Dynamoelectric Amplifier for Power Control

Discussion and authors' closure of paper 40-7 by E. F. W. Alexanderson, M. A . Edwards, and Κ. K. Bowman, presented at the AIEE winter convention, New York, Ν. Y. January 22-26 ,1940 , and published in AIEE TRANS-ACTIONS, 1940, pages 937-9 .

P. L. Alger (General Electric Company, Schenectady, Ν. Y.): The advance of engineering in recent years has been espe­cially marked by the joint use of widely different branches of engineering knowledge in a single structure, as, for example, the combined electrical, mechanical, and ther­mal theories underlying such developments as the Magne-Blast breaker and the hydro­gen-cooled machine.

Heretofore, the experts in the communi­cation and electronic fields have seemed to live almost in a world apart from their fellow engineers who have dealt in industrial types of power equipment, very few of either branch being interested in the work of the other. Now, however, it has been shown that the circuits and concepts of the elec­tronic art can be built into power equip­ment; and that in fact there is a remarkable similarity between amplification and stabil­ity problems throughout these fields.

These papers on amplidynes, therefore, seem to me to be of historic importance. They mark for the first time the union of the talents of the electronic and industrial motor engineers, resulting in the creation of power amplification systems of the greatest variety and potential value to industry. Hereafter designers of d-c machines will take a real interest in negative-feedback power amplifiers, and communication engineers will think of d-c machines as possessing far more than the prosaic character with which they have generally been endowed in the past.

Κ. M, Sparrow and E, C Watson (nonmem-ber; both of Westinghouse Electric and Manufacturing Company, East Pittsburgh, Pa.) : The amplidyne described is very interesting. Since 1928 our company has been manufacturing a rotating regulator and are using it extensively on variable-voltage elevator and planer sets.

In Mr. Alexanderson's paper he mentions the difficulty caused by back coupling and he states that it required a rather delicate adjustment. In most industrial plants we have found it is best to avoid delicate ad­justments. The paper also states that the tendency toward regenerative oscillation is present when the amplifier is used to feed highly inductive loads such as field windings on large machines. He comments that they have always been able to obtain stable opera­tion in "one way or another". I t appears that this would tend to complicate the ap­plication as compared with a simple easy-to-maintain regulator.

Mr. Alexanderson's paper mentions that the electromechanical resonance is apt to be a real problem in certain applications of this amplifier. Does this mean that it cannot be applied to an existing drive unless new mo­tors and generators are also supplied? There are many applications where it is desired to

add a regulator without having to change the present drive.

The regulator as described in the papers should find its place in industry where there is a growing demand for all types of regula­tors.

E. F, W. Alexanderson: The question asked by Κ. M. Sparrow and E. C. Watson is very appropriate, and I am glad to have an opportunity to make the situation clear.

The delicate s tate of balance of forces which make possible a high ratio of amplifi­cation is a problem for the designer, but if the machine is once correctly designed this does not involve any complication or diffi­culty in operation. Quite the contrary, the better the design the less will the operator be aware that there ever was a problem. In most cases, it is only necessary to have a specially designed generator, whereas exist­ing motors can be used. I t is also usually possible to use an amplidyne generator as an exciter for an existing generator, which in its turn controls a motor or performs what­ever function is desired.

Now we have endowed the dynamoelectric machine with a new property, tha t of an amplifier, and the implications of this new requirement must be understood if the principle is to be successfully applied. We have also hoped that our analysis of the problem will convince those who wish to be­come interested in the practical use but find it hard to believe that a ten-kw dynamo can be controlled directly by a small radio tube.

Industrial Applications of Amplidyne Generators

Discussion and authors' closure of paper 40-23 by D. R. Shoults, M. A . Edwards, and F. E. Crever, presented at the AIEE winter conven­tion, New York, Ν. Y., January 22 -26 ,1940 , and published in AIEE TRANSACTIONS, 1940, pages 9 4 4 - 9 .

Κ. M. Sparrow and E. C. Watson (non-member; both of Westinghouse Electric and Manufacturing Company, East Pittsburgh, Pa . ) : Since 1928 our company has been ap­plying a rotating regulator to certain appli­cations. We have had many problems arising out of its application.

In many industrial plants where they have their own power station we have found that the frequency is allowed to vary. This variation in frequency is objectionable on this type of regulator especially in cases where it is used against a matching voltage supposedly supplied by a constant-poten­tial exciter such as in the paper-mill drive. Would the authors please give their ex­perience with the effects of variation of frequency? I t appears that the amplidyne would amplify the changes in frequency.

In some cases this amplidyne has been designed without interpoles, reliance being put on the brush drop to commutate the currents involved. In applications where close regulation is required and where the control voltage must reverse, the brush drop is objectionable for it causes an error in the linear response. This regulator has two sets of brushes and so it appears that

this error will be amplified. Would the authors please give their experience with the brush drop?

The paper states that in the simple cir­cuit, figure 1, the accuracy obtained is within a few per cent. From further discussions and diagrams it appears that to get better accuracy it is necessary to use a "composing amplifier". Nothing is said in the paper about the "composing amplifier". Is the simple amplidyne accurate only within a few per cent? Our experience has been that most applications requiring a regulator must have greater accuracy.

Can this regulator be applied to a con­stant-voltage circuit such as an adjustable speed motor or must it always be used on variable or modified variable voltage?

The authors are to be complimented on their work. The trend in industry is toward a greater use of regulating devices and this type of machine doubtless will find a number of applications.

R. W. Porter (General Electric Company, Schenectady, Ν. Y.) : Messrs. Shoults, Edwards, and Crever have pointed out the fact that the amplidyne generator, because of its low control-power requirement, can often be used as a regulator or closed-cycle controller without any additional ampli­fiers. Although the amplidyne character­istics differ from those of an ordinary ma­chine only in order of magnitude, this differ­ence opens up an entire new field of appli­cations which previously have been im­practical.

For example, in the scheme shown by the authors in figure 1, there are two opposing control fields. The magnetomotive force which controls the amplidyne is the differ­ence in magnetomotive force of these two fields. If the per cent regulation of the controlled quantity is to be R, the reference field must produce (l / i?)(A.T.) and the control field must go from 1/R to (1/R)-( l+ i? ) (A.T. ) , where (A.T.) is the number of ampere turns required to control the am­plidyne over the necessary output range when only the one control field is excited. Since the watts dissipated in a field winding con­taining a fixed amount of copper varies as the square of the ampere turns, the total dissipation in the two bucking fields will be approximately 2/R2 times the power re­quired for one field alone, or 4/R2 times the power required if the copper space in both fields were used in a single large control field.

An ordinary generator requires about one per cent of its rated output for excitation. If it were to be used in a differential-field scheme such as this, designed for ten per cent regulation, it would have to dissipate 400X0.01 or four times its rated output in the form of field heating. The amplidyne, however, needs only one Vioo as much power for excitation and therefore will dissipate only four per cent, instead of four times its rating in the two opposing fields.

In the differential-voltage scheme, shown by the authors in figure 2, field heating is no longer a problem, but still an amount of power approximately equal to 1/R times that required for excitation must be trans­ferred either from the primary source Β into the load, or in the reverse direction. Furthermore, if the range of control is great, the potentiometer across Β must be of low resistance, and will absorb a t least

1136 Discussions A I E E TRANSACTIONS