paper conf iemc09 full-paper final

8/3/2019 Paper Conf IEMC09 Full-Paper Final

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Experiments to Observe the Impact of Power

Quality and Voltage-Source Inverters on the Temperature of

Three-Phase Cage Induction Motors using an Infra-Red Camera

Fernando J. T. E. Ferreira1,2, Anbal T. de Almeida2, Joaquim F. S. Carvalho1, and Mihail V. Cistelecan31 Dep. of Electrical Engineering, Engineering Institute of Coimbra (ISEC), Coimbra, Portugal

2Institute of Systems and Robotics, University of Coimbra (ISR-UC), Coimbra, Portugal

3Research Institute for Electrical Machines (ICPE-ME), Bucharest, Romania

E-mail: [email protected]

Abstract Nowadays, voltage-source inverters (VSIs) arewidely used to control speed, torque and/or position of three-phase, squirrel-cage, induction motors (IMs). However, theharmonic distortion inherent to the VSI output PWM voltagesleads to the increase of the motor operating temperature and,consequently, to the reduction of the electric insulation system

lifetime. Additionally, the most common long-duration voltageanomalies in the power network, such as voltage magnitudedeviation, unbalance and harmonic distortion, have a well-known

impact on IMs performance and lifetime. The operatingtemperature increase due to poor power quality is particularlycritical in the totally-enclosed, fan-cooled IMs. In order to allow a

direct observation of the temperature distribution in IMssubjected to different voltage supply conditions, a set ofexperiments are proposed, using a totally-enclosed, fan-cooled IM,

an end-shield-less, fan-less IM, and an infra-red camera toacquire the temperature maps in the frame for the first mentionedmotor and in the stator core, stator windings and rotor for the lastmentioned motor. The motor steady-state temperature is directlyrelated with the losses being produced in it under differentoperating conditions, including ambient temperature, load, speedand input power quality. The proposed laboratorial experimentscan be used to easily observe and study that relation, includingthe impact of PWM voltage supply and of individual voltage

harmonics in the IMs operation, being a contribution in the scopeof electrical engineering education, particularly for electricmachines and power electronics courses.

I. INTRODUCTIONNowadays, power quality is of major importance, particularly in

the industrial and tertiary sectors. Power quality is related with the

mains supply reliability and quality of the respective voltage

waveforms, in terms of shape and amplitude. The most common

long-duration voltage anomalies in the power network, such as

voltage magnitude deviation, unbalance, and/or harmonic

distortion, have a significant impact in the performance and

lifetime of three-phase, squirrel-cage, induction motors (IMs)[1, 2], which are, by far, the most used electric motor type in

industry. Therefore, it is important to evidence that impact in the

electric machines and power electronics courses, as well as the

importance of derating motors operating in such conditions in

order to avoid internal overtemperature in steady-state operation,

since it can lead to a significant reduction in their lifetime.

The use of voltage-source inverters (VSIs) to control speed,

torque and/or position of IMs is increasing significantly. It is

expected that, in the European Union, in the next decade, of the

IMs will be fed by VSIs. Besides their well-known benefits, such

as process improvement and energy savings, VSIs can lead to a

significant increase of the IMs operating temperature due to the

additional losses associated with the harmonic distortion inherent

to output PWM voltages (even when output filters are used), and,

in the case of totally-enclosed, fan-cooled (TEFC) motors, due to

the reduced cooling capability when the motor speed is reduced

below nominal speed. The operating temperature increase in VSI-

fed IMs, associated with the voltage transients at the motor

terminals, due to the voltage reflection effects in the power cables,

can lead to a significant reduction in the motor electric insulation

system lifetime.

In IMs directly fed from the mains supply (line-fed IMs), a

significant increase in the operating temperature can occur due to

voltage unbalance and/or voltage harmonic distortion in the power

network. The voltage harmonic distortion is provoked by

nonlinear loads (such as VSIs, which typically incorporate diode

rectifiers at the input stage). The 5th-order harmonic is particularly

baneful, since it produces a negative-sequence stray rotating

magnetic field, dragging the motor and heating the rotor [2]. Ingeneral, all negative- and positive-sequence current harmonics,

resulting from voltage harmonics at the motor terminals, lead to

extra losses in the motor stator core and windings.

Thermal phenomena are of major importance in the electric

machines courses, particularly from the perspective of

experimental demonstration and basic explanation of the related

causes and effects. In thermal steady-state1

(or thermal

equilibrium), the capacitances (representing the thermal capacity

of the different parts) disappear from the lumped-parameter

thermal equivalent circuit based model, yielding only current

sources (representing the loss production in the different parts)

and thermal resistances (representing the thermal resistance

between the different parts), which are constant, assumingconstant motor speed and ambient temperature. Therefore, in

general, after thermal stabilization/equilibrium, the temperature

increases linearly with the motor losses. However, the lumped

parameters mesh can be very complex, depending on the

refinement of the motor thermal model, as explained in [3].

1 Typically, the thermal equilibrium state is considered when the temperaturevariation is lower than 0.5 K/30 min.


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In a motor, the temperature rise, , between two points with

different temperatures, caused by the heat produced in the hottest

point by Joule effect losses, Plosses, dissipated through the

equivalent thermal resistance,Rth, to the surrounding ambient with

temperature, amb, and stored in the equivalent thermal capacity,

Cth, is shown in Fig. 1, assuming that, at the initial instant, all the

motor parts are at the ambient temperature2. After reaching

thermal equilibrium, which occurs 6 to 7 times de thermal

constant, th, the thermal capacity effect ends, and it can beremoved form the equivalent circuit for thermal steady-state

analysis purposes, yielding the circuit in Fig. 2a. On that basis, the

steady-state temperature rise is simply given by (1).

=Rth Plosses (1)

If three different points (including ambient) are considered, the

same principle can be applied, as presented in Fig. 2b, in which

the intermediate temperature rise, between points 2 and 3 is given

by (2), similarly to a voltage divider.

2 = (Rth2/(Rth1+Rth2)) =Rth2 Plosses (2)

In fact, it is obvious the analogy between the thermal

quantities and the electric quantities the temperature rise (in Cor K) is equivalent to the voltage drop (in V), the losses (in W) are

equivalent to the current (in A), and the thermal resistance (C/W

or K/W) and thermal capacity (W.s/C or W.s/K) are equivalent to

the ohmic resistance ( ) and electric capacitance (in F),

respectively.

Fig. 1. Temperature rise and thermal time constant in a motor.

2 After the motor starting transient period, at rated load, the motor equivalent

lumped thermal capacity and resistance can be both approximately determined

by simply recording over time the frame temperature rise (e.g., using an infra-

red thermometer with USB communication to the PC, with proper data

visualization software), motor efficiency, and input power (allowing for thelosses calculation), and then fitting to the temperature rise vs. time data a curve

with the form in which and .

(a) (b)Fig. 2. Approximate lumped-parameter thermal equivalent circuit of a motor.

In this paper, the heating phenomenon in IMs is addressed and

laboratorial experiments are proposed using an infra-red camera, a

conventional IM and a special low-power end-shield-less IM.

II. EXPERIMENTS AND THEORETICAL EXPLANATIONSA. Experiments with a Totally-Enclosed Induction Motor

In the first proposed experiment, the IM temperature relation

with its load and voltage supply harmonic distortion is analyzed.

At this stage, it is important to define the motor load, which is

given by (3), where is the motor load (in %), Pshaft is the motor

output shaft or useful power (in W), PN is the motor rated or

nominal power (in W).

= 100% Pshaft/PN (3)

In this experiment, a 3-kW, 4-pole, 400-V, TEFC IM is directly

fed from the line/mains and by a 2-level PWM VSI (2-kHz

carrier/switching frequency, 50-Hz fundamental frequency, 400-V

fundamental voltage), in order to allow the motor frame

temperature comparison in both cases, for different load levels. It

is used a high-accuracy hysteresis dynamometer (which includes a

load cell and an encoder) [4], a power analyzer, an oscilloscope,

and an infra-red camera (IRC), as can be seen in Fig. 3.

Fig. 3. Test-bench and IRC used for the experiments with the 3-kW IM.

Firstly, the 3-kW IM was fed directly from the line/mains and

its temperature and losses were measured for 0%, 25%, 50%, 75%

and 100% load. The results for no-load and full-load operation are

presented in Fig. 4, clearly evidencing the well-known motor

temperature dependency on load. It should be noted that, during


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this experiment, the mains supply voltage was slightly distorted,

as shown in Table I.

(a)

(b)

Fig. 4. Side-view IRC images of a line-fed 3-kW, 4-pole, TEFC IM, at

different loads: (a) no-load; (b) full load.

TABLE I

Line-to-Line Voltage and Line Current Harmonic Distortion (f1 = 50 Hz).

Load (%) 100 75 50 25 0

THDu (%) 2.6 2.6 2.6 2.7 2.8

THDi (%) 3.6 4.8 5.9 6.7 6.5

U5 | I5 (%) 2.4 | 2.9 2.2 | 4.3 2.5 | 5.5 2.6 | 6.4 2.7 | 6.4

U7 | I7(%) 0.9 | 2.0 0.8 | 2.0 0.7 | 2.0 0.6 | 1.8 0.6 | 0.7

U11 | I11 (%) 0.2 | 0.1 0.2 | 0.1 0.1 | 0.2 0.1 | 0.1 0.1 | 0.4U13 | I13 (%) 0.4 | 0.2 0.3 | 0.1 0.3 | 0.2 0.3 | 0.3 0.1 | 0.2

U17 | I17(%) 0.1 | 0.1 0.1 | 0.0 0.1 | 0.0 0.1 | 0.0 0.0 | 0.0

U19 | I19 (%) 0.1 | 0.0 0.1 | 0.0 0.1 | 0.0 0.1 | 0.0 0.0 | 0.1

Note: and .

Obviously, since the total losses are not proportional to the

motor load, and no-load losses (i.e., core plus mechanical losses)

can be significant, the motor temperature rise will not decrease

proportionally with the motor load. However, the frame

temperature roughly varies proportionally to the losses responsible

for internal heat production, which includes all the losses except

the windage losses (note that friction losses also produce heat in

the bearings). Therefore, in order to explain the observed

temperature variation in the frame, two equivalent thermal

resistances can be considered, one between an internal virtual loss

production point (lumped heat/losses production) and the frame

surface, and other between the frame and the ambient, as

represented in Fig. 5. It should be noted that the last resistance

actually varies with the motor speed because it depends mainly on

the heat dissipation by convention (i.e., by means of air movement

in the frame surface). In fact, in frame-ambient heat-exchange

process, only a minor part of the heat is dissipated by conduction

and radiation. However, since the speed in single-speed IMs only

slightly vary due to the slip dependency on motor load, the frame-

ambient equivalent thermal resistance can be considered constant

[3]. On the basis of Fig. 5 and (2), the frame temperature rise in a

given point (which is considered as reference for all calculations),

frame, is given by (4), where Rframe is the frame-ambient

equivalent thermal resistance.

frame =Rframe Plosses (4)

The losses component that does not contribute to internal heatproduction (heatless losses, Pheatless) can be estimated by means of

intersecting the curves of the frame-ambient thermal resistance, as

a function of that losses component, for different motor loads, as

can be seen in Fig. 6, corresponding to the intersection point (in

this case, 26 W at 0% load speed). The heatless losses are properly

compensated as a function of the speed squared. The estimation of

heatless losses is important for the temperature rise calculation as

a function of the losses component actually producing heat in the

interior of the motor and, therefore, the respective thermal

resistance. The theoretical demonstration of the experimental

results is summarized in Table II, in which the estimated Rframe is

presented for different load points, with a very small variation,

validating the proposed methodology. Typically, the steady-state

winding temperature rise variation as a function of the motor loadis higher than that for the frame, but approximately equal in

percentage.

Fig. 5. Approximate lumped-parameter thermal equivalent circuit of a motorwith separated interior-to-frame and frame-to-ambient thermal resistances.

TABLE II

Demonstration of the Experimental Results for Different Loads ( amb = 20C).

Load

(%)

Plosses*

(W)frame

(K)

Rframe

(K/W)

2(p.u.)

Pheatless

(W)

0% 190 13.8 0.072 1.00 26.0

25% 194 14.1 0.073 0.98 25.4

50% 246 17.8 0.072 0.96 24.9

75% 354 25.7 0.073 0.93 24.2

100% 687 49.9 0.073 0.89 23.0

* Plosses is equal to total losses minus heatless losses, Pheatless.

When an IM is fed by power-electronic-based devices (such as

VSIs and electronic voltage regulators) its efficiency is negatively

affected due to the additional harmonic losses. In the case of VSIs,

these additional losses are not relevant when their well-knownadvantages are taken into account. Most manufacturers often

recommend a derating up to 10% in the VSI-fed IMs. In order to

evaluate the referred effects, the 3-kW IM was fed by a VSI (fc = 2

kHz, f1 = 50 Hz) and tested at different loads. The harmonic

distortion of the VSI output PWM voltages and currents, for a

carrier/switching frequency fc = 2 kHz, is shown in Table III. It

should be noted the significant 5th

order voltage harmonic

(negative-sequence harmonic, mainly due to the inverter operation

in the overmodulation region, i.e., amplitude modulation index


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higher than 1), which can produce significant additional losses in

the motor. In Fig. 7, the motor operation at 50-Hz, 400-V

fundamental voltage when it is fed directly from line or by the

VSI, can be seen. It is possible to observe a general temperature

rise increase in the frame.

Fig. 6. Heatless losses component estimation.

In Table IV, the experimental results are summarized and

explained with the approximate thermal circuit. The frame-

ambient thermal resistance practically matches for both cases, as

expected. For the 27-W additional heat-producing losses (or Joule

effect losses), a 2-K temperature increase (+14.5%) was observed.

TABLE IIIVSI Output Harmonic Distortion (f1 = 50 Hz,fc = 2 kHz).

Load (%) 100 75 50 25 0

THDu (%) 37.0 37.7 39.7 38.4 39.4

THDi (%) 9.9 9.7 9.9 9.6 11.0

U5 | I5 (%) 3.8 | 6.4 2.9 | 4.1 0.8 | 3.1 1.3 | 1.3 0.7 | 11.4

U7 | I7(%) 1.7 | 3.2 2.3 | 2.6 1.4 | 2.0 1.1 | 1.3 0.7 | 10.7

U11 | I11 (%) 1.0 | 0.9 0.6 | 0.3 0.4 | 0.2 0.5 | 0.2 0.4 | 10.1

U13 | I13 (%) 0.4 | 0.2 1.4 | 0.5 1.7 | 0.3 0.3 | 0.2 1.3 | 10.1

U17 | I17(%) 0.6 | 0.1 0.9 | 0.1 0.8 | 0.1 1.3 | 0.2 0.7 | 10.2

U19 | I19 (%) 0.3 | 0.1 0.2 | 0.1 0.8 | 0.1 1.0 | 0.1 0.3 | 10.1

Note: and .

TABLE IVDemonstration of the Experimental Results for No-Load Operation.

Supply Plosses*(W)

frame(K)

Rframe(K/W)

2

(p.u.)

Pheatless(W)

Line 190 13.8 0.072 1.00 26

VSI 217 15.8 0.073 1.00 26

* Plosses is equal to total losses minus heatless losses, Pheatless.

On the basis of the presented results, it is possible to conclude

that the proposed strategy to relate the thermal steady-state frame

temperature in a given point with the total heat-producing losses,

by means of the average equivalent frame-ambient thermal

resistance, is valid and simple to apply. Moreover, the impact of

PWM supply on IMs temperature can be also evidenced and

explained with the proposed methodology.

(a)

(b)

Fig. 7. Side-view IRC images of a 3-kW, 4-pole, TEFC IM for different

supplies at no-load operation: (a) Mains/line supply; (b) VSI supply.

To avoid position-related temperature measurement errors in

particular points of the IRC 2-D images, the temperature map can

be processed in order to extract an average temperature in a

predefined area in the motor frame side (for that purpose

MATLAB software tools can be used), and that value can be used

for the proposed calculations, instead of using only one point.

Alternatively to the IRC, a low-cost, single-point, infra-redthermometer can be used.

With the described set-up it is also possible to test the motor

average temperature rise (in %) under unbalance voltage supply,

which reflects the overall losses increase, although there will be a

thermal asymmetry in the per-phase windings, which as to be

taken into account in the motor derating.

B. Experiments with an End-Shield-Less Induction MotorIn this section, experiments using a 150-W, 4-pole, three-

phase, end-shield-less, squirrel-cage, induction motor (ESLIM)

are described. In Fig. 8, the setup used for that purpose is shown,

in which the ESLIM is coupled with an electromagnetic brake,

which has a load cell and an encoder. It should be noted that this

motor testing setup is indicated only for didactic/teachingpurposes, since the accuracy of the torque measurement is

medium-low.

The purpose of the first experiment with this set-up was to

evaluate the voltage unbalance impact on the IMs performance.

The supply voltage unbalance was achieved by means of a series

resistance in one phase line, as shown in Fig. 9. Alternatively, if

three separated autotransformers are available, they can be used to

directly unbalance the voltage system. In this experiment, three

different cases were analyzed, namely, balanced voltage supply


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(Case 1), unbalanced voltage supply with voltage amplitude drop

in one phase (Case 2), and conditions of Case 2 with motor

derating on the basis of the rotor speed (Case 3). In Figs. 10 and

11, the IRC images obtained for the three cases are shown for the

ESLIM. The general operating conditions are summarized in

Table V, where Ud, Id, Ui and Ii are the direct and inverse

symmetric components amplitude of the line-to-neutral voltage

and line current.

Fig. 8. ESLIM test-bench (top-left), IRC (top-right), ESLIM non-drive-endview (bottom-left), and ESLIM side view (bottom-right).

Fig. 9. Diagram for the unbalanced voltage supply test set-up.

TABLE V

ESLIM Operating Conditions in the 1st Experiment.

CaseURN

(V)

USN

(V)

UTN

(V)

IR

(A)

IS

(A)

IT

(A) (r/min)

Pin

(p.u.)

Pshaft

(p.u.)

1 30.4 30.4 30.4 0% 3.82 3.82 3.82 0% 0.55 1451 1.00 1.00

2 16.7 30.4 30.4 15% 2.82 5.55 3.75 48% 0.61 1432 1.02 0.97

3 21.5 30.4 30.4 13% 2.51 4.94 3.33 48% 0.53 1451 0.80 0.75

Case 1Balanced voltage (f1 = 50 Hz), rated load;

Case 2Unbalanced voltage (f1 = 50 Hz; Rseries = 4.25 , hot), rated load;

Case 3Unbalanced voltage (f1 = 50 Hz; Rseries = 4.25 , hot), derated.

The IRC images evidence the asymmetric impact of unbalance

in the per-phase winding temperatures (because of the asymmetric

per-phase currents) and, due to the negative-sequence (or inverse)

symmetric component, the rotor heats much more and the speeddecreases significantly. Derating the motor (reducing the shaft

load torque) in order to reach the initial speed obtained in the

balanced situation, the temperature of the rotor and of the hotter

phase windings is still higher in relation to the balanced case, as it

can be observed by comparing Figs. 10a and 10c. Moreover, in the

unbalanced cases, the rotor temperature remains approximately

equal because, although the load/torque decrease in the derated

case, the consequent speed increase leads to an increase of the

rotor losses due to increase of the rotor current components

produced by the inverse air-gap rotating magnetic field (opposite

direction to the direct rotating field) created by the inverse

symmetric component of the stator currents. For that purpose, the

NEMA derating curves can be applied and even validated with the

used setup. It should be noted the 6.9-K temperature increase in

the hottest per-phase winding heads from the balanced to the

unbalanced cases (Figs. 10a and 10b). Since, in general, per each

10-K increase in the winding temperature, the insulation system

lifetime is shortened to half (Arrhenius law), in the presented case,

the motor lifetime will be significantly shortened. This experiment

clearly shows the importance of properly derating the motor when

its voltage supply is unbalanced, in order to avoid lifetime

shortening.

In order to demonstrate (once more) the impact of PWM

voltage supply in the motor operating temperature, the ESLIM

was fed by a 2-level VSI (fc = 1-kHz,f1 = 50 Hz), as shown in Fig.

12. Since the VSI was operating in the linear region (i.e.,

amplitude modulation index lower than 1), the low-order

harmonics at the output voltages are relatively low. Thus, the

additional losses are mainly due to the high-order harmonics

(carrier frequency related harmonics), although the corresponding

current harmonics are strongly filtered by the motor impedance, as

can be seen in Table VI. In Figs. 13 and 14, the IRC images of the

ESLIM can be seen.

On the basis of the presented images, it is evident the

significant impact of PWM voltage waveforms on the motor

performance, leading to an increase of the core and copper losses

and, therefore, of the temperature of all motor parts (a 7.5-K

temperature increase in the winding heads). Tables VI and VII

summarize the operating conditions.

The 5th

harmonic is commonly found in most industries due to

the operation of nonlinear loads, such as those incorporating diode

rectifiers, and its impact on the IM operation is strong. Therefore,

it is very important to derate IMs operating in such conditions.

NEMA derating curves (defined as a function of the voltage

harmonic distortion) are recommended for that purpose. The next

experiment deals with the 5th

harmonic effect on the ESLIM.

TABLE VI

ESLIM Operating Conditions in the 2nd Experiment.

CaseURN

(V)

URN_1

(V)

IR

(A)

IR_1

(A)

THDu

(%)

THDi

(%) (r/min)

Pin

(p.u.)

Pshaft

(p.u.)

1 30.4 30.4 3.82 3.82 4.6 1.5 0.53 1451 1.00 1.0

2 90.5 30.4 3.85 3.82 40.0 1.7 0.20 1451 1.18 1.0

Case 1Sinusoidal voltage supply (f1 = 50 Hz), rated load;

Case 2PWM voltage supply (f1 = 50 Hz;fc = 1 kHz), rated load.Balanced three-phase system, i.e., URN= USN= UTN and IR =IS = IT.

TABLE VII

ESLIM Supply Low-Order Harmonics in the 2nd Experiment.

CaseU5

(%)U7

(%)U11

(%)U13

(%)U17

(%)U19

(%)I5

(%)I7

(%)I11

(%)I13

(%)I17

(%)I19

(%)

1 1.14 0.85 0.16 0.19 0.05 0.15 1.12 0.67 0.04 0.08 0.01 0.042 4.93 5.79 4.43 4.28 0.98 5.39 1.28 0.44 0.16 0.09 0.11 0.12

The 5th

harmonic voltages superposition to the fundamental

line voltages was achieved by means of a VSI feeding at 250-Hz

fundamental frequency (properly filtered to eliminate carrier-

frequency related harmonics) three single-voltage transformers

(T1, T2 and T3), connected in star/wye (which contribute to the

carrier-frequency related harmonics elimination) whose secondary

windings are connected in series with the 50-Hz sinusoidal


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variable-voltage supply provided by the autotransformer, as can be

seen in Fig. 15. This low-cost set-up is innovative and can be used

for teaching purposes.

It should be noted that the VSI output voltage has to be

carefully regulated, and it is important to ensure that the 250-Hz

three-phase voltage system has a negative phase sequence and the

50-Hz three-phase voltage system has a positive phase sequence,

in order to produce indirect and direct rotating air-gap MMFs in

the motor, respectively. This can be previously verified by

supplying the motor with each of the sources individually, and

ensuring that it rotates in opposite wises. Otherwise, two phases

must be exchanged in one of the three-phase voltage systems. In

Figs. 16 and 17, the IRC images of the ESLIM fed by a distorted

voltage supply with a dominant 5th

harmonic (5.4%) can be seen.

The motor operating conditions during this experiment are

summarized in Tables VIII and IX.

(a)

(b)

(c)

Fig. 10. Non-drive-end-view IRC images of the ESLIM for different operating

conditions: (a) balanced voltage supply; (b) unbalanced voltage supply; (c)

unbalanced voltage supply and motor derating.

On the basis of the images presented in Figs. 16 and 17, it is

evident the significant impact of the 5th-order harmonic on the IM

performance, leading to a decrease on its speed (note the 1.7%

decrease in motor speed), and increase of their internal

temperature, as a consequence of the stator core and copper losses

increase and, particularly, of the rotor losses increase, even with

the significant decrease of the output shaft power and input active

power. Itshould be noted the 15% decrease in the phase-to-neutral

fundamental voltage, which also contributed to the motor slip

increase. Therefore, the motor average internal temperature

increases (note the 1.2-K increase in the rotor cage ring and 0.5-K

in the coil-heads temperatures). This experiment can also be used

to validate the NEMA motor derating curves, considering one of

the most critical situations voltage distortion considering only

the 5th

harmonic (which, of course, is not likely to actually

happen). The same setup can be used to study the impact of other

harmonics alone, evidencing their individual impact in the motor

performance.

(a)

(b)

(c)

Fig. 11. Side-view IRC images of the ESLIM for different operating

conditions: (a) balanced voltage supply; (b) unbalanced voltage supply; (c)unbalanced voltage supply and motor derating.

Fig. 12. Diagram for the voltage-source inverter supply test set-up.


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(a)

(b)

Fig. 13. Non-drive-end-view IRC images of the ESLIM for different operating

conditions: (a) sinusoidal voltage supply; (b) PWM voltage supply.

(a)

(b)

Fig. 14. Side-view IRC images of the ESLIM for different operating

conditions: (a) sinusoidal voltage supply; (b) PWM voltage supply.

TABLE VIII

ESLIM Operating Conditions in the 3rd Experiment.

CaseURN

(V)URN_1

(V)THDu

(%)IR

(V)IR_1

(A)THDi

(%) (r/min)

Pin

(p.u.)Pshaft

(p.u.)

1 30.4 30.4 0.0 3.82 3.82 0.0 0.53 1451 1.00 1.00

2 34.6 25.7 8.6 3.62 3.61 5.4 0.43 1427 0.85 0.75

Case 1Sinusoidal voltage supplyf1 = 50 Hz), rated load;

Case 2Distorted voltage supply (f1 = 50 Hz;f5 = 250 Hz; fc = 16 kHz).

Balanced three-phase system, i.e., URN= USN= UTN and IR =IS = IT.

TABLE IXESLIM Supply Low-Order Harmonics in the 3 rd Experiment.

CaseU5

(%)

U7

(%)

U11

(%)

U13

(%)

U17

(%)

U19

(%)

I5

(%)

I7

(%)

I11

(%)

I13

(%)

I17

(%)

I19

(%)

1 1.14 0.85 0.16 0.19 0.05 0.15 1.12 0.67 0.04 0.08 0.01 0.04

2 5.37 1.19 0.39 0.48 0.73 0.19 5.33 0.15 0.09 0.06 0.06 0.05

Fig. 15. Diagram of the set-up to test the effect of individual harmonics.

(a)

(b)

Fig. 16. Side-view IRC images of the ESLIM under different operating

conditions: (a) sinusoidal voltage supply; (b) distorted voltage supply.

C. Thermographic Images of a VSIThe presented analysis can also be applied to power-electronic

devices such as VSIs. Just for curiosity, in Fig. 18, the IRC images

of a 2-level, 3.7-kW, 400-V VSI, supplying 7-A output line

current (RMS value), with a carrier frequency of 2 kHz and 15

kHz are shown. Excluding the power associated with the

control/processing board and cooling ventilation, the inverter

losses can be divided into two major components conduction


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and switching losses. The first are mainly load dependent and the

last mainly carrier frequency dependent. Therefore, for a given

output current, if the carrier frequency is increased, the VSI

temperature will also increase, assuming constant ventilation (i.e.,

constant heat dissipation by convection). In fact, the increase of

the carrier frequency from 2 kHz to 15 kHz led to an increase of

2 K in a particular internal spot of the inverter, indicated in

Fig. 18.

(a)

(b)

Fig. 17. Side-view IRC images of the ESLIM under different operating

conditions: (a) sinusoidal voltage supply; (b) distorted voltage supply.

(a)

(b)

Fig. 18. Side-view IRC images of a 2-level VSI under different operating

conditions: (a)Iline = 7 A,fc = 2 kHz; (b)Iline = 7 A,fc = 15 kHz.

III. CONCLUSIONSThe proposed experiments are easy to implement, although

most of the required equipment is expensive, particularly the

dynamometer, the power analyzer, and the infra-red camera. A

new approach to study the impact of individual harmonics in the

IMs is proposed, which can be used either for didactic/teaching

purposes or for advanced research, depending on the quality and

accuracy of the equipment used. The use of an end-shield-less IMand an infra-red camera for demonstration of the power quality

(voltage unbalance, magnitude deviation, and harmonic distortion)

impact on IMs is also novel and useful for teaching activities, once

the results are very didactic or easily understood. The IRC images

are appellative and clearly demonstrate the losses variation in the

machines under different operating conditions. In general, the

provided theoretical demonstrations seem to be appropriate to

explain the experimental results, since both fairly match. In

particular, the proposed methodology to estimate the motor

heatless losses and the equivalent frame-ambient thermal

resistance offers good results. Since the proposed setups are

simple to implement, and the respective theoretical background is

relatively simply to explain, they are indicated for electricmachines and power electronics courses, as well as to set case

studies for analysis during two or more classes. As future work, an

analysis ofNEMA induction motors derating curves for voltage

unbalance and harmonic distortion (for one or two points), on the

basis of the experimental thermographic images in the end-shield-

less IM, taking into account the highest temperature rise in the

per-phase windings, will be made.

REFERENCES

[1] F. J. T. E. Ferreira, A. de Almeida, and G. Baoming: Comparative Studyon 2-Level and 3-Level Voltage Source Inverters , 5th Inter. Conf. on

Energy Efficiency in Motor Driven Systems (EEMODS'07), Conf. Proc.,

Beijing, China, June 2007.[2] F. J. T. E. Ferreira, A. de Almeida, W. Deprez, R. Belmans, and G.

Baoming: Impact of Steady-State Voltage Supply Anomalies on Three-Phase Squirrel-Cage Induction Motors,Inter. Aegean Conf. on Electric

Machines, Power Electronics and Electromotion Joint Conf.(ACEMP07), Conf. Proc., Turkey, Sept. 2007.

[3] F. J. T. E. Ferreira, A. de Almeida, and G. Ba oming: Three-PhaseInduction Motor Simulation Model Based on a Multifrequency Per-PhaseEquivalent Circuit Considering Stator Winding MMF Spatial Harmonicsand Thermal Parameters, 17th Inter. Conf. on Electric Machines(ICEM06), Conf. Proc., Crete, Greece, Sept. 2006.

[4] de Almeida and F. J. T. E. Ferreira: User-Friendly High-PrecisionElectric Motor Testing System, 4th Inter. Conf. on Energy Efficiency in

Motor Driven Systems (EEMODS05), Conf. Proc., pp. 149-157,Heidelberg, Germany, Sept. 2005.

[5] A. de Almeida, F. J. T. E. Ferreira, and Both, D.: Technical andEconomical Considerations to Improve the Penetration of Variable SpeedDrives for Electric Motor Systems, IEEE Trans. on Industry

Applications, Vol. 41, No. 1, pp. 188-199, Jan./Feb. 2005.

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