CA1125854A - Traction motor current control apparatus - Google Patents

Abstract

47,287 TRACTION MOTOR CURRENT CONTROL APPARATUS

ABSTRACT OF THE DISCLOSURE

A multiple phase current control apparatus, including a plurality of interconnected chopper units, is connected to control the energization current supplied to one or more propulsion traction motors of a train vehicle operating in an environment including vehicle operation control signals such as vehicle speed control signals and the like. The operation of a two phase chopper apparatus is determined by conduction control pulses that are supplied to the respective chopper units, and which control pulses are positioned in an adjacent relationship with the ON conduction period of one chopper unit being adjacent and immediately ahead of the ON
conduction period of the next chopper unit of the chopper apparatus.

Description

The present application i~ related l;o a pre-viously f'iled and copendin~ Canadla~ pa~ent application - Serial Mo~, 303,699 whieh wa~ fi~ed on M~y 1~, 197~ b~
F~ J~ P~nes et al and is erltitlad 'ITraction ~tor Curr~nt Control Apparatus ~
BAC~ROUND OF T~E I~VENTION
~t is }cnown ~n th~ p~ior ark to c~ntrol ~he cu~rent and volta~e, and t~us the speed and torque" o~
direct current propul~ion motors for a mass transit train ~ehicle using chopper appar~tu3 includlng ~olid ~ate thyristors a~ con~rolled re~i~ier devices. An artlcle e~itled "Alte~ate Systems for ~pid rran~it Propulsion And ~lectricaï ~reaking~ th~ was published ~n ~he - ~ ~or ~rch9 19739 a~ pages 34~41 describe~ ~he opera~ion o:E such c~o~er ~ ~. ~ S 3 ~ ' 1 47, 287 apparatus.
In an effort ~o reduce the noise interference w:Lth wayside train control s:lgnalling ~yStem~, a b~o~,d b~nd ~o. l~e spectra limitation may be desired for propulsion control equipment on the train. The broad band nolse specLficatlon typically will depict the peaks of each harmonic that exists as a con-tinuous curve instead of the line spectra that really exist such that the worst case chopper noise environ-ment is thereby specified.
SUMMARY OF' TXE INVENTIO_ An improved multiple phase chopper apparatus is provided for controlling the energization of a load such as the one or more propulsion motors of a train vehicle, with a reduced noise characteristic resulting from that chopper apparatus. A train vehicle load operates in an environment including operation control signals, such as speed control signals, that require a minimum noise disturbance caused by that chopper apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a single phase prior art chopper apparatus;
Figure 2 shows a two phase prior art, chopper apparatus including two chopper units;
Figure 3 shows the equivalent circuit f'or the single phase chopper apparatus o~ Figure l;
Figure 4 shows the equivalent circuit for the two phase chopper apparatus of Figure 2;
Figure 5 shows sig~al waYeforms to illustrate the operation of the single phase chopper apparatus of Figure l;
Figure 6 shows current ~ave~orms to ilustrate the S~54 1~7, 2~7 operakion of the two phase chopper apparatus of ~igure 2; t Figure 7 shows the general waYeform of a typical rectangular pulse provided in the output of a chopper appa~
r~tus;
Figure ~ shows current wave~orms ~or the two phas~
chopper apparatus o~ Figure 2, ~or the prior art l~0~ pha~
relationship operation Figure 9 illustrate~ a chopper apparatus embodi-ment of the present in~ention;
Figure 10 ~llustrates the conduction control signal~ ~or the chopper apparatus of Figure 9 in accordance with the present invention~ ~
Figura ll illustra~es a chopper apparatus o~ the present invention connected to energi~ a common load;
Figurc 12 shows cu~rent waveforms for the two phase chopper apparatus o~:Figure 1l, when operated in : accordance with the present~inven~ion;
Figure 13 is a graph showing the enYelope of the peak nolse spectra for a`tw~ phase chopper apparatus con-trolled in a~1~0 phase relationship operation in accorfiance with Pigure ~ as oompared to a two phase chopper apparatus controlled wikh adjacent phase rela~ionship control pulses in accordan~e with Figure 12; and : Figure 14 (on the :same~ sheet as Pigure Il) shows a suitable circuit arrangement ~or providing the control pulses shown in Figure lO.
: ~ESCRIPT N N OF THE`PREFERRE~ ~MRODIMENT
In Figure 1 there is sho~n a prior a~ sîng1e phase chopper apparatu~: operati~ ~ith a DC vo1tage source lO and including a line resis~ance 12, a co~mu~ating inductor 14, a comm~ta~i~g~capacitor 16, Q commuta~ing resistor l~ a ~3~

P,5~3~
47, 287 main thyristor swltch device or controllable rect.l~ier 20, a motor reactor 22, a load 24, illustrated to inc~ude ~ mo~or armature 26 and a mo~or field 28, and ~ ~eewh~el.in~ diode 30. ~ conduction control apparatu~ 32 is opera~.iye ~o provide control pulses to determine the ON conduc~ion period of the th~ristor 20.
In Figure 2 there is shown a prior art two phase chopper apparatus operative with the DC voltage source 10 and including the line resistor I2, the commutating inductor 14, t.he commutating capacitor 16 and the commutating resistor 18. The first chopper unit includes the main thyristor or controllable rectlfier 20, the motor reactor 22 and the freewheeling diode 30 operative with the load 24, whereas the second chopper unit i~ncludes the main thyristor or controllable rectifier 21~ the~motor reactor 23 and the freèwheeling diode 31 operative with a load 25. Each of the loads 24 and 25 are 111ustrated as a motor armature and a series field winding, with the load 24 including the armature 26 and the field winding 28 and the load 25 including the armature 27 and the field winding 29. The conduction control apparatus 32 is operative to turn ON one of the chopper units and when that one chopper unit has been commutated OFF
then to control the turn ON of the second chopper unit, and when the second chopper unit has been commutated OFF to control the turn ON of the first chopper unit, with this operatlon being repeated as:desi~ed to determine the current supplied to the respective loads 12 and 13.
~n Figure 3 there is sho~n the equiva~ent circuit for the single phase chopper apparatus of Figure 1.
3 In ~igu~e 4 there is sho~n the equivalent circuit .

. ~p;J~ ~ ~ 4 47,2g7 for the t~o phase chopper apparatus of Figure. 2.
~ n Flgu.re S ~here ~re sho~n the si~n~ Y5f~X~
to illustrate the operation of the pr~or art.single pha~e chopper apparatU5 shown in ~igure 1. The ahoppe~ contro:l signal shown in curve 5A determines the ON condition o~
operation, where the main thyristor 20 is conducting lvad current, and the OFF condition Or operation~ where the main thyristor 20 i.s not conducting load current and the freewheeling ; : : diode 30 provides load current during the OFF condition of ~ 10 operation of the main thyristor 20. The resulting load : current sho~n in curve 5B passes through the load 24. The : ~ current flow through the main thyristor 20 is shown in curve 5C:and the current~suppl~ed by the line including the DC
voltage source~lO is shown by ~the curve 5D.
In Figure 6:there are shown the current waveforms to illustrate the~ o~perat~ion of the prior art two phase~:~
chopper apparatus~as~shown:in:Figure 2. The curve 6A~shows the chopper current supplied by the first thyristor 20 and : the curve 6B shows the ~current~ supplied by the second thyristor 20 device 21. The current:passing through the line filte~r, including the resis'tor 12 and the inductance 14, is shown by the curve 6C. The curve 6D shows the current supplied by . . ~
the line including the~DC~voltage~source lO. It should be~
understood that the current control apparatus 32 prov1dès~ON:
control pulses to:the respective thyristor switches 20 and~
~: ~ 21, which control puls~es are 180V out of phase such that the position of a current pulse of;~the first'thyristor~swi:te~h 20 is halfwa~ bçtween the current pulse~ provided.P~ the thyristor switch devicç 21, as~sho~n b~ ~he respecti~e curves 6A and ~: 3Q. 6B, to in e:~fec't~pull pulses o~ current from the' 1lne that : 5 ~ ' . ' , ' : :

47,287 .
, are double the frequency o~ the ~N control pulses proYided by the conduc~io~ contro~ apparatus 32 to each of the t~is~or switch devices 2q:and 2I. This creates ],lne ~ol.se in rel~t~on to the AC current flow ln the inductive branch of ~hiS pro- :
pulsion motor power supply circuit. ~hen the chopper appara~us shown in Figure 2 is connected to ener~ize the propulsion ~ ~ motors of a train vehicle operating in a mass transit system, : including train control signals in the track rails and which ~ train control signa}s have various~coded frequencies, it is ~: lO ~desired that the chopper~apparatus:shown in Figure 2 not:
interfere with t:hose traln control~signals and otherwise : potentially result~in safety problems in relation to the :
operatlon of the train~vehicle.
In the normal~operation~of a chopper apparatus, the thyristor switch~devices when~:closed switch the current ~ : into the load from the~line,:and;~when the thyristor swit:oh is~open~the load~current~is~provlded through the operatlon of~t:he freewheèling diode. :The filter, including the line:
resistor and llne~inductance, lsolates the line source from~ :
-: `20 the switching action going on inside the chopper apparatus;, but the actual line~current has a lesser peak to peak ampli~
: tude because of~the~provided~filtering action. When the ;
; : chopper apparatus~is ~applying current to a large motor~
reactor, the induct~ance in the motor fields in effect ~draws current from the line,:which~includes the filter consisting of~the~line resistance,~the~line inductance and.the commutatlng~
: re~i~stor, assumi~ng that~the voltage~sourcè is a~zero impedance sQurce~as~far as looking~t t:he ~C:characteristics are concerned. It l.s known~ln~the~prior art that ~a two` phase : .
:30 chopper, such as sho~n~in Figures 2.and l~, shquld provide ~ .
~ ; -6- . : :

:~ ~ : :.

47,287 some reduction in the line noise. The prior art te~c~es operating the two chopper un-Lts, of the t~o ph~se chQpper apparatus shown in Figure 2, 180~ out of phase such that the position of the current pulse in one chopper unit is hal~y between the current pulses of the other chopper unlt, which in effect pulls pulses of currents from the line ~ilker that are double the frequency and can create the undesired line noise in relation to the AC current flow in the inductive branch of the circuit.
If it is assumed that the chopper apparatus operates in a continuous current mode, such that if the thyristor switch is ON, it is supplying motor currentS and if the thyristor switch is OFF, then the freewheeling action will maintain the load current until the thyristor switch is turned ON again, then the equivaIent circuit arrangement shown in Figures 3 and 4 can be made.
In Figure~7 there is illustrated the typical rectangular waveform of a general rectangular pulse series having an amplitude AB, having a pul~e repetition rate of fBASE = l/T, where T is a time period between the leading edges of the current pulses and the pulse width t = T/K, where t is the width of a given current pulse and K = T/t.
A Fourier analysis can mathematically illustrate the current pulse thàt the chopper apparatus is drawing from the equivalent line filter circuit and permits calculating the amplitude ILINE of the nth harmonic which, for a propulsion motor control system normally run at 218 Hz, would be the 2Qth or 30th harmonic. The a~mpli~ude of this harmonic by using these equations is a maximum for a 50~ 0~ duty cycle, 3Q and assu~ing ~he provided typical values for the line reactors 47,287 .

in terms of inductance, ~esistance, line capacitor and filter inductance, resistance and a t~pical load cur~ent, when the impedance Or the inductive branc,h iS calculated, this determines what the line current noise in the line is by the ratio of -the two impedances times the harmonic cur~ent amplitude. The calcultion for various n's provides a plot of poin~s showing the peak noise current limit.
An = amplitude of harmonic n.
AB = amplitude of base current.

A A 2 Sin nR K = 2 A Sin nR - , for n = 1~2~ 3, 4 -- (1) As stated above, the generated noise will be a line spectra, so n will have integer values only. To calculate the amount of noise current flowing in the line, the current is calculated at each harmonic. This current is~then applied to the line filter, whose attenuation at the harmonic frequency is completed. The result is the noise spectra. Assume the follo~ing typical values Line reactor LL = 3 henry RL = .0011 ohm 1ine cap CL = .014 farad RC = 75 ohm Base current AB - 1350 amps.
The impedance of the parallel resonant circuit can be determined as (RL2 t ~2 L2~ (R 2 ~ 1/w2 c2~ 1/2 tRL ~ Rc~ ~ (WL ~ C~ ~2~
:
and the impedance of the LR branch can be determined as . . .. .

'~ ~" 7 J 5 ~
47,287 ZL = (R2L2 ~ ~2L2~ 1~2 ILIN~ ZL Qn (4~

The above mathematical calcula,tion determines the amount of noise in the current pulses flowing in the track rails.
Figure 8 shows the current waveforms ~or t,he two phase chopper of Figure 2, operated with the ON control pulses to the respective thyrlstor switch devices being supplied in a 180 phase relationship. For example, the curve 8A for a 10% ON condition of operation of the thyristor switch 20 could illustrate the current flow through the ..
thyristor switch 20, with the cur~e 8B showing the current : pulse ~low through the thyristor switch 21 for a 10~ ON duty cycle of operation. The curve 8C could show the current flow through the thyristor switch 20, with the curve 8D
.
showing the current flow through the thyristor switch 21, for a 25% ON duty cycle of the chopper apparatus in relation to energizing the propulsion motors represented by the loads 24 and 25. ~ :
In Figure 9, there is shown a two phase direct ~.' current chopper apparatus connected for controlling the : average current supplied from a direct current source 50 to -:

DC motor loads 52 and 54:, and a first chopper unit includes ; ' a s,eries mai.n thyristor switch deYice 5~ that repetitively operates in an 0N condition and an OFF condition.to chop the ::
voltage appli.ed to the load 52. ~ second chopper unit includes a series main thyristor switch device 58 that repetitively~ operatés in an ON condition and an OFF condition '~

5B5~
47,287 to chop the voltage applied to the load 5LI. The control of this voltage to earth load ls determined by changin~ the ~N
operation time in relation to the O~'F operation time o~ the conduction by the main thyristors 3 ~or controlling the ~peed B of the respective motor ~ loads 52 and 54. The speed of a DC series field traction motor is proportional to the arma-ture voltage and inversely proportional to the field current or field flux in accordance with the relationship S propor-tional to E/~. To reduce the speed of such a motor, the armature voltage is decreased, and to increase the motor speed the armature voltage is increased. The power supply for a typical traction motor application can ke a relati.vely constant direct current voltage source such as 900 volts ~rom a central power station. Each main thyristor switch 56 and 58 is gated to close and become conductive and subse-quently commutated to open and blook as required to provide an average voltage across the respective motors_52 and 54 which determines the motor speed. Each of the chopper units shown in Figure 9 operates in accordance with the disclosure of the above referenced related patent application, the disclosure of which is incorporated herein by reference.
Each of the main thyristor switches 56 and 58 requires a commutation circuit to become blocked when it is desired that the main thyristor switch no longer conduct current to the motor. A commutation circuIt for th~ main thyristor switch 56 is shawn to include a parallel connected com~utating capacitor 6Q and a commutating thyristor switch 62 operatiYe to shunt the m~tor current and stop conduction by the main thyristor switch 56. To block the main thyristor 3 switch 56 and st~op conduction ~of motor current b~v the main -10- `

r~
1,7,287 thyristor switch 56, the commutating thyrlstor 62.is g~ted by the conduction co~rol apparatus 32 ~o conduct ~d ~pplle~
a reverse bias voltage from the capaçitor 6~.across the maln thyristor switch 56 to block the thyristor switch 56.
Similarly~ the capacitor 64 and commutating thyristor s~itch 66 operate to shunt the motor load current and stop conduction of the motor current by the main thyristor switch 58. The thyristor 66 is gated by the conduction control apparatus 32 to conduct and apply a reverse bias voltage from the capacitor 10 64 across the main thyristor switch 58 to block the thyristor switch 5 8.
The conduction control apparatus 68 shown in Figure 9 provides in accordance with the present invention ON conduction control pulses to each of the main thyristor switch devices 56 and 58 and provides OFF commutation control pulses to each of the commutating thyristor switches 62 and :
66 as shown by the signal waveforms of Figure lQ. The conduction control apparatus 68 provides an ON conduction control pulse to the main thyristor switch device as shown 20 by the curve 80 to initiate the current conduction by the ~
main thyristor s~itch device 56 in accordance with the curve ::
82 showing the ON conduction time period of the thyristor switch device 56. The conduction control apparatus 68 then provides an OFF control pulse 84 to the commutating thyristor switch device 62 for terminating the ON conduction time period 82 Qf the main thyristor switch device 56. The conduçtion control apparatus provides the~ON control pulse ~:
86 to the maln thyristor switçh device 58 to initiate the current conduction by the main thyristor switch device 5~ as 3 shown by the curve~of the O~ conduction time period of the :

~2~
1l7,287 main thyristor switch deYlce 58. The conduct~ con~rol apparatus 68 then provides an OF~ cont~ol pulse ~ ~o ~he commutating thyristor switch de~ice 66 ~or ~er~ln~ting khe ON conduction time period of the main thyrls~or Swltch device 58.
In Figure 11 the two phase chopper apparatus of Figure 9 is shown connected to energize a common load, whlch can include traction propulsion motors connected as shown at page 36 in the above mentioned published article.
In Figure 12 there are shown current waveforms to illustrate the operation of the two phase chopper apparatus shown in Figure 11, when operated in accordance with the teachings of the present invention. For a 10% ON duty cycle condition of operation, the curve 12A shows the current flow through the thyristor switch device 56 and the curve 12B
shows the current flow through the thyristor switch device 58, and the curve 12C represents the resulting current supplied to the load 68. For a 25% ON duty cycle condition of operation, the curve 12D shows the current flow through 2Q the thyristor switch device 56 and the curve 12E shows the current flow through~the thyristor switch device 58, such that the curve 12F represents the load current supplied to a load device 68 by the chopper apparatus. For a 50% ON duty cycle of operation, the curve 12G shows the current flo~
through the thyristor switch device 56 and the curve 12H
shows the current flow through device 58, such that the curve 12I represents the load cu~rent supplied by the chopper apparatus. For a 75~ ON duty cycle condition of ope~ation, the curve 12J shol~s the current supplied by the thyr~stor 3a switch device 56, the curve 12K shows the curren~ supplied ~ 5~-~ 47,287 by the thyristor switch device 58, and the curve 12L sho~s the current .supplied to the load 68 k~ the ch~ppe~ appa~rat To reduce the AC nolse provided from ~h~e po~er supply line, the respective chopper units ln the chop~er apparatus are fired ON to become conductive by control pulses arranged as shown in Figure 12 which are ad~acently positioned. When supplying 1,300 amperes to energize a propulsion motor, the chopper apparatus is already typically built with multiple paths o~ components which may typically include six main thyristor switch devices. These:can be split up into multiple phase chopper units, without:adversely changing the operating stresses on each individual component~
~ith some slightly additional wiring complexity and control complexity. However, thls chopper~apparatus arrangement :~
will operate to reduce the noise and permit working with a signal rail system having longer signaI block track rails :
and longer runs between signal blocks. It is believed that prior art propulsion motor controlling chopper apparatus : will not be able to satis~y more restrictive speciflcations in relation to noise limits that are permitted to be caused by the line AC noise. This could necessitate a multiple phase chopper apparatus arrangement as shown in Figures 9 and ll and controlled in the manner set forth in Figure 12 to reduce the noise lntroduced into the signal track rails by the.propulsion motor po~er supply line. The chopper ~;
apparatus in accordance with the present inventlon pr.ovides mult~ple chopper units as sho~n in Figures 9 and ll and operated by adjacent contrQl slgnals, and can include ~

c~opper units ~here ~ is: a re~li.s~ic and practical number 3Q from .th.e: vie~point o~ cost and complexity.

~ -~ 5~ Ll 7,287 In Figure 13 a graph is ~hown providln~ the envelope of the peak nolse spectra ~or a two phase chopper appara~us controlled by each Or the pr:Lor ar~ 18~ ou~ o~
phase relationship operatlon and the adJ~cen~ phase rel~tion ship operation of the present inventi~n. The abscis$~, represents the frequency times a thousand and the ordinate represents the AC noise current in amperes, with the scale being logrithmic. A typical more restrictive specification for noise current above 5,000 cycles can be in the order of' 15 milliamps or less. The curve 13A shows ~he noise current envelope ~or the prior art 180 out of phase conduction operation as illustrated in ~igure 8. The curve 13~ shows - the noise current envelope for the present adjacent con-duction operation illustrated in Figure 12. With the prior art chopper apparatus operatlon at 5,000 cycles, the noise current is about 32 milliamps as shown by curve 13A. The present chopper apparatus operation at 5,0Q0 cycles as shown by curve 13B provides a noise current of about 15 or 16 milliamps.
As illustrated in ~igure 12, increasing the duty cycle from 10% to 251 and then to 50% shows that at a 50% ON
duty cycle a continuous current is drawn from the line, with substantially no AC line noise except ~or a small transient overlap. If the duty cycle is then increased to 75% ON, the same AC line noise is present as ~as present for a 25% ON
duty cycle. The normal operation of a propulsion motor controlling chopper apparatuS is to start at a ~ery low duty cycle and increase t~e du~y cycle as required to keeP ~he current increasin~ as it is desired for the motor speed to 3Q increàse. ~t a 25% ON dut~ cycle, the maximum ~C line noise 5 ~ 5'~
47~2~7 wlll occur because this pro~ldes the biggest amount of ripple and then at 75% ON duty cycle a maximum noise condi~
tion again oocurs. A typical operatlon ~or propul~lo~ motor chopper apparatus is at about a 50% ON duty cycle~ with mos~
of the running time being at this 50~ duty cycl~. The curves in Figure 13 show the signi~icant improvement in noise in~er~rence obtainable, with an improved noise factor o~ two or more, with the propulsion motor chopper apparatus being run with ad~ace~k control pulses as sho~m in Figure 12 In Figure 14 there i~ ~hown one suitable arrange-ment that can be:~provided within the conduction control apparatus 6~ to de~elop the conbrol pulses shown ~n Figure 10. The first ON pulse ~0 is pro~ided by a pulse source 100. This first ON pulse ~0 ~s fed into a DC flip-flop deYice 102 which changes state in its output~ That output i~ then fed into an opera~ional amplifier integrating de~ice 104 to result in a posltive going ~oltage change signal.
When the pulse ~4 is provided by the pulse source 100, the flip-flop 102 will be reset and its output goes to zero.
The integrator 104 then s~arts a negative~goin~ voltage change signal, a~d ~hen this voltage signal returns to zero the operational ampli~ier 106 triggers a one-shot device 10 to generate the OFF eontrol pulse 90. ~n this wa~ the time period between the control pulses ~O and ~ can be made to e~ual the ti~ne per~od betwaen the co~rol ~ulses ~6 a~d 90.
The pulse source 100 ie responsive to the current flowing in the:motor power equipment as well kno~ to persons ~-skilled in this artO The ti~e period between the ON control ; pulse ~0 and the OFF control pulse ~4 is establ~shed ~or -~17,287 regulating the cu~rent in the propulsion motor, such that a desired average voltage i5 provided. A known reference level is compared for adjustlng the deslred current leve~ in the motor circuit.

Claims (7)

47,287 CLAIMS:

1. Chopper apparatus having at least first and second switch means and coupled with a direct current voltage source and having an output for controlling the energization of at least one load, wherein said first switch means is connected to provide a first load current path be-tween said voltage source and said output, said second switch means is connected to provide a second load current path between said voltage source and said output, said apparatus comprising: conduction control means for pro-viding a first control pulse to cause a first ON condi-tion of operation of the first switch means to supply current to said output and after a selectable time interval for providing a second control pulse to cause the OFF condition of operation of the first switch means and to cause simultaneously a second ON condition of operation of the second switch means to supply current to said load; and means responsive to a percentage of full load required to be supplied to said load, to control a second ON time by select-ing a required time interval between said first control pulse and said second control pulse whereby, by a prede-termined sequential operation of said conduction control means to control the ON and OFF operation of said first and second switch means, a suitable percentage of full load is delivered to the load, and at the same time any electrical noise generated by said first and second switch means is minimized.

2. The chopper apparatus of claim 1, with said first ON condition of operation having a time period sub-stantially equal to the time period of said second ON condi-tion of operation.

3. The chopper apparatus of claim 1, with the time period of said first ON condition of operation being in sucession to the time period of said second ON condition of operation.

4. The chopper apparatus of claim 1, including commutation means coupled with the first switch means to terminate said first ON condition of operation, with said conduction control means providing said second control pulse to said commutation means for controlling the time period of the first ON condition of operation.

5. The chopper apparatus of claim 1, with said one load being a direct current motor and with said ener-gization comprising controlling the speed of the motor.

6. The chopper apparatus of claim 1 with said first control pulse being supplied to the first switch means for initiating the first current conduction time period and said second control pulse being supplied to the second switch means for initiating the second current conduction time period.

7. Chopper apparatus as in claim 1, having first, second and third switch means, wherein said conduction control means controls ON and OFF conditions of all the three switch means such that their conduction times are adjacent and opposite to each other without any time gaps therebe-tween.