Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE FOR CONTROLLING POWER BY PHASE CONTROL, AND
METHOD FOR REDUCING HARMONIC WAVES
The present invention relates to an apparatus for controlling
the power of an AC voltage supplying an electrical consumer by
phase control and for reducing harmonics caused by said phase
control, particularly within a range of 4 kHz, preferably
within a range of the third harmonic, comprising a first
switching element connected in series with said consumer and
driven by a control device for performing phase control. The
invention further relates to a method for reducing harmonics in
the range of 4 kHz, preferably in the range of the third har-
monic, in a power control performing a phase control.
An apparatus and a method for power control by means of phase
control and phase adjustment, respectively, is for example
known from the application DE 198 50 905 Al of the present
applicant. Devices for phase control serve to control the power
supplied to an electric consumer by periodically switching the
consumer on and off by means of a switching element. Typically,
a TRIAC is used as the switching element, which is connected in
series with the consumer and which is fired via a control unit.
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The control unit comprises a series connection of a resistor and a firing
capacitor, the series connection being provided parallel to the TRIAC (Triode
for
Alternating Current). The firing signal is tapped off between the resistor and
the
firing capacitor and is supplied to the control input (gate) of the TRIAC via
a
series connection of a resistor and a firing element in form of a DIAC. Beside
these analogue designed control devices, also integrated circuits are nowadays
available which perform this driving function.
The power control via phase control of the AC voltage supplied
to the consumer causes undesired harmonics, and there are Euro-
pean standards regulating the level of such harmonics. These
standards have to be fulfilled by the power control apparatuses
which is the reason why approaches for reducing these harmonics
have been proposed in the past.
When reducing the harmonics, particularly the third harmonic
plays a special role, therefore, former approaches have concen-
trated to reduce the third harmonic. An example of an apparatus
for reducing the third harmonic is for example disclosed in
EP 0 859 452 B1 of the present applicant. In the solution dis-
closed therein, the phase angle and firing angle, respectively,
is changed by the control device around a predetermined value
in succeeding full waves. Such an asymmetry in the firing angle
results in only slowly increasing even numbered harmonics and
highly reduced odd numbered harmonics.
Although this approach proved successful in practise and al-
lows, to meet the existing standards readily, of course it
further exists the request for finding a solution even less
expensive. Moreover an apparatus would be desirable, the hum of
which is slightly reduced in the range of 25 Hz compared to the
-~:Laratus shown in the mentioned document.
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In view of the above it is an object of the present invention
to provide an apparatus for power control by phase control,
which may be manufactured at a reasonable price and which has
an improved hum performance.
According to the present invention, there is provided an apparatus for
controlling
the power of an AC voltage supplying an electrical consumer by phase control
and for reducing harmonics caused by said phase control, particularly within a
range of the third harmonic, comprising a first switching element connected in
series with said consumer and driven by a control device for performing a
phase
control, characterized in that a second switching element connected in series
with a resistor element is provided, a series connection of said second
switching
element and said resistor element is provided electrically parallel to the
first
switching element, and said control device is adapted to drive the second
switching element shortly before the first switching element and switches the
second switching element in a conducting state for a short time period.
That is in other words that caused by prior switching the sec-
ond switching element into the conducting state, a current
flows which is taken over by the first switching element after
its firing. Caused by the resistor element, the current passing
through the second switching element is less than the current
passing through the first switching element. Altogether, by
means of the second switching element a softer increase of the
current in total may be achieved so that the harmonics caused
thereby are reduced. Particularly, switching the second switch-
ing element on generates harmonics which partially extinguish
harmonics occurring upon firing the second switching element.
Due to the few additional components which are necessary for
the apparatus for power control according to the present inven-
tion, manufacturing costs may be saved, however, without dete-
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riorating the quality with respect to the reduction of harmon-
ics.
Generally, the present invention is based on the idea to have
an amount of current flow before the actual firing procedure
and the switching on of the TRIAC, the amount of current being
smaller than the real current flowing through the first switch-
ing element. Due to that, as mentioned before, further harmon-
ics are generated previously which harmonics partially extin-
guish harmonics caused during the real phase control.
Preferably, a TRIAC is used as a switching element. It is fur-
ther preferred that the resistor element is provided as a sim-
ple ohmic resistor. Of course other resistor elements may also
be used, for example inductive or capacitive resistors. How-
ever, it has been proven that ohmic resistors are suitable well
with respect to their dimensions and costs.
The object underlying the present invention is also solved by a
method for reducing harmonics in the range of 4 kHz, preferably
the third harmonic, in a power control performing a phase con-
trol, wherein the method comprising the steps:
- driving a first switching element on the basis of a first
predetermined firing angle, as to perform a phase control
method;
- shortly switching a connection into a conducting state
before firing the first switching element, said connection
having a resistance and bridging said first switching element.
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As described before, a TRIAC is used as the switchable connec-
tion having a resistance which is connected in series with an
ohmic resistor parallel to the first switching element.
It is to be understood that the features mentioned above and
those yet to be explained below can be used not only in the
respective combinations indicated but also in other combina-
tions or in isolation, without leaving the scope of the present
invention.
The invention will now be described in detail on the basis of
an embodiment with reference to the drawing. In the drawings:
Fig. 1 shows a schematic block diagram of an apparatus for
power control according to the present invention, and
Fig. 2 shows a diagram for illustrating the voltage- and
current time curves.
In Fig. 1, an apparatus for power control is shown and indi-
cated with reference numeral 10. The power control is performed
by a phase control method which is generally known and has not
to be described again here.
For performing the phase control, the apparatus 10 comprises a
TRIAC 12 which is connected in series with the consumer 14 to
be controlled. The series connection of TRIAC 12 and consumer
14 is supplied by an AC voltage supply U,,atZ, wherein both elec-
tric poles are indicated with L and N.
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In the present embodiment the consumer 14 is provided as a
motor 15 which represents an inductive load.
The TRIAC 12 is connected with a control unit 20 via its con-
trol input (gate) 16), which control unit 20 generates respec-
tive firing pulses corresponding to the desired power and sup-
plies these pulses to the control terminal 16.
This control device 20 may either be provided as a discrete
circuit, comprising for example a firing capacitor for generat-
ing the firing pulses. Of course, the control device may also
be provided as an integrated circuit.
The firing pulses causes the TRIAC 12 to switch in a conducting
state so that the consumer 14 may be supplied with power. The
TRIAC 12 remains conductive until the end of the half wave of
the supply voltage and switches then in the non-conductive
state. By a respective selection of the firing time within a
half wave of the supply voltage it is possible to control the
power.
With this kind of power control harmonics are generated acting
back into the AC voltage supply network. Due to European stan-
dards these harmonics may not exceed predetermined values. It
is therefore necessary to take measures for reducing these
harmonics.
In the apparatus 10, a switching unit 30 is therefore provided.
This switching unit 30 has the task to cause an amount of cur-
rent to be passed through the consumer 14 before the real fir-
ing time instant, which amount of current is less than the real
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nominal current. In Fig. 2, this amount of current is indicated
with reference numeral 41. It is clearly shown that this amount
of current 41 starts flowing before the real flowing current
after firing the TRIAC which is indicated with reference nu-
meral 43. The time difference between the instant tl of the
smaller amount of current 41 and the instant t2 of the nominal
current 43 is about 1-2 ms and may be varied dependent on the
firing angle. Particularly, the difference may be set to zero
with very small and/or with very high firing angles having the
result that the switching unit 30 remains ineffective.
By means of a small amount of current 41 flowing before the
real nominal current 43, a total current passing through the
consumer 14 is generated, which rise is significantly flatted.
This flattening has the effect that the generation of harmonics
is reduced, particularly by destructive interference of the
harmonics which are assigned to the amount of current 41 and
the nominal current 43.
Altogether, this type of control is able to reduce the lower
harmonics and particularly the third harmonic here.
The function of the switching unit 30 is achieved in the pre-
sent embodiment by a series connection of a resistor 32 and a
TRIAC 34, which series connection is arranged parallel to the
TRIAC 12. Hence, a current path between a terminal of the sup-
ply voltage via the consumer 14, the resistor 32 and the TRIAC
34 to the second terminal N is formed when the TRIAC 34 is in
the conducting state. In this case, the TRIAC 12 is bypassed.
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The TRIAC 34 is driven also by the control unit 20, which sup-
plies firing pulses to the control input 36 (gate) of the TRIAC
34. As mentioned before, this firing pulse is generated at the
instant ti, which lies before the instant t2 of the real firing
pulse supplied to the TRIAC 12. The respective difference
t2 - t1 may be a fixed value or may be adjusted by the control
unit 20 dependent on the firing angle of the firing pulses to
the TRIAC 12.
The resistor 32 provided in the switching unit 30 is preferably
an ohmic resistor and serves to keep the current 41 below the
amount of the nominal current 43. A resistor having a resis-
tance in the range of for example 10 Ohm has been proven as
particularly suitable. However, it is to be noted that the
limitation of the current 41 may be achieved differently and
with different components.
As already mentioned before, the TRIAC 12 is bypassed shortly
before its firing through the switching unit 30, so that a
current may pass through the consumer 14. This procedure is
periodically repeated every half wave of the supply voltage.
It has been shown that a reduction of harmonics is possible
with a very simple and cost effective switching unit 30 so that
the existing standards may be met. Furthermore, it has been
shown that the humming noise which occurs with prior switching
units for reducing harmonics, may be substantially reduced.
