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Improvement of the Terminal Characteristics of a Slightly Modified d.c. Machine with an Electronically
Assisted Commutaion
A. Ihaddadene, G. Quichaud
To cite this version:
A. Ihaddadene, G. Quichaud. Improvement of the Terminal Characteristics of a Slightly Modified d.c.
Machine with an Electronically Assisted Commutaion. Journal de Physique III, EDP Sciences, 1995, 5 (1), pp.59-76. �10.1051/jp3:1995110�. �jpa-00249295�
J Phys. III France 5 (1995) 59~76 JANUARY 1995, PAGE 59
Classification Physic-s Abstracts
41.90 86.90
Improvement of the Terminal Characteristics of a Slightly
Modified d.c. Machine with an Electronically Assisted Commutation
A- Ihaddadene and G- Quichaud
Laboratoire d'Electrotechnique des Universitds Paris VI et XI, Unlversitd Paris XI, Bitiment 214~
91405 Orsay, France
(Receii~ed 13 Apii/ 1994, ret>ised 28 September J994, accepted14 Oclobe; J994)
Rdsumk. Dans
un prdcddent article [[al, nous avons pr4sent4 une machine h courant continu dont le collecteur
a subi des modifications qui permettent de mieux l~adapter h l'assistance dlectronique h la commutation. L'expdrimentation a rdussi et la machine modifide a fonctionnd h
une puissance (gale h 1,4 fois la puissance nominate de la machine initiate sans aucune production d'dtincelles sons les balais. Dans le prdsent article, nous prdsentons les rdsultats obtenus.
Abstract, In [[al, we have presented a new d-c- machine arrangement which is especially designed to operate with an electronically assisted commutation. The experiment has succeeded and the modified d-c- machine operates at a power equal to 1.4 times its initial nominal power
without any spark under brushes. We present, in this paper, the obtained results.
1. Introduction
The commutating poles are usually used to assist the commutation process in d-c- machines.
But their limitations have led to research for other means to ensure a satisfactory commutation.
The idea of using an external circuit to generate the commutating emfs appeared in the thirties. In the sixties, Bates [1~4] experimented the assisted commutation by the addition of an external 60 Hz emf of a constant amplitude to those generated by the commutating poles.
Nevertheless, a complex commutator was necessary. To achieve its experiments in pulsed power devices, our Laboratory studied the electronically assisted commutator [5~8]- To improve the collector capabilities and the specific power of the machine, it suggested the
suppression of the commutating poles and their replacement by an extemally emf which must be continuously adapted to the operation. A scheme which can be used with a commutator
whose technology is purely conventional is proposed and built [9~12]. The operation has been successful for a power equal to 1.4 times the nominal power of the original machine. In the
following, we present the experimental results.
QLes Editions de Physique 1995
2. The Original Machine
The original machine which is modified to operate with an electronically assisted commutation had the following characteristics :
Stator 4 poles.
Rotor : 31 slots, a wave winding with two parallel circuits (2 a
= 2), 31 coils, two coil sides per slot.
Nominal power 8 kW (U
= 135 V, I
=
60 A).
Separated excitation : (U~~~ = 135 V, i~~~ = 1.6 A).
Nominal speed : 3 000 rpm.
3. The New Collector-Winding-Brushes-Arrangement [10]
Let us consider a conventional collector of, for example, 18 bars (Fig. I) and a wave winding having 9 sections. The nine extremities of the winding can be connected to nine bars of the collector as shown in Figure 2. The nine other unconnected bars, represented by a black
rectangle, are used to separate two successive connected bars. Four brushes Al, A2, Bl, 82
are used and placed on the machine neutral lines. To make the electronically assisted commutation possible, the brush width must be slightly smaller than the width of one bar.
By this arrangement, we have access to the two extremities of the commutating section and
we can, then, apply an extemal emf which will cancel and reverse the current in this section.
To have a satisfactory commutation (with no sparks at the brush-bar contact), we must ensure
that the current is zero through any brush which is leaving a conducting bar.
Lulks win4~ewuldulg
Fig. I. A conventional commutator.
C d
c
c a
iBl=1
iB2~ BI Al 82 A2
iAI+iA2=I iBl iA2
Fig. 2. The new machine collector-winding-brushes arrangement.
4. The External Commutation Aid Circuit
The machine and the extemal commutation aid circuit (ECAC) are presented in Figure 3. In order to control the commutation process, four thyristors Tl, T2, T3, T4 are mounted with the
N° I A D-C- MACHINE ELECTRONICALLY ASSISTED 61
T4 D4
E2
M u
Ti
Di
n n
Fig. 3. The machine and the extemal circuit.
brushes. To avoid an excessive increase of the current density in the brushes, it is necessary to delay the gating of the thyristors. To avoid the oscillation phenomenon, four diodes Dl, D2, D3, D4 are used. Two capacitors Cl and C2 constitute the generators which deliver the two emfs needed to assist the commutation. They store a quantity of electrical energy during the time when no commutation occurs before releasing if into the commutating section. The two wound magnetic circuits, which have air gaps to prevent saturation, are used to control the
capacitors voltages amplitudes and to split the machine current I in two approximately equal parts.
To detail the operation of the system, it is enough to consider the brushes of negative polarity Bl and 82. The cycle starts when brush Bl is on segment I and brush 82 on an
unconnected segment (Fig. 4a). Thyristor Tl is on and thyristor T2 is off. Dl and D2 are on and Cl is charged to a positive voltage V~~-VBj. When the contact occurs between 82 and segment 6, the commutation of the section Sl becomes, then, possible (Fig. 4b). Firing T2
starts that commutation. Cl is discharged in the commutating loop T2-82-Sl-Bl-Tl-Cl. Tl
is, then, turned off. The current in brush Bl cancels. If this cancellation happens before Bl leaves segment I, the commutation in Sl is then achieved without sparks. The next step is shown in Figure 4c. 82 is on segment 6 and B I is on an unconnected segment. Tl is off and T2 is on. Cl is charged to a negative voltage V~~-VBI. This situation ends when Bl touches
segment 2 (Fig. 4d). 82 is still on segment 6. The commutation of 56 becomes, then, possible.
Firing Tl discharges Cl into the commutating loop Tl-Bl-56-82-T2-Cl so that T2 is tumed
off and the current in 82 is cancelled. This must happen before 82 leaves segment 6 in order to
achieve the commutation in section 56 without sparks. The next step during which BI and 82
are respectively on segment 2 and on an unconnected segment is described by a figure similar to Figure 4a. The cycle is then closed.
5. The External emf
The external emf must have the form presented in Figure 5. It is alternating and its frequency is
synchronous with the brush-conducting bars contact. If n~ is the number of the conducting bars, the extemal emf have n~ periods per revolution of the rotor. So, for a rotor speed N, given in rpm, the emf frequency is f (Hz
= n~/N/60. For example, for a machine with 31
~, ~ ~~ ~~ ~~ ~~ ~, ~ ~~
C' ,d e'
i i
C
i
C
~, , i
, ,
Rotcrsmd ,
,
Rotorsmd
, , i
~_, ~,,
~ ~ 'f
iBl=I Lil~
iB2~ Bl 82 &i2=1 Bl 82 A2
LLl+J2=j iBi 1B2 1A2 iBi+1B2=1 iBl 1B2 A2
Tl T2 Tl
~
T2
~i cl D2 Di cl D2
L L
i i
al b)
d 8 d 57
e, ',' '~
,
e e
~ l
,
Rotor s1med
,
Rotor swed
,
,
s~
c a c
~
iBlW iAl=1
iB2=J Bi Al A2 A2=0 Al A2
iA1+1A2=1 al '" iBl+1B2=1 lA' 1B2 A2
Tl T2 ~ T~
+ ~
Dl Cl D2 ~j ~~ ~~
L ~
i j
c) ~~
Fig. 4. The cycle of operation.
bars and a speed of 3 000 rpm, case of the experimental machine, the emf frequency must be 550 Hz.
6. Gating
To generate the gating pulses, it is necessary to use an encoder which detects the rotor position and so the positions of the brushes on the commutator bars. A thyristor must be, fired at a
certain time after its associated brush has been in contact with a conducting bar. To obtain the
gating pulses, we have experimented successfully two methods. The first one (Fig. 6) uses an
N° I A D.C. MACHINE ELECTRONICALLY ASSISTED 63
Extenw f d n
t
Fig. 5. The extemal emf wave form.
E%Wdmmtalmachine pc
PCTIO-10
M. National
Inswwnmt
~~j~,
Isol%lion
Encoder Arnplification
Fig. 6. Numerical gating.
industrial timing board (PCTIO-10 designed by National Instruments Corporation) placed in a
PC and an encoder which delivers two synchronous TTL signals Oi and O~ which are
constituted by respectively 744 pulses and one pulse per rotor revolution (Fig. 7). The collector is constituted, as seen above, by 31 conducting bars and 31 unconnected ones. Consequently,
a brush is in contact with a conducting bar 31 times per revolution. The signal
cB simulates the contact between a brush and the conducting bars. The contact begins between B and a conducting bar at tj, ends at t~ and begins again with the next conducting bar at t~. The two instants tj and t~ representing two beginnings of the contact are separated by
744/31
=
24 pulses of the signal Oj. The PC-TIO board possesses 10 programmable counters.
If we send Oi to its input source, a counter is able to deliver a single pulse (signal O~) at every 24th pulse of Oj. By the use of the caphbilities of the PC-TIO board we can bring forward the signals delivered by the counters in order to have a delay time a between the instant of the
beginning of the contact between a brush and a conducting bar and the instant of firing the associated thyristor.
The second method is simpler and cheaper than the numerical one. It is analogical. It uses a disc which turns with the rotor and which has 31 reflecting sectors and 31 black sectors. Four
optical sensors (one for each thyristor) mounted, as shown in Figure 8, deliver a pulse at every transition from a black sector to a reflecting one. This pulse is then amplified and isolated before being sent to a thyristor gate. The delay time between the instant of the beginning of the
contact between a brush and a conducting bar and the instant of firing the associated thyristor
is, here. obtained by adjusting the position of the optical sensor on the disc.
To determine the optimum delay between the moment when the contact between a brush and
a conducting bar occurs and the instant of firing the associated thyristor, we have realized the
a rcvolulion ifthc rotor
< >
~
B position,I at tl
~
position 2 at Q B
~
positiin3at ~
l j ~j ~j_ j ~j
cB ~
~ ii t2 (~
oi
~ 744 pulses ~
02
oJ
G i i i_ i i
S k-
Fig. 7. The principle of the numerical gating.
Black sector
Optical sensor
Fig. 8. Analogical gating.
experiment presented in Figure 9a. The experimental machine is driven by a motor and excited. The voltage between two complementary brushes precisely located on the neutral line is shown in Figure 9b. The instants of gating are, then, chosen when this voltage cancels.
N° I A D-C- MACHINE ELECTRONICALLY ASSISTED 65
fi
~i~~
Bl
"~
lf R
VBI-VB2
~ ~
Motor 82
Experimental machine al
VBI-82
Gi
~
G2 t
~
b) Fig. 9. Deterrnination of the instants of gating.
7. Experimental Results
The composition of the test bench is summarized in Figure IO. The machine designed for a
nominal power of 8 kW has operated from 0 to 3 000 rpm and from 0 to 11.2 kW (150 V,
75 Al without sparking. Its previous nominal power has been then multiplied by 1.4 and we
Excitation Excitation
~°~~ gen~$nor ~~~~u~~~~~ ~~~~~~
ECAC Fig. 10. The test bench.
have not reached its Iimitations yet. The machine has operated as a motor and as a generator. In the following, we present the experimental results.
In Figure ii, we present the two brush currents iBl and iB2 with the two gating pulses Gl and G2 for a test with the following parameters 1 = 64 A, U
=
135 V, C
=
400 ~F and a
speed of 2 200 rpm. The commutated power is then P
=
8.6 kW which is slightly higher than the nominal power. In Figure 12, the two brushes current are shown with the signals cBl and cB2 simulating the brush-conducting bars contact duration. We observe that the current begins
to build up some time after the beginning of the contact between the associated brush and the conducting bars. This allows to avoid an excessive current density in the brushes. We can also notice that the currents cancel before the contact separation, ensuring a successful commutation
Fig. ii. The current wave forms in two complementary brushes and gating pulses. Scale Current
20 A/div ; voltage : 5 V/div ; time : 250 ~s/div.
(. ..I
Fig. 12. The current wave forms in two complementary brushes and the signals simulating the contact
between the brushes and the conducting bars. Scale : Current : 20A/div voltage 5 V/div time 250 ~s/div.
N° I A D.C. MACHINE ELECTRONICALLY ASSISTED 67
process. In Figure13, the same currents are shown with, this time, the capacitor voltage
U~ which is the emf assisting the commutation. It has a well adapted frequency and we observe that the commutations occur at its maximums and minimums which are equal, in this test, to
= 25 V. In Figure 14, a zoom of the commutation, at I1.2 kW, is presented.
iBl
Fig. 13. The current wave forms in two complementary brushes and the capacitor voltage. Scale :
Current 20 A/div voltage 20 V/div : time 250 ~s/div.
iB2
UC
Fig. 14. A view of the commutation. Scale : Current 20 A/div voltage : 20 V/div ; time
: 50 ~s/div.
The voltage across a thyristor is shown in Figure15. We observe that a thyristor must
support a voltage equal to U~. At high power and nominal speed, the maximum capacitor voltage
U~~~~ is, for this experiment, around 18 V. The root mean square current through a
thyristor and through a diode is approximately il ,fi. The maximum voltage
across a diode is
~~n<,x~~
Uc
Fig. 15. The voltage across a thyristor. Scale
: Voltage : 20 V/div time 20 ms/div.
8. Simulation ill]
The machine and the CEALC constitute a very complex circuit. But, if we neglect the
resistances and the emfs in the commutating loop, we can obtain the simplified pattem presented in Figure16, where f et L represent respectively the commutating coil and the
wound magnetic circuit inductances. The operation of such a circuit depends on the parameters
f,L, C and
on the rotor speed. The experimental values (f
=
30 ~H, L
=
2 mH, C
=
300 ~F) led us to distinguish four types of operation corresponding to four speed intervals. The detailed
calculations are presented in ii Ii.
is(t) Gl
Tl ~ T2
ic(t)
Dj
L
Fig. 16. The simplified pattern.
8, I. OPERATION FOR A SPEED CONTAINED BETWEEN 0 AND 484.6 RPM. The operation is,
in this case, described by the four configurations of the pattem presented in Figure17. At
to, the simulation starts with the capacitor voltage u~(to) 0. The situation is described by Figure 17a. The capacitor voltage and current are given by
ucj (t)
= (I/C w ~) sin w
~ t
,
( I) ici(t)
=
I cos w~ t (2)
N° I A D.C. MACHINE ELECTRONICALLY ASSISTED 69
is(t)
C
icl(t) 1/2 j/2
~ ucl(t)
L
a)
I
is(t)
c
I/2 1/2
~ uc2(t)
L
b)
'
is(t)
~ ic3(t)
1/2
L
C)
c
~
f/2 uc4(t)
L
d)
vc2 vc3
vcl vc4
to tI t2 t3 t4 ' e)
Fig. 17. (a b, c, d) : Configurations of the pattern ; (e) form of uc(i) for a speed contained between 0 and 484.6 rpm.
where, w
~ =
II fi. At time ti
= (gr/2 fi, the diode D2 is off. The next configuration is
presented in Figure17b. During this step, the capacitor voltage uc~(t) is equal to
uci(ti) while the current ic~(t) = 0. At time t~, the thyristor T2 is fired. The commutation process starts. The new configuration is presented in Figure 17c. The capacitor C is discharged
into the commutating loop. The capacitor voltage and current are given by uc~(ti
=
A sin iwi(t t~i + Bi
,
(3) ic~(t) = CAW cos [wj(t t~) + B
,
(4)
where wj
=
II,~. The
constants A and B are calculated so that uc~(t~)
= itc~(t~) and
duc~(t~)/dt
= duc~(t~)/dt. The current in the commutating coil is given by
i
is (t
=
1/2 (1/f) uc~(t) dt, (5)
t~
with is (t~)
= +1/2 and is (t~)
=
1/2. At time t~, the thyristor Tl is off. The commutation is achieved. The next step is described by the configuration presented in Figure17d. The capacitor voltage and current are given by :
ic~(t)
=
-1
,
(6) uc4(t i
= uc~(t~i I (t t~i/C (71
We have, then, described the half of a period of the operation. The form of uc(t is shown in
Figure 17e. The commutation ends at t~ and the capacitor voltage cancels at t~. This kind of operation lasts until tj = ti. The calculations give, then, t~
=
1.9971501 rpm and a rotor speed of 484.6 rpm.
8.2. OPERATION FOR A SPEED CONTAINED BETWEEN 484.6 AND 487.7 RPM. The
operation, for those speeds, is described by four configurations. Three of them are already presented above. The thyristor T2 is, here, fired at an instant ti after the beginning of the
simulation like mi(gr/2) fi< tj
~
(gr/2),$, with
mj = (2/gr) ar cos ((IL). At to, the simulation starts. The first step is described by the configuration already presented in
Figure 17a. The capacitor voltage and current are, also, given by the equations (I) and (2). At time ti, the thyristor T2 is fired. The commutation starts. D2 is still on. This step is described by Figure18a. The capacitor voltage and current are given by
uc~(t
= D sin [wi~(t tj) + El (8)
icz(t " CDW
i~ cos [Wi~(t tj) + El
,
(9)
with wj~
=
if ,fi and L'
=
f L/ (f + L). The constants D and E are calculated to have
ucj(tj) uc~(tj ) and ducj(tj)/dt
= ditc~(tj )/dt. At time t~, the diode D2 becomes off. The
next step is. then, described by the configuration presented in Figure 17c. The voltage and the
currents are given by three equations similar to (3), (4) and (5). At time t~, the commutation
ends. The Figure 17d describes the next step. the capacitor current and voltage are given by
two equations similar to (6) and (7). At t~, the capacitor voltage cancels. A half of the period is described. The form of tic(t) is shown in Figure18b. This type of operation lasts until t~ = t~. The calculations give, then, t~ = 1.9855268 ms and a rotor speed of 487.4 rpm.
8.3. OPERATION FOR A SPEED CONTAINED BETWEEN 487.7 AND 596.4 RPM. The
thyristor T2 is fired, in this case, at an instant ti like m~(gr/2) fi
< tj mm i(gr/? ) ~ LC, with m~ = (2/gr)arcos ((f +L)/2 L]. The first step is described by the configuration
