IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, VOL. IECI-27, NO. 3, AUGUST 1980 137

the final word size which expresses the motor drive e2 may betruncated to 8 bits, the D/A converter size.

VII. CONCLUSIONSA procedure is discussed for designing a PID controller for

any motor/gearbox system. A general purpose graph is pre-sented from which a stable set of control coefficients can beselected. A simple technique for determining the parametersof the motor is shown. The effect of the choice of the controlcoefficients on the number of bits required in the digitalcontroller is also examined.The stability curves apply to a system with proportional and

derivative control utilizing error prediction. Variations on thistype of controller can be made and a similar set of curvesgenerated. One such feasible variation could be the removal ofthe error prediction if the control law calculation time is small

relative to the sample interval. Another variation would beto include the integration coefficient in cases where high-speedfloating-point arithmetic capabilities are available and/ormultiple processors are used.

REFERENCES

[1] M. Reed and H. W. Mergler, "A Microprocessor-based controlsystem," IEEE Trans. Electron. Contr. Instrum., vol. IECI-24,pp. 253-25 7, Aug. 1977.

[2] B. C. Kuo, Analysis and Synthesis of Sampled-Data ControlSystems Englewood Cliffs, NJ: Prentice-Hall, 1963.

[3] A. B. Corripio, C. L. Smith, and P. W. Murrill, "Evaluating digitalPI and PID controller performance," Instrum. Contr. Syst., vol.46, no. 7, pp. 56-58, July 1973.

[4] R. A. Mollenkamp, C. L. Smith, and A. B. Corripio, "Designing adigital controller for fast processes," Instrum. Contr. Syst., vol.46, no. 8, pp. 47-49, Aug. 1973.

[5] R. L. Ramey, J. H. Aylor, and R. D. Williams, "A microcomputer-aided eating for the severely handicapped," Computer, vol. 12, no.1, pp. 54-61, Jan. 1979.

Motor Thermal Protection by ContinuousMonitoring of Winding Resistance

DEREK A. PAICE, SENIOR MEMBER, IEEE

Abstract-Simple practical techniques which sense ac line currentafford protection of motor windings for many applications. A morecomplete protection system is described which continuously monitorsaverage winding resistance and hence temperature by means of an easilyapplied dc injection technique. No additional motor connections arerequired.

I. SUMMARYIN numerous applications, it is desirable to protect the stator

windings of an ac motor against damage by overheating. Insome cases, this protection is achieved by schemes which de-tect when the i't passed through the motor exceeds a certainvalue. This can be done, for example, by means of a bimetalsensor which is responsive to the heating effect of the current,providing the bimetal has transient thermal characteristicsanalogous to the motor.This protection technique is effective but limited. For ex-

ample, if the motor cooling air is inadequate, a bimetal locatedoutside the motor does not detect it. In other more completeschemes, temperature sensitive elements may be embedded inthe stator winding. These techniques are more universally ef-fective since they more directly sense the motor winding tem-perature; however, they require additional wiring to the motorand may exhibit substantial thermal delays.The technique described in this paper achieves the desired

Manuscript received September 19, 1979; revised February 12, 1980.The author is with the Research and Development Center, Westing-

house Electric Corporation, Pittsburgh, PA 15235.

protection without additional motor wiring and responds rap-idly to the average temperature of the stator windings. Theconcept uses the fact that the resistance of copper increaseswith temperature and by measuring resistance the temperaturecan be determinedi 1, 121J. Resistance is measured by means ofa small dc current which is caused to flow through the motorwindings. This concept is not new; however, its applicationhas previously been difficult to implement practically. In thescheme described in this paper, a simple technique has beenused to provide the dc current. This method, which is prac-tical, and relatively low cost, uses a nonlinear resistor in serieswith the motor winding such that alternate half-cycles of cur-rent are different. Thus a small dc current is developed in themotor windings. Tests with a breadboard unit were very en-couraging in that over the normal range of loads and voltageexpected for a practical motor installation, namely no load to1.5 times full load and 220 V ±10 percent, it was possible tomeasure a temperature of 650C with an accuracy of withinabout ±1.50C.To successfully implement the concept, it was necessary to

accurately measure low values of dc signal with high amplitudeac signals present, for example, 0.5-A dc with 45-A ac and0.4-V dc with 220-V ac. Simple circuits were developed thatdo this with sufficient accuracy and stability.

II. PRINCIPLE OF OPERATIONIhe principle of operation is to cause a small dc current, typ-

ically 3 percent of the rated ac current, to flow through the

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138 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, VOL. IECI-27, NO. 3, AUGUST 1980

- Vda ] dc r 3 Phase Motor

r r rw

Fig. 1. Method of dc current injection into motor.

motor windings by means of a series asymmetric resistance inone line. Measurement of the corresponding dc current andvoltage enables the winding resistance and hence temperatureto be evaluated. The signal developed can then be used inmotor overload and thermal protection circuits.A practical form of the asymmetric resistance consists of

antiparallel diodes as shown in Fig. 1. Using available diodes,the equivalent of a mean dc voltage of about 0.4 V is injectedin series with the ac supply. This is analogous to having a bat-tery of 0.4 V in series with one line. It is noted that suitablesingle chip structures could be developed to replace the twodiodes or alternatively if solid-state ac switches are used tocontrol the motor, some deliberate asymmetry could easilybe introduced by control of the switch gating signals.

Referring to Fig. 1, it is evident that if the resistance of the3-phase source is negligible compared to the motor windingresistance, then voltage Vdc which has a large ripple contentis approximately equal to Vd,. which has much reduced rippleand thence is easier to measure. For most practical cases, Cr ismuch less than r-; however, to develop a scheme for universalapplication, it was decided to measure the motor voltage Vdc.Referring to Fig. 1 we get

Vdc = Idc [rw + rw/2]

.37W .36

'A .35..3

c- X

c I,. 33* 0

M .331-u-',

- -

- Estimated based on coppertemperature coefficient andaverage of 3 thermocouples

I 1I I I .I I25 30 35 40 45 50 55 60

Motor winding temperature -degree C(frnm avPranp nf I thormnrn inlac I

Fig. 2. Motor winding resistance as a function of temperature whenmeasured by bridge (0) and by proposed dynamic technique (0).

voltage and current. In the test the dc components were gen-erated by means of antiparallel diodes as shown in Fig. 1; dccurrent and voltage were measured by techniques describedin Section IV.For comparison Fig. 2 also shows the expected winding resis-

tance based on the known temperature coefficient of resis-tance for copper (0.004 per °C). It is observed that in thisrespect significant differences occur between resistance andtemperature. By allowing time for thermal equilibrium to bereached it was determined that the discrepancies were in partcaused by thermal lags in the thermocouple sensors and partlybecause the thermocouples do not read average temperaturefor the complete windings. These differences occur despitethe fact that three thermocouples were used in the tests, eachfitted as snugly as possibly with the stator winding and awayfrom possible cooling air drafts. These results highlight thedifficulty of using thermal sensors for motor protection andmake the technique of measunng winding resistance seem evenmore advantageous.

from which

2 Vdc

3Idc

and winding resistance and hence temperaturemined by measuring Vdc andIdc.

can be deter-

III. PRACTICAL VERIFICATIONTo determine whether the principle of operation defined in

Section II was practically true, measurements were made on a

3-phase 220-V 7.5-hp induction motor. The motor was oper-ated at various loads and- the average temperature was mea-

sured by means of thermocouples embedded in the motorwindings. The motor winding resistance was measured bymomentarily disconnecting the motor and applying a measur-

ing bridge and also by measurement of Vdc and Idc. The re-

sults are shown in Fig. 2.It is noted from Fig. 2 that there is excellent agreement be-

tween the value of resistance determined from the bridge read-ings and those obtained by dynamic measurement of motor dc

IV. MEASURING TECHNIQUES

In the proposed technique for measuring resistance andhence temperature of an ac load such as a motor or trans-former, only a small dc current is caused to flow through theac load. To successfully implement the technique requiresthe ability to measure, for example, 0.5-A dc in a 45-A ac sig-nal and 0.4-V dc in a 220-V ac signal. The measurementsmust be accurate and have good long term stability. For ex-ample, to measure temperature within ±50C requires that resis-tance be measured within ±2 percent.The techniques that were developed for doing this are illus-

trated in Fig. 3. Referring to Fig. 3 the principle of operationis quite simple, for example, a 1 :1 current transformer CT isconnected to subtract the ac component of current such thatthe current flowing through Rc (which is the sensing element,e.g., a magnetic amplifier winding) is almost ripple free, like-wise a 1 :1 voltage transformer VT is connected to subtract theac voltage such that the voltage across Rcv is substantially rip-ple free.The exact accuracy is dependent upon the transformer char-

PAICE: MOTOR THERMAL PROTECTION

CTn n Rs

dcVoltageSignal

Fig. 3. Arrangement for measuring dc voltage and current with largesuperimposed ac signals.

Current Transformer

3 Phase Motor

Fig. 4. Motor thennal protection by winding resistance monitor.

acteristics as calculated in Appendix I, however, excellent re-sults are obtained using practical materials.

V. PROTOTYPE AND SYSTEM CONCEPTTo further evaluate the feasibility of the dynamic measuring

technique, a prototype monitoring circuit was built and testedin conjunction with a 7.5-hp 220-V motor. The importantcomponents and their function are illustrated on the block dia-gram shown in Fig. 4. A detailed circuit description is not pro-vided since the circuit used to verify the overall feasibility wasnot a fully developed low cost design.An asymmetry is obtained by antiparalel diodes (type

INI 198A) and the dc current component produced is about0.65 A (note-the motor full load ac current is 23 A). The dccurrent component is measured by a magnetic amplifier suchthat an isolated low drift output of about 8-V dc is obtained.This signal is compared with a dc signal produced by an oper-ational amplifier from the voltage across the voltage trans-former and active filter. The current and voltage signals devel-oped are shown on Fig. 4 as KiIdc and KvVdc, respectively.The two signals are added algebraically and applied to a high-gain operational amplifier such that an output trip signal is

obtained when K, Vdc - KiIdc > 0, i.e., when Vdc/Idc > Ki/AK%Since Vdc/Idc is dependent upon the resistance of the motor

windings it is possible to set a value of resistance at which atrip signal is obtained, by simple adjustment of the amplifiergain constants Kg and K,. In the prototype model the signalwas used to activate a lamp and in order to prevent spuriousindication a 5-s time delay was built into the high-gain opera-tional amplifier.

VI. TEST RESULTS AND PERFORMANCEThe problem foreseen in determining the overall accuracy of

the resistance monitoring circuit was in relating its perfor-mance to the actual average winding temperature at trip. Thisproblem is partly created by measuring lags associated with thethermocouples and is also due to the practical limitation thatthermocouples read only the temperature at their point ofplacement. In summary the thermocouple results are onlyconsidered approximate. By allowing time for the thermo-couple readings to become reasonably steady the thermalmeasuring lags were minimized during testing. In contrast theresistance monitoring scheme instantaneously measures theaverage temperature of all three windings.

139

140 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, VOL. IECI-27, NO. 3, AUGUST 1980

TABLE IPERFORMANCE OF MONITOR CIRCUIT AS FUNCTION ofAC CURRENT

TripTemperature Degree C 66 65 64 65

Motor ac Current Amperes 8. 5 17 26 35. .. . _ .~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ R5 = CT Winding Resistance

Iac + dc IC

RrFig. 6. Current transformer operation.

A . 1 2

Tc XMotorVdc

C -5--

Switch No.Opened

1

CurrentSensitiveSlgnal0

Vdc

0

2 0,1 0.233 5.5 0.174 5.8 0.355

Kv vdcK t Idc

Indeterminate

27

0. 38

0.74

WhetherUnitTripsYes/ No

Yes

No

Yes

Comments(OperatesTransiently)

(No power supply)

5 5.8 0.355 0.74 YesFig. 5. Characteristics of resistance monitor for different open-circuit

lines.

The measured (by thermocouple) average temperature at themonitor trip point is given above in Table I as a function ofmotor ac load current. For these tests, the trip point was arbi-trarily set at 660C, a temperature that could easily be achievedwith a wide range of load current. It is noted that the resultsindicate excellent stability of the measuring circuits.

It was further determined at this time that variations ofsupply voltage over the range of 220 V ±10 percent made lessthan 1-percent variation in the indicated temperature at trip.

VII. OTHER PROTECTIVE FEATURES OF

RESISTANCE MONITORING CIRCUITIt is noted that the resistance monitoring circuit will, under

some conditions, detect other failures, for example, an opencircuited supply line. Functional tests were made on theprototype unit and the results are indicated in Fig. 5 above.Although not in all instances is loss of supply line indicated, itis considered that relatively simple improvements could makethis feature available.

VIII. CONCLUSIONSA means has been developed for detecting motor tempera-

ture by measuring the change in winding resistance with tem-perature. The technique may be useful in other applicationswhere it is not practical to incorporate sensing devices withinthe equipment; also, other protective functions can be achievedby the means described.

APPENDIX IPERFORMANCE OF RIPPLE REDUCING

AND MEASURING CIRCUITS FOR DETERMININGDC VOLTAGE AND CURRENT

In the text, Fig. 3 illustrates the complete measuring scheme.This appendix analyzes the detailed performance of the cur-rent and voltage transformers used to reduce the ripple presenton the dc signals.

A. Current Transformer OperationReferring to Fig. 6, the current Ic has two components,

namely, direct IcdC. and alternating Ica,. The dc componentis given simply as

Icdc = IdcRsl(Rc + Rs)In a practical circuit Rc would be made approximately equal toRs in order to transfer maximum power to the measuring cir-cuit.The ac component Ica, of the current Ic can be derived as

follows:

ICacRc = Il acRs + IiacIC )L - IaciWM [where Ijac is the accomponent of current Ii]

also

Iac = Icac + Iiac*If we assume good coupling such thatM = L, we get

IC IacRscac (Rc +RsXl +jcoL(Rc +Rs)

Thus

Icac 1ac I

ICdc Idc (1 +IcL/(Rc +Rs)Thus, providing &L/(Rc + Rs) is »1, as is readily obtainedpractically, useful filtering is achieved.

B. Voltage Transformer OperationReferring to Fig. 7 the voltage Vc has direct and alternating

components, namely Vcdc and Vcac. The dc component isgiven simply as

Vedc = VdcRcv/(Rcv + Rw)The ac component Vcac can be derived as follows:

Vcac jiM +im(Rw +jL)=Vac (Primary)Rc +

Vcac (Rcv + Rw + jwL) + im (fcoM) = Vac (Secondary).Rcu

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, VOL. IECI-27, NO. 3, AUGUST 1980 141

and

vcLV cdc Vcac

Rw = Windi ng Resistance ofTransformer

Vcac Vac 1

Vcdc Vdc (1 +jWL/RW)Thus providing &L/R w is >>» as is readily obtained prac-tically, useful filtering is achieved.

Fig. 7. Voltage transformer operation.

Thus

Vcac = 2M2 IRVVac(k)M- (Rw +j(AL))co - (1 + R wIRcv + i@LIRcv)(Rw + jwL).

Assuming good coupling such thatM = L, this reduces to

Vcac = Vac/(l + RWIRcv +jcLIRw + 2j.LIRcv).Further assuming that Rw is much less than RCU this reducesto ____________ 1

veaVaRv

_ .

(Rw + RCv) 1 +j/LIRw

AcKNOWLEDGMENT

The author is indebted to the Westinghouse Electric Corpo-ration for the opportunity to publish this paper and acknowl-edges the contributions of co-worker R. L. Ivey in the designand testing of a prototype.

REFERENCES[1 ] R. E. Seely, "A circuit for measuring the resistance of energized ac

windings," AIEE Trans. Commun. Electron., vol. 74, pp. 214-218,May 1955.

[21 C. J. Johnson, "Determination of temperature rise of energizedtransformer by the resistance change method,"IEEE Trans. PowerApp. Syst., vol. PAS-89, Feb. 1970.

[31 United States Patent 4 083 001.

An Application of Distributed Computer Controlof Railroad

ESREF ADALI

Abstract-There are three generations of railroad control: 1) earlyelectromechanical control; 2) central computer control; and 3) distrib-uted computer control. In the second-generation, tracks were dividedinto numbers of track sections and applications had been aimed towarddetecting individual track section as to whether or not there is a trainon the track and dispatching necessary information to trains at the dis-patching points.This project aims to establish a third-generation railroad control sys-

tem. The basic idea of the project is to identify every train and switch.This method eliminates the unpowerful track detecting and discretedispatching method. In order to build a railroad system in terms ofdistributed computer control, trains will be furnished with a computeron board, switches will be detected and controlled by a switch controlprocessor and a host computer will be able to communicate with trainsand switches.This paper represents the main ideas and objectives of distributed

computer-controlled railroad (DCCR), as well as the result of experi-ments that have been done. DCCR was realized on the HO scale-model

Manuscript received September 2, 1978; revised May 18, 1979.The author was with the Systems and Computer Engineering De-

partment, Case Western Reserve University, Cleveland, OH. He is nowwith the Department of Electrical Engineering, Techninical Universityof Istanbul, Istanbul, Turkey.

train and railroad as much as possible. But, nevertheless, the goal ofthis project was addressed to the actual railroad system.

INTRODUCTION

THE WORD railroad in this paper covers the interstate andlocal transportation on the railroad. On interstate railroad,

one track is usually used for both direction and more than onetrain may run at any given time, location, speed, and directionof train(s) must be known. On local railroad system, the prob-lems are slightly different since one track is dedicated for onedirection. The major problem of local railroad system is sched-uling of whole local traffic.In order to establish more reliable, more efficient, and safer

railroad, a central computer-controlled railroad system hasbeen proposed [1] - [5]. The central computer-controlledrailroad system is a second generation railroad control. Theobjective of this system is as follows. One main computer canmonitor the whole railroad, gather the information aboutspeed and location of train(s), and dispatch the necessary in-

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