International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012)
Performance Comparison of Permanent Magnet Synchronous
Motor and Induction Motor for Cooling Tower Application
Prof. H.K.Patel1, Raj Nagarsheth2, Sharang Parnerkar3
1
Assoc. Professor, Instrumentation & Control Engg. Dept., Institute of Technology, Nirma University, INDIA.
2,3
Student, Electrical Engg. Dept., Institute of Technology, Nirma University, INDIA.
The design is performed in order to achieve a sinusoidal
back EMF without changing the stator geometry and
winding as sinusoidal excitation used with PMSM,
eliminates the torque ripple caused by the commutation.
PMSM are typically fed by voltage source inverter, which
cause time-dependent harmonics on the air gap flux. [1][5]
Permanent magnet synchronous machines can be realized
with either embedded or surface magnets on the rotor, and
the location of the magnets can have a significant effect on
the motor’s mechanical and electrical characteristics,
especially on the inductances of the machine. As the
relative permeability of the modern rare-earth magnets is
only slightly above unity, the effective air gap becomes
long with a surface magnet construction.[6][7]
This makes the direct-axis inductance very low, which
has a substantial effect on the machine’s overloading
capability, and also on the field weakening characteristics.
As the pull-out torque is inversely proportional to the daxis inductance, the pull-out torque becomes very high.
Typically, the per-unit values of the d-axis synchronous
inductances of the PMSM vary between 0.2−0.35 p.u., and
consequently the pull-out torque is in the range of 4−6 p.u.,
which makes them well suitable in motion control
applications.
The drawback of a low Ld –value is the very short field
weakening range, as the armature reaction with a surface
magnet construction is very weak. This means that a high
demagnetizing stator current component would be required
to decrease the air gap flux, and consequently, there would
be very little current left on the q-axis to produce the
torque.
Direct-axis inductance of a machine having embedded
magnets becomes high, as the rotor magnets per pole form
a parallel connection for the flux, while with a surface
magnet construction they are connected in series. With
equivalent magnets, the rotor reluctance of the surfacemagnet construction is therefore double compared to an
embedded-magnet construction, and the inductance is
inversely proportional to the reluctance [8][9]. With
embedded-magnets, the direct-axis inductance is further
increased because of the higher rotor leakage flux [10]
[11].
Abstract - The paper discusses the basic construction and
types available in Permanent Magnet Synchronous Motor
(PMSM) on the basis of the arrangement of the permanent
magnets. The various pros and cons of each arrangement are
discussed in brief. The paper elucidates the application of
PMSM in industry for cooling towers, with statistical data and
various practical results concerning various important
parameters such as efficiency, power factor and load current.
It clearly justifies and supports the efficient use of PMSM
over conventional induction motors (IM) with high efficiency,
with an in-depth analysis.
Keywords - Permanent Magnet Synchronous motor, cooling
tower, high efficiency, low power density.
I.
INTRODUCTION
The use of variable speed drives in industry is on
increase. Highly efficient drives are costly to manufacture
as well as provide difficulty in maintenance.
The
conventionally used 3-phase induction motor is a constant
speed motor, and with the help of drives the motor can be
used for variable speed applications, but at the cost of
reduced efficiency.[1][2][3]
The recent development in Permanent Magnet machines
has provided a solution for the variable speed applications,
which offer easy design for controller as well as operate at
higher efficiency [1] [4].
In this paper, the basic introduction of Permanent
Magnet Synchronous Motor (PMSM), its types and basic
constructional features are discussed [4]. The use of PMSM
in cooling tower for any industry is discussed using the
various graphical data, which provides an insight of its
operation in comparison to the usually used induction
motor. The advantages associated with PMSM and a
concept toward efficient energy systems by implementing
PM machines can be concluded from the discussion in the
paper.[3]
II.
T YPES AND CONSTRUCTION OF PMSM
In principle, the rotor of PMSM is constructed based on
the stator frame of a three-phase induction motor. It has
rotor structure similar to motor which contain permanent
magnets in rotor.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012)
Three basic configurations of PMSMs are shown in
Figure1.
As the direct-axis inductance is typically high with a
buried magnet construction, the overloading capability will
be poor, which makes this motor type incompetent in
motion control applications. Typically, the embedded vshape magnet machine can have Ld approx. 0.7 p.u, which
means only 1.4 p.u. overloading capability according to the
load-angle equation P=3EaVasinδ/Xs of a synchronous
machine with the assumption that the field EMF(EPM) EPM
= 1 p.u. and Ld = Lq. If there is a reluctance difference in
the machine, the maximum torque can be somewhat larger.
[14] It must, however, be borne in mind that despite the
embedded magnets, it is of course possible to increase the
physical air gap large enough, and thereby to decrease the
direct axis inductance of the machine remarkably from the
value given above. However, the consumption of the
magnet material is increased remarkably in such a case.
Figure 1: Three different types of rotor constructions for PMSM.
In addition to the good overloading capability, another
reason that makes the surface magnet construction
favorable in servo applications is the lower inertia. With
multi-pole machines, the rotor and the stator yokes can be
made very thin, and all the additional iron can be removed
from the rotor to provide a lower inertia. These large holes
also improve the heat transfer from the rotor, as the high
frequency flux pulsations generate heat on the magnets and
on the rotor iron.
The rotor in Fig. 1(b) with inset surface magnets has
better mechanical characteristics, but on the other hand, it
has higher leakages between two adjacent magnets. In
addition to the higher leakage, the torque production
decreases more as the motor must operate at higher pole
angle due to increased q-axis inductance compared to a
non-salient rotor.
Typically, the construction of commercial PMSM is
somewhere between the two typical topologies presented in
(a) and (b) in Fig. 1, that is, the magnets are slightly
embedded in the rotor. This improves the mechanical
strength of the rotor and introduces a reluctance differencebased term in the torque. According to measurements made
at Look up table for eight different commercial PMSM in
the power range of 3−5 kW, the values for the q-axis
inductances were 10−20 % higher than the values in the ddirection.[1][12][13]
With buried magnets and flux concentration, a sinusoidal
air gap flux density distribution is possible with simple
rectangular magnets. A sinusoidal air gap flux distribution
significantly decreases the cogging torque especially with
low-speed multi-pole machines that have a low number of
slots per poles per phase number. Also, it is possible to
increase the air gap flux density beyond the remanence flux
density of the magnets with a flux concentration
arrangement, and the machine can produce more torque at a
given volume. This is especially desirable in low speed
applications, such as in wind generators and in propulsion
motors where the space is limited.
III.
APPLICATION O F PMSM IN COOLING T OWER
The concept of cooling tower is based on circulation of
air, through fans via proper ventilating channels, which in
turn help in maintaining the balance of heat generated to
the heat dissipated. Since long, induction motors have been
employed in controlling the fan, which is used for
providing air circulation and hence eventually registering
temperature control.
In general, while cooling tower design was finalized, the
application of induction motors became inevitable due to
rugged construction, reliability, easy operation, and their
ability to run from the same supply as in comparison to DC
series motors. Also, with the advent of power electronics
development, the possible construction of converter drives
for the speed control of IM was possible and hence cooling
tower application was justified by IM.
Induction motor, invariably incurs conduction losses in
stator circuit and reaction and reactance losses in the rotor
circuit. The magnetization losses provide for a very poor
power factor at no load and light load conditions. The
operation of IM at poor power factor is a failed concept and
limits the speed control to a small range.
The excitation current and current needed to set up the
field flux in IM is completely overshadowed by the
permanent magnet used in rotor in PMSM. This provides
for a confinement for the losses to occur in only the stator
circuit. These losses are conduction losses and the reactive
power absorbed from the supply is largely decreased with
the introduction of a permanent magnet rotor. Thus an
improved efficiency and better power factor are the two
major rewards provided by PMSM over IM on the very
basic level.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012)
This same concept has been benevolently elaborated in
the mentioned below case study, which has proven the use
of PMSM over IM in a closed loop control of a cooling
tower with better speed range, easier control, high
efficiency and improved power factor.
Cooling tower applications follow fan affinity laws
which state that horse-power of motor (H.P) varies by the
cube of the fan speed. To have more efficient operation two
motors are used and as soon as the heat load increases, the
other motor with drive is brought into operation. This
lowers the horsepower required to only 12.5% of the rated
value [15].
Two speed motors do provide some energy savings, but
still must be cycled on and off to maintain the desired water
temperature.
Here the use of variable frequency drives is inevitable, in
order to provide an analogy to the efficient energy system
approach. It is all due to the energy savings associated with
the fan affinity laws. Additionally, most towers being
upgraded or refurbished are also being equipped with
VFDs [15][16]. These drives have the advantage of a soft
mechanical start, no large starting current draw, and the
ability to run the fan at any desired speed from zero to the
maximum design speed for the application. The energy
savings realized by using a VFD are well recognized and
documented. [17].
The installation for induction motor in cooling tower is
shown in fig. 2. [15][18]. For the same cooling tower
application fig.3 shows the installation of PMSM, which
provides variable speed at high efficiency and power factor
[15,19].
Figure 3: PMSM Motor installation
It is the improved cooling method, along with the higher
efficiency and power factor achieved with the PM
technology that allows for increased power density in
PMSM designs. Power density is the key for being able to
match the height restriction of the existing gearbox [15].
This case study involves the retrofit of an existing
cooling tower constructed in 1986 at Clemson University in
South Carolina. The existing tower had:


Fan Diameter: 18 ft
Flow Rates: 4,250 gal per minute per cell; 8,500 gpm
total.
 Motor Information: Frame - 326T, hp - 50/12.5; speed
- 1765/885 rpm
 Gear Ratio - 8.5:1
The tower is comprised of two identical cells. For this
study, one cell was retrofitted with the new slow speed PM
motor and VFD while the other was left intact as originally
constructed. This allows for a direct comparison of the two
fan drive solutions.
Figure 2: Induction Motor installation
169
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012)
Again, power measurements were made and a third party
testing service was engaged to verify the manufacturer's
results. [20]
Figure 4: the curve of power factor of a PMSM in comparison with an
IM, when used together in cooling tower. The p.f. maintained by
PMSM is higher than IM and is load independent.
Figure 5: Typical Partial Load Efficiencies of 75 HP, TEFC, 1800
RPM Motors
Fig. 4 shows the curve related to the comparison of
power factor with load for PMSM and IM. The PMSM at
loads is able to maintain high p.f, while the IM fails to
maintain the same power factor, for reduced loads or initial
loading. This is similarly reflected in Fig. 5, with the
operation of PMSM at efficiency above 95% and the full
load efficiency reaching to 97%. The similar efficiency
curves for conventional induction motor, for operation in a
cooling tower application, fall below the PMSM
characteristics.
Prior to the installation, the current being drawn by the
two original induction motors was measured with the fans
running at full speed. An ammeter was used and the current
was measured to be 47A, rms on both induction motors. As
the induction motors are identical, this is a good indication
that both cells were operating under the same load
conditions.
After the PM motor and VFD installation was complete,
the current was again re-checked and found to be only 41A
for the PM motor. The induction motor on the original,
identical,
tower
was
still
drawing
47A.
From this data, it was determined that both cells were
running at less than full load and that the load should be
increased on each cell. To this end, the pitch of the blades
on each fan was increased to 12 degrees. This change of
pitch caused the fans to draw more air, thus increasing the
load on each motor. Further, the increased air flow
improved the effectiveness of the overall tower
performance.
For the final blade pitch, 4.5 kW less power
consumption was observed on the cell with the PM motor
installed. The PM motor solution requires less input power
for each load point (blade pitch) because of the PM rotor of
the machine, there is no need to make current flow in the
rotor, and hence less energy is utilized from the primary or
the supply.
IV.
ADVANTAGES O F PMSM
The above industrial application has broadened and
enlightened the application of PM technology and its
advantages over the conventionally used induction
principle. The main points which we can consider while
summarizing this advantage are as:
170

PMSM provides higher power density for their size
compared to induction machine. This is because with
an induction machine, part of the stator current is
required to "induce" rotor current in order to produce
rotor flux. These additional currents generate heat
within the motor whereas, the rotor flux is already
established in a PMSM by the permanent magnets on
the rotor.

With the low power density it aids compactness. This
results in development of a PMSM with low rotor
inertia, which is capable of providing faster response.
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012)


[3 ] Downing, S., Reunanen, A., Saari, J. and Arkkio, A. 2005. Losses,
Cooling and Thermal Analysis of Electrical Machines, Lecture notes
of the postgraduate seminar, Espoo, Otamedia Oy, ISBN 951-227991-6
[4 ] Engelmann, R. H,. Middendorf, W. H, 1995. Handbook of Electric
motors, New York: Marcel Dekker Inc, 801 p
[5 ] Gieras, J.F. and Wing, M., 1997. Permanent Magnet Motor
Technology –Design and Applications. New York: Marcel Dekker
Inc, p. 444.
[6 ] Heikkilä, T., 2002. Permanent Magnet Synchronous Motor for
Industrial Applications – Analysis and Design. Dissertation, Acta
Universitatis Lappeenrantaensis 134, ISBN 951-764-699-2, 109 p.
[7 ] Hendershot Jr, JR., Miller, T.J.E., 1994. Design of Brushless
Permanent-Magnet Motors. Magna Physichs Publishing and
Clarendon Press, Oxford, ISBN 1-881855-03-1. 574 p
[8 ] Atallah K., Howe D., Mellor P.H., Stone D.A., “Rotor loss in
permanent–magnet brushless AC machines”, IEEE Transactions on
Industrial Applications, 36(6), pp. 1612 ~ 1618, 2000
[9 ] Carr W.J., Magnetic Properties of Metals and Alloys, America
Society of Metals, 1959.
[10 ] Deng F., “An improved iron loss estimation for permanent magnet
brushless machines”, IEEE Transactions on Energy Conversions,
14(4), pp. 1391 ~ 1395.
[11 ] Jahns T.M., “Flux-weakening regime operation of an interior
permanent magnet synchronous motor drive”, IEEE Transactions on
Industrial Applications, 23, pp. 681 ~ 689, 1987.
[12 ] Kurronen, P., 2003. Torque Vibration Model of Axial-Flux SurfaceMounted Permanent Magnet Synchronous Machine. Dissertation.
Acta Universitatis Lappeenrantaensis 154, ISBN 951-764-773-5,
123p.
[13 ] Mohammed Rakibul Islam, Phd thesis ,Laappeenranta University of
Technology “Cogging torque, torque ripple and radial force analysis
of Permanent Magnet Synchronous motors”,
[14 ] Y. K. Chin, J. Soulard, “A permanent magnet synchronous motor for
traction applications of electric vehicles," Royal Institute of Tech,
available online.
[15 ] Roman Wajda, Robbie McElveen, Bill Martin, and Steve Evon,
Baldor Electric Compan, “Permanent Magnet Technology within
Direct Drive Cooling Tower Motors Creates System Energy
Savings” 2011 ACEEE Summer Study on Energy Efficiency in
Industry
[16 ] Benjamin Cohen, “Variable Frequency Drives: Operation and
Application with Evaporative Cooling Equipment”, Cooling
Technology Institute Paper No. TP07-22, 2007
[17 ] William F. Immell, “Variable Speed Fan Drives for Cooling
Towers”, Cooling Technology Institute Paper No. TP96-03. 1996
[18 ] Rick Foree, “Cooling Towers and VFDs”, Cooling Technology
Institute Paper No. TP01-07,2001.
[19 ] M.P. Cassidy and J.F. Stack, “Applying Adjustable Speed AC
Drives to Cooling Tower Fans,” Pulp and Paper Industry Technical
Conference, 2008. PPIC , 1988.
[20 ] Steve Evon, Robbie McElveen and Michael J. Melfi, "Permanent
Magnet Motors for Power Density and Energy Savings in Industrial
Applications," IEEE Transactions on Industry Applications - IEEE
TRANS IND APPL , vol. 44, no. 5, pp. 1360-13. 66, 2008.
It is operating at a higher power factor compared to
induction motor (IM) due to the absence of
magnetizing current.
The design of controller required for the design of
speed control of the fan operated by PMSM is simple.
The PMSM also provides a key feature of operating at
high efficiency with low speeds, thus giving all round
efficient operation for the cooling tower at high and
low speeds.
V.
CONCLUSION
The discussion so far on PMSM can be finally
summarized in a brief manner that PMSM has certain
advantages over the preferred induction motor. The
operation of PMSM discussed in context with cooling
tower has proved the overall compatibility and advantages
over the IM’s. This survey does not certainly limit to the
application at cooling towers but also to various other
applications, including domestic applications like washing
machines. In other utilities where variable speed drives
prove costly and inefficient, for e.g. electric traction, where
speed control needs to be flexible and again with the
provision of less weight and wear and tear. There are
certain other advantages apart from those discussed above,
which were for a particular application.

PMSM does not require regular brush maintenance
like conventional wound rotor synchronous machines.
 The PM rotor does not require any supply nor does it
incur any loss.
 Low noise and vibration than switched reluctance
motors (SRM) and IMs.
Hence, from the discussed application with its in-depth
graphical analysis, it procures a clear picture that the use of
PMSM is inevitable so as to meet the current energy
efficient systems and develop a smarter, compact and
effective grid altogether.
REFERENCES
[1 ] Jussi Puranen, “Induction motor versus permanent magnet
synchronous motor in motion control application”.
[2 ] Deng, F., 1999. An Improved Iron Loss Estimation for permanent
Magnet Brushless Machines. IEEE Transactions on Energy
Conversion. Vol. 14, No. 4, pp. 1391-1395.
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