Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
-
Upload
frankgriffith -
Category
Documents
-
view
228 -
download
0
Transcript of Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 1/15
IMPROVING THE EFFICIENCY OF SQUIRREL CAGE
INDUCTION
MOTORS: TECHNICAL AND ECONOMICAL
CONSIDERATION
INTRODUCTION
There are several methods of decreasing the energy consumption in electrical machines, for
example: reorganising the production lines, using adjustable ac-drives choosing the motor size
correctly and decreasing the losses of machines. The first three are mainly dependent on the
processes used in industry, but the fourth is both up to industry and motor producers.
Purchasing motors as cheaply as possible might not be the optimum solution for the whole
company in all cases. Especially in three shift industry where a great deal of the motors run
7000 - 8000 hours a year it might be wiser to buy more expensive motors with better
efficiency and thus decrease the motor energy costs. The decreased losses in industry also
lead to lower power transmission losses, reduction in the need for cooling and lower power
reservation costs.
2 LOSSES IN INDUCTION MACHINES
Maximising motor efficiency is equivalent to minimising motor losses. Motor losses consist
of ohmic-, iron- and mechanical losses. Fig. 1 shows the sharing of losses in a standard 2-pole
4 kW motor. These losses are explained in more detail in the following sections.
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 2/15
Fig. 1. Losses in a standard 2-pole 4 kW motor 2.1 Iron Losses
The main factors of iron losses are the hysteresis losses and the eddy current losses. The
hysteresis losses depend on the magnetic properties of the iron used and the eddy current
losses on the lamination thickness τ , frequency f , flux density B and the resistivity ρ lam of the
iron. Iron losses occur all over the iron and they are a function of (local) flux density and
frequency. In addition, there are also harmonic losses which mainly occur on teeth and iron
surfaces and are caused by harmonics in the flux density.
2.1.1 Hysteresis losses
The area inside the hysteresis loop represents hysteresis losses in the material. The increasing
of the amount of silicon in iron reduces the area of the hysteresis loop. Silicon decreases the
friction between the Weiss domains, but also impairs to some degree the saturation fluxdensity. The losses correspond also to the frequency f .
2.1.2 Fundamental eddy current losses in lamination
The alternating magnetic flux creates electro motive forces (emf) inside the material it flows
through. These emfs fluctuate at the same speed as the flux and similarly create eddy currents
normal to the flux path, i.e. the eddy currents circle around the flux. The eddy current power
loss can be presented as follows [2]
. (1)It can be seen from Eq. (1) that there are several factors that affect the eddy current losses, but
usually the outer dimensions of the motor, flux density and the frequency are constants and so
there are only two variable factors left. The thickness of the lamination seems to have the
strongest influence in eddy current losses. Some problems arise if the plate is very thin. First
it is very difficult to manufacture and handle and second the filling factor becomes small
because at least one side of the laminated plate has a non magnetic insulation layer which
prevents eddy currents from travelling between the plates. Another possibility is to increase
the resistivity of the plate. One way of achieving this is to add some silicon to the iron [3]. In
small motors magnetic materials with quite poor properties are us when considering power
losses.
2.1.3 Harmonic eddy current losses
There are two main causes of harmonics in the air-gap flux density distribution of the machine
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 3/15
when used with a sinusoidal supply: the permeance harmonics and the winding harmonics.
Air gap permeance function generates flux density fluctuations on the rotor surface. These
flux density fluctuations induce high frequency eddy currents that generate losses on the
surface of the rotor.
One method to flatten the permeance function is to use some semi-magnetic material to close
the stator slot opening. The purpose of the slot-wedge is to lead the flux under the slotopening. The magnetic properties of the wedge should be somewhere between stator iron and
air and its electric conductivity should be low. Choosing the material is a question of
optimisation between the power factor of the machine and harmonic losses on the rotor
surface. Increasing the permeability of the slot-wedge flattens the permeance function, but
also increases slot leakage flux. The effect of the slot-wedge, compared with a normal slot
opening is presented in Fig. 2 [4].
a) b)
Fig. 2. a) Flux plot in a normal semi-closed stator slot section. b) Flux plot in a stator slot
section as a result of using a wedge made of Aluminium-Iron dust (8 w-% of Al) impregnated
with epoxy resin.
Since the windings of induction machines are usually placed in stator slots and cannot thus be
spread smoothly over the stator inner surface we get, in addition to permeance harmonics,
winding harmonics, too. The ordinals of the winding harmonics ν in an ms-phase winding are
, (2)
where g 1 is any positive or negative integer. The stator winding factor for the harmonic ν, ξ sω
in an ms-phase winding is [5]
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 4/15
, (3)
The ratio ws/τ p describes the deviation of the coil span ws from the pole pitch τ p and Qs is the
number of stator slots. Two different winding types are used in different test motors, a full
pitch winding ws/τ p = 1 and a short pitch two layer winding ws/τ p = 5/6. Two layer windings
have not usually been used in small machines, but they could be a good way to reduce the
effect of the harmonics, even though they are more difficult to manufacture than single layer
windings.
Actually the ratio ws/τ p = 5/6 happens to give an overall minimum for the harmonic content of
the three-phase stator winding magneto motive force (mmf).
2.2 Winding lossesThe stator windings are usually made of enamelled copper wires and the rotor cage of cast
aluminium. The main winding losses are caused by the fundamental and harmonic currents,
but also skin and proximity effects contribute to the losses.
The stator winding dc-resistance Rs as a function of temperature T is
, (4)
where l s is the length of the stator winding, Asb is the cross-section area of a stator bar and Cu
is the resistivity of copper (1.72 10-8 m at 20 °C) and the coefficient of resistivity is = 410-3°K-1. As can be seen from Eq. (4) there are four parameters that affect the stator dc-
resistance. If the material and the temperature are not changed the conductor cross-section
area can be increased or the length of the winding decreased. Both acts decrease the stator
resistance. But these also affect several other factors.
One factor that affects the ohmic losses of the stator is the obligatory slot insulation. The
insulation, made normally of polyester, takes more than 10 % of the effective slot area in
small motors. For example using 0.15 mm thick polyimid instead of 0.30 mm thick polyester
would make it possible to increase the conductive area and decrease the coil resistance both
by about 8 %.
The calculation of the rotor resistance is much more complicated than that of the stator.
Because of the shape of the ring, the current path on the outer part of the ring is much longerthan in the inner part. This leads to unequal current density in the ring. Levi [6] presents an
equation for calculating the rotor resistance Rr per phase
. (5)
The resistance of a rotor bar Rrb can be calculated by Eq. (4) Rotor end ring equivalent
resistance is approximately
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 5/15
, (6)where Aer is the average cross-section area of the end ring and the average path length in the
ring is
. (7)
Qr is the number of rotor slots. The squirrel cage is usually made of aluminium. If the end
rings and rotor bars were made of copper the rotor resistance would be only 63 % of the
original which means that the rotor ohmic losses would drop correspondingly. Because the
casting of copper is difficult, the copper rotor cage with short-circuit rings would require other production techniques and thus increase the production costs. This is the main reason why this
technique has not been used in small motors.
Both iron and winding losses are greatly affected by the size of the motor. Usually an energy
optimal motor is larger than the present standard motors.
2.3 Mechanical losses
In small induction machines the mechanical losses are usually about 10 % of the total losses.
The main factors that affect on windage losses are the rotors peripheral velocity, the
smoothness of the rotor and stator surfaces and the length of the air-gap. The smaller the air-
gap, the bigger the windage losses and also the permeance harmonic losses presented earlier,
but on the other hand increasing the air-gap increases the need for magnetising current. The
air-gap length has to be optimised between these three factors. The earlier mentioned stator
semi-magnetic slot-wedges smoothen the stator surface, and thus also decrease the windage
losses.
2.4 Summary of the actions that can improve the efficiency
The summary of the actions that can improve the efficiency of induction motors is presented
in table I.
Table I. Summary of the actions that can improve the efficiency of induction motors.
LOSS POSSIBLE
DESIGN
CHANGES
POSITIVE
EFFECTS ON
LOSSES ADVERSE
EFFECTS
STATOR LOSSES
- Ohmic loss I 2 R
1. Increase amount
of copper wire in
slot.
2. Increase stator
slot size & amount
of copper wire in
slot.
3. Decrease length
of coil extensions.
1. - 3. Decreased
stator resistance R.
1. -2. Difficult to
build, increased
costs.
3. Possible increase
of inrush current &
difficult to build.
May increase iron
losses.
ROTOR LOSSES
- Ohmic loss I 2 R
1. Increase flux
density in air gap.2. Increase rotor bar
1. Decrease in slip
& resulting ohmiclosses.
1. Increased inrush
current.2. -3. Possible
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 6/15
/ end ring size.
3. Increase rotor bar
/ end ring
conductivity.
2. - 3. Decreased
ohmic losses.
increase of inrush
current & decrease
of starting torque.
IRON LOSSES
- Hysteresis losses
- Fundamental eddy
current losses in
lamination
- Harmonic eddycurrent losses
1. Change to better
lamination steel.
2. Decrease
lamination steel
thickness.
3. Improve
coreplating /
annealing processes.
4. Use a semi-
magnetic slot-
wedge
5. Increase the airgap length
6. Modify the shape
of the slot opening
7. Use two layer
windings instead of
single layer
windings
1. Decreased
hysteresis losses.
2. - 3. Decreased
fundamental eddy
current losses.
4. - 6. Decreased
permeance
harmonics7. Decreased
winding harmonics.
1. - 2. Increased
costs & reduce
available of
materials.
3. Increased cost
and usage of energy.
4. Increased costs.
5. Increases the
magnetising current6. - 7. Difficult to
build, increased
cost.
MECHANICAL
LOSSES- Windage losses
- Friction losses
1. Optimise the fan
design2. Optimise bearing
selection.
1. Reduced
operatingtemperatures.
2. Reduced friction
losses.
1. Can increase
noise levels. May
result in
unidirectional fans.2. May affect noise
level or impose
speed or bearing
loading restriction.
STRAY LOAD
LOSSES
1. Insulate rotor
bars.
2. Increase air gap
length
3. Eliminate rotor
skew.4. Strand depth.
5. Transposed turns.
1. Reduced bar to
lamination current.
2. Reduced high
frequency surface
losses.
3. Reduction in W r .4. - 5. Reduced eddy
currents.
1. Increased costs.
2. Reduced power
factor
3. May increase
noise level & affect
speed torquecharacteristics.
4. - 5. Difficult to
build, high cost.
3 TEST MOTORS AND RESULTS
In order to evaluate that it is economically reasonable to produce induction motors with better
efficiency, Lappeenranta University of Technology (LUT) has examined methods and costs ofincreasing the efficiency of small squirrel cage induction motors. As an example we have
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 7/15
manufactured several versions of a 4 kW two-pole induction motor and compared them with a
standard 4 kW motor called later as a reference motor. The construction of the motors is
shown in Table II. These changes are explained in more detail in the following chapters.
Table II. The construction of the test motors
Motorno.
Construction
(4 kW, two pole ( p = 1), 400 V motors,Qs = 36, Qr = 28 Dd = 104 mm, Dr =
103.2 mm, Dex = 180 mm )
1 Standard motor
- ls = 0.082 m
- motor core material DK-70, 0.63 mm,
- full pitch winding
2 Standard motor + semi magnetic slot-
wedge
3 Standard motor with 5/6 short pitch
winding
4 Modified construction
- ls = 0.120 m
- motor core material CK-37, 0.50 mm,
- full pitch winding
5 Modified construction as motor no. 4
- W = 5/6 τ p
6 Modified construction as in motor no. 4
- rotor end ring material copper
7 Modified construction as in motor no. 4- end ring material copper + Fe.
3.1. Stator material
In the initial 4 kW standard motor the iron losses were about 13 % of the total losses at
nominal load. Almost 80 % of these occurred in the stator yoke. Only about 11 % of the rotor
losses were iron losses. That makes 1.5 % of the total losses, so it does not seem to be very
economical to try to reduce them. Replacing the 0.63 mm DK-70 (3.1 W/kg, 50 Hz, 1 T) with
0.50 mm CK-37 (1.45 W/kg, 50 Hz, 1 T) reduces the stator iron losses and results in an 1 %-
unit improvement for the efficiency at the nominal point.
3.2 The semi-magnetic slot-wedge
Despite the effect of the permeance harmonics which are assumed to be quite small in a
machine with laminated rotor the effect of a slot wedge was examined. Slot openings in one
of the standard motors were filled with a mixture of Aluminium-Iron powder (8 w-% of Al)
and epoxy resin.
3.3 Winding
A standard stator was equipped with a 5/6 short pitch two layer winding. The effect of
winding harmonics at low slip, which is the normal operating area, was rather small.
3.4 The stator core length
As mentioned earlier the ohmic losses were the most noticeable in the standard motor. When
searching for a practical modification one of the starting assumptions is to keep the shaft
height constant. This means that the diameter of the motor can not be change, which leaves
the core length the only possible varying dimension. In order to get the best possible
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 8/15
efficiency, we had to find an optimum between the iron ( P Fe) and ohmic losses ( P el), in other
words to change the core length so that
. (8)The electro motive force (emf) E in a non saturated machine is
, (9)
where s is the angular velocity of the voltage supply, N s is the total number of the turns in
series per stator phase, is the peak value of the flux density in the air-gap,
p is the pole pitch and l sc is the length of the stator core.
The standard motor was tested with different voltages. These tests proved that the best power
factor - efficiency combination was reached at nominal voltage (U ph = 230 V). This means
that the flux density for this material was suitable. The flux density as well as the emf and the
angular velocity are held constant in the calculations what leads to an equation between two
constructions
. (10)
If the structure of the winding is not changed, the pole pitch and the winding factor ξ can be
eliminated and so
. (11)
Thus if the length of the stator core is increased, the number of stator bars can be decreased
and if the shape and the dimensions of the slot cross-section area are unchanged, the cross-
section area of the stator bars ( Asb) can be increased. As Eq. (11) shows, the total length of the
stator coil in the slots (l cs) does not change, but the total length of the stator coil ends ( l ce)
corresponds only with the number of the bars. So if the coil span is constant the total length of
the stator winding decreases
, (12)where l se is the average length of one conductor in one stator end.
3.5 Resistances
The stator winding dc-resistance was presented earlier in Eq. (4) as a function of temperature
T . Both the decreased length of the stator winding and the increased conducting area of the
stator bars decrease the stator ohmic losses. If the temperature remains unchanged the new
resistance of the stator is
. (13)
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 9/15
The length of the rotor core should be the same as the length of the stator core what increases
both the length of the rotor bars and the rotor bar resistance ( Rrb). The resistance of the end
rings ( Rre) remains the same. The new rotor resistance is
, (14)
where rr is the ratio of rotor end ring resistance to total rotor resistance. The transformation
ratio az for referring one rotor phase reactances and resistances to the stator in a squirrel cage
machine is [7]
. (15)
Both the number of phases (ms) and the number of rotor bars or rotor phases (Qr ) are constant.
r1 is the rotor skew leakage factor. The decrease in the rotor resistance from the stator side
corresponds to the square of the transformation ratio or the number of the stator bars
, (16)
where Rr1´ is the original rotor resistance seen from the stator.
While the flux density, the axial cross-section area, materials and the emf are set to be
unchanged the iron losses change corresponds to the ratio of the stator core lengths
. (17)
4 THE ENERGY OPTIMAL MOTOR
If high efficiency and a constant shaft height are the only criteria when choosing the motor for
a certain drive, we are able to calculate the optimal length of the stator stack by using the
equations presented in the previous chapter. From calculations it can be seen in Fig. 3 that theoptimum stack length is about 0.185 m. A better motor electrical steel (for example CK-37)
still increases the optimum stack length. The problem with the increased stack length is that
while ohmic losses diminish, the power factor deteriorates. This increases the need for current
and decreases the advantage of diminished resistances.
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 10/15
Fig. 3. The calculated electrical losses as a function of stator core length.
Fig. 4. The calculated electrical efficiency and power factor as a function of stator core
length
There were several reasons that led to the chosen stack length (lsc = 0.12 m) for the test
motor. First, as stated earlier one of the requirements was that the existing motor could be
replaced directly with the new one. Secondly, the increment of the stack length 0.12 mm leads
to a considerable deterioration of the power factor while the improvement of the efficiency
remains quite small.4.1 The measurements
To evaluate the effects of the changes made, all motor configurations were tested in
laboratory. The load tests were made with sinusoidal supply.
The measured efficiencies of the motors are presented in Fig. 5. which shows a remarkable
increase in the efficiencies of motors 4, 5, 6 and 7 compared to the initial motor. It should be
noticed that the efficiency is much better throughout the whole presented scale and that the
peak of the efficiency curve is very flat.
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 11/15
Fig. 5. The measured efficiencies of different test motors
The calculated loss components of the standard and test motor 4 at the rated load are
compared in Fig. 6.
Fig. 6. The calculated loss components for the standard and test motor 4 at rated power
As can be seen in table III, the starting current of some high efficiency versions is higher than
the standards allow. The starting current of the best motor, however, is reduced to a level
accepted also by the NEMA standard. This reduction of the starting current is mainly due to
the changes in the rotor construction that increase the locked rotor resistance and reactance.
Table III. Some motor data measured with sinusoidal supply.
Motor number 1 2 3 4 5 6 7
Efficiency at 4 kW output / % 85.3 85.9 85.4 90.6 91.4 91.8 89.0
Best efficiency / % 86.7 87.4 87.1 90.9 91.7 92.1 90.2
I st / I N (rotor locked, coldmotor)
6.9 7.1 7.7 10.9 10.4 8.7 8.0
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 12/15
Power factor at 4 kW output 0.914 0.912 0.915 0.860 0.840 0.856 0.77
Slip at 4 kW output / % 4.42 4.45 4.36 2.31 2.31 1.85 2.15
5 REDUCING HIGH EFFICIENCY INDUCTION MOTOR
STARTING CURRENT
The disadvantage of decreasing losses is that the starting current can get higher than the
standards allow as a result of the decreased starting impedance Z . The reduction of the ohmic
losses decreases stator and rotor resistances and the reduction of iron losses by using silicon
has made the values of the leakage reactance X s and X ́ r somewhat lower at starting than at
normal speed. This is because the relatively high rotor and stator current during starting may
saturate portions of the iron in the rotor and stator teeth in the leakage-flux paths.
At starting the slip s = 1 and therefore the magnetising impedance Z m is high compared to the
rotor impedance Z r. By using the values of the equivalent circuit and line-to-line voltage U we
get a good approximation of the line current I st at starting
, (18)
where Rs is the stator winding resistance, Xss is the stator leakage reactance, R´ r is the rotor
resistance and X´sr is the rotor leakage reactance referred to the stator. When the motor is
operating at normal speed (i.e., at a slip of about a few percentages), the line current is
limiting mostly by the magnetising impedance.The reduction of the starting current or the increase in the starting impedance without large
changes the running values can be done by taking advantage of the skin effect which increases
the conductor impedance. This idea is normally used in double-squirrel-cage and deep-bar
motors.
The skin effect means an uneven current distribution in a current carrying conductor. The total
current density distribution in a conductor determines its dynamic impedance which is the
result from system operating frequencies, conductor's shape and the electromagnetic
properties of the materials used.
The equation for skin effect can be derived from Maxwell´s equations. If it is assumed that the
end ring leakage flux Φ has only a component normal to the conductor cross-section and the
current density J has only a component tangential to the conductor, the skin effect can bederived from Ampere´s law as:
(19)
where J is the sectional current density, σ the electrical conductivity, L the tangential length
of the conductor and ∆Φ the leakage flux of the conductor. The leakage flux of the conductor
can be expressed as follows:
(20)
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 13/15
where µ 0 is the permeability, ht the height of the conductor, b p the cross-sectional width of the
conductor and i the sectional current.
At starting the frequency of the rotor currents is relatively high so the influence of uneven
current distribution both in the rotor bars and end-rings increases thus the rotor impedance.
Skin effect appears mainly in the rotor bars which are surrounded by the ferromagnetic rotor
core. In order to increase the skin effect in the rotor end rings the end rings were partlysurrounded by iron. This was done by adding extra ferromagnetic rings beside the aluminium
end ring and modifying the shape of rotor end-rings.
The idea of sophisticated squirrel-cage end ring area design has been know a long time. One
of the earliest inventions about this has been made in year 1926 by K. L. Hansen and W. J.
Oesterlein, Milwaukee, Wis. (Fig. 7a). Since those days more advanced inventions have been
patented like starting disk in 1981 by Gabor Kovacs, Budapest, Hungary (Fig. 7b) or squirrel-
cage rotor having end rings of double structure in 1980 by Makio Sei, Nakaminato, and Kunio
Miyashita, Hitachi, both in Japan (Fig. 7c).
a) b) c)
Fig. 7. a) Self-starting induction motor patent, b) Squirrel-cage rotor having starting disc, c)
Squirrel-cage rotor having end rings of double structure
Referring to Eq. (20) the dimensions of the end-ring shape that affect the skin effect are the
radial depth h and the axial length b.To maximise the skin effect the radial depth of the end-ring should be as high as possible and
the end-ring should be made of copper, whose electrical conductivity is higher that of
aluminium. The axial length b of the end-ring should also be as small as possible because it
increases the leakage flux Φ through the conductor
The task of the extra ferromagnetic rings around the aluminium end ring is to decrease the
reluctance in the leakage flux path and thus increase the leakage flux. To reduce iron losses,
the ferromagnetic rings were made of lamination sheets.
6 THE COSTS OF INCREASING THE EFFICIENCY
When the core length of a machine is changed the amount of the materials and the work
needed in production changes too (Fig. 8). The amount of electrical steel in a motor is
comparable to the core length. In the rotor, the aluminium rotor bars also correspond to the
stack length, but the short circuit rings remain unchanged. For this reason the slope of
aluminium is lower than that of iron. As was previously seen the amount of conductors in one
slot can be reduced when the stack length is increased and the air-gap flux density is kept
constant. This means that the "useless" part (length) of coils at stack ends is reduced. If the
cross section area of one conductor were unchanged, the total amount of copper could be
decreased. To reduce ohmic losses, the cross section area of each conductor is increased in
such a way that the filling factor of a slot remains about the same.
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 14/15
Fig. 8. The change in the amount of the materials as a function of the stator and rotor core
lengths
The only change in manufacturing costs is caused by the motor electrical steel processing,
which is relative to the stack length. The work needed in winding the machine, casting,
composition and testing are constant. The price for the decreased power losses is presented in
Fig. 9.
Fig. 9. Increased production cost of the 4 kW test motors divided by the decreased losses.
The manufacturing costs are based on a research [8] made for a 2 hp motor. In that study, 65
% of the total production costs were due to the materials and 35 % due to the other costs. Inthe calculations above, the other costs are assumed to increase 2.5 % for each increased 10 %
of the stack length. The material prices used in the calculations are: Cu 30 FIM/kg, Al 20
FIM/kg, DK-70 3.50 FIM/kg and CK-37 5.10 FIM/kg.
7 CONCLUSIONS
If only the rise in the production costs is considered, the payout period of the investment for
example with 5000 h annual usage time is less than a year.
From the results reached we can notice that the harmonic field effects in small inductionmotors are quite insignificant, especially around the nominal point. It seems that the short
8/13/2019 Improving the Efficiency of Squirrel Cage Motors-elmotorer Beaktande 803970
http://slidepdf.com/reader/full/improving-the-efficiency-of-squirrel-cage-motors-elmotorer-beaktande-803970 15/15
pitch winding, which is much more complex to manufacture than the normal full pitch
winding, is not economical, at least not with sinusoidal supply or when the number of slots
per pole and phase is large (6 in our axample). Instead of that, the semi-magnetic slot-wedge,
which could quite easily be joined with the slot insulation at the slot opening, might be a
useful alternative. Anyway, the most important factors in the motors' efficiency are the correct
dimensioning of the cores and the motor electrical steel used. However the reduction of lossesincludes some problems. First, the resistance and reactance values diminish, which causes an
increased starting current when the motors that are directly connected to the network are used
periodically.
The reduction of the starting current without changing the running values very much can be
done by using either deep slot rotors or by modifying the rotor end rings as in motor 7. Both
of these actions would increase the production costs to some extent, but not remarkably.
REFERENCES
[1] T. Jokinen, Reduction of Losses in Electrical Machines and Transformers. Helsinki
University of Technology/Laboratory of Electromechanics. Report nr. 17 (in Finnish), 1983.
[2] L. W. Matsch, J. D. Morgan, Electromagnetic and Electromechanical Machines. New
Mexico/USA. Harper & Row, Publishers Inc, 1986, p.51.[3] C. Heck, Magnetic Materials and their Applications, London, Butterworth, 1974, 770 p.
[4] J. Pyrhönen, P. Kurronen, Research on Solid-Rotor Induction Machines, part 1. Research
report EN B-73 Lappeenranta University of Technology, 1991, 24 p. (in Finnish)
[5] K. Vogt, Elektrische Maschinen, Berechnung rotierender elektrischer Maschinen. Dritte
bearbeitete Auflage, Berlin, Veb Verlag Technik, 1983, 500 p.
[6] E. Levi, Polyphase Motors; A Direct Approach to Their Design. New York/USA: John
Wiley & Sons, 1984. 438 p.
[7] R. Richter, Elektrische Maschinen, Volyme IV. Die Induktionsmaschinen.
[8] Tucci, C. L., Lang, J. H., Tabors, R. D., Kirtley, J. L. 1994. A Simulator of the
Manufacturing of Induction Motors. IEEE Transactions on Industry Applications, Vol. 30 ,
No 3, pp 578 - 584.