A completely automatic method for testing of magnetic materials

I SA TRANSACTIONS °

ELSEVIER

ISA Transactions 34 (1995) 245-252

A completely automatic method for testing of magnetic materials V i v e k N. J a m b h e k a r ,

N.C. Ray

*

Department of Electrical Engineering, hldian Institute of Technology, Kharagpur-721 302. India

Abstract

This paper presents a completely digital method for fast and accurate magnetic testing under cyclic excitation condition. The data generation, data acquisition, data analysis and data display - all built around a PC with the aid of software packages and a bit hardware circuitry - has made the system a powerful virtual instrument dedicated for magnetic measurements. The performance is demonstrated with experimentation.

1. Introduction

The knowledge of magnetic behaviour of magnetic materials is essential for designing electrical machines, transformers and instruments. It is extremely important in industries where samples from lots used for manufacturing electrical equipment have to be tested for quality. For this purpose, the users become very much concerned with its characteristic features like magnetization curve, hysteresis loop, total loss, etc. Unfortunately, the determination of these characteristic features were sensitive from two points - the standardization of test core and the ability to sense the flux inside the core. The difficulty caused by the test core is overcome by standardization of test cores either in the form of toroidal or square (Epstein) with predefined geometry [1]. This ensures that a closed magnetic circuit and a well defined magnetic circuit condition exists. The difficulty in measuring magnetic flux inside the

* Corresponding author.

core is directly related to the difficulty in obtaining accurate magnetic flux transducers. Hall transducers can give a direct measure of the flux [2], however, its insertion in the test core changes the condition of the test core. Additionally, these transducers being very small in section do not give a correct information about the average flux in the test core unless a great number of such (thin pieces) transducers are used. The use of a search coil and an ac excitation do solve the problem if the electromotive force of the search coil is treated correctly. The E M F can be utilized to measure the flux by numerical integration technique and one does not require a direct flux sensor to be inserted in the magnetic path. This simple but efficient and flexible method was proposed by Carminati and Ferrero in 1992 and developed around L A B V I E W software package [3]. However, there was no proposal for controlling the operating condition at the user desired choice like setting of maximum operating flux density and operating frequency. The work has been extended by the present authors to develop a virtual instrument which can give total losses,

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ld N. Jambhekal, N.C. Ray / ISA Transactions 34 (1995) 245-252

246

plot hysteresis loops, normal magnetization curve, etc., at user defined frequency of excitation and magnetic induction. The user may also disconnect the excitation circuit and drive excitation current of nonsinusoidal nature of his choice. For finding the hysteresis loop, at the user defined maximum induction Bm,~×, the system goes by a step by step approach in which B,,~,× and H,,~,~ are determined for different peak to peak excitation current. The time spent by the virtual instrument in finding B ..... at different excitation values is not a waste, because these result in data for normal magnetization curve. The present work of virtual instrument with automatic test control facility is conceived around a low cost IBM X T Personal Computer. In the later sections, the theoretical basis of the method has been recalled, accompanied by the instrument architecture in brief and finally the test results are given to illustrate the efficacy of the concept.

2. M a t h e m a t i c a l

basis

The instantaneous E M F e(t) developed across a search coil placed around the test core with an average flux 4~(t) developed inside the test core is related as d

(1)

where N a is the search coil turns. The flux can be estimated as 1

q~(t) -

f'e(t) dt + 05o.

N a t,,

&0 -

l f"e(t)dt,

2N,,

,

ch( t ) = ~ho - ~ f ' e ( t )

fie(t) dt = E o = 0.

(5)

If E o is not zero, that is offset error is there due to ADC, then the data points e(t) may be corrected as

An accurate and fast method for the determination of flux ,;bo is to use alternating symmetrical flux variations. These variations can be obtained by applying an ac voltage to the excitation winding at the test core. In this condition, the

(6)

where N is the number of samples collected over one period. The error correction is thus very simple and can give a more reliable flux estimation c o m p a r e d to the analog one.

2.2. Hysterisis loop determination The main crux in the problem of the B - H loop determination is the ability to determine flux efficiently. For a particular geometrical configuration of test core with cross sectional area A and magnetic length l, the induction value B(t) is obtained as 1

B(t)

t 0•

(4)

Use of ac excitation helps to reduce the off-set error likely to occur due to the A D C used. If flux variations are symmetrical, then induced emf is also symmetric, and over one period T,

(2)

Here, t o is the time when the excitation current is zero and 4'0 is the value of magnetic flux at time

(3)

where t o and t I are the successive zero crossing points of the excitation current. The magnetic flux at any time t can be calculated as

e'(t) = e ( t ) - E o / N ,

2.1. Magnetic flux measurement

e(t) = - N a - ~ - ~ b ( t ) ,

hysteresis loop is symmetrical and 4'0 can be evaluated by

= Bo - N --fi-

f'e'(t) dt. ,,,

(7)

The corresponding magnetic field strength H(t) is evaluated as

H(t) =Nei(t)/l,

(8)

where N e is the number of turns of exciting coil,

i(t) the corresponding exciting current and l the effective magnetic length of the closed circuit test

IZ~ Jambhekar, N.C. Ray / ISA Transactions 34 (1995) 245-252

piece, Each pair of H(t) and B(t) represents a point in the dynamic B - H loop, corresponding to the excitation. It may be noted here that this dynamic loop is different from the usual static loop in the sense that the excitation current i(t) includes both magnetization as well as eddy current component [4,5].

247

II /

<,e"

I

DATA ACQUISITION AND DAC BOARD

2.3. Normal magnetization curve determination It is well known that if the vertices of different loops are joined together they form the normal magnetization curve. In the present set up, there is provision for continuously changing the excitation current step by step controlled in a preprogrammed manner. So the data sets (peak values) stored can be utilized for the purpose. This is an attractive feature of the present set up.

-

! POWE R [ AMPLIFtER t

-

SHUNT

-

+

OIL

TOROIDAL RING SPECIMEN

2.4. 7 otal loss determination

Fig. I. Schematic diagram of virtual instrmnent.

The loop generated based on e(t) and i(t) is a dynamic loop and the area of the loop is representative of the total loss, that is, sum of hysteresis and eddy current loss. This is given as w =

(9)

The line integral can be evaluated based on the N discrete points of the B - H curve by the numerical integration method [6] and the total power is given as t, = w/r,

(to)

3.1. Hardware realisation Data acquisition We have seen in our earlier discussion of the theoretical basis of the method, that only two informations, viz. the excitation current and the search coil EMF, are necessary for the computation of the B - H values and the dynamic hysteresis loop. So, we take in the data from two channels of the A D C on the PCL-207 card. This is shown schematically in Fig. 2.

where T is the period of excitation current.

3. Instrument architecture

A schematic diagram of the virtual instrument is shown in Fig. 1. The entire virtual instrument is based on an Extended Technology Personal Computer and a PCI-207 D a t a Acquisition Card. The virtual instrument realisation may be divided into two categories: (i) hardware realisation and (ii) software realisation.

Fig. 2. Data aquisition system.

I
248

T

1/JF

A NA LOG LATCH 1 ~ LE 398

1

DDRESS

ANALOG OUTPUT TO FUNCTION GENERATOR

SELECT

!

CLOCK FROM RS 232C SERIAL P O R T

ANALOG I LA~CH 2 I LF 398 J

L

OUTPUT F'ROM DAC OF PCL 207

ANALOG OUTPUT TO ANALOG MULTIPLIER

± lpF

L

Fig. 3. Voltage controlled oscillator and gain amplifier.

The data acquition is done from the A D C using software polling method. Since we are taking data from two points, the sampling time achieved per channel is about 82 ~.s. With this sampling rate we can easily carry out the tests on the test core for frequencies from 30 Hz upto 250-300 Hz without dangerously compromising m e a s u r e m e n t accuracy. The test range, however, can be extended using faster sampling rate A D C cards with Direct Memory Access capability.

A u t o m a t i c test control

The system has been designed so as to give the user a choice of frequency of the excitation current at which to carry out the tests and the maximum value of magnetic induction. These choices are made available by implementing an elaborate circuit comprising of analog latches, linear voltage to frequency converter and an analog multiplier and efficient control from the D A C of the PCL 207 card. The automatic

o V + =12V

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~l

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0

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i o V =-12V

Fig. 4. |CL 8038 function generator.

TO ADC

~

CHANNEL

TO ANALOG MULTIPLIER

V.N. Jambhekm; N.C. Ray / ISA Transactions 34 (1995) 245-252

ZIN

C--]3

XIN o

OUTPUT =

Z,

YIN o 7

XI N

10

:

240

XY -10 -

INPUT FROM FUNCTION GENERATOR OUTPUT .SINE WAVE

YIN : DC INPUT FROM ANALOG LATCH 2

X0

Y0

Z0

Fig. 5. Voltage controlled gain amplifier.

test control system works through the following steps: (i) The PCL207 card has a single D A C with an output range of 0-10 V available for controlling the test. However, we require two D A C outputs. This is achieved by using two analog latches (LF 398 sample hold ICs) which latch two analog data for the control of the test automatically. The enabling of the latches is achieved by a J - K Flip Flop with a clock generated by the RS 232 C serial communication post. The entire switching operation is software controlled. The circuit diagram for switching of channels is shown in Fig, 3. (it) Linear voltage to frequency converter: The ICL 8038 function generator 1C gives an wide range of output of variable frequency and constant amplitude. This can be used for generating the user specified frequency of excitation. The R - C values in the circuit are chosen to obtain the frequency range from 30 Hz to 250 Hz. So with a voltage input from 6 V to 12 V, the linear voltage controlled oscillator gives a frequency output from 250 Hz to 30 Hz. To ascertain that a correct frequency is chosen, a feedback is given to the amplifier through one more channel of ADC. On operating a single channel for data acquisition, a sampling time of 41 p~s is achieved. The scheme is shown in Fig. 4. (iii) Linear voltage controlled gain amplifier: The maximum value of magnetic induction can be controlled by selecting appropriate value of excitation current. However, we have no idea of test core permeability. So, we go step by step, performing tests on the core for different values of excitation current, till we find a suitable value. By carrying out tests for different values of excitation, we are not wasting the test time, because for

each excitation current level we get corresponding values of B ..... and Hm:,x which are utilized for plotting the normal magnetisation. The level of excitation current can be altered by the use of a voltage controlled gain amplifier. An ICL 8013 analog multiplier has been used as a voltage controlled gain amplifier as shown in Fig. 5. (iv) Power amplifier: Finally, :l signal of specific frequency and voltage level is applied to a commercial power amplifier STK 459 with maximum power delivering capacity of 300 Watts for driving the exciter coil of the magnetic test system.

4. Software realisation The virtual instrument software controls the test equipment at everystage and gives the computed results of the B - H loop, normal magnetization curve and total loss in the core. It gives the graphic display of all curves and finally gives the specification of the test core material. The software modules controlling the test have been mostly written in assembly language. The entire software for the virtual instrument has been written in assembly language. The data processing functions and graphic modules have to be written in a high level language. The entire software for the virtual instrument has been written in ANSI C in T u r b o C programming environment to give the software the flexibility of assembly language and versatility of a graphics package. The software package performs the following functions: (i) Data acquisition from the two channels of

V.N. Jambhekar, N.C. R a y / I S A Transactions 34 (1995) 245-252

250

A D C has to be done as fast as possible by software polling method to achieve as small sampling time as possible. So, this module has been written in assembly language to avoid wasting computing time in calling high level functions available with Turbo C. (ii) The test control module initially has to ascertain which analog latch is active high. So it takes in data from that channel. It sends a pulse through RS 232 C to J - K Flip Flop to activate the channel corresponding to frequency control. The D A C sends out its analog data to select the frequency. The input from A D C channel corresponding to output of linear voltage controlled

oscillator is checked for frequency, and DAC output to the VCO is adjusted to get user selected frequency. Then some multiplier gain is chosen by giving a D A C output to that channel. In this way the entire hardware is controlled. This again has been written in assembly language, to facilitate correct register selection and data loading of the 8251A U S A R T and DAC on PCL 207 card. (iii) The data processing modules compute the B and H values for different values of excitation current at different time. The dynamic hysterisis loop is plotted and area inside the curve is computed. The normal magnetisation curve is also

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Fig. 6. (a) Hysteresis loop; (b) normal magnetic induction; (c) excitation current and (d) search coil voltage at 60 Hz.

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K N. Jambhekar, N.C. Ray /ISA Transactions 34 (1995) 245-252 0.8 0.6g

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plotted. This m o d u l e specifications.

also gives the test core

5. E x p e r i m e n t a t i o n T h e virtual instrument d e v e l o p e d was used to carry out the total loss tests on a toroidal ring s p e c i m e n of mild steel (density = 7.8 g m / c c ) with an exciting coil o f 370 turns and a search soil of 10 turns. A w a t t m e t e r was also c o n n e c t e d with current coil carrying exciting current and pressure coil sensing the excitation voltage. T h e tests

Specification of the test material The Length of the magnetic c o r e = O / , 5 2 m The Qreo of cross section offhe con = 0.000050 s q m The f r e q u e n c y

of e x c i t a t i o n

= 60.0 Hz

The f a t a l enercjy Loss per cu. m e t r e p e r cycLe = 8 7 0 1 . 6 3 ' / c u . m - c y c t e The f a t a l losses per cu. metre per sec. = 5 2 2 0 9 3 . 2 W Q t t s / c u . m The loss in the core = 11.B5 W a f t s The m a x i m u m value of field sfrencjth

= 3725.2 A-T/m

The maximum value of macjnefic induction = 0.898 f e s t a The toss per unit weicjhf = 1 0 1 . 2 5 W a t t / kcJ.

Fig. 8. Test sheet at 60 Hz.

252

V.N. Jambllekal, N.C. Ray/1534 Transactions 34 (1995) 245 252

Specification of the test material The tength of the magnetic core

: O.Z~52m

The area of cross section of the core = 0.000050 sq.m The frequency of excitation = 100.0 Hz The total energy loss per cu.metre per cycle =7337.5 J / c u . m - c y c l e

The total losses per cu. metre per see.= 7337500 Watfs/cu.m 1"he loss in the core = 16.65 Watts The maximum value of field strength = 3992.0 A - T / m The maximum value of magnetic induction =

0.686 teslo

The Loss per unit weight = 1/,2.30 W a t t / k g

Fig. 9. Test sheet at 100 Hz.

were done at different frequencies with specific magnetic induction setting. The wattmeter readings closely agreed with the computed total loss, confirming the usability of the method. Though tests were performed at various level of magnetic induction, and frequency, the hysterisis loop, normal magnetic induction operating excitation current and search coil voltage have been shown for the frequencies 60 Hz and 100 Hz in Figs. 6 and 7. The print out test sheets for the two cases are also given in Figs. 8 and 9.

6. Conclusion

The proposed method of magnetic testing is a completely digital one, based on digital process-

ing of two signals excitation current and induced EMF, which can be sensed fairly accurately. The user can perform the test at desired magnetic induction and frequency. Current and EMF waveforms of normal magnetic curve, dynamic hysterisis loop, etc. together with the test results in the form of a test report sheet can be given very fast. Repeated tests with the same level of accuracy release the burden of experimentors which is commercially a very attractive proposition.

References [1] M.U. Reissland, Electrical Measurements Fundamentals, Concepts, Applications (Wiley, New York, 1989). [2] W.D. Cooper, Electronic h~strumentation and Measmement Techniques (Prentice-Hall, Englewood Cliffs, N J, 1978). [3] E. Carminati and A. Ferrero, "A virtual instrument for measurement of the characteristics of magnetic materials", IEEE Trans Instrumentation and Meastaements 41(6) (December 1992). [4] M.B. Stout, Basic Electrical Meastuements (Prentice-Hall, Englewood Cliffs, N J, 1981). [5] E. Frant, Measurement Analysis (McGraw-Hill, New York, 1989). [6] F.J.J. Clark and J.R. Stockton, "Principles and theory of Wattmeters operating on the basis of regularly spaced sample pairs", J. Phys. E: Scientific Instruments 15(6) (1982).