025 kanchan finalpaper eemods13

8/9/2019 025 Kanchan Finalpaper EEMODS13

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Experience with efficiency measurement of VSD fed inductionmotors using calorimetric loss measurement technique

Rahul S. Kanchan1, Ingo Stroka1, Ville Särksmäki 2

1 ABB AB Corporate Research, Sweden, 2 ABB Oy, Finland

Abstract

Accurate measurement of the motor and/or motor drive system losses is very important in view ofupcoming stringent standard requirements. The EU eco-design directives 2005/32/EC throughCommission Regulation (EC) No 640/2009 is pushing for use of higher efficiency class motors forindustrial applications through the upcoming efficiency standards for motors and drive systems. Formotors of higher efficiency classes, the measurement of efficiency becomes more critical since smallerrors introduced by measurement system have large effect of estimated efficiency. There are wellknown established procedures to measure the efficiency of motors fed from sinusoidal supply asoutlined by IEC 60034-2-1 standard. The efforts are made in to draft a new technical specification IEC60034-2-3

1 to address the methods to measure the additional losses incurred associated with

Variable Speed Drives (VSD) supplied motors [10] [21]. Similarly, the work is ongoing to describemethods for measuring the system efficiency of complete motor-drive system for VSD fed motor

drives [7][6]. This draft specification describes either electrical input-output or calorimetric lossmeasurement as preferred methods to measure efficiency of motor drive system.

This paper addresses our experience with measuring the VSD fed motor efficiency using calorimetricloss measurement technique and comparing the measurements with other preferred efficiencymeasurement methods. A calorimetric loss measurement system is built for measuring the losses ofmotor supplied from a sinusoidal supply mains or VSD. The calorimeter is an open type two phasesystem. The thermal equilibrium is established by running the test motor inside the calorimetricchamber, and then the same thermal equilibrium is re-established using a heating resistor elementsupplied from a DC power source. The power fed to the heating resistor at the thermal equilibrium issubsequently interpreted as the motor losses. A series of tests were performed using calorimeter tomeasure the motor losses at different load conditions. The measured losses are then compared withlosses measured with the direct input-output efficiency measurement method. The uncertainties ofcalorimetric loss measurements are also established by considering all possible heat leakage points

and instrumentation error sources. Finally both electrical and calorimetric loss measurement resultsare compared.

Keywords:- IEC60034-2-1, IEC 60034-2-3, Direct input-output efficiency measurement, calorimetricloss measurement technique

1 The technical specification is still in draft stage, the publishing date is scheduled to be in late 2013. http://www.iec.ch


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Still calorimetric loss measurements often involve more complex and time consuming tasks and thusthis method cannot be used for routine testing but rather should be used as an evaluation tool forconfirming the accuracy of efficiency measured by other methods. This is the main motivation for thispaper. The paper presents construction of a test setup which is capable of performing bothcalorimetric loss measurements and electrical efficiency measurements simultaneously. The paperaddresses some common issues with calorimetric loss measurement systems involving electricmotors as test objects. The paper is organized as follows. First a literature overview of the calorimetric

loss measurement method is presented, with more focus on application to measurements involvingthe electric motor as test object. Then the common issues, and difficulties associated with lossmeasurements are discussed, and solutions are proposed. The main cause of heat leakage whichdeteriorates the accuracy of measurements, and methods to account for these losses is described.Then the actual measurement results by calorimetric loss measurement method performed on a testmotor at different load conditions are presented and compared with the losses measured by electricaldirect input-output power measurement method.

Overview of calorimetric loss measurement

The calorimetric loss measurement technique has been in use for loss measurements for many years.The initial usage can be found for chemical engineering processes. In recent years, its applicationshave been reported for measuring losses on electrical machines and power electronic systems. Agood overview about earlier calorimeters and subsequent attempts for measuring losses in electricmachine can be found in [14]. The calorimeters can be classified into different categories based onthe principle of their operation and cooling medium used. Direct calorimeters measure the heatproduced by the test object directly, whereas indirect calorimeters uses an auxiliary resistor andcreate a similar heat generation. Thus indirect calorimeters may perform two tests-first using the testobject for heat generation and second for mimicking the similar thermal condition using a resistanceheating element. Double chamber calorimeters are also reported in literature- one chamber forhousing the test object while a second chamber is used for heating the resistor element. The coolingmedium also differs, open type generally uses air as heat transfer medium. When liquid or gascoolants are used, the calorimeter also consists of a heat exchanger to take out the heat from coolant.The reference [14] provides an overview of different calorimeter types and also presents advantagesand dis-advantages of the same. The accuracy of calorimetric system is of prime importance to justifyinvestments in such a complex system and the time required for testing.

Important requirements for calorimetric loss measurement

The main challenges in calorimetric loss measurements for measuring motor efficiency can bedescribed as below-

- Minimize and estimate the heat loss from the walls and other sealing places

- Temperature control and precise measurement

- Mounting of motor inside calorimeter and arrangement for shaft opening and sealingarrangement.

- Accurately control and measure the air (or other mass) flow rate, minimize disturbance bymotor fan

The last two requirements is very specific to the measuring motor losses. Since the heavy motor masshas to be securely mounted inside the calorimeter and the shaft has to be brought out. It is very

important to contain any heat leakage either from shaft opening or through the motor mounting boltsin such situation. This heat leakage from different sources has to be calculated and used to readjustthe measured motor losses through dissipated heat loss. The next section of the paper presentsconstruction of the calorimetric measurement system and describes novel solutions to minimize theheat leakages from both of the above mentioned points.


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Calorimetric loss measurement system

The calorimetric loss measurement system is based on an open type calorimeter. The main controlfunction during calibration phase is to attain a steady temperature rise between air inlet to outlet bycontrolling the blower fan speed. This is followed by the balance phase during which the blower fanspeed is kept constant, equal to the blower speed recorded during calibration phase and then thepower to the heating resistor is controlled until the same temperature rise is obtained.

The main assumption for power balance during calibration and balance phase is that the roomambient conditions remain unchanged; else the results will have significant errors. In past, somecalorimeters have been reported which use a preheater stage to control air inlet temperature. In ourcase, the room temperature is controlled by a separate air conditioning system, thus there is no needfor a separate preheater stage. Although utilizing a separate preheater does reduce the risks of errorsdue to room temperature variation.

Mechanical construction-

The schematic diagram of the calorimeter is shown in the Figure 1. The calorimeter chambers insidedimensions are 1000 x 1000 x 1000 mm, designed to handle power losses up to 3 kW. Thecalorimeter walls consist of 150 mm of polyurethane sandwiched in between two fiberglass plates.The box houses the test motor, heating resistor and an array of temperature sensors.

Figure 1: A schematic of the calorimetric loss measurement system

The motor is supported on a metal plate which is bolted to the metal table structure on which thecalorimeter is placed. Heating resistor is mounted on the aluminum plate and the PT100 sensorelements are supported on the ceiling. There are 6 sensors on top level and 6 sensors at middle ofthe chamber. The chamber has openings for motor shaft, inlet and outlet air pipes. The calorimetercan be opened from two sides for easy mounting and dismounting of the test motor. The inlet andoutlet air pipes houses four PT100 temperature sensors for accurately measuring the air temperature.

Motor and chamber mounting

The schematic of motor mounting is shown in Figure 2(b). A 5 mm thick aluminum baseplate is usedbelow the chamber and inside on the chamber floor. The aluminum plates help in reducing risks ofdamage to the calorimeter chamber due to uneven weights and uneven pressures by mounting bolts.The heating resistor is secured to the inside aluminum plate. The motor is mounted on a 20 mm thicksteel plate placed on top of aluminum plate using normal steel bolts as shown in Figure 2(b). Themounting arrangement for the steel plate is critical since any mismatch will result into shaftmisalignment causing extra stress on the shaft assembly.


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The motor shaft assembly

The mechanical assembly of the calorimeter should minimize the heat leakage from the box. Thepossible leakage points are the motor shaft hole and the chamber (motor) mountings which must bedesigned carefully to minimize any heat leakage. Motor shaft assembly consists of a long shaftsupported by two elevated bearings, one inside the calorimeter and one outside the calorimeter. Thisshaft is coupled to the motor shaft through a flexible backlash free compact elastomer coupling which

minimizes any heat conduction through the motor shaft to connecting shaft. The other end of the shaftis connected to torque transducer through a flexible bellow coupling. The load machine is connectedto the torque transducer at the other end.

To avoid any heat leakage through the shaft hole, special rubber sealing are inserted in the shaft holeas shown in Figure 2(a). The calorimeters reported in literature, as in [12], mount the test motor firstand then place the calorimeter on top of it, which causes large unsealed portions around the shaftopening and thus are major source of heat leakage. The steel plate is secured to the mounting tableusing four fiber glass bolts as shown in Figure 2. The holes are reinforced with fiber glass tube toavoid any heat leakage and hard fiber bolts are used instead of metal bolts. The use of fiber glassbolts as described above also minimizes any heat flow through motor baseplate.

The shaft height is maintained such that 160 frame size motor can be coupled directly to theinterconnecting shaft. Any lower frame motor can also be connected by using additional baseplate,but a larger than 160 frame size motor cannot be tested on existing setup.

Figure 2: (a) Details of shaft ho le, (b) Chamber suppor t arrangement

Air inlet and outlet pipes

The air enters the calorimeter chamber from the top on one side and leaves from the bottom on theopposite side. The blower fan is connected at the outlet pipe which draws the air from inside thechamber. The blower fan is controlled by a separate VSD which runs at the reference speeddetermined by the control system. Four temperature sensors are mounted on the inlet and outletpipes to measure the air temperature. The long inlet pipe ensures that the air is thoroughly mixedbefore entering the chamber.

Electrical cable connections and motor control

The electrical wiring schematic is shown in Figure 3. The load machine is connected to a variablespeed drive (ABB ACS800 drive) which is configured in torque control mode. The drive controls theload motor to apply a constant torque to the motor shaft. The test object (motor) can be connecteddirectly to the supply mains for performing loss measurements under sinusoidal supply conditions oralternately it can be connected to a test drive (ABB ACS850) which can rotate the test motor atvariable speeds. Two power meters are utilized to measure test converter input and output powersrespectively. The current sensors are used to provide current interface to the power meters whereasthe voltage inputs are directly connected. A torque sensor and associated acquisition system provides


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measurement of the mechanical speed and torque at the motor shaft. Both test and load motorconverters can be controlled from the central control system.

Torque, Speed

Acquisition Precision poweranalyzer

L1 L3L2

U

Test

Converter

LoadConverter

Precision poweranalyzer

Current

Sensors

L1

L3

L2

IU

Supply

mains

Supply

mains

I

Current

Sensors

Calorimeter

Load machine

Test Object

DC power

supply

Heating

Resistor

Figure 3: Electrical wir ing schematic

Communication interface

The overall instrumentation system is as shown in Figure 4. All the peripheral instruments, as well aspower converters are connected to standard Ethernet network. 4 wire PT 100 elements are used to

measure temperature at various places of interest like, various points inside calorimeter, air inlet-outlet pipe, motor housing and motor winding temperatures and ambient temperature. Thetemperature sensing elements are connected to temperature data acquisition instrument, which sendthe temperature data to main ‘data acquisition and control computer’ over Ethernet network. Blowerfan is driven by a separate variable speed drive (ABB ACS355). The drive receives the commandsignals from main data acquisition and control system over Ethernet network and also sends the drivedata for control and monitoring purposes. The test motor drive also configured to communicate withthe main control system computer over Ethernet. The load motor drive receives the command interms of analog signals from test motor drive.


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Figure 4: Communication i nterface between dif ferent i nstruments and control computer

The power meters used in the calorimeter system are Precision power analyzers [23], which areconnected to Ethernet network. The speed and torque signals from torque transducer are fed to oneof the power meter as analog and digital signals respectively. The DC power supply which is used toprovide power to heating resistor elements is also connected to Ethernet network and receivescommands from main computer through Ethernet network.

Control system

The control system is designed on LabVIEW platform which runs on a control and data loggingcomputer. The LabVIEW comes with basic library function to establish interface with wide variety ofsensing, data acquisition and monitoring equipments and instruments. The interface to temperaturedata logger, DC power supply, power analyzer is designed using these standard functions. Theinterface with the test, load and blower fan drive is build using standard Modbus over Ethernet IP

protocol. The main tasks of the control system are described as below-

- Data acquisition from peripheral sensors, instruments drives.

- Data logging at specified interval

- PID control for controlling speed of blower during calibration phase

- PID control for controlling heating resistor power during balance phase

- Basic protection functions for chamber, motor over temperature and motor over speed

Experimental tests on an induction motor

The calorimetric measurement system is used to measure the losses of the test induction motor at

different load conditions. The induction motor used in the experimental study is a 15 kW, IE2efficiency class (400 V, 50 Hz, 1500 rpm) induction motor. The motor is tested for efficiency undersinusoidal supply conditions. The motor efficiency is measured as per test methods specified inIEC60034-2-1. First the heat run test performed at different loading conditions and the efficiency ismeasured as per direct input-output method. This is followed by indirect efficiency measurement asdescribed in method 2- summation of losses, residual losses estimated from stray losses.

Electrical efficiency measurement

Table 1 summarizes the results from direct and indirect efficiency measurement tests with sinusoidalsupply. As per IEC 60034-2-1, the motor efficiency from indirect loss measurement can be expressed


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in two ways- first as function of input power (P in) and losses (PL) and secondly as a function of losses(PL) and output power (Pout). The efficiency calculation using both methods is shown in Table 1.This isfollowed by calorimetric loss measurements at rated load point.

Table 1: Summary of motor efficiency measurement using electrical methods

Load Torque [%] 115 100 75 50 25

Efficiency (%)

Indirect method (Pin - PL)/Pin 90.5 91.2 91.9 91.6 88.1

Pout/(Pout + PL) 90.4 91.2 91.9 91.6 88.0

Direct input-output power measurement 90.2 90.2 90.9 91.6 91.2

Figure 5: Efficiency of induction motor measured from standard measurement methods

Calorimetric loss measurement tests

The motor is started by directly connecting to line supply and nominal load is applied from the loadmotor. The reference temperature gradient of 30

0 C is set between inlet and outlet air. The calibration

phase is continued until the air blower fan speed is stabilized at a constant value. The room ambientconditions are maintained constant during the test. Since the test object is iron mass, it takes longtime to bring it to a constant temperature. In order to speed up the process, the power is also fed tothe heating resistor during the initial time to rapidly increase the chamber and motor temperature. Theelectrical power to the motor during calibration phase is also logged using the data logging system.The P L* in Table 2 denotes the motor loss (1598.2 W) obtained by subtracting output power P m measured by torque transducer from electrical input power P e_in during calibration phase.

The balance phase is carried out right after calibration phase. As the motor is already at thermalequilibrium, it helps to speed up the balance test. The DC power is supplied to heating resistor andthe blower fan is set to run at the speed recorded during the calibration phase. A PID controllercontrols the power to the heating resistor in such a manner that the temperature rise between inletand outlet is equal to the temperature gradient during calibration phase. The recorded power P res atthe thermal equilibrium is be equal to 1651 watts as show in Table 2.


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Table 2: results summary of calorimetric loss measurement at rated load

Calibrationphase

Balance phase 1(motor standstill)

Balance phase 2(motor running at nominal speed)

Temp Diff, ∆T C 30.0 30.0 30.0

Blower speed, N blower RPM 3199.6 3198.1 3198.1

Blower freq Hz 53.30 53.30 53.30

P L* (P e_in-P m) W 1598.2

Heating res. Current, I res. A - 17.8 17.1

Heating res. Voltage, V res. V - 92.9 89.7

Heating res. Power, P res. W - 1651 1534.8

Air circulation inside calorimeter during calibration and balance phase

There is one fundamental difference between the air circulation inside calorimetric chamber duringcalibration phase and balance phase. During calibration phase, there is very good air circulationbecause of the test motor fan inside the calorimetric chamber. But during balance phase, there is noair circulation inside the calorimetric chamber. This results in the large temperature gradient inside thechamber.

Figure 6: Temperature profile at various points during balance phase (a) outlet pipe, (b) input

pipe, (c) motor, (d) heating resistor power, (e) motor side temperature inside calorimeter, (f)

air blower speed

This is shown in Figure 6 (e), the motor side temperature sensors show around 12 degrees differencebetween the top and middle temperature sensor. This even creates a temperature gradient inside


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motor body which is evident from Figure 6(c). The temperature sensors mounted inside motor atdifferent locations show different temperatures. One option to create similar air circulation duringbalance phase is to rotate test motor with the help of load motor in the same direction. The load motordrive is now configured in speed control mode and controls the speed of the motor equal to the speedduring calibration phase. The temperature gradient diminishes as the test motor is started (at 18:00 hras shown in Figure 6). The balance phase after this point is referred as “balance phase 2”. Theheating resistor power during both balance test phases was found to be equal to 1651 W and 1552

W, the different between these two readings is ~100 W.

The main reason for this variation is that the thermal equilibrium (and air circulation) under bothbalance phase conditions is very different to each other. When the test motor is driven from loadmachine, the friction losses in the test motor are supplied by load machine through motor shaft. Thiscauses reduction in the power in the heating resistor. Thus the friction losses supplied by loadmachine plus heating resistor power shall be considered as total losses during balance phase 2. Theshaft power recorded by torque transducer is equal to 107 watts in balance phase 2. As mentionedabove, the difference in the heating resistor power during balance phase 1 and balance phase 2 is~100 W. Thus it can be concluded that when the test motor is rotated using the load drive to avoidtemperature gradients, part of the losses (which are mainly friction and windage losses inside testmotor) are supplied by load motor. This power should be considered together with heating resistorpower is determine total power loss inside calorimeter during balance phase.

Still the mechanical power measured by torque transducer (as described above) can be very

erroneous as the torque value in above case is a few percent of the rated torque for the transducer.This can lead to significant errors in torque or power measurements under this condition.

Determination of friction and windage loss

It is possible to estimate the power input through shaft during the balance test 2 using the calorimetricloss measurement system. A separate set of calibration and balance test are performed for this asdescribed below

- During the calibration phase, the test motor is rotated using load machine but no power is fedto the heating resistor. During this phase, the losses inside the calorimetric chamber are onlyfriction and windage losses inside test motor. The temperature rise and blower fan speed isrecorded when the thermal equilibrium is reached. It should be noted that in this phase, sincethe power losses inside the chamber are very small, the temperature gradient is set to very

low value, to achieve reasonable blower speed. Otherwise the blower fan may run at very lowspeed. For better accuracy, the calibration phase is first performed with temperature gradientsetting of 4

0C and then again with 2.7

0C to obtain two different blower fan speeds for thermal

equilibrium. The recoded blower speed at thermal equilibrium is both cases were 1440 rpmand 2170 rpm respectively.

- This is followed by a balance phase in which the temperature gradient and blower speedobtained from calibration phase are set as reference and the DC power to heating resistorelements is controlled to obtain desired temperature gradient. The resulting heating power,blower speed and temperature gradient for two independent temperature gradients of 4

0C

and 2.70C (i.e. the friction losses) are equal to 94 watts and 113 watts. Again this closely

matches with the 100 W power difference recorded during balance phase 1 and 2 describedin previous section.

The main conclusions from above tests are summarized as below

- The test motor can be rotated using load machine during the balance test to maintain thehomogeneous temperature distribution inside calorimeter. This avoids large temperaturegradients which otherwise can be generated inside the box because of no air circulationduring balance phase.

- In this condition, the total power loss in the balance test is the sum of power input to heatingresistor and motor friction and windage losses which are fed by the load motor through motorshaft.


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- This part of loss i.e. motor friction and windage loss, fed by load motor can be measured fromtorque transducer or a separate set of calibration and balance test can be performed. Theresults from this balance test closely match with the power recorded by torque transducer.

Summary and comparison of motor efficiency measurement at rated power

Table 3 shows the summary of the determined efficiency using electrical and calorimetric lossmeasurements at rated load under sinusoidal supply conditions. The motor efficiency in case ofcalorimetric loss measurement is determined assuming the electrical input power during calibrationphase. It can be seen that the results are in very close agreement using both type of balance tests.The real advantages with Balance test 2 is that the possibility of large temperature gradients insidecalorimeter is eliminated by utilizing motor fan for air circulation.

Table 3: Summary of efficiency measurements at nominal operating condition

Test Input power, [W] Measured loss, [W] Motor efficiency [%]

Electricalmeasurements(Calibration phase)

16715 1598.2 90.4

Balance phase 1(test motor standstill)

--- 1651 90.1

Balance phase 2

(test motor rotated atnominal speed)

---- 1647.8(1534.8 heating

resistor + 113 frictionloss from load motor)

90.1

Separate fans for air circulation

An alternate way to maintain good air circulation inside the calorimeter chamber is using separatefans mounted at suitable location on the calorimeter chamber ceiling, as shown in Figure 7. In suchsituation, power fed to the air circulation fans also result in additional losses inside chamber and thusshall be subtracted from losses measured in balance phase.

A balance test is performed wherein the air circulation fans were used for air circulation both duringcalibration and balance phase. The motor is loaded with nominal torque and speed as before. In thissituation, the power measured by calorimeter is approximately 1670 watts, thus measuringapproximately 20 watts more than the previous results shown in Table 3. The power to the auxiliaryfans is measured using power meter during the test, which was equal to 22 watts. This additional 22

watts gets reflected in the total power loss measured during balance test.

Motor

Aluminium plate

Heating

resistor

Steel

plate Air circulation

fans

Motor sidetemp.

sensors

heater sidetemp.

sensors

Middle temp.

sensors

Figure 7: Use of separate fans on calorimeter ceiling fo r air c irculation inside the calorimeter

The difference in temperature between upper and lower temperature sensors during balance testswith use of auxiliary fans is compared with “balance test 1” described in pervious section. Figure 8


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shows the temperature gradient between top and bottom temperature sensors at three differentlocations shown in Figure 7; "motor side" refers to the temperature sensors towards motor side,"heater side" towards heater side, and "middle" is in between heater and motor. The reduction in thetemperature gradient is clearly evident. The largest reduction in temperature gradient is obtaineddirectly under the circulating fan (for ex. Heater side temperature difference is reduced from 22.3

0C to

1.80C. On the other hand, the reduction in temperature is only 6.1

0 (from 17.5

0C to 11.4

0C) in the

central part. Thus placement of the circulation fans is very important. It is recommended that more

number of small fans shall be used rather than using one big fan.

Figure 8: Effect of air circulation fans on temperature gradient inside the calorimeter at three

locations

Motor efficiency measurement under VSD supply

The similar procedure is followed to measure the motor efficiency under VSD supply conditions. Thissection summarizes the results obtained from direct input-output power measurement and calorimetric

loss measurement for different four load conditions.

Calibration phase

First calibration test is performed for four different load conditions at rated speed with suitabletemperature gradient. The results are summarized in Table 4. A higher value of temperature gradient“ ∆T ” is required for nominal load conditions to handle higher loss inside calorimeter and it is graduallydecreased for partial load conditions. Thus the motor ambient temperature for all load conditions isnot similar. The blower speed N blower at thermal equilibrium is summarized in Table 4 for different loadconditions. The electrical measurements like motor input power P e_in, output (mechanical) power Pout,and so the motor loss P Loss, motor is also recorded during the test so that the motor efficiency as perdirect input-output methods is known instantaneously.

Balance phase 1

The balance phase 1 is performed right after calibration phase and the test motor is rotated using loadmotor in this phase to maintain the air circulation. The temperature gradient “ ∆T ” and blower speedsNblower obtained in calibration phase are used as input variables and the calorimeter is allowed tosettle at thermal equilibrium by controlling the DC power fed to heating resistor elements. The heatingresistor power Pres. at thermal equilibrium condition is taken as the heat loss inside the calorimeter. Asthe motor is rotated using load motor, the mechanical power from torque transducer is also recordedwhich indicates the friction and windage losses for the test motor. Instead the second approach (asdescribed in previous section) is followed to estimate this power flow into the calorimeter throughmotor shaft (i.e. the friction and windage loss supplied by load motor) by performing a separate set ofcalibration and balance phase experiment. During the calibration phase, the tests motor is driven by


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load motor and thermal equilibrium is established. As the friction and windage loss is a smallpercentage of the total loss handling capacity of the calorimeter, the errors in the measurement areanticipated. To increase the measurement accuracy, a constant power of 500 W is fed to heatingresistor as a bias power. The thermal equilibrium at temperature gradient of 5

0C is established at

1800 rpm. The blower speed value obtained at thermal equilibrium is used as reference value duringbalance phase. The corresponding DC resistor power was found to be equal to 613 W under thermalequilibrium. Thus the friction and windage loss power supplied from load motor is equal to the 113 W

(500 W being the bias power). This is used as mechanical power input “P mech” during calculation oftotal losses during balance phase 1.

Balance phase 2

A second set of balance tests is performed in which the air circulation inside calorimeter is maintainedusing separate fans. The DC power fed to these auxiliary fans “Pfan” is also measured with powermeters, and kept constant around 22 W. The resulting heating resistor power “Pres.” at thermalequilibrium for blower speed “Nblower ” and temperature gradient “ ∆T ” values obtained from calibrationphase for different load conditions is summarized in Table 4.

Table 4: Summary of calorimetric loss measurement tests with PWM supply conditions

Load Torque [%] 100 75 50 25

Calibration phase Pe in kW 16.8 12.6 8.6 4.3

Pout kW 15.0 11.4 7.8 3.7

PLoss, motor W 1837.3 1156.0 812.4 608.7

∆T 0C 20.0 9.0 5.0 5.0

Nblower RPM 1447.7 2008.4 2516.9 1628.0

Balance Phase 1 ∆T 0C 20.0 9.0 5.0 5.0

Nblower RPM 1444.9 2006.9 2507.9 1614.0Pres. W 1729.9 1069.6 691.0 434.7

Pmech W 113.0 113.0 113.0 113.0

Ptotal_1 W 1842.9 1182.6 804.0 547.7

Balance Phase 2 ∆T C 20.0 9.0 5.0 5.0

Nblower RPM 1445.8 2010.3 2501.8 1614.0

P res. W 1793.3 1163.4 804.4 515.7Pfan W 22.0 22.1 22.1 22.3

Ptotal_2 W 1815.3 1185.5 826.5 538.0

Motor loss

Direct input-output PLoss, motor W 1837.3 1156.0 812.4 608.7

Calorimetric- Balance phase 1 Ptotal_1 W 1842.9 1182.6 804.0 547.7

Calorimetric- Balance phase 2 Ptotal_2 W 1815.3 1185.5 826.5 538.0

Efficiency

Direct input-output % 89.1 90.8 90.6 85.9

Calorimetric- Balance phase 1 % 89.0 90.6 90.7 87.3

Calorimetric- Balance phase 2 % 89.2 90.6 90.4 87.5

The summary of motor loss and motor efficiency values determined based on above calibration andbalance phase tests are also given in Table 4. The results obtained with direct input-output andcalorimetric loss measurement system are in close agreement with each other. The only exception isat 25% load torque where the electrical measurements deviate from calorimetric loss measurementsby approximately 50 W. The power loss inside calorimeter in this situation is small, thus small errors inloss measurement leads to large variation in efficiency value.


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Estimation of heat leakage from the calorimeter

The accuracy of calorimetric loss measurement greatly depends upon the minimizing any heatleakage from the calorimeter. The main source of heat leakage is through the motor shaft andmounting bolts along with the heat dissipated through the calorimeter surface. It should be noted thatthe accuracy of calorimetric loss measurement is independent of actual value of heat leakage. But it isdependent upon the difference between heat leakage during calibration and balance phase. The heat

leakage through the above three locations is determined as described below

Motor shaft

The heat flow through the motor shaft occurs due to different surface temperature inside and outsidethe calorimeter. Two temperature sensors are mounted at these locations and used to monitor thetemperatures during the tests. Then the actual heat conduction through motor shaft is determinedbased upon the temperature gradient, cross section area, and length of shaft between thetemperature sensors as

shaft

shaft

T p kA

x

(1)

Whereshaft

p is heat conduction through shaft, k is the thermal conductivity (Wm –1K –1), A is the area of

cross section (m2),shaft

T is the temperature difference (K), and x is the distance of heat flow (m) [12].

For steel shaft, the diameter is 42 mm, the thermal conductivity of the steel is 50 (Wm –1

K –1

) [20], andthe distance between the inner and outer temperature sensor 150 mm (insulation width). Thecorresponding values of heat flow through shaft are given in Table 5 for both calibration and balancephase (the analysis is only performed for balance phase 1 described in previous section). It should benoted that the temperature gradient across the shaft is a negative value for 20%, 50% and 75% loadcondition. This is due to the local heat produced by the supporting bearing on the interconnectingshaft. It is found that the outer bearing is producing more heat than the bearing inside calorimeter andthus the actual heat flow is in reverse direction.

Mounting bolts

The motor baseplate is secured to the foundation using four glass fiber bolts. Two temperaturesensors are mounted on top and bottom of the one of the bolts to measure the heat conductionthrough these bolts and the temperature measurements at these points are recorded duringcalorimetric tests. Since the glass fiber bolts are good resistors of heat (thermal conductivity of glassfiber is 0.04 (Wm

–1K

–1)), the heat flow through the bolts is very small. Thus the temperature recorded

by temperature sensors mounted on outer side of bolt does not show any influence from thetemperature inside calorimeter (although it is changing with respect to change in ambient roomtemperature, this is indicated by ∆ Bolt_ambient in Table 5). It can be concluded that the heat leakagethrough the mounting bolts can be neglected.

Calorimeter surface

The heat flow through calorimeter surface can be expressed as,

. .

surface surface surface surface p U S T

(2)

wheresurfaceU is heat transfer coefficient of calorimeter wall material, surfaceS is the surface area and

surfaceT is the temperature gradient between inner and outer surface of the calorimeter [12]. The

temperature of the inner surface is very difficult to measure. Even though the proper heat circulationinside the calorimeter chamber is maintained, the inner surface temperature at different points is notconstant due to local vertical temperature gradients. Therefore average of the temperature measured


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by sensors inside calorimeter is considered as inner surface temperature. The total heat leakage

leakage p from calorimeter is determined as below

leakage surface shaft p p p

(3)

The estimated value of heat dissipation according to the above described procedure is summarized inTable 5 for calibration and balance phases.

Table 5: Estimation of heat leakage through different points

Load Torque [%] 100 75 50 25

Calibration phase Temp, Shaft inside0C 55.8 46.1 41.3 39.2

Temp, Shaft outside0C 49.2 47.0 44.8 46.8

shaft T 0

C 6.6 -1.0 -3.5 -7.6

shaft p W 3.1 -0.4 -1.6 -3.5Temp, Bolt inside

0C 42.4 31.7 26.8 24.5

Temp, Bolt outside0C 20.6 21.3 21.0 19.9

Temp, inlet0

C 21.5 22.6 21.7 21.1∆ Bolt C 21.8 10.4 5.9 4.6

∆ Bolt ambient 0C 1.0 1.3 0.8 1.2

Surface

surfaceT 0

C 20.3 9.2 5.4 5.1

surface p

W 50.2 22.9 13.4 12.6

Balance phase 1 Temp, Shaft inside0C 51.6 40.6 36.5 37.4

Temp, Shaft outside0C 51.2 46.8 45.9 46.9

∆T 0C 0.4 -6.2 -9.4 -9.5

shaft p

W 0.2 -2.9 -4.4 -4.4

Temp, Bolt inside 0C

43.4 29.0 22.8 23.6

Temp, Bolt outside 0C

21.4 20.6 20.4 20.2

Temp, inlet0C 22.7 22.2 21.1 21.1

∆ Bolt 22.0 8.4 2.3 3.4

∆ Bolt ambient 0C 1.3 1.6 0.7 1.0

Surface

surfaceT 0C 20.8 7.4 3.6 4.9

surface p W 51.6 18.4 9.0 12.1

leakage p Calibration phase W 53.3 22.4 11.8 9.0 Balance phase 1 W 51.8 15.5 4.7 7.7

Difference 1.5 6.9 7.1 1.3

It can be seen from Table 5 that the biggest portion of the heat leakage is from the calorimetersurface whereas the heat leakage from motor shaft and mounting bolts is comparatively small. Asmentioned earlier, the accuracy of the calorimetric loss measurement is dependent upon thedifference between the heat leakage during calibration and balance phase. This difference is verysmall compared to the actual loss measured during the tests as shown in Table 5. Thus its addition tothe actual measured loss will not result into big variation in the efficiency values which are presentedin Table 4. This also confirms the superior construction of the calorimeter.


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Conclusions

The main purpose of the work is to validate the electrical (direct input-output) efficiencymeasurements procedures being used extensively in ABB using alternate efficiency measurementmethods like calorimetric loss measurement. A calorimetric loss measurement system is built for thispurpose and series of the tests were performed to measure the efficiency of the induction motor atdifferent load conditions. The special construction methods are followed to avoid any heat leakage

through motor shaft and motor mountings. These are the main concerns (points of heat leakage)when measurements on electric motor are performed inside calorimeter. The electrical efficiency isdetermined as per direct input-output method and this was followed by calorimetric loss measurementon rated load condition under converter supply. Because of the cooling fan of the motor, the aircirculation inside calorimeter is gets affected during calibration phase. To maintain the similarconditions as calibration phase, the test motor is rotated with the help of load machine during balancephase. The additional power flows inside calorimeter through motor shaft during this condition whichis the friction and windage losses inside test motor and this power should be accounted in the totallosses during calibration phase. A separate set of calibration and balance phase is performed tomeasure this power flow from motor shaft. The measured friction and windage loss is then consideredtogether with heat resistor power to determine the total loss under different load conditions. Theresulting loss and efficiency values are in close agreement with the direct input-output method.

The heat leakage by conduction through shaft is calculated based on measured shaft temperatureinside and outside of calorimeter. Similarly, the non-variation of temperature recorded by temperaturesensor at outer side of mounting bolt indicates that there is no heat leakage through the mountingbolt. It is seen that the estimated value of leakage loss is smaller as compared to the power lossmeasured by calorimeter and thus considering the leakage value in total loss will not change theestimated efficiencies considerably.

It is also observed that the calorimeter accuracy can be further enhanced by controlling thetemperature of the inlet air to the calorimeter. Since the calorimeter stability is based on temperaturegradient between inlet to outlet temperature, any slight variation in inlet temperature will cause thewhole motor mass inside the calorimeter either to absorb or dissipate the heat. This introduces errorsin the loss measurement. Clearly, the situation can be avoided by use of external inlet temperaturecontroller. Another source of error is the friction loss produced by the supporting bearing used forinterconnecting shaft. One of the support bearing is inside the calorimeter. The heat produced by thisbearing also contributes to the heat inside calorimeter. Thus the losses measured by calorimeter arehigher than the actual motor losses. Suitable methods to measure and compensate for these bearing

losses are the next steps towards accurate loss measurements using calorimetric method.

References

[1] IEC 60034-30, Rotating electrical machines – Part 30: Efficiency classes of single-speed,three phase, cage-induction motors, Edition 1, 2008

[2] IEC 60034-30 Ed. 2: Rotating electrical machines – Part 30: Efficiency classes (IE-code),Committee draft, 2011-xx-xx

[3] IEC 60034-2-1, Rotating electrical machines – Part 2-1: Standard methods for determininglosses and efficiency from tests (excluding machines for traction vehicles), Edition 1.0, 2007-09

[4] IEC 60034-2-3, Rotating electrical machines – Part 2-3: Specific test methods for determininglosses and efficiency of converter-fed AC induction motors, draft edition

[5] IEC 60034-31, Rotating electrical machines – Part 31: Selection of energy-efficient motorsincluding variable speed applications – Application guide, Edition 1.0, 2010-04

[6] prEN 50589-1 Procedure for determining the energy efficiency indicators or motor drivenapplications by using the extended product approach and semi analytical model, CENELEC,20xx.


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[13] Lindström, J.: “Calorimetric methods for loss measurements of small cage induction motors”.Master’s thesis, Helsinki University of Technology, 1994

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[16] Cao, W., Bradley, K.J., Ferrah, A.: “Development of a high-precision calorimeter formeasuring power loss in electrical machines”, IEEE Trans. Instrum. Meas., 2009, 58, (3), pp.570–577

[17] Cao, W., Huang, X., Fench, I.: “Design of a 300-kW calorimeter for electrical motor lossmeasurement”, IEEE Trans. Instrum. Meas.,2009, 58, (7), pp. 2365–2367

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[19] Bowman, J.K., Cascio, R.F., Sayani, M.P.,Wilson, T.G.: “A calorimetric method formeasurement of total loss in a power transformer”. PESC “91 Record. 22nd Annual IEEEPower Electronics Specialists Conf., June 1991, pp. 633–640

[20] J.P. Holman, “Heat Transfer”, McGraw-Hill publications, 7th ed., ISBN-0-07-112644-9

[21] Aldo Boglietti, Andrea Cavagnino, Marco Cossale, Alberto Tenconi, and Silvio Vaschetto,“Efficiency determination of converter-fed Induction Motors: Waiting for the IEC 60034-2-3

Standard”, IEEE Energy Conversion Congress & Exposition (ECCE), Denver, CO, USA, 15-19 September 2013

[22] Kosonen, A.; Aarniovuori, L.; Pyrhonen, J.; Niemela, M.; Backman, J., "Calorimetric conceptfor measurement of power losses up to 2 kW in electric drives," Electric Power Applications,IET , vol.7, no.6, pp.,, July 2013

[23] “IM 760301-01E” WT3000 Precision Power Analyzer User's Manual

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