
following description. Description of well-known components and processing
techniques are omitted so as to not unnecessarily obscure the embodiment
herein. The examples used herein are intended merely to facilitate an
understanding of ways in which the embodiment herein may be practiced and to
further enable those of skill in the art to practice the embodiment herein.
Accordingly, the example should not be construed as limiting the scope of the
embodiment herein.
The following description of the specific embodiment will so fully reveal the
general nature of the embodiment herein that others can, by applying current
knowledge, readily modify and I or adapt for various applications, such specific
embodiments without departing from the generic concept, and therefore, such
adaptations and modifications should and are intended to be comprehended
within the meaning and range of equivalents of the disclosed embodiment. It is
to be understood that the phraseology or terminology employed herein is for the
purpose of description and not of limitation. Therefore, while the embodiment
herein has been described in terms of the preferred embodiment, those skilled in
the art will recognize that the embodiment herein can be practiced with
modification within the spirit and scope of the embodiment as described herein.
9
Variable frequency drives (VFDs) generally deploy semiconductor devices such
as SCR, Triac, MOSFET, IGBT's to convert a fix frequency input to variable
frequency output to motors. VFDs pose a challenging environment for motor
operation in the form of increased heating due to reduced motor cooling and
harmonic current heating. Listed herein below are few of the challenges
associated with VFD applications in motor.
• VFD applications increase dielectric stress on motors.
• Operation of motors at lower speeds result in increased heating of motor.
• Motors used in VFD applications experience additional voltage stress.
• Motors used in VFD applications are particularly subjected to dielectric
stress.
• Motors used in VFD applications are subjected to higher rate of voltage
change, wherein the higher rate of voltage change causes:
o premature failure of motor winding;
o electro-magnetic emission in cables connecting motors to inverters,
thereby affecting the operation of nearby sensitive electronic
equipment;
o a voltage doubling effect at the rising and falling edges of motor
voltage waveform;
o insulation puncture, thereby reqmrmg frequent insulation
upgrading.
• Motors used in VFD applications may be subjected to bearing currents,
vibration and noise.
• Winding and overhang portions of motors used in VFD applications are
subjected to increased double frequency mechanical forces.
• Motors used in VFD applications are subjected to higher thermal stresses.
10
Figure 1 illustrates a variation in heat transfer coefficient as a function of fan
speed. It is observed that the fan speed cannot be increased beyond a rated speed
for providing effective cooling and heat transfer coefficient reduces linearly
with fan speed.
Figure 2 and Figure 3 illustrate graphs depicting the variation of various losses
such as iron losses (represented by "IL") including hysteresis losses
(represented by "HL") and eddy current losses (represented by "ECL") and
mechanical losses (represented by "ML") including friction losses (represented
by "FL") and windage losses (represented by "WL") with respect to variation in
frequency. It is concluded from Figure 2 and Figure 3 that there is increase in
various losses with respect to increase in frequency. Copper loss is dominant
amongst all losses and depends on the type of load handled by the motor. In
case of centrifugal pumps/ fans, copper loss typically reduces proportional to
square of the motor speed. In case of motors used in constant torque
applications such as screw compressors, copper loss nearly remains same and is
detrimental for the efficient operation of motors.
Furthermore, the effectiveness of insulation systems used in induction motors
rapidly deteriorate over a period of time and the lifetime of induction motors is
shortened in case the motor is running at a temperature more than its rated
temperature due to ill effects of thermal stress. Figure 4 illustrates graphical
representation depicting a variation of insulation life with respect to the
operating temperature. It is derived from the graph that the normalized
insulation life decreases with increase in operating temperature (normalized at
25°C). Generally, an increase in temperature to 130 °C leads to a decrease in
insulation life by about 83 percent from normal insulation lifetime. Therefore,
VFD environment requires better insulation systems.
11
The method of winding magnetic wires through the slots in the stator of an
electrical machine will now be explained with reference to FIGURES Sa to 5h;
the key components being referenced generally by numerals as indicated in the
accompanying drawings.
The method of winding magnetic wires through the slots 12 in the stator of a
low voltage electrical machine includes the following steps:
• selecting spike resistant magnetic wires 16, 20 having a dual
coating of polyester imide and polyamide imide insulation;
• providing a slot liner 14 along the inner periphery of each of the
winding slots 12 for insulation and providing mechanical strength;
• winding the spike resistant magnetic wires 16,20 through the
plurality of winding slots 12 until each phase is completely
wound;
• providing a top stick 22 for preventing wires near the top of the
slot from contacting the stator which is grounded;
• providing silicon sleeves (not specifically shown) on the first tum
of the winding;
• providing support to the end turns of the winding using at least
one of a supporting member, thermal insulation and electrical
insulation for mitigating vibrations exerted on the windings;
• increasing voltage and thermal stress withstand capabilities by
covering the mouth of the winding slots 12 with a formulation
comprising mica powder blended with epoxy resin, typically
Dbeckot 520F;
• impregnating the stator with Elmotherm H-71 varnish 26 by
multiple dip and bake process at a predetermined controlled
temperature; and
12
• engaging a holder 24 with each of the winding slots 12 for holding
the magnetic wires 16, 20 in respective winding slots 12.
In accordance with an embodiment of the present disclosure, the step of
selecting spike resistance magnetic wires 16, 20 (constituting two phases) is
preceded by cleaning the stator and ascertaining core healthiness. The spike
resistant winding wires 16, 20 are provided with dual coating of polyester imide
and polyamide imide for addressing issues related to increased dielectric stress
and increased thermal stress, thereby increasing overall capabilities of the
machine and enabling use of the machine in Variable Frequency Drive (VFD)
environments. The spike resistant winding wires 16, 20 can endure temperature
up to 200 °C and hence are appropriate for sustaining the thermal stress
generated by VFD environment. The step of winding the spike resistance
magnetic wires 16, 20 further includes the step of disposing phase separators 18
between phases in case of polyphase machines. The phase separators 18 in each
winding slot serve as dielectric barriers between the phases. Further, insulation
typically a center stick of an insulating paper constituting aramid and polyester
in predetermined proportions is provided between the turns of the winding. The
center sticks insulate one winding from the other.
Typically, the holders 24 in the form of wedges are provided over the top stick
22 to reduce relative motion between the winding and the stator. Such wedges
are made up of hot pressed cotton cloth impregnated with a thermosetting
phenol-formaldehyde based binder. The wedges are disposed to reduce relative
motion between the winding and the stator core due to double frequency
mechanical forces acting on the windings. Typically, the slot liner 14, the top
stick 22 and the phase separators 18 are insulating papers. The insulating paper
is made up of a class H insulating material with a temperature rating of
approximately 180 °C. Due to the high temperature rating of the insulating
paper, the insulating paper is appropriate for high temperature operation and is
13
free from blisters at high temperature. The insulating paper is a composite of
aramid and polyester in predetermined proportions. The insulating paper is
typically Nomex, Nomex-Mylar-Nomex, Nomex-Polyester-Nomex and the like.
In accordance with an embodiment of the present disclosure, the phase separator
18 is an insulating paper wrapped in a glass cloth.
Further, silicon sleeves are provided on the first tum of the winding to reduce
dielectric stress on the tum of the winding due to uneven distribution of applied
voltage. Typically, sleeves are H class silicon sleeves. Space for
accommodating the silicon sleeves are created by reducing a tum in the winding
of each phases .. During VFD operation of the machine, vibrations caused by
pulse width modulating (PWM) signals occur in the machine may damage the
insulation between the winding turns. Therefore, supports are provided to make
the end turns a solid structure and prevent movement in the over hand portion of
the winding. Supports to the end turns of the winding are provided by bracing
the overhang portion of the winding using polytetrafluoroethylene tape or
Teflon tape having a thickness typically in the range of 2 to 3 mm. The supports
also provide thermal and electrical insulation to the overhand portion of the
winding.
The stator is impregnated with varnish 26 typically by triple dip and bake
process. The stator is further kept in a hot environment for curing the varnish
26. Curing is performed for a time period of about 12 hours, in which firstly the
varnish 26 is cured at a temperature of 120 °C for 4 hours and subsequently at a
temperature of 150 °C for 8 hours. The varnish 26 provides insulation between
the windings. The varnish 26 also provides mechanical strength to the electrical
machine by filling the void between the winding wires 16, 20.
14
TEST DATA
Following tests have been performed to check the performance of the electrical
machine (low voltage induction motor) wound with magnetic wires m
accordance with the process of the present disclosure described herein above.
1) Core healthiness test: Core healthiness test on the machine was performed
on rated flux. Temperature was recorded after it stabilized at 4 different points
with 90 degrees angular difference to test healthiness of motor core. The
temperature variation was found to be less than 5 deg C that indicates a healthy
core.
2) Dielectric Stress Testing:
1. The low voltage electrical machine was tested with suitable VFD with cable
length of 250 meter. The maximum voltage pulse observed was 953 Volt with
rise time of 570 ns. The motor performance was found satisfactory.
2. The low voltage electrical machine was tested as per IS: 325 and motor
performance was found to be satisfactory and in line with the requirements of
the IS.
TECHNICAL ADVANCEMENTS AND ECONOMIC SIGNIFICANCE
The technical advancements offered by the present disclosure include the
realization of:
• a method that eliminates the disadvantages associated with the low
voltage electrical machines with VFD applications and subjected to a
15
higher rate of voltage change, higher dielectric stresses, higher thermal
stresses, and higher voltage stresses;
• a method for winding magnetic wires in low voltage electrical machines
that enhances dielectric capabilities of the machines;
• a method for winding magnetic wires in low voltage electrical machines
that enhances thermal capabilities of the machines;
• a method for winding magnetic wires in low voltage electrical machines
that enhances physical capabilities of the machines;
• a method for winding magnetic wires in low voltage electrical machines
that enables low voltage electrical machines to sustain VFD environment;
• a method for winding magnetic wires in low voltage electrical machines
that enables low voltage electrical machines to withstand increased
voltage at motor terminals;
• a method for winding magnetic wires in low voltage electrical machines
that enables low voltage electrical machines to withstand higher dielectric
stress at first tum;
• a method for winding magnetic wires in low voltage electrical machines
that enables low voltage electrical machines to bear higher vibration due
to fast changing magnetic field;
• a method for winding magnetic wires in low voltage electrical machines
that increases reliability of the motor;
16
• a method for winding magnetic wires in low voltage electrical machines
that provides better insulation system thereto for sustaining VFD
environment; and
• a method for refurbishing low voltage induction motors and making them
compatible for VFD applications.
Throughout this specification the word "comprise", or variations such as
"comprises" or "comprising", will be understood to imply the inclusion of a stated
element, integer or step, or group of elements, integers or steps, but not the
exclusion of any other element, integer or step, or group of elements, integers or
steps.
The use of the expression "at least" or "at least one" suggests the use of one or
more elements or ingredients or quantities, as the use may be in the embodiment of
the disclosure to achieve one or more of the desired objects or results.
Wherever a range of values is specified, a value up to 1 0% below and above the
lowest and highest numerical value respectively, of the specified range, is
included in the scope of the disclosure.
The numerical values mentioned for the various physical parameters, dimensions or
quantities are only approximations and it is envisaged that the values higher/lower
than the numerical values assigned to the parameters, dimensions or quantities fall
within the scope of the disclosure, unless there is a statement in the specification
specific to the contrary.

We Claim:
1. A method of winding magnetic wires through a plurality of winding slots
in the stator of a low voltage electrical machine, said method comprising
the following steps:
• selecting spike resistant magnetic wires having a dual coating of
polyester imide and polyamide imide insulation;
• providing a slot liner along the inner periphery of each of the
winding slots for insulation and providing mechanical strength;
• winding said spike resistant magnetic wires through the plurality
of winding slots until each phase is completely wound;
• providing a top stick for preventing wires near the top of the slot
from contacting the stator which is grounded;
• providing silicon sleeves on the first tum of the winding;
• providing support to the end turns of the winding using at least
one of a supporting member, thermal insulation and electrical
insulation for mitigating vibrations exerted on the windings;
• increasing voltage and thermal stress withstand capabilities by
covering the mouth of the slots with a formulation comprising mica
powder blended with epoxy resin;
• impregnating the stator with Elmotherm H-71 varnish by multiple
dip-bake process at a predetermined controlled temperature; and
• engaging a holder with each of said winding slots for holding the
magnetic wires in respective winding slots, said holder being in the
form of wedges provided over said top stick to reduce relative
motion between the winding and the stator;
said slot liner and said top stick being insulating paper comprising an
aramid and a polyester in predetermined proportions.
18
2. The method as claimed in claim 1, wherein the step of selecting spike
resistant magnetic wires is preceded by at least one step selected from the
group consisting of cleaning the stator and ascertaining core healthiness.
3. The method as claimed in claim 1, wherein the step of winding said spike
resistant magnetic wires further includes the step of disposing phase
separators between phases to provide a dielectric barrier therebetween.
4. The method as claimed in claim 3, wherein the step of disposing phase
separators between phases is carried out by providing insulating paper
comprising an aramid and polyester in predetermined proportions.
5. The method as claimed in claim 3, wherein the step of disposing phase
separators between phases is carried out by using insulating paper
wrapped in a glass cloth.
6. The method as claimed in claim 1, wherein the step of winding includes a
step of providing an insulation between the turns of the winding using
insulating paper comprising an aramid and a polyester in predetermined
proportions as a center stick.
7. The method as claimed in claim 1, wherein the step of providing silicon
sleeves further comprises the step of reducing a winding tum to create
space for accommodating said sleeve.
8. The method as claimed in claim 1, wherein the step of providing support
is carried out by bracing the overhang portion of the winding using
Polytetrafluoroethylene tape having a thickness in the range of 2 to 3 mm.
19
9. The method as claimed in claim 1, wherein the step of increasing voltage
and thermal stress withstand capabilities includes the step of preparing a
formulation comprising mica powder blended with Dobeckot 520F.
10. The method as claimed in claim 1, wherein the step of impregnating the
stator with varnish includes a triple dip and bake process.
11. The method as claimed in claim 1, wherein the step of engaging a holder
is carried out by using a wedge made of hot pressed cotton cloth
impregnated with a thermosetting phenol-formaldehyde based binder.
12. The method as claimed in claim 1, wherein the step of impregnating the
stator further includes the step of curing the varnish at a temperature of
120 °C for 4 hours and subsequently at a temperature of 150 °C for 8
hours.