Note: Descriptions are shown in the official language in which they were submitted.
` 1061403
This invention concerns an electrical drive arrange-
ment, and more particularly relates to the dynamic braking of
A.C. induction motors by capacitive self-excitation, either
alone or advantageously together with other braking methods
such as D.C. injection. Such braking systems are known to
be effective particularly for use with polyphase A.C. motors;
our British Patent Specification 1231067, which issued May 5,
1971 to Dewhurst and Partner, discloses and claims one such
system employing capacitors together with short circuiting
of some of the stator windings of the motor and our British
Patent Specification 1473327, which issued on May 11, 1977
also to Dewhurst and Partner, discloses another system employ-
ing capacitors with D.C. injection. The present invention is
suitable for use with the braking systems of our above men-
tioned specifications, but is not exclusively suitable for
use therewith.
The present invention provides an arrangement in
which the state of the braking capacitor is monitored to
determine whether it will be safe to operate the motor and
subsequently rely upon the capacitor to perform its braking
function.
According to the present invention there is pro-
vided an electrical drive arrangement comprising an alternat-
ing current induction motor, a capacitor, means for connect-
ing the capacitor to the motor in such a manner as to cause
braking thereof, means for testing that the capacitor is in
a condition to effect said braking, and means to disable
operation of the motor upon said caplcitor not being found
b~ said testing means to be in a condition to effect said
braking.
In the particular examples of the invention
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described hereinafter,the capacitor is tested upon an
operator attempting to start the motor. If the result of
the test is satisfactory, the motor is permitted to be
started, but otherwise the motor is disabled.
In order that the invention may be more fully
understood and more readily carried with effect, two
particular examples thereof will now be described by way of
illustration with reference to the accompanying drawings
in which
Figure 1 is a circuit diagram of an electrical
drive arrangement in accordance with the present invention.
Figure 2 is a circuit diagram of another example
of an electrical drive arrangement in accordance with the
present invention,
Figure 3 is a circuit diagram of a phase sensitive
detector of the arrangement of Figure 2, and
Figure 4 illustrates various waveforms produced
during operation of the arrangement of Figures 2 and 3,
Figure 4A illustrates the rectified waveform on
the bus bars 12, 13,
Figure 4B illustrates the charging and discharging
waveform of capacitor C1,
- Figure ~C illustrates the waveform applied to
terminal 2 of the PSD, and
Figure 4D illustrates the output waveform of the
transistor TR3.
Referring firstly to Figure 1, a three-phase A.C.
supply L1, L2, L3 is coupled to ~e stator winding terminals
A, B, C of a three-phase induction motor through the contact
pairs MC1, MC2, MC3 of a main contactor relay MC having
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other contact pairs MC'~ and MC5 the function whereof will
be explained hercinafter. The relay contact as illustrated
in all of the figures of the drawings follow the convention
that the contacts move towards the left-hand side, thus
closing MC1 and opening MC4 for example, when the relay is
energized.
The braking capacitor is designated C, and as shown,
is connected to phases L2 and L3, and to stator terminals
B and C, via contact pair MC4 of the main contactor relay MC.
These contacts are normally open during operation of the
motor and close only when the capacitor braking action is
initiated.
Coupled to the capacitor C is capacitor proving
circuitry for determining whether the capacitor C will
perform its braking function. The circuitry includes a
capacitor prove relay CP having contact pair CP1 connected
in the energization circuit of a relay RR having contact
pairs RR1, RR2 and RR3 connected as shown. Relay RR has
two functions, as will become more apparent hereafter, ihe
first of which is to "remember" during the running time of
the motor that thecapacitor C was tested to be satisfactory, `
and the second of which is to disconnect the capacitor after
- proof of its condition has been obtained since it is con-
sidered that the maintenance of a potential on the capacitor
would be counter productive to safety considerations by virtue
of the possibility that the capacitor might be damaged by
the maintained potential.
The capacitor proving circuitry also includes
diodes DI and D2 and resistors R1 and R2 connected as shown,
the functions of which will become apparent from the
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following e~plan~tiona, and the arrangement is completed by
"start" pushbutton S1 and "stop" pushbutton S2 coupled
between a D.C. supply and the main contactor relay MC as
shown.
In operation of the illustrated circuitry, the
"start" pushbutton S1 is depressed by the machine operat,or
and main contactor relay MC is energized from the D.C.
supply. Contacts MC1, MC2 and MC3 close to connect the
A.C. supply L1, L2, L3, to the motor terminals A, B, C
and the motor starts to run. Contacts MC4 of main contactor
relayMC open and contacts MC5 close to establish a
"potential" holding circuit for the main contactor relay
MC which depends for its operation upon the condition of
capacitor prove relay CP and of relay RR.
Assuming that the capacitor C is in good condition
and that capacitor prove relay CP is therefore energized
(as explained in the following) which causes its contacts
CP1 to close, relay RR is then energized via normally closed
S2, contacts MC5 now closed and contacts CP1 now closed.
The energization of relay RR closes its contacts RR2 thereby
,completing a holding circuit for relay MC, and also closes
its own self-holding contacts RR3. At the same time contacts
- RR1 open thereby disconnecting the capacitor prove circuit.
The motor can now run even if the "start" button S1 is
opened, and in practice it is necessary only that the "start"
button S1 be held by the operator for some,what less than
100 m secs.
Had the capacitor C not been in good condition at
the start of the above-described sequence, the relay CP
would not have been energized and the holding circuit for
main contact relay MC would not have been completed. The
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motor would have stopped running as soon as "start" push-
button S1 was released.
Considering the operation of capacitor prove relay
CP, it will be seen that during the period when relay RR is
not energized (i.e. the period immediately following
depression of ~'start" pushbutton S1) the capacitor C receives
a charge via the path including closed relay Contacts RRls
resistor R1 which serves a current iimiting function, and
diode D1 which sets the terminal of the capacitor C which is
connected to terminal B of the motor at a relatively negative
potential and the other capacitor terminal at a relatively-
positive potential. Diode D2 is then reverse biassed. In
the subsequent half cycle of the A.C. supply, the capacitor
C is able to discharge from its positive terminal through
the coil of relay CP to the junction point of diodes D1 and
D2 and resistor R1 and, at this instant, this junction is
held so that it cannot be more negative than the negative
capacitor terminal. This discharge of the capacitor C
serves to "prove" the operative condition of the capacitor;
if a short circuit exists on the capacitor, or if it presents
an unusually low impedance, the relay CP will not be
energized. If an open circuit condition exists in the
capacitor, then the relay CP cannot be energized. Otherwise,
if the capacitor is in good condition, the relay CP will be
energized.
The holding process on diode D2 results in the
resistor R1 being connected directly across the two motor
phase wires B and C and the power rating of R1 must ~e such
as to accommodate this. By virtue of this arrangement, the
possibility is precluded of a voltage doubling condition,
which would result in unnecessarily high potentials being
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1061403
offered to the coil of relay CP, occurring as ~ e result of
the combination of the potential sbored in the capacitor
supplemented by the potentials at the motor terminals.
~ esistor R2 provides a discharge path for capacitor
C after relay RR has been energized.
Another embodiment of an electrical dr-ve arrange-
ment including apparatus for testing a braking capacitor
will now be described with reference to Figures 2, 3 and ~.
The apparatus operates on the principle that when the
braking capacitor is connected across the windings of the
motor, the capacitor and the windings define a resonant
circuit. The capacitor is tested by applying a test signal
to the resonant circuit, and the phase of an output signal
from the circuit is monitored to provide an indication of
the condition of the braking capacitor. Any change in the -
condition of the capacitor is manifested as a change in the
phase of the output signal, which upon detection can be used
to inhibit operation of the electric motor.
In the circuit arrangement shown in Figure 2,
a polyphase electric motor having windings W1, W2, W3, is
connected to a three-phase A.C. supply from terminals L1,
L2, L3- The motor is provided with a braking capacitor C
- which can be selectively connected to the motor windings W
to brake the motor. As in the arrangement of Figure 1, the
motor is switched on by operation of switch S1 and is
switched off and braked by operation of switch S2. An
electromechanical relay arrangement is provided which is
arranged so that upon operation of the on switch S1, a single
phase A.C. test signal derived across the terminals L1, L2,
is applied to the resonant circuit comprising motor coils
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1061403
W1, W2 and braking capacitor C. A phase sensitive detector
circuit P.S.D. is connected to the output of the resonant
circuit to compare the relative phases of the input test
signal and the output of the resonant circuit. If the
phases of the signals accord with a predetermined allowable
relationship, the relays are so arranged to switoh on the
motor, but if the phases do not accord with the relationship,
the motor is not switched on.
An A.C. signal for energizing the relays is derived
from the phases L1 and L2 by means of a transfor~er T1, and
is fed to the start and stop switches S1 and S2. Upon
operation of the start switch S1, the relay energizing signal
is fed to the coil Q1 of a relay P.P. which operates switches
S3, S4, S5 and S6. Upon operation of switch S3, the relay
energizing signal is fed to switch S7 which is a part of a
relay P.O. that is controlled by the phase sensitive detector
P.S.D. The arrangement is such that when the P.S.D.
indicates that the capacitor C is in a good order to effect
motor braking, the relay P.O. is triggered so that switch S7
applies the relay energizing signal`to a coil Q2 of a further
relay MC that controls switches S8 and S11. The relay MC
also has a self-holding switch S12. Operation of the relay
- MC thus causes the three-phase supply to be connected through
switches S9 - S11 to the motor windings W to start the motor.
Operation of the relay P.P. also causes a single
phase A.C. test current to be applied to the resonant circuit
comprising capacitor C and the windings W1, W2. The test
current is derived from the terminals L1, L2 and is fed
through switches S4, S5, resistors R1, R2 and switch S8.
The input to the resonant circuit is monitored by the P.S.D.
106~403
circuit; the voltage across resistor R2 being applied to
terminals X and Y of the P.S.D. An output from the rcsonant
circuit is applied to terminal Z of the P.S.D. through switch
S6.
Thus to start the motor, an operator depresses the
start switch S1, causing the relay P.P. to be operated such
that an A.C. test signal is applied to the capacitor C and
the windings W through the switches S4 and S5, and the relay
energizing signal is applied to the switch 57. If the
capacitor C is in good condition, the phase sensitive detector
causes the relay PØ to operate in such a manner that switch
S7 directs the relay energizing signal to the coil Q2 of relay
MC which is thus energized. As a result, the switch S8,
which was previously closed, is opened so as to disconnect
the braking capacitor C from the windings W, and the
previously open switches S9 - Sll are closed to connect the
motor to the power supply termina]s L to start the motor.
The switch S12 holds the relay MC in its energized state and
hence the motor starts and continues to rotate until the
stop switch S2 is actuated, at which time, the relay MC is
released to connect the braking capacitor across the windings
W1, W2 and to disconnect the power supply from terminals L.
~ It will be appreciated that upon starting the motor, the
P.S.D. and the relays must respond fast enough to operate
the relay MC whilst the switch S1 is held instantaneously
depressed by the operator. In practice, the switch is
- typically held depressed for only a few hundred milliseconds.
A phase sensitive detector circuit having a fast enough
~, response for this purpose will now be described with reference
to Figure 3.
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The circuit has inputs X, Y, Z which correspond to
those shown in Figure 2, the input test signal being applied
to the circuit between terminals X and Y, while the output
from the resonant circuit is applied to terminal Z. The
single phase input signal applied across X and Y is fed to a
four way rectifier D and then to bus bars 12, 13. A
transistor TRl is connected in series with a resistor R3
between the bus bars; switching on of the transistor being
controlled by the output signal applied to terminal Z, which
is connected to the transistor's base through diodes D3, D4 .
A slave transistor TR2 has its base connected to the collector
of TRl to be ~witched on when TRl is switched on.
The transistors control a charging and discharging
circuit for a capacitor Cl which, as will be explained here-
inafter in more detail, is charged when the transistors are
switched off, through a resistor R4 connected to the terminal
X. The capacitor Cl is discharged when the transistors are
; switched on, and as a result the capacitor is charged and
; discharged sequentially in dependence upon the signal
applied to the terminal Z. A transistor TR3 is connected
between the bus bars 12, 13 with its base connected to the
capacitor Cl to compare the relative phase of the rectified
A.C. signal on the bus bars with the phase of the charging
cycle of capacitor Cl. The transistor TR3 is arranged to
fire a thyristor S.C.R. if the compared phases lie within a
given relationship, the thyristor then causing energization
of a coil Q3 of the relay PØ, *o actuate the switch S7 of
Figure 2. The transistor TR3 is baissed to a switched-off
state during positive-going half cycles of the signal applied
to terminals X, Y through a current path between terminal X
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and the base of the transistor, the path including a resistor
R5 -
Operation of the circuit of Figure 3 will now be
described in more detail with reference to the waveforms
shown in Figure 4.
As shown in Figure 4B, during positive-going half
cycles of the input waveform appl.ied to terminals X and Y,
the capacitor is charged such that its left hand terminal
assumes a positive potential, the charging being effected
A 10 - through resistor ~. When, however, the potential of the
waveform applied to the terminal Z passes through zero, the
transistors TR1 and TR2 are switched on, and the left hand
terminal of the capacitor is connected via the conducting
~/
transistor T~ to the emitter of transistor TR3 and in
consequence the right hand terminal takes up a negative
potential with respect to the emitter of TR3, as shown at 14
in Figure 4B. As a result, transistor TR3 cannot conduct
until the charge on condensor C1 has leaked away to zero
as is indicated at 15 in Figure 4B. The time taken for the
capacitor C1 to discharge to zero is a function of the phase
difference between the signal applied across terminals X and
Y, and the sig~al applied to terminal Z, all other factors
- which could effect the discharging of the capacitor remaining
substantially constant throughout each cycle of operation
of the circuit. Thus, by determining whether the discharge
time for capacitor C1 and hence the phase difference, exceeds
a given value, the condition of the braking capacitor C
(Figure 2) can be established. This determination of the
: discharge time is effected by the transistor TR3 which is
arranged to be switched on only if the discharge of capacitOr
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106~403
C1 extends in time from the positive going half cycle of the
waveform applied to X, Y in which the discharge was initiated,
into the next negative going half cycle of the waveform.
The output of the transistor TR3 is shown in Figure 4D. The
positive going half cycle of the waveform applied to X, Y
corresponds to the half cycle numbered 1 of the rectified
waveform applied to the bus bars 12, 13, and during this
half cycle, the transistor TR3 is biassed off by a signal
from X through the resistor R5. If, however, as is shown in
Figure 4, the capacitor C1 has a charge remaining therein
upon the occurrence of the next half cycle (2) of the
rectified waveform of Figure 4A, the transistor is switched
on until the charge on the capacitor C1 is dissipated, and
hence the output of transistor TR3 is as shown in Figure 4D.
The output from transistor TR3 causes the thyristor
S.C.R. to be fired, and consequently the relay PØ is
operated indicating that the capacitor C of Figure 2 is in
a condition to perform its braking function.
, The coil Q3, of relay PØ has a significant level
of self inductance and, since no alternative path is provided
for the inductive currents, they flow through the thyristor
S.C.R. and maintain the thyristor in a fired condition
between the periods during which transistor TR3 is conductive.
In the event of the braking capacitor C being short
circuited, there will be less of a phase difference between
the signals applied to the phase sensitive detector circuit
of Figure 3, and the discharge of the capacitor C1 will not
ex'end into the half cycle 2 of Figure 4. Hence the tran-
sistor TR3 will not be switched on and the relay PØ will'
not be energized. As a result relay MC will not be triggered
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1061403
and the motor will not start.
Conversely, in the event of the capacitor presenting
an open circuit, the capacitor C would not provide a resonant
circuit with the motor windings, and hence the relay P.O.
would not be operated to start the motor.
The sensitivit~- of the circuit can be altered by
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adjusting a variable resistor VR1 to adjust the rate of
discharge of the capacitor Cl.
It is estimated that an average squirrel cage motor
has an impedance when stalled that contains a power factor
better than 0.4, and at this power factor, the displacement
of voltage relative to current would be 66 . The dimensions
of the braking capacitor C that is connected ~o the motor
to effect braking are chosen to bring the power factor of the
stalled motor to a value approaching unity. -Hence the phase
sensitive detector circuit must be capable of detecting
between power factors around 0.4 and unity for the circuit
comprising the motor windings and the braking capacitor. We
have found that the circuit arrangement described with
reference to Figure 3 fulfils this requirement.
In some motor installations, it may prove necessary
to calibrate the variable resistor VRl by the process of
setting the resistor VRl to its maximum value, and with the
capacitor C connected to the motor windings, operating the
stop switch S22 and moving the resistor slider of VRl until
the relay PØ operates. The setting of the resistor can
then be checked by disconnecting the capacitor C from the
motor, since if the resistor VRl is set correctly, the relay
should fail to operate. Any apparently undue sensitivity of
the variable resistor VRl can be expected to indicate that
the capacitor C is presenting an inadequate capacitance to
brake the motor.
Clearly many modifications and alterations could be
made to the above described arrangemcnt; and for example the
phase sensitive detector could be of other forms than that
described. The P.S.D. could for example operate digitally.
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1061403
Although, as above described, the invention con-
templates that it would be sufficient to prove the condition
of the braking capacitor only once, at the beginning, of each
start to stop routine of the machine, in some circwnstances
it could be desirable to interrogate the braking capacitor
additionally at intervals in the running of the machine.
Such an arrangement could readily be accommoda~ed by the
invention simply by incorporation of a timer circuit, for
example, arranged to perform the capacitor interrogate
routine at intervals as desired after the first interrogate
routine performed on starting up the machine. Each additional
interrogation of thecapacitor could be arranged to determine
whether the machine was allowed to continue running or
whether it was disconnected from its power supply.
Many other modifications and variations lying
within the scope of the claims will be apparent to those
skilled in the art.
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