A new electric hoist drive for CranesA new system of electric hoist drive has been developed ,and its success has been demonstrated in practical use. For the types of hoist for which it is particularly intended this system provides a closer approach to the ideal characteristics than has previously been available. Although the system is not intended for use in all of the important types of hoist, it is expected that further experience and additional study of details and refinements will broaden its field of application beyond that which has, to date, been established in practice.The Ideal CharacteristicsAmong the several hoisting applications for which the new system is believed to be suitable, the high-grade heavy-duty indoor cranes in steel mills and heavy machine shops may be considered typical. A study of the requirements indicates that the ideal characteristics for a crane of this type and of certain other types are principally as follows:1.The same manipulation of the controller to any position in thelowering direction should cause the motor either to deliver power for lowering an empty hook or to absorb power for the proper lowering ofa load, whichever is required by the conditions at that moment.2.Maximum hoisting speed at empty hook should be a definite value,approximately twice the speed at rated load.3.At the full-speed hoisting position of the controller the stalled torqueshould be limited to a reasonable overload value.4.At the first-speed hoisting position the speed-torque curve should berelatively flat, having a very low speed at empty hook but a substantial stalled torque.5.The maximum lowering speed at empty hook should be a definiteselected value, between 160 per cent and twice rated hoisting speeds 6.Maximum speed of lowering rated load should be substantially lessthan at empty hook, and maximum speed of lowering maximum load should be less than that of lowering rated load,7. The minimum lowering speeds, that is, the speeds at the firstlowering-speed position of the controller should be nearlyalike .irrespective of load. If readily attainable, it is preferable thatthis lowering speed be less at heavy loads than at partial loads orempty hook.8. The electric equipment must have in the lowering direction asubstantial margin, so that, when the maximum load which can belift is handled, the torque capacity available in lowering direction is ample to maintain dependable control of the load under the mostadverse conditions9. A safe and reasonably smooth retardation and stop must occur automatically in the event of failure of incoming power and other emergencies.10. The regulating means by which the sixth, and seventh requirements are met must not be capable of stalling a load which is being lowered, however great that load may be, at any lowering position of the controller.11. The electric equipment should be self-protective against abase, either of itself or of the mechanical equipment, under the condition of rapid and unrestrained movement of the controller handle.12. It is usually important that the holding brake be so controlled automatically that in regular service it is not required to supply any large part of the retardation effort.The New SystemThe ideal characteristics for the applications under approximated closely by the development of a radical improvement of the Ward-Leonard system, the principal feature of which is the addition of an exciter of unusual design embodying a cross-flux principle. By this means, a characteristic is obtained in which the voltage of this cross-flux exciter is responsive in a unique manner to the variations of magnitude and polarityof the current in the "loop" circuit, that is, the local circuit comprised by the armatures of the generator and hoist motor. At zero "loop" current the voltage of this exciter is at its maximum. At substantial increases of "loop" current, irrespective of polarity, the voltage of this exciter decreases, and at the maximum overload values of the "loop" current the voltage closely approaches zero. The voltage generated by this exciter provides the excitation of the generator field and modifies the excitation of the motor field. By means of the generator and motor- excitation characteristics thus provided, characteristics of speed versus load are obtained of the kind illustrated later in the paper.The 9th and 12th requirements listed in the foregoing for the ideal crane hoist are met respectively by a simplified arrangement of an emergency dynamic-braking resistor in the hoist-motor armature circuit and by an improvement in the control details of the magnet brake. These two features are not dependent upon the cross-flux-exciter principle .they were, however , developed as contributions to the same project , namely, the attempt to achieve the al-most perfect crane-hoist drive. Because of lack of space, these features are not described.Construction of the ExciterFigure 3 illustrates diagrammatically the principles of construction of thecross-flux exciter in its basic form. Although multipolar machines are possible,the exciters built to date are bipolar, by which is meant that the armature-winding and commutator connections are those of a normal bipolar machine. Four pole pieces are provided. In Figure 3 the two upper pole pieces constitute one“pole” insofar as relates to the generation of the output voltage. The two lower pole pieces constitute the other “pole ”.For reasons which will appear, a degree of artificial saturation is introduced in the pole pieces, for example, by notches the sides of the pole pieces as indicated in Figure 3.In one design which has been used, the diametrically opposite pole pieces p1 are duplicates ,each having a high degree of artificial saturation; the diametrically opposite pole pieces P2 are duplicates but have a much less degree of artificial saturation. For the general explanation let all pole pieces and their respective air gaps be considered duplicate.To avoid a possible misunderstanding it should be noted that in Figure 3 the positions of the brushes as shown are diagrammatic only and represent the positions of the armature slot conductors at which theyundergo commutation.Characteristics of the ExciterIn the circuit arrangement in which this exciter is used in the system, armature reaction and armature IR drop exciter tend, if not offset, to be of more than negligible effect, but they can be compensated sufficiently to make them almost negligible; consequently the explanation of the operation of the cross-flux exciter can be based upon a study of theno-load saturation curves of the respective flux paths. The upper lower part of Figure 4. If compensation usually important to offset or minimize part of Figure 4 represents the no-load saturation curve, not of the entire machine but of each of the two duplicate and practically independent flux paths, P1 and P2, respectively.H1 represents the selected field strength of the separately excitedmain-field winding which remains substantially constant throughout. When the current in the cross field is zero, each flux path of the exciter causes the generation of a voltage E0, and the total voltage generated is 2Eo plotted in the lower part of Figure 4 at zero cross-field ampere turns. Now assume a cross-field strength of H2. In flux path P1 the value of H2 is additive to H1, hence the voltage generated in this flux path is now Ea. In flux path P2 the value of H2 is subtractive from H1; hence the voltage generated in this flux path is now Eb. The total voltage generated isEa+Eb, as plotted in the lower part of Figure 4. Since saturation increasesbeyond point E0 and decreases below E0, voltage Ea+Eb is substantially less than 2Eo. At a cross-field strength of 2H2 the further increment of voltage generated in path P1 is very small, but the decrement of voltage generated in path P2 is large-in fact the voltage of path P2 reverses. The corresponding total voltage EC+Ed accordingly is much reduced. When the two magnetic circuits are duplicate, and when the numbers of turns of the respective windings are equal upon all pole pieces, the negative voltage generated by path P2 cannot become quite so great as the voltage generated by path P1; hence the o total voltage of the exciter never reaches zero. When the polarity of the cross-filed current is opposite to the foregoing, path P2 behaves as did path P1 in the foregoing and conversely. Hence the characteristic of voltage generated versuscross-field (that is "loop" circuit) current tends to repeat symmetrically about the left-hand side of the vertical axis in the lower part of Figure 4. If compensation for armature reaction is not provided, this characteristic shows a departure from symmetry about the two sides of the vertical axis. The dissymmetry caused by armature reaction may or may not be practically disadvantageous, according to the specific design and application. To date, a design of exciter having closely symmetrical characteristics about the vertical axis has not been built. In the first design the armature reaction was not compensated. In a later design of a larger exciter the armature reaction was compensated, but a dissymmetry ofcharacteristics was purposely introduced by means of unlike proportions of the respective flux paths.If a moderately greater value of separately excited field is held, represented by a higher value of H1 in Figure 4, a voltage characteristic is obtained of similar shape having higher voltages throughout, and conversely for a moderately smaller of separately excited field.Design and Arrangement of SystemThe generator is of normal design. Its main field is wound with the maximum practicable cross section of copper in order to provide the desired high no-load voltage with the least oversize of generator. Such a field winding provides a generator time constant which, in combination with the circuit arrangement and cross-flux- exciter-characteristics, limits the peak currents to values which are suitable for the electric equipment and the hoist system.The hoist motor has a nonstandard main-field winding for a variable separate excitation but in other respects is of the type which is standard for high-class crane-hoist installations.In order to provide the desired variation of the hoist-motor field, the cross-flux exciter is designed for a maximum voltage substantially less than that of the constant-voltage main-excitation bus. One terminal of the cross-flux exciter, negative as shown in Figure 5, is connected permanently to the main-excitation bus terminal of like (that is negative) polarity , Thus between the positive terminals of these two sources of excitation a variable voltage is available which is the difference of the two voltages.Because the cross-flux exciter is used for two purposes, its armature current is the difference of the generator-field and motor-field currents. Thus at zero "loop" current the exciter armature current is at its maximum value as output. At rated "loop" current the generator-field current and motor-field current are nearly equal; hence the exciter armature current is not far from zero. At maximum "loop" current, when the exciter voltage is low accordingly, most of the hoist-motor field current becomes input into the exciter which accordingly acts regeneratively. The uncorrected effect of armature IR drop would be to decrease the maximum effective voltage of the exciter by a more than negligible percentage, but also to increase the minimum voltage of the exciter by a large percentage over the desired low value. It is principally for these reasons that the exciter is compounded as has been mentioned.Speed-Torque CharacteristicsThe characteristics of the system are determined principally by the manner in which the generator voltage varies, subject, however, to the modifying effect of the IR drop of the "loop" circuit and the additional modifying effect of the motor-field variation .when the generator field is excited and controlled as described, the characteristics of generator voltage and of motor counter electromotive motive force with respect to "loop" current will resemble those of Figure 6, in which the curves of generator voltage represent not terminal voltage but voltage generated within the generator.The characteristics shown in Figure 7 and figure 8 are considered to be particularly suitable for cranes in steel plants and heavy machine shops, subject to the possible exception that the highest-speed lowering characteristics there shown may be faster than considered desirable.it is a simple matter to real just so as to reduce the speeds of the fastest lowering characteristic and to redistribute the intermediate characteristics accordingly.With the possible exception of an extraordinary combination ofconditions a crane is never called upon to lower a hook load greater than that which it is capable of picking up and hoisting. However, in the factory tests of the develop mental sample electric "live" loads were applied equivalent to hook loads substantially greater than any which could be lifted, as shown in Figure 8. At such excessive loads curves 1L show increases in speed. These, however, do not involve any tendency toward instability but are merely the result of increased IR drop of the "loop" circuit throughout the excessive overloads at which the generator voltage is at a nearly constant minimum. Throughout the entire over load range the hoist-motor excitation increases until at maximum overloads including those which are impossible in practice the motor is highly saturated, and hence in a very stable condition.ConclusionThe hoisting-machinery art includes many different application. Some of these types of hoist have relatively unexacting requirement which are met acceptably by various simple systems of electric drive. several systems of electric-hoist drive have been developed , each of which has certain specialized modifications which are particularly advantageous .for certain exacting applications but which are irrelevant or disadvantageous for certain other hoisting applications whose requirements are equally exacting. but different. The electric system described in this paper is not to be considered as preferable or even applicable for every important typeof hoist, but it is intended for use in several types of hoist in which, notwithstanding certain substantial differences of design and use, the speed-torque requirements are similar.No installations have yet been made in large heavy-duty indoor cranes such as are typical of steel plants and heavy machine shops. However, the results first demonstrated by the developmental sample and confirmed in practice by the Fontana Dam installation indicate the suitability and advantages of the system for heavy-duty indoor cranes. It seems reasonable to expect that with further experience and additional study of details and refinements the field of application will be broadened further.一种新的电动葫芦驱动起重机一个新的电动葫芦驱动器的系统已经研制成功，已经在实际使用中证明了它的成功。