Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to apparatus and methods
for controlling a power supply circuit for an induction motor to
optimize power consumption.
The power consumption W of a single phase induction motor
is W = VI cos ~, where V and I are the r.m.s. supply voltage and
current drawn, and ~ is the phase lag of the current behind the
voltage waveform. At rated load the phase lag ls small so -tha-t
the power factor cos ~ ls approxlmately equal to one but at well
below rated load, the power consumption decreases partly because
iO the motor draws reduced current but more lmportantly because the
phase lag ~ increases and reduces the power fac-tor. Thls reduces
the effi~lency of the motor because losses from resistive heating
and hysteresis are not reduced in proportion to power consumption.
Similar effects occur in three phase inductlon motors.
According to a first aspect of the present invention
there is provided a power controller for an lnduction motor com-
prising one or more switching means for connection between an
alternating current electrical supply and an induction motor which
is to be energised from the supply, there being one switching
~0 means for each phase of the supply, and each swltching means be-
coming conductive when a trigger signal is applied to that swit-
ching means and remaining conductive until the current supplied
thereto ceases, means for generating a time-reference signal in
the form of a ramp waveform which has a repetition frequency twice
that of the supply voltagQ and has minima synchronised with zero
crossings in the said voltage, monitor means for deriving a moni-
toring signal representative by its magnitude of respective in-
~ '
~8'7~i~
--2
tervals between a zero in the voltage waveform of at least onephase of the supply and the next final cessation of current in
that phase before current reversal -therein by sampling the said
ramp waveform each time a said final cessation of current occurs,
control means for generating the trigger signals, the control
means generating the trigger signal for one switching means each
tirne the magnitudes of the time reference signal and the monitor-
ing signal reach a predetermined relationship with one another, and
means for adjusting the said predetermined relatlonship, the con-
txol means being arranged -to change the time relationship between
the supply waveform and the trigger ~ignals in that sense which
shortens the conduction period of the switching means when the
said intervals tend to increase and lengthens the said conduction
period when the said intervals tend to decrease.
~ ccording to a second aspect of the present invention
there is provided a method of controlling an induction motor com-
prising -the steps of supplying -the induction motor from an alter-
nating current electrical supply by way of switching means connec~
t.ing the motor to the supply when trigger signals are applied -to
the switching means, there being one switching means for each
phase of the supply, each switching means becoming conductive when
triggered and remaining conductive until current supplied thereto
ceases, generating a time-reference signal in the form of a ramp
waveform which has a repetition frequency twice that of the sup-
ply voltage and has minima synchronised with zero crossings in the
said voltage, generating a monitoring signal representative by its
magnitude of respective intervals between a zero in the voltage
'7~j~
-2a-
of at least one phase of the supply ancl the hext final cessation
of current in that phase before current reversal therein by samp~
ling the said ramp waveform each time a said final cessation of
current occurs, and generating the trigger signals for one switc-
hing means each time the magnitudes of the time reference signal
and the monitoring signal reach an adjustable predetermined re-
lationship with one another, the time relationships between the
trigger signals and the supply waveform being changed, in response
to the monitoring signal, in that sense which shortens the con-
duction period of the switching means when the said intervals -tend
to increase and lengthens the said conduction period when the said
intervals tend to decrease.
The main advantage of the present invention is that it
effectively produces the same result as a reduction in the sup-
ply voltage V when the motor powex requirement falls so that the
power factor and efficiency of the motor remain high, irrespec-
tive of motor loading. Motors have been observed to consume up
to 70% less energy depending on loading, when the invention is
used.
For a single phase motor the monitor means may comprise
means for detecting steps in the voltage across the switching
means which occur when the switching means ceases to conduct, and
means for providing a signal which is representative of the -time
interval
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between a zero crossing in the supply voltage and the time at
which the said voltage step occurs. A signal so provided i5 sub-
tracted from a reference signal before application to the control
means and adjustment of the reference signal se-ts the phase lag.
Corresponding arrangements ma~ be made for three phase motors to
provide either a monitoring signal derived from one phase, or
preferably an average monitoring signal derived from all three
phases.
The method, used in the second aspect of the invention,
of phase lag measurement by sampling the ramp waveform has impor-
tant advantages over methods in which pulse length or the mark/
space ratio of a pulse signal initially represents the phase lag
and integration or capacitive smoothing is used to provide a fur-
ther signal in which amplitude represents phase lag. Methods
involving integra-tion or smoothing introduce phase lag in the
feedback system formed by the power controller and the motor and
-this can make stabilisation of the feedback control loop impos-
sible. Using the new sampling technique, phase compensation
(mentioned below) can be used to introduce phase lead (increase of
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gain with frequency) in the loop at higher frequencies (for example
above 6Hz) to neutralize the effects of cumulative phase lag in
the response of the motor and controller.
According to a third aspect of the present invention
there is provided a power controller for an induction motor com-
prising
one or more switching means for connection between an
alterna-ting current electrical supply and an induction motor to be
energised from the supply to connect the motor to the supply when
trigger signals are applied to the switching means, there being
one switching means for the, or each phase of the supply, and the
or each switching means becoming conductive when triggered and
remaining conductive until the current supplied thereto ceases,
monitor means for deriving a monitoring signal represen-
tative of the interval between a zero in the voltage of the, or at
least one of the supply phases and the next final cessation of
current in that phase before current reversal therein9
control means for generating the trigger signals, the
control means being responsive to the monitoring signal to change
the time relationship between the supply waveform and -the trigger
signals in that sense which shortens the conduction period of the
switching means when the said interval tends to increase and leng-
thens the said conduction period when the said interval tends to
decrease, and override means for changing the said time relationship
in that sense which lengthens -the said conduction period the over-
ride means being responsive to decrease in the back e.m.f. of the
motor and coming into operation when the said back e.m.f. decreases
to a value which indicates at least that the motor is stationary
or tending to stall.
According -to a fourth aspect of the present invention
there is provided a method of controlling an induction motor
comprising
supplying the motor from an alternating supply by way of
one or more switching means, the, or each of the, switching means
connecting the motor to the supply when a trigger signal is applied
-to the switching means, there being one swi-tching means for each
respective phase of the supply, and the, or each, switching means
:lO becoming conductive when triggered and remaining conductive until
the current supplied thereto ceases,
deriviny a monitoring signal representative of -the phase
angle between a zero in the voltage of the, or at least one of the,
supply phases and the next final cessation of current supplied in
that phase before current reversal therein, and
generating trigger signals having a time relationship
relative to the supply waveform which is responsive to the monitor-
ing signal to shorten -the conduction period of the switching means
when the said interval increases to lengthen the said conduction
period when the said interval decreases unless the back e.m.f. of
the motor decreases to a value which indicates at least that the
motor is stationary or tending to stall and then -the said time
relationship is changed in response to the said back e.m.f. in
that sense which lengthens the said conduction period.
In all four aspects of the invention more than one motor
may be connected in parallel to be supplied by way of the switching
means. References to 'the motor' in the four aspects of the inven~
tion thus include the plural, references to time and phase rela-
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-5a -
tionships and phase anyles are overall parameters, and references
to stalling or a motor being stationary relate to any one motor
where motors are connected in parallel.
The override means overrides the power reduction provided
by the control means and applies -the full supply voltage to a motor
which is to be started and this voltage is applied while the motor
speeds up. The override means thus enables a single power con-
troller circuit to run many single phase motors in parallel and
still save power. It also ensures the necessary full power for
starting -transients when one or more single phase motors are
started from rest. Any tendency of any of the motors to stall due
to sudden abnormal loading or supply voltage fluctua-tion causes
the override means to operate and prevents stalling. Arrangements
for starting a number of three phase motors in parallel are des-
cribed later.
The override means may comprise a rectifying diode and
a Zener diode connected to the junction between the, or one of
the, switching means and the motor, and a capacitor in series
with the
Zener diode. When the voltage applied to the Zener diode exceeds
-the Zener voltage the capaci-tor s-tarts -to change, and the voltage
across the capacitor provides the override signal. The rec-tifier
diode prevents discharging and reverse charging of the capaci-tor
05 during -the reverse half cycle of the supply.
The override signal may be combined with the moni-toring
signal a-t the input to a differential amplifier, their amplified
difference being applied to control -the time re:Lationship between
-the supply voltage and the trigger pulses.
Problems arise in the control of -three pha~se motors connected
by three wires only (the neutral connection being omitted) -to a
controller comprising switching means and -to a supply, such as the
controller of the first and third aspects of the present invention.
The present inven-tor has found one of these problems -to occur
when whilst one switching means, such as a -triac, is non-conducting
the current in a second switching means also becomes zeroO With
two non-conducting switching means and only a three wire connec-tion
-there is no continuous path for current through the -third swi-tching
means and current in this switching means also ceases. Since all
three switching means are now non-conducting the motor is switched
ofE and even if subsequently the three switching means are indivi-
dually fired in sequence -there remains no current path and the
mo-tor remains off. Thus the moment -two switching means become
non-conducting, the third switching means is switched off and the
motor ceases to operate.
This si-tuation occurs whenever -the off periods of two switching
means overlap during reduction of power to the motor by phase
colltrol such as is described above and is also used in other known
methods of induction motor control. The situation can also occur
if one switching means fails to conduct jus-t before a second
switching means becomes non-conducting, for example due to a
momentary interruption of the supply from a sliding pick-up contact.
The presen-t inventor has also discovered that another of the
above mentioned problems is due to circumstances which may arise
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and cause erroneous measuremen-t of phase lag between the supply
voltage and supply current. Variations in phase lag accompany
changes in the motor load and are used to determine when the
switching means should be triggered to meet motor load variations.
These circumstances arise when two switching means turn
off, since -then the resulting turn off and cessation of current in
the third switching means does not occur at -the natural phase lag
value. Rather, it is imposed by the off state of the other two
switching means and leads to inaccurate phase lag measurement and
subsequent failure of control circuits.
According to a fifth aspect of the present invention
therefore there is provided a power controller for three phase
induction motors supplied by way of three wires only, comprising
three switching means, each for a respective supply phase
and being, in operation, connected hetween that phase of a three
phase supply and a respective terminal of a star or delta connected
induction motor, each switching means being capable o~ passing
current from the moment when a primary or secondary trigger
signal reaches that switching means until current applied thereto
ceases,
monitoring means for generating a monitoring signal
representative of the interval between a zero in the voltage of at
least one phase o~ the supply and the next final cessa-tion of
current in -that phase before current reversal -therein,
control means for generating primary trigger signals for
the switching means of respective supply phases, the control means
being responsive to the monitoring signal to change the time rela-
tionships between the supply waveform and the primary trigger
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signals in that sense which shortens the conduction period of the
switching means when the said interval tends to increase and
lengthens the said conduction period when the said interval tends
to decrease, and
the control means also being constructed to generate
secondary trigger signals at the same time as primary trigger
signals, each secondary trigger signal being a.pplied to that
switching means which received the last primary trigger slgnal.
According to a sixth aspect of the present invention
there is provided a method of controlling a three phase star or
delta connected induction motor when the motor is supplied by way
of three wires only, comprising the steps of
supplying the induction motor from a three phase alter-
nating curxent electrical supply by way of three switching means,
each for a respective supply phase and connecting that phase to a
respective terminal of the star or delta connection of the motor
when primary or secondary trigger signals are applied thereto,
each switching means conducting from the moment when a primary or
secondary trigger signal reaches that switching means until current
0 applied thereto ceases,
generating a monitoring signal representative of the
interval between a zero in the voltage of at least one phase of
-the supply and the next flnal cessation of current in that phase
before current reversal therein,
generating primary trigger signals for the respective
switching means in the reverse phase sequence to that in which the
supply voltages attain maximum magnitude, in response to the moni-
toring signal to change the time relationships between the supply
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g
waveform and the primary trigger signals in that sense which
shortens the conduction period of the switchiny means when the
said in-terval tends to increase and lengthens the said conduction
period when the said interval tends to decrease, and
generating secondary trigger signals for the switching
means o a supply phase at the same time as the primary trigger
signal for the switching means of the next phase in the said
reverse phase sequence.
The first of the above mentioned problems is overcome by
generating a secondary trigger signal each tirne a primary trigger
signal is generated. In this way a path is created for current
through the two switching means receiving the simultaneous primary
and secondary trigger signals. The switching means which receives
the secondary triggering signal is that switching means which
received the previous primary trigger signal.
Accordiny to a seventh aspect of -the present invention
there is provided a power controller for a three phase induction
motor supplied by way of three wires only, comprising
-three switchiny means, each for a respective supply phase
and being, in operation, connected between-that phase of a three
phase supply and a respective terminal of a star or delta connected
induction motor, each switching means being capable of passing
current from the moment when a trigger signal reaches that switch-
ing means until current applied -thereto ceases,
monitor means for generating a monitoring signal rep-
resentative of the interval between a zero in the voltage of at
least one phase of the supply and the next final cessation of
current in that phase before current reversal therein,
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-- 10 --
control means for generating trigger signals for the
switching means, the control means being responsive to the moni-
toring signal to change the time relationships between the supply
waveform and the -trigger signals in that sense which shortens the
conduction period of the switching means when the said interval
tends to increase and lengthens the said condllc-tion period when
the said interval tends to decrease, and
means for suppressing generation or use of the monitoring
signals whenever current -through a switching means ceases but next
flows agains in the same direction.
According to an eighth aspect of the present invention
there is provided a method of controlling a three phase star or
delta connected induction motor when the motor is supplied by way
of three wires only, comprising -the steps of
supplying the induc-tion motor from a three phase altern-
ating current electrical supply by way of three switching means,
each of which when triggered by a trigger signal connects one phase
of the supply to a respective terminal of the star or delta con-
nection, each switching means being capable of passin~ current
from the moment when a trigger signal reaches that switching means
until current applied thereto ceases,
genera-ting a monitoring signal representative of the
interval between a æero in the ~oltage of at least one phase of
the supply and the next final cessation of current in that phase
before current reversal therein,
generating trigger signals for the respective switching
means in response to the monitoring signals to change the time rela-
tionship between the supply waveform and.the primary trigger signals
- lOa -
in that sense which shortens the conduction period of the swi.tch-
ing means when the said interval tends to increase and lengthens
-the said conduction period when the said interval tends to de-
crease, and suppressing the generation, or the use, of the moni-
toring signals whenever current through a switching means ceases
but next flows again in the same direction.
Use of the seventh and eighth aspects of the invention
allows the second problem to be overcome since erroneous phase lag
measurements are suppressed.
Preferably, one monitoring signal is genera-ted for each
supply phase, and then the generation, or the use, of the monitor~
signal for each respective phase is suppressed when the preceding
phase in the sequence in which the phase supply vol-tages go posi-
tive, is not conducting.
~ lthough power controllers and methods o.-f control accord-
ing to the invention are suitable for three wire only connections
to motors, they may be used in four or six wire connections.
Certain embodiments of the invention will now be described
by way of example, with reference to the accompanying drawings in
which:-
Figure 1 is a block circuit diagram of a power controlleraccording to the invention,
Figures 2a to 2j are waveforms used in explaining the
operation of Figure 1,
Figures 3a to 3d show various ways in which an induction
motor can be supplied from a three phase supply,
Figures 4a to 41 show waveforms which arise in operating
an induction motor connected as shown in Figure 3c,
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-- 11 --
Figure 5 i8 a block diagram of another controller according
to the invention,
Figure 6 is a circui-t diagram of an alternative arrangement
for the amplifier 19 and the sample circuit 23 of Figure 1, and
05 Figure 7 is a circuit diagram of threshold detec-tors which
may be used in a modifi.ed form of Figure 5.
In Figure 1 an induction mo-tor 10 is supplied from an a.c.
mains supply 11 by way of a triac 12. One side of the motor 10 is
connected by way of a switch 13 to a terminal 14 of the power
supply 11. The other terminal 15 of the power supply ll is connected
to the triac 12.
Trigger pulses are supplied to the triac 12 by a trigger
pulse generator 16 which generates trigger pulses when -the output
of a comparator 17 indicates that the voltage from a ramp
generator 18 has just exceeded the output voltage of a differential
amplifi.er 19. The trigger pulse generator 16, the comparator 17
and the ramp generator 18 are commercially available in a number
of integrated circuits designed for triggering triacs in power
controllers.
It is required to delay trigger pulses when the power factor
of the motor falls and this is achieved by means of a comparator 21,
a monostable circuit 35, a sample and hold circuit 22 and -the
difference amplifier 19. The comparator 21 detects the time
instant X (see Figure 2c) when the triac 12 ceases to conduct.
25 The lag ~ between the supply voltage falling to zero at Z (see
Figure 2a) and the point X depends on the load on the mo-tor and on
the firing instant F.. If the triac is fired earlier, the cessation
of current at X occurs later and vice versa. Also increase in
load on the motor reduces the lag and vice versa.
The voltage waveform across the triac (see VT in Figure 2e)
is used to determine the time instant X. The voltage across the
motor is shown in Figure 2d and it is the same as the supply
voltage when -the triac is conducting. At o-ther times -the voltage
does not fall to zero because the motor is still rotating and a
~ 8~6~
voltage is induced in the mo-tor s-tator. A-t such times -the vol-tage
across the triac VT (see Figure 2e) is equal to the difference
between the supply voltage and the induced voltage. It will be
seen from the waveform of Figure 2e that steps occur in the triac
05 voltage when conduction commences and ceases. The latter of -these
steps is used to indicate phase lag ~ (see Figure 2b) in the way
which is now described.
The voltage across the -triac is applied to the compara-tor 21
giving an ou-tput for the comparator 21 as shown in Figure 2f.
Each time the output of the comparator 21 changes from posit-ive to
nega-tive the monostable circuit 35 generates one of the sampling
pulses shown in Flgure 2g and the sample and hold circui-t 22
samples the output voltage of the ramp generator 18 (shown in
Figure 2h). (Alternatively, a sampling spike may be more simply
obtained by differentiating the output of comparator 21 with a CR
circuit.) However the output voltage of the sample and hold
circuit 22 is obtained by subtracting the ramp voltage (Vx) a-t -the
time the sample is taken from the reference voltage (VREF) obtained
from a potentiometer 2~.
The output of the sample and hold circuit 22 is applied to
the inverting (-) input of the differential amplifier 19.
In opera-tion, when the lag ~ (between the points Z and X of
Figures 2a and 2c) increases due to reduction in load on the
mo-tor, the sampled ramp voltage Vx increases in magnitude since it
is taken at a higher magnitude point in the outpu-t voltage of the
ramp generator 18 (as shown in Figure 2h). Vx is substracted from
VREF so that the error signal outpu-t of the sample and hold
circuit 22 falls but this is applied to -the inverting input of the
differential amplifier 19 so that the output of this amplifier
increases. Trigger pulses are thus produced later in each supply
vol-tage cycle since such pulses are produced at the point F in
Figure 2h when the ramp generator jus-t exceeds the voltage applied
by -the differential amplifier 19. Thus the triac then fires later
in the supply cyc]e which shor-tens its conduction period, reduces
-the power inpu-t to -the mo-tor, and opposes the increase in lag ~
due -to the change in load on the motor. This maintains -the power
factor and efficiency of -the motor and also acts to return the
power factor and the lag ~ -to normal operating values which are
05 set by VREF - Vx = since any difEerence is amplified by the full
d~c. gain of the differential amplifier 19. The circuit acts to
stabilise the motor power factor and phase lag ~ at the phase
angle where the sample voltage Vx = VREF and VREF determines the
operating phase angle of the motor.
A.c. response is damped by a feedback network consisting
of capacitor 27 and -two equal resistors 25 and 26 connected to the
differential amplifier 19. The d.c. gain of the amplifier 19 is
unaffected (typically 60 to 100 dB) but the A.C. gain is reduced
to maintain stability of the overall feedback system and produce a
smooth response to sudden changes in motor loading without hunting.
The amplifier gain at high frequency (above approximately 2Hz) is
unity and gives an immediate response to sudden changes in load
and motor phase angle.
The integrated circuit containing the trigger pulse generator 16
is selected from circuits having control logic to prevent the
trigger pulse from occurring after the supply voltage crosses zero
at Z in Figure 2a but before the current turns off at X in Figure 2c.
Triggering must occur after the point X otherwise current will
turn off at X and not be retriggered until the following half
cycle. The integrated circuit may require a connection from a
poin-t, such as the output of the comparator 21, where a signal is
available indicating whether or not current is flowing in -the
triac 12. If it is required to use an integrated circuit which
does not include such control logic, an equivalent logic circuit
should be provided externally.
Further motors can be connected in parallel with the motor 10
up to a maximum of about eight and two such motors 28 and 29 are
shown in Figure 1.
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The circuit of Figure 1 as so far described requires modifica-
tion for starting and when a number of motors are connected in
parallel and started at random. If one motor is running normally
at a predetermined phase angle and a second motor is then connected
05 in parallel with the first, the circuit will sense any change in
phase angle between the applied voltage and the overall current
supplied to the -two motors. However, connecting the second motor
may increase or decrease the lag depending on the phase lag at
which the circuit is operating and on the phase characteristics of
the two motors.
In the case of an increase in phase lag the circuit as so far
described would respond by further delaying firing of the triac
and by reducing power to the motors, which might fail to start the
newly connected motor, and the current drawn by the stalled motor
could lead to dangerous overheating. Even if a decrease in phase
lag resulted from connecting a further motor so that the circuit
responded by supplying increased power, -this might still not be
sufficient or rapid enough for satisfactory starting. Thus measure-
ment of phase lag is insufficient to guarantee reliable independent
starting of motors in parallel.
To overcome this problem an override system 3l is provided
and is operated by the voltage across the triac 12. When the
triac is conducting the voltage across it is less than 2 volts and
may be considered as essentially zero. When the triac is non-
conducting the triac voltage equals the difference between themains voltage and the voltage across the motor. The latter is not
zero because, as stated above, although the current inpu-t to the
motor stator may have ceased, current is still flowing in the
b..ck e. m.f~)
rotor. As it rotates, its magnetic field induces an e.m.f.~in the
stator winding which appears across the motor terminalsO The
voltage across the triac in the non-conducting period is thus the
difference between the mains voltage and the induced motor e.m.f.
The magnitude of this voltage depends on the rotor current and
rotor slip which both depend strongly on the load on the motor and
~3L9~'769
on the motor speed so that the peak value of tile voltage across
-the triac increases immediately on sudden extra loading on the
motor or if it shows any tendency -towards stal:Ling. For a single
stalled motor, for example, or a motor startingT from resty there
05 is no induced e.m.f. and the vol-tage across the triac equals the
full mains voltage. Likewise, when a second motor at res-t is
connected across a running motor, the voltage across the -triac
rises immediately in the non-conducting period to a value close to
the mains voltage because the very low impedance of the stationary
motor loads the induced e.m.f. of the running motor very heavily.
The peak value of the voltage across the triac is thus a
sensitive indicator for sensing the connection on-line of a motor
for starting and is used to actuate a starting sequence which
applies the full supply voltage for a few seconds to start the
motor reliably.
Any rise in the peak value of the voltage VT across the triac
above a predetermined value is sensed by means of a Zener diode 32
The voltage VT is applied by way of a potentiometer 33 and when
the potentiometer voltage rises above the Zener conduction voltage
a current flows through the Zener diode 32 and charges a capaci-tor 34
to create a negative voltage transient at the non-invertlng input
to the differential amplifier 19. A rectifying diode 36 prevents
the capacitor 34 discharging and the charging in reverse during
the reverse half cycle of the supply, and Zener diode 37 limits
the override signal across capacitor 34. The effect is to reduce
the output of the differential amplifier 19 and thus cause the
triac to fire earlier in the supply cycle~ Subsequently, when,
for example, a newly connected motor has speeded up~ VT falls to a
lower value and current through the Zener diode 32 ceases~ The
-transient then decays exponentially back to zero and smoothly
reduces the motor power to the normal reduced running levelO A
t-lme constant of a few seconds is suitable for discharge of the
capacitor 34 through a resistor 38. A motor which is usualLy slow
in starting results in VT remaining high throughou-t the starting
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- 16 -
period and this sustains the transient applied to the differential
amplifier 19 allowing -triggering pulses to be applied immediately
after current ceases a-t X (see Eigure 2c) during each supply half
cycle throughout the starting period, thus giving -the full supply
05 voltage for startingO
Since VT increases with motor slip, that is decreases in
rotor speed, -the same circuit allows the full supply voltage
waveform to be applied if motor speed drops excessively for any
reason, such as abnormal load or operation at too small a lag.
This effectively eliminates the possibility that power reduction
by the control system may cause a motor to slow excessively or to
stall. The override system 31 thus acts as a starting/stalling
sensor and as a safety device to override the phase lag controller.
Instead of causing the override circuit to come into operation
when the peak voltage across the triac exceeds the voltage of a
Zener diode, it may be arranged for the override circuit to operate
when the triac voltage exceeds the peak mains voltage less -the
voltage of a Zener diode. The effect of this is tha-t if the
supply voltage drops -then so does the threshold triac vol-tage at
which the override circuit operates. This ensures adequate safety
margins under conditions of reduced supply vol-tage~
A preferred alternative arrangement to replace items 19, 22
and 24 to 27 is shown in Figure 6. The ramp signal from the
generator 18 passes to a sample-and-hold circuit 23 which receives
a sampling pulse from the monostable 35. The sampled ou-tput of
the circuit 23 is applied to the non-inverting input of a
differential amplifier ]9~ which has a similar function to the
amplifier 19. The reference signal VREF is applied by way of a
resistor 25' to the inverting input of the amplifier 19', and the
output signal of this amplifier is therefore Vx ~ VREF amplified,
as is that of the amplifier 19 of Figure 1. In a similar way to
Figure 1 the output VsTART of the threshold circuit 31 is applied
to the amplifier 19' by way of the resistor 25'.
~8~7~i~
Because -the motor 10, the amplifier 19', -the comparator 17,
the trigger pulse generator 16 and the triac 12 form a feedback
loop there is a danger of instability in the overall response.
The phase lag ~ of motors responds in a complex way to rapid
05 changes in load and firing point F of the supply triac, and the
response also depends on motor construction. It has been Eound
that the response of -the amplifier 19' should be one of phase lag
at frequencies up to about 2 Hz with gain decreasing at 6db/octave
from high value at zero frequency to around unity at abou-t 2 Hz.
]0 The phase lag is imparted by a resistor 26' and a capacitor 27'.
As is explained later a resistor 86 and a capaci-tor 87 are used in
controllers for three phase motors.
As has been mentioned the controller can opera-te many motors
in parallel and this is so even with different motors driving
different loads such as pumps, drills, compressors and variable
loads.
Identical motors driving identical loads operate with the
same phase lag and the power saving is the same as operating each
motor with a separate controller. Different motors or iden-tical
motors with different loads operated in parallel tend to operate
with different phase lags but a triac supplying the motors will
cease conduction when the total current reaches zero. The circuit
described operates as though this cessation of current indicates
overall phase lag and the setting of VREF determines the value of
this lag.
The overall phase lag will not necessari~y be optimum for
each motor but by considering the waveforms for two motors in
parallel (Figures 2i and 2j for a first and second motor, respec-
tively) it is seen that the combined current (IM~ Figure 2c which
is the sum of the currents IMi and IM2 of the first and second
motors, drops to zero at the point ~ and conduction through the
triac ceases. However, equal but opposite currents continue -to
flow in the two motors and as shown the first motor draws curren-t
Erom the second motor. In fact, when the triac does not conduct,
7~
the second mo-tor is acting as a generator supp]ying current to the
first motor. Thus the firs-t motor which demands power over a
greater portion of the main cycle, ob-tains this during the non-
conductive period of the triac from the stored energy in the
05 second motor. In operating the second motor alone, the circuit of
Figure 1 would fire the triac earlier to lengthen the conduction
period and cause the current to reach zero at the predetermined
phase lag. For the first motor alone the circuit would act in the
opposite way. When operating in parallel the requirement for
differen-t conduction periods is balanced out by the mo-tors exchanging
energy. As long as the conduction periods required are not too
different, the improvement in power consumption is not very different
from that when the motors are controlled individually.
It will be realised that the control system described witb
reference to Figures 1 and 2 can be put into practice in many
other ways. For example, the circui-t for sensing motor phase
angle or the total phase angle of a group of motors may be con-
structed differently, for example by sensing the voltage and
current supplied to the motors. Similarly other override circuits
may be adopted.
The control system can also be applied to motors connected to
three phase supplies, and in one arrangement each phase has a
separate circuit similar to that shown in Figure 1 except tha-t a
single common difference amplifier 19 and a single common override
circuit 31 may be used. The difference amplifier is connected as
a summing amplifier with three inpu-ts from the three sample and
hold circuits 22 and provides a single output signal which is
applied to the three comparators 17. Using the single difference
amplifier prevents imbalance between power supplied to the various
phases. For example an increase in power to one phase may result
in a reduc-tion in power to another phase and tend -to cause imbalance
between the phases, The single override circuit responds to any
peak triac phase voltage which exceeds the reference voltage set
by the Zener diode 37. To enable grounded common connections and
~L~9~3~76~
a single override circuit to be used, the triacs are fired via
isolating pulse transEormers, and the voltages VT across the
triacs, from which the control voltages are derived, are transmi-t-ted
to the override circuit and the comparators 21 by way of optical
05 isolators driven from the voltages VT by respective full wave
rectifiersO (Similar isolating arrangements may be used in the
arrangment of Figure 1.)
In another arrangement the difference amplifier may be driven
by signals from one phase only but its output signal may be used
10 for all phases by way of separate comparators 17, trigger pulse
generators 16 and triacs 12.
Various ways of connecting a three phase induction motor are
shown in Figure 3. For example in Figure 3a a star connection is
shown with triacs 40, 41 and 42 connected in series with star
15 connected induction motor windings 43, 44 and 45. Three line
connections Ll, L2 and L3 and also a neutral c:onnection ~ are also
shown. Four wires are required for connection to motor terminalsl52
to 155 with use of the neutral -terminal 155. In Figure 3b a six
wire connection to six motor terminals 146 tol51 is used between a
20 controller containing the triacs and the motor itself since in
this case the triacs form part of the del-ta connectionO In
Figures 3a and 3b each triac conducts independently of the others;
for example if the triac 40 in Figure 3a is conducting there is no
need for the triacs 41 and 42 to conduct since curren-t can pass by
25 way of the neutral connection ~. Similarly in Figure 3b if one
triac conducts, a current path is established between two of the
supply lines L1, L2 and L3~
In Figures 3c and 3d, three wire connections are used, with
only three wires connecting to the three terminals of the motor.
30 Two problems arise when only three supply wires are used.
The first problem is explained in more detail with reference
to the waveforms of Figure 4 which are simplified waveforms showing
t:ime relationships between supply voltage waveforms and the "off"
periods, firing points, turn off points and sampling points. In
.~L9~
- 20 -
one condition shown in Figures 4d, 4e and 4f -the non-conductlng
period of each triac is such that there are always two -triacs
conducting so that for example when phase A is conduc-ting s-trongly
at 60 phases R and C are each taking small amounts of current as
05 shown at 61 and 62. If however the non-conducting periods are
increased the condition shown in Figures 4g, 4h and 4i is reached
where the current in waveform A falls to ~ero at 63 as the current
in phase B ceases at 64 and phase C is fired at 65. The waveforms
of Figures 4g, 4h and 4i are a transition to the state shown in
the waveforms of Figures 4j, 4k and 41 when the conducting periods
have been reduced to the point where phase A cannot conduct during
an interval 66 because the triac in phase B cease to conduct at
instant 67 and the -triac in phase C has not yet been fired at 68.
The phase A triac therefore switches off and cannot conduct, when
required at the end of the interval 66 and the motor would come to
rest.
This problem is overcome by causing the triacs to fire in
pairs. For example when current is to flow in phase C and the
triac 52 is to be fired at point 68 then the triac 50 in phase A
is also fired as indicated by the arrow 70. Similar firing pairs
can be deduced from Figures 4j, 4k and 4~ and are shown in Table 1.
Table 1
Primary triac A B C
Secondary triac B C A (Supply sequence -L1-L2-L3-Ll-)
Secondary triac C A B (Supply sequence -L1-L3~12-Ll-)
The primary triac in row 1 of -this table indicates the triac which
would norma]ly be fired, the secondary triac in row 2 indicates
the triac which should also be fired when the supply sequence is
as given in the fiEth column, and the third row of the table gives
the secondary triac which should also be fired when the supply
sequence is reversed.
I~hen the triacs are fired according to this scheme, it is
apparent from Figures 4j, 4k and 4~ that the triac currently
769
- 21 ~
considered as a secondary triac was -tha-t -triac :Last considered as
a primary triac. Thus each pair of triacs Eired consists of the
one which would "normally" be fired plus the one which was -the
las-t to be fired "normally".
05 Figure 5 includes the connections necessary to implement
Table 1, where three controllers 71, 72 and 73 are shown, one for
each of phases A, B and C respectively, colmected to respective
supply lines Ll, L2 and L3. Only the controller 71 is shown in
detail, the other two controllers 72 and 73 being similar. The
controllers 71, 72 and 73 are each the same as the controller of
Figure 1 excep-t that a single common difference amplifier 19 and
associated components, a common threshold circuit 31 and a common
detector for direction of rotation 74 are provided, together with
additional circuits 75, 76, 83, 84 and 85. The component circuits
of the controller 71 are designated in the same way as Figure 1
where their function is the same.
The voltage output held by each sample circuit 22 is applied
by way of one of three resistors: the resistor 25 and corresponding
resistors for the other controllers. The average error signal is
developed a-t the junction oE the three resistors and applied to
the inverting input of the amplifier 19.
Table l shows that with the first supply sequence when the
triac 50 for phase A fires it should also fire the triac 51 for
phase B. This is arranged by passing the triggering signal for
the phase A triac 50 from the trigger pulse generator 16 to an AND
gate 75. The normal direction of rotation is taken to be the
sequence in the second row of Table 1 and this is detected by the
direction detector 74. For the normal direction of rotation~
then, the AND gate 75 is opened and the triac 51 is triggered at
the same time as the triac 50. Another AND gate 76 receiving
inputs from the trigger pulse generator 16 and the reverse terminal
of the direction detector 74 is connected to trigger the triac 52
of phase C for the other supply sequence as shown in Table 1~
3'716~
- 22 -
The other phase controllers 72 and 73 have respective pairs
of gates corresponding to the ga-tes 75 and 76 coupled to their
trigger pulse generators (TPGs) and these gates have outputs to
the trigger terminals of the triacs of other phases as indica-ted
05 in Figure 5. The -trigger pulses are applied by way of respective
isolating pulse transformers (not shown) having secondaries connected
-to the trigger terminals of the triacsO
The direction detector ci.rcuit 74 receives two of the supply
voltage waveforms. These sinusoidal waveforms are limited to
produce symmetrical square waves, in phase with the respec-tive
supply waveforms. The square waves are applied to logic in order
to produce the normal (N) and reverse (R) signal. The logic may
comprise a D-type clocked flip-flop which is connected to receive
one square wave at its data input and the other after differentiation,
at its clock input.
The second problem which arises when only -three supply wires
are used, concerns the measurement of phase lag between the instant
the supply voltage reaches zero (relative to neu-tral) and the
instant when the triac current reaches zero. As is explained
above variations in this phase lag accompany changes in motor load
and are used by the controller to fire the triacs to meet motor
load variations.
Unfortunately when two -triacs turn off, the resulting turn
off and cessation of current in the -third triac does not occur at
-the natural phase lag value for cessation of current in this
phase. Rather the turn off is imposed by the cessation of current
in the other two triacs. In measuring phase lag this premature
turn off must be ignored and this is achieved by omitting -to
sample the phase lag during periods when the corresponding triac
has been prematurely turned off.
For example in Figure 4j the current waveform of phase ~ is
prematurely termina-ted at the beginning of the interval 66 and
since the comparator 21 uses the voltage increase across the
triac 50 when i-t goes open circuit to cause phase measurement by
.~g~6~
- 23 -
sampling the waveform from the ramp 18, a false phase measurement
will occur at the beginning of the interval 66.
This problem is overcome by inhibiting sampling a-t cer-tain
times, for example in the normal phase sequence when the triac 52
05 of phase C ls off it inhibits sampling in the controller 71 for
phase A as indicated by a broken arrow 80 in Figure 4k. The full
scheme for inhibiting sampling is shown in Table 2 for both the
normal direction of phase sequence (row 2) and the reverse sequence
(row 3).
Table 2
Signal causing inhibiting A off B off C off
Phase sampling inhibited B C A
(Supply sequence -Ll-L2~L3-L1-)
Phase sampling inhibited C A B
(Supply sequence -L1-L3-L2-L1-)
However a further problem arises in that since the beginning of an
interval 81 in waveform 41 and the natural end of conduction in
phase A shown in waveform 4j at point 82 coincide, the cessation
of conduction in phase C which inhibits sampling in phase A would
also inhibit sampling at the point 82~ This problem is avoided by
delaying the inhibiting waveform long enough to allow s~mpling to
occur at the point 82 but not long enough to prevent sampling at a
time such as the beginning of the interval 66~ Such an arrangement
functions correctly because current always ceases in the phase
which is to control inhibition well before inhibition is required
but ceases at the same time at which undesired inhibition could
occur.
In Figure 5 an inhibiting signal for the controllers 72 and
73 is generated from the comparator 21 of the controller 71. The
outpu-t of this comparator is 'low' when the triac 50 is not conduc-
-ting (see -the waveform of Figure 2f) and it is delayed in a
resistance capacitance delay circuit 83 before being passed to the
controllers 72 and 73. Corresponding inhibiting signals are
g~7~
- 2~ -
received by A~D gates 84 and 85 from delay circui-ts oE the
con-trollers 72 and 73, respectively. The gates 85 and 84 also
rece-ive enabling signals Erom the direction detector 74 corresponding
to normal and reverse rota-tion respectively. The output of -the
05 monostable circui-t 35 indicating when sampling of the ramp waveform
Erom the generator 18 should take place is passed -to the gates 84
and 85 and only reaches the sample circuit 22 when one of these
gates is enabled.
Since intervals of non-conduc-tion by triacs occur af-ter they
have been initially triggered, such as the interval 66 in Figure 2j~
further -triggering by the trigger pulse generator 66 must be
prevented although the ramp voltage from the generator 18 exceeds
the output of the amplifier 19. This can be achieved by including
logic which inhibits the generator 16 af-ter each trigger pulse
until a zero crossing occurs in the power supply. Such logic is
available in some of the aforementioned integrated circuits contain-
ing the generators 16 and 18 and the comparator 17. The logic
mentioned above for inhibiting primary trigger pulses while the
triac is conducting is also required.
Although the invention has been specifically described with
reference to the arrangement of Figure 5 it will be clear that it
can be put into use in many other ways, for example with -the motor
connected in three-wire delta (see Figure 3d). The connections to
the triacs are then as shown in Figure 5, that is each comparator 21
is connected across a respective one of the triacs, with the
trigger connected to the corresponding trigger pulse generator 16,
and each ramp generator 18 and each input of the direction
detec-tor 74 connected to a respective one of the line terminals L],
L2 and L3. Terminals 90, 91 and 92 of Figure 3d are then the same
as terminals 93, 94 and 95, respectively, of Figure 5.
It is an important advantage of the controller of Figure 5
that it can be used with a motor of the type shown in Figure 3d
where the mo-tor may have only three external terminals. This
advantage arises because in the present invention phase lag may be
g~7~
measured in the supply line as opposed to the motor windings (the
latter requiring four or six wire connection to the motor).
A conventional starter may be interposed between the triacs 50
to 52 and the motor windings A, B and C but in many applications
05 the controller described makes such starters unnecessary. The
controller may also be placed between a starter and the motor.
It is usually preferable to replace -the threshold circuit 31
of Figure 5 by a threshold circuit of the type shown in Figure 7,
which responds to -the voltage across the triacs in each phase of
the controller. A photo-transistor 96 which forms part of an
optica] isolator in the connection between the triac 50 and the
comparator 21 is connected by way oE a relatively low value
resistor 97 (for example 2 Kohms) to the non-inverting input of an
amplifier 98 forming the comparator 21. A relatively high value
resistor 99 (for example 200 Kohms) connects the non-inverting
input to a positive supply and the junction between the resistors 97
and 99 is connected to earth by way of a diode 100. The photo-
transistor 96 is also connected by way of a diode 101 -to a
differential amplifier 102 whose input is also fed Erom similar
circuits for phases B and C.
In normal operation the current through the pho-to-transistor 96
is low corresponding to the low voltage VT (see Figure 2d) across
the triac 50. The voltage across the resistor 99 varies in accor-
dance with this current since the diode 100 remains back biassed
and the output of the amplifier 98 is low (negative) when the
-triac 50 is not conducting (see Figure 2f). However when the
voltage across the triac 50 rises considerably due -to, for example,
a tendency towards stalling, the non-inverting input of the
amplifier 98 is held to the forward bias voltage across diode lO0
and voltage Ls developed across resistor 97 proportionate to the
voltage across the triac 50. When the voltage at the junction of
resistor 97 and photo~transistor 96 becomes more negative than the
override threshold voltage se-t by a po-tentiometer 103, the diode lOl
conducts and the output of the amplifier 102 goes negative to
provide a voltage VSTART which is fed to amplifier 19 (Figure 6)
~9~3~7~
- 26 -
to cause increased voltage to the motor. Thus if a tendency to
stall is indicated by any phase, a voltage is applied to the
amplifier l9 to cause the trigger pulses -to be applied immedia-tely
after a zero crossing occurs in the supply waveform.
05 Preferably the circuit of Figure 6 is used instead of the
amplifier l9 and the circuits 22. To overcome instability problems
in three phase systems, phase lead is advantageous and sometimes
essential at frequencies between about 6H~ and 70Hz (gain increasing
at 6db/octave). This may be introduced by the resistor 87 and the
capacitor 86.
Circuits other than those of the type shown in Figure 1 may
be used for measuring phase lag.
Other switching means such as parallel connected opposi-tely
poled thyristor pairs or a thyristor connected .o the d.c. terminals
of a full wave rectifier may be used instead of triacs.
The or each switching means may be in the form of a single
thyristor and may then be used in parallel with a diode, so that
triggering is carried out in half cycles having one polarity but
current flows automatically through the diode in half cycles
having the other polarity without triggering.
The invention is also applicable to three phase motors connected
in parallel when al] corresponding terminals of all star or delta
connected motors are supplied by way of one switching means which
may comprise a triac or a group of triacs in parallel all receiving
triggering signals at the same time. Thus three switching means
are required, for each group of parallel connected motors. If
more than two three phase motors are to be connected in parallel
and started independently it is advisable to ensure that for an
interval when each motor is started, the triacs are fired immediately
zero crossings in the power waveform occur. Motor starters usually
have a pair of extra contacts which are closed on starting and
-these contacts in all starters may be connected in parallel to
cause a voltage similar to that obtained from the threshold circuit 31
of Figure l to be applied to the non-inverting input of the
amplifier l9 for a suitable interval.
