Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to drive circuits for direct
current electric motors.
According to the invention, a drive circuit comprises
a direct current motor having two parallel oppositely wound field
windings, a pair of controlled rectifiers, each connected in series
with a respective one of the said field windings, a commutating
capacitor coupling the controlled rectifiersl and means for applying
gating signals to the two controlled rectifiers alternately for
causing conduction of direct current through the two field windings
~o in alternate complementary conduction periods.
In such a circuit each of the aforementioned controlled
rectifiers (which are preferably thyristors) acts as a main current
switch for the winding with which it is in series and an exting-
uishing switch for the other controlled rectifier. The periods
of conduction of one controlled rectifier are complementary
to those of the other. If the periods of conduction are e~ual,
~ the opposed field windings are energised alternately with substant-
: ially equal pulses of curre-nt so that, provided that the frequency
of commutation is high er,ough, the motor is at a standstill.
Equalisation of the conduction periods occurs when the successive
intervals between the gating signals applied to the rectifiers
alternately are equaL for instance when the gating signals are
rectangular waveforms of unity mark-to-space ratio but in antiphase.
; The two fiald windings constitute a transformer, the action of
, 25 which keeps the currents through the windings very low at stand-
; still. The transformer also inhibits the appearance of over-
voltages across the controlled rectifiers. Variation of the
relative timing of the gating signals will alter the average -~
currents flowing through the field windings differentially so
that the motor will be driven in one or other direction
according to which winding reçei~-es the greater average current.
Because tne starting, stopping and reve~sing of the
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motor can be effected merely by adjusting the relative timing
of the gating signals, the use of contactors for these operations
can readily be avoided. -
In order ~o permit the appliction of reverse voltages
to the thyristors and to limit the discharge current of the com-
mutating capacitor, a choke may conjointly couple the cathodes
of the controlled rectifiers to the appropriate terminal of the
;~ source of power. However, the armature of the motor, in the
case of a series wound motor, may perform the same functions
' 10 as the choke and therefore the armature of the motor may couple
~:; the cathodes of the controlled rectifiers to a terminal of the
source of power.
There follows a description of two embodiments of the
invention by way of example, reference being made hereinafter
to the accompanying drawings, of which Figures 1 and 2 each show
- in simplified form a drive circuit for a direct current electric
~` motor.
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The circuit shown in Figure 1 includes a positive terminal
1 and a negative terminal 2. These terminals would normally
be connected to the appropriate terminals of an electric storage
battery. I'he motor has two parallel field windings 3 an~ 4 which
are disposed to produce, when energised normally with current
, from the batter~, electric fluxes in opposite directions. In
;, this embodiment the motor is a series motor, of which the armature
5 is connected between the terminal 1 and the field windings
' 3 and 4.
! A Eree wheeling diode 6 is disposed, in accordance
; with ordinary practice, across the armature 5 of the motor.
l~ The commutating network for the motor basically comprises
;i~ 30 two con'~rolled rectiflers, in this embodiment the thyristors
~; 7 and 8. Each thyristor is in series with a respective one of
;~ ~ the field windings 3 and 4. The anodes of the thyristors are
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connected by a commutating capacitor 9O In this network each
:: thyristor acts as a main thyristor for the passage of current
j. through the respective field winding and.also, when rendered
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conductive, as an extinguishing thyristor for the main thyristor
of the other winding.
The circuit shown in Figure 1 also includes a blocking
diode 10 interposed between the field winding 3 and the thyristor
-:; 7 and a blocking diode 11 interposed between the field winding
- 4 and the thyristor 8. A free wheeling diode 12 is connected
:~ 10 between the negative terminal 2 and to a point on the circuit
- path between the field winding 3 and the blocking diode 10.
A free wheeling diode 13 is connected between the terminal 2
:
and a point on the circuit path betw~en the field winding 4 and
the blocking diode 11. The cathodes of the thyristors are connec~ed
.~ 15 together and conjointly through a choke 14 to the negative terminal
: . .
~ The gates of thristors 7 and 8 are connected to output
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terminals of an astable multivibrator 15, which produces at its
. output terrllinals two complementary rectangular wave switching
,` 20 wave forms of which the mark-to-space ratio is variable i.n a
co~..plementary manner. The particular construction of the astabl-
multivibrator is not important. The operating frequency of the
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astable multivibrator is likewise not important provided tha~, :
when the mark-to-space ratios of its complementary outputs are
25 equal, so that the periods of conduction of the thyristors 7 .
and 8, and the periods of current flow in the respective windings
.. . .
are likewise equal, the motor is stationary. Motion of the motor
in one or other direction can be produced by varying the mark-
to-space ratio of one of the outputs of the multivibrator, and
~0 correspondingly varying the mark-to-space ratio of the other
output in a comp.lentary manner so that the periods of conduction
as aforesaid alter differentially so as to increase the average
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current flowing through one of the field windings at the expense
of the average current flowing through the other.
The circuit shown in Figure 2 differs from the circuit
shown in Figure 1 by the ommissiion of the choke 14 and the re- -
placement of the armature of the motor in the position occupied
' by the choke. In this embodiment the free wheeling diode 6 is
unnecessary and is omitted.
It would be possible to reduce the currents flowing
through the windings when the motor is at a standstill more by
the incorporation of the windings of a transformer (not shown)
in series with the field windings.
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