I N V E S T I G A T I O N OF THE ERRORS OF S E L S Y N - T Y P E E L E C T R I C

L. P, B e i l i n , M. S. G o r o d e t s k i i , a n d V. S. K o l g a n o v

M A C H I N E S

UDC 621.314.228.001.5:531.7

When selsyns, rotating transformers, inductosyns, etc. , operating under conditions of phase shifters or trans- formers with a var iable transformation ratio, are used in measuring systems as angular or l inear d isplacement con- verter it is necessary to have exact information on their errors.

In the indica ted nonstandard connection systems the measurement accuracy is determined by the e lec t r ica l asymmetry of the machines, which is not indicated for a l l types of lo t -produced machines; the methods for de te r - mining it do not reveal asymmetry caused by higher harmonics in the emf curve [1].

A description is given below of a sufficiently s imple and re l iab le method and scheme for determining the error both of individual e lec t r ic machines in the entire range of angular and linear displacements of their moving

parts and of measuring systems in which selsyns are used in a transformer regime with a var iable transformation ra - tio or in a phase shifter regime. These schemes do not require a s tabi l ized reference vol tage source or standard

phase shifter. The indicated method is essential ly the same as the method of determining the e lec t r ica l asymmetry of selsyns, but it permits recording an arbi trar i ly large number of points of the output vol tage curves [2]. According

to the proposed method, the exci ta t ion winding of the selsyn is powered from an unstabil ized al ternating vot tage source (s and the synchronizing windings are connected to a symmetr ic load (Fig. 1). The character of the load

does not play a role, since in the case of symmetry of this load there are no violations of the dependence of the l inear voltages at the outputs of the synchronizing winding on the angle of turn of the rotor and consequently the

accuracy character is t ic of the selsyn is not distorted. The load is determined by the power of the invest igated machine.

The measuring instrument is connected to the load at points A-O so that the output vol tage

/Ion t = b, - bo. (1)

The voltages at the outputs of the synchronizing winding of the selsyn, as is known, are equal to [Jl = 0 cos (a +240~ D2= [J cos (a +120~ t33= 12T COS a, and the voltages between points O and C are

0 2 - 30 = (b2 - 53) ~ , (2) R

where R is the resistance of the l inear load; R a is the resistance of part of the l inear load (BC) between points C and O.

Equating Dout to zero, we obtain with consideration of (1) and (2)

b~-&+ ( O ~ - b a ) ~ . = 0 R

and after transformations

R a 2

T 1/ -ctg + 1

Translated from Izmer i t e l ' naya Tekhnika, No. 2, pp~ 27-29, February, 1972. Original ar t ic le submitted

April 10, 1969.

01972 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New

York, N. Y. I0011. All rights reserved. This article cannot be reproduced for any purpose whatsoever

without permission of the publisher. A copy of this article is available from the publisher for $15.00.

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TABLE 1 Type of selsyn- c onv er t er

SS 405

BS 404A

BS 404A

Type of selsyn- receiver

SS 405 BS 404A

SS 405

Maximum error, miD

• 45 +35 J:35

Maximum scatter of results, rain

15

i I0

i 30

o

-0 s

8(0,)

Fig. 1 Fig. 2

Thus by changing R a and the order of connecting the synchronizing winding of the selsyn to the load we can

obtain zero values of the output voltages for any preassigned a~gle of turn of the rotor cx within the l imits of the full

turn of the rotor.

In the measuring system (Fig. 1) used in the investigations the output vol tage is sent from the load bridge to the measuring instrument v ia a symmetr ic phase-sensi t ive ampl i f ie r - rec t i f ie r . The vol tage of the supply source

of the selsyn Lls will be its reference voltage. This construction of the scheme made it possible to do away with a s tabi l ized power source, to e l imina te the effect of even harmonics and residual stress, and to use highly sensitive

dc vol tmeters for measurements.

The angular error is measured by setting a zero reading of the vol tmeters Vcoarse and Vfin e (instead of r e - cording the min imum voltage at a given point), which increases the measurement accuracy. Since each measure-

ment of the angle of turn is taken at the instant the output vol tage across the load equals zero, the input impedance

of the phase-sensi t ive rect i f ier and vol tmeters Vcoarse and Vfin e, the resistance of the wires, and the error of the

measuring vol tmeters do not affect the measurement accuracy.

To check the method and scheme we invest igated the accuracy of a group of BS 404A and BS 405 selsyns of

extra fine quali ty and an expanded selsyn developed by the Efremov Novosibirsk Heavy Machine and Hydraulic Press

P lan t

During testing the BS 404A and BS 405 selsyns were placed on an ODG-10 opt ica l index head with a resolution of 10" and the expanded selsyn was placed on a special stand with the use of an opt ica l scale and spiral microscope

with a resolution of 1 ~ m.

The accuracy of placing the selsyns was determined by the min imum error provided by the s ix-point method.

The potent iometr ic bridge was made up of standard class 0.2 R83 resistance boxes. The resistance of each

arm of the bridge was selected on the basis of the nominal power of the selsyn being tested and amounted to 300 f~

for the BS 404A selsyns, and 700 ~ for the BS 408 and expanded selsyns. A check of the effect of the inductance

of the resistance boxes on the measurement accuracy showed that the resistance of the boxes in the ac circuit changes

p rac t i ca l ly proportional to the change of their resistance. Therefore the measurement accuracy does not decrease, and only a shift of the reference point is observed, which if necessary can be e lminated by a slight phase shift b e - tween the reference vol tage of the phase-sensi t ive rect if ier and supply vol tage of the selsyn 0 s.

The symmetr ic phase sensitive rect i f ier shown in the diagram in Fig. 1 provided a sensit ivity not worse than

1000 tl V/rain.

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The appropriate error curves were obtained as a result of the experiment . A comparison of the results ob- tained with the rating data showed that the error of the selsyns when connected in a transformer regime with a var i - able transformation ration was less than the rating error in ai t cases. Thus, for the BS 404A selsyns the error was

35' (versus ~ 45') and for the B8 405 selsyns ~ 13' (versus t 15').

When investigating the errors of the selsyns in phase shifter regimes and phase measuring systems we used a scheme (Fig. 2) which provided a measurement error of order 20'. The measuring system consisted of two pulse shapers and an e lec t ronic oscil lograph EO. The pulse-shaping circuits converted the two sinusoidal voltages 0in , and lJinz being compared into pulses at the instants the sinusoidal voltages passed through zero (pulse duration 20 /~ sec, s tabil i ty of the instant of pulse shaping 5-10 u see).

Using the indicated scheme we measured the s tat ic error of a circui t with a master selsyn and se lsyn-con- verier supplied from the same three-phase, ac system, and also the s tat ic error of a system with master stable phase- shifting circuits and selsyn-converter (the s tabi l i ty of the phase shifts of the phase-shifting circuits in the pres- ence of change of the ampl i tude of the supply voltage from 20 to 80 V and temperature fluctuations of the ambient environment from + 20 to + 55~ was 20'). We invest igated SS 405 and BS 404A selsyns of extra fine quality.

The results of measuring the s ta t ic error of the system with the master selsyn are given in Table 1.

In the second group of measurements the selsyn's signal was sent to one input Uin 1 of the scheme in Fig. 2 and the reference vol tage was sent to the other input t)in 2 via the phase-shi f t ing circuit; the selsynTs :rotor was turned

until the pulses matched comple te ly on the screen of the oscil lograph EO.

The measurements showed that in the presence of the usual fluctuations of the power line vol tage (• 10% with respect to ampl i tude and • 1 ~ with respect to phase) the error of the three-phase phase shifter can reach 5-6 ~ this error being a function of the angle of turn of the selsyn's rotor. In addition, it was found that the error depends

also on the type of selsyn.

If the measuring system is made up of two ident ica l selsyns, its error corresponds to the error of the individual

selsyns (see Table 1). In this case fluctuations of the supply vol tage cause ident ica l changes of the output voltages of the setsyns, which are mutual ly compensated. In the case of different types of selsyns the error of the measuring

system is greater than the error of individual selsyns, since there is only par t ia l compensation of the error caused by

fluctuations of the supply voltage. When the phase-shift ing circuits are supplied by the reference vol tage of the

selsyn (via an in te rmedia te selsyn) the error of the entire measuring circui t is just as low as in the absence of this selsyn. This is explained by the fact that the error from the fluctuations of the supply vol tage is a function of the angle of turn of the selsyn's rotor. The errors of the selsyns, even those of the same type but having dissimilar an-

gular positions of the rotors, are different and in the general case are not compensated.

L I T E R A T U R E C I T E D

1. Yu. M. Pul 'er , Inductive Elect romechanical Elements of Computer and Remote Follow-Up 8ysten~ [in Russian],

Mashinostroenie, Moscow (1964). 2. H. Arbinger, Feinwerktechnik, Jg. 65, H. 5 (1961).

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