Commutation Period Research Articles

Dynamic Performance of Converter-Fed Synchronous Motors

Transfer function models of converter-fed synchronous motors have been derived, and the effects of motor parameters on the performance of the systems are discussed. The transfer function models have been derived on the basis of the average behaviors which are represented by continuous dynamic equations. The phenomena during commutation periods are taken into consideration equivalently as average voltage drops in the circuits. The effects of motor parameters such as the size of DC choke, constants of damper windings and so forth have been examined.

Optimum Design of Commutation Circuit in a Thyristor Chopper for DC Motor Control

This paper deduces the expressions for the commutating capacitor voltage and current during the commutation period of the main and auxiliary thyristor in a chopper for a dc motor control system. After a successful commutation, a certain amount of energy is ``stored'' in the circuit components, and for best component utilization this stored energy has to be minimized. The procedure of minimum stored energy in the commutation circuit has resulted in the selection of optimum values of commutating capacitors and inductors, and the values so obtained are more precisely defined than those indicated by early workers.

Newly Developed Thyristor Chopper Equipment for Electric Railcars

In the chopper control system for electric railcars, it is effective to adopt high operational frequency of the chopper for achievement of the following: 1) decrease of higher harmonic current induced in the trolley wire, 2) reduction of weight for reactors and capacitors in the traction circuit, and 3) improvement of control response. Through development of reverse-conducting thyristors with very short turn-off time and a repulsion type two-phase chopper, we finally realized production of a new standard high-frequency chopper equipment with regenerative braking for 1500-V dc railcars. Technical achievements mentioned previously were completely realized as a result of adoption of high frequency, 660 Hz, in the equipment. The newly developed standard high-frequency chopper equipment for 30 cars were delivered to the Chiyoda Line of Teito Rapid Transit Authority in Tokyo, and they have been operated satisfactorily in revenue service since March 1971. This paper also describes: 1) the chopper circuit using fast-switching reverse-conducting thyristors and series saturable reactors, 2) analysis of commutation circuit and methods of suppressing reapplied forward voltage increasing rate (dv/dt) and shortening commutation period, 3) the composition of the traction circuit including the protection system, such as protection for overvoltage at the regenerative braking, and 4) test results on the Chiyoda Line of Teito Rapid Transit Authority.

Theory of the Three Phase Salient Pole Type Generator with Bridge Rectified Output-Part I

The theoretical equations, describing the characteristics of a three phase salient pole type AC generator with asymmetrical damperwindings and bridge rectified output are derived. All resistance and rectifier drops are neglected; only the first winding MMF's and the first two permeance harmonics are considered. All winding inductances are defined in detail in App. I and are expressed in absolute and per-unit values. A set of equations is derived, defining the mutual relations between DC output voltage and current, start and duration of commutation, commutation current, constant and variable field current, damper circuit currents and instantaneous DC voltage fluctuations. By the use of two simplifying assumptions, this set is converted into explicit equations for the above values. These show that the inductance, determining the length of the commutation period is the equivalent of the average between the direct and quadrature subtransient inductances. Both a Star as well as a Delta connected stator winding is discussed. All equations can be expressed in generalized machine parameters. A numerical example is included.

Design Criteria of Commutation Circuit in a DC Chopper

Commutating capacitor voltage and current equations are derived for the commutation period of the main and auxiliary SCRs. The circuit turn-off time is computed as a function of x, where x is the ratio of the maximum load current to be commutated to the peak discharge current of the commutating capacitor. A suitable choice of x is suggested. In addition to the worst case design for L1, L2, and C, a nomograph for the selection of L1, L2, and C is given as a function of x.

Improving commutation in d.c. machines by the use of flux traps

It is suggested that sparking in a d.c. machine may be reduced by incorporating thin sheets of copper (‘flux traps’) to arrest the rapid changes of flux, at the end of the commutation period, that induce the high e.m.f.s that cause the sparking. The sheets need only be thin (typically 0.01 in) since they are intended to be effective only for the short periods (up to 10 μs) when sparking would otherwise occur. Skin effect makes thick sheets unnecessary. The thinness of the sheets eliminates the disadvantages of damper windings (substantially increased losses, and reduction of space available for active copper). Interpoles are retained. Bridge measurements on circular coils and armature coils show that the correct choice of flux traps can reduce the inductance at 20 kHz (corresponding to sparking periods) by a factor of up to 5, while leaving the inductance at 200 Hz (corresponding to commutating periods) substantially unchanged. Measurements on a test rig confirm that sparking is reduced. Measurements on a 300 kW traction motor fitted with flux traps show that, for a given width of black band, the running speed can be doubled. Losses appear to be increased by about 20%.

Artificial Commutation of Converters Through Injection Technique

The conventional converter consumes reactive power by virtue of the lagging power factor inherent in normal operation. Operation at leading power factor requires commutation in the region (α+ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> ) > π. This can be achieved through artificial commutation by reversing the valve-to-valve voltage during the commutation period. In a two-converter station the vars so generated can be supplied to a conventional converter operating in parallel on the ac side and, ideally, the station capacitive reactance can be reduced to that required for satisfactory harmonic filtering.

Analysis of the commutation curve of the d. c. machine

As the contact resistance between the brush and the commutator is nearly constant for the pulse current, the equation of commutation (1) and its general solution (3) is obtained. Figures 2, 3 and 4 represent typical commutation curves.On the inclination of the commutation curve at the commutation period x=1, calculations express that if r is smaller than unity, then the expression (7) holds and if r is equal to or greater than unity, then the expression (8), (9) and (10) hold, and they do not coincide with the expressions introduced by some authors.In case of r≥1 i.e. L≥RbT, the relation (11) which is written in many books does not hold. Sparking at the trailing edge of the segment commences when vi arrives at 2.7-4 volts. The ratio υ1 to 2 IRb are given in Figs. 5, 6 and 7.The effect of the resistance lead upon the commutation curve, is given in the figure 8 in case of r=2, a0=4, which shows that the resistance lead does not improve the commutation but injures it. Fig. 9 shows a tandem brush and Fig. 10 shows the brush having a rectangular trapezoidal contact surface with an acute trailing angle.While the linear commutation had been believed to be best, Dr. T. Ichiki pointed out that the best commutation curve had its zero inclination at x=1. The writer has proposed the best commutation curve and given the necessary commutating emf. to realize it (fig. 12).List of symbols and abbreviationsL, Coefficient of inductance of the coil2I, Brush currenti, Coil currentt, Timeυ1υ2, Voltage drops between the brush and the segmentec, Commutating emf.Rb, Contact resistance of the brushT, Commutating periodm=σII/σI, Ratio of two surface conductivities of a tandem brushγ=L/RbT= (mean reactance drop) / (brush voltage drop)γ1=γ/alpha;+ (1-m), γ2=γ1mb=1/rα, (Thickness of the leading part of a tandem brush) / (Total thickness)alpha;0= (Rc+2Rl) /RbF=∫x0σ (x) f0 (x) dx/∫10σ (x) f0 (x) dx, f=dF/dxf0 (x), Height of the brush contact surface, see the figure 11.

Resistance Commutation

Thorburn Reid considered the commutation problem in rotating machines as arising from reactance in the coils being commutated by the varying brush contact resistance. This paper extends Reid's treatment to include the effects of reactance in the coils which are not being commutated, and shows that these noncommutating coils are an important factor in commutation performance. An equivalent circuit for a simplified commutator machine is developed and used as the basis for the analysis given in Appendixes I through VI. By means of this equivalent circuit, the average brush terminal voltage is seen to approach the value of the commutating reactance voltage under severe commutating conditions. Voltage pulses near the end of the commutation period are shown to be produced by a parallel influence of the noncommutating and the commutating coils. A testing device based on the equivalent circuit is described in Appendix VI, and consists of a mechanical inverter connected so as to separate the functions of the brushes in reversing current through the commutating coils, and in passing current through the noncommutating coils. When connected to the armature of a particular machine, the testing device is used to measure the commutating reactance and the brush contact resistance under simulated operating conditions. By means of this device, a correlation between brush electrical wear and severity of commutating conditions is established, which helps to confirm the basic theory.

Nonlinear commutating reactors for rectifiers

ONE of the prime objectives in rectifier development during the past 20 years has been the decrease in frequency of arc-back. The factors involved in arc-back have received considerable attention, and it now appears that arc-backs of typical mercury arc rectifiers are caused by the following: 1. Breakdown of the mercury vapor because of inverse voltage. 2. Conditions favorable for cathode spot formation on the anode at the end of the commutation period.

Nonlinear Commutating Reactors for Rectifiers

ONE of the prime objectives in rectifier development during the past 20 years has been the decrease in frequency of arc-back. The factors involved in arc-back have received considerable attention, and it now appears that arc-backs of typical mercury arc rectifiers are caused by the following: 1. Breakdown of the mercury vapor because of inverse voltage. 2. Conditions favorable for cathode spot formation on the anode at the end of the commutation period.

Heat losses due to load current in direct-current armature windings

Designers of alternating-current machinery have long been able to make rapid estimates, by means of published curves, of the heating effect due to current displacement in slot-wound conductors. The wave-shape of the current in the windings of direct-current armatures is complex, since it is rich in harmonics. The estimation of the heating effect due to current displacement in direct-current armatures has hitherto necessitated laborious calculation, for each case on its own merits. The busy designer rarely has the time to carry out such a long process, and it is probable that in most cases guesswork is resorted to.In this paper an attempt is made to provide curves, by the use of which the heating of direct-current armatures may be estimated rapidly. It is believed that results are obtainable with fair accuracy, and with almost the same rapidity as is associated with the estimation of the loss in an alternating-current machine. It is not possible to construct one curve for use in all cases; a “standard” curve is given, which refers to a single-turn full-pitch winding, having 15 slots per pole and 45 commutator segments per pole. Two additional curves give coefficients which enable other full-pitch single-turn windings to be dealt with. The heating ratios so obtained do not differ from those obtained from individual calculations by more than 2 per cent in any practical case. Curves are also given for windings which differ from the standard winding in having (a) two turns per coil, (b) three turns per coil, (c) chording. The question of the critical height of conductor, and the question of the use of unequal heights for the two layers in a slot, are alsotouched upon.It is, perhaps, not generally realized that current displacement is present at all times in a direct-current armature winding; it is not confined to the period of commutation. A set of current-distribution curves has been calculated for one case, to show this.The author hopes to deal with the problem of the rotary convertor, and the problem of unusual winding arrangements designed to reduce heating losses, in a subsequent paper.

Contact resistance of metal electrodes

The reactive power due to phase displacement caused by controlled line commutated convertors can be avoided by the use of forced commutation whereby the line current is cut off before the voltage reverses. The basic problems of forced-commutation devices are presented by examples of wellknown circuits. Using these circuits as a basis convertors are described and analysed, whose forced commutation is assisted by a supporting capacitor. It is paralleled to the AC side during the whole commutation period. This effects a soft “cut-off” of the line current with comparatively low over-voltage or markedly lower capacitor expenditure. The reactive power loading of the supporting capacitor permits the use of smoothing capacitors. The principle is equally suitable for application in convertors during inverter operation.