Parallel oscillatory circuit frequency converter with safety circuit

The invention relates to a parallel oscillatory circuit frequency converter comprising: a rectifier (1); an inverse rectifier (4) having thyristors disposed in a bridge circuit; a parallel oscillatory circuit (5) having an inductance (L) and a series connection of two capacitors (C, C.sub.K) whereby one capacitor (C) is connected to the load points of the bridge circuit; and a gate-controlled turn-off time control device (6) for the alternate firing (switching) of the thyristors a, b forming the particular diagonally adjacent bridge branches with a cycle frequency adapted to maintain a minimum turn-off time, a safety firing circuit (24, 25) controlled by a detector circuit (11-23) to detect incorrect switching delivering firing pulses without delay to all the thyristors at least until all the thyristors are current-carrying.

The invention relates to a parallel oscillatory circuit frequency converter 
comprising: a rectifier; an inverse rectifier having thyristors disposed 
in a bridge circuit; a parallel oscillatory circuit including an 
inductance and a series connection of two capacitors, the terminals of one 
of the capacitors being connected to the load points of the bridge 
circuit, and a gate-controlled turn-off time control device for the 
alternate firing (switching) of the thyristors forming the diagonally 
adjacent bridge branches with a cycle frequency adapted to maintain a 
minimum turn-off time. 
A frequency converter of the kind specified, disclosed in German OS 34 25 
324, is preferably used in the inductive heating of workpieces, the 
elements of the load circuit consisting of that ohmic/inductive proportion 
of induction coil and workpiece which changes in the course of the heating 
operation, and also of the capacitive proportion of a generally 
fixed-value series connection of two capacitors. A gate-controlled 
turn-off time control device is also provided which operates the 
thyristors in the inverse rectifier bridge with a cycle frequency higher 
than the specific frequency of the parallel oscillatory circuit, so that 
the bridge current can be switched by the thyristors in the diagonally 
adjacent bridge branches of the inverse rectifier. The cycle frequency is 
so selected as to be as little as possible greater than the specific 
frequency of the load circuit, so that the turn-off time of the thyristors 
differs only slightly from the required minimum turn-off time. As a result 
the inverse rectifier gives off the maximum power to the load circuit. 
To determine the actual value of the turn-off time, this prior art 
arrangement uses a measuring device (already disclosed in German OS 32 37 
716) which determines the signal for the turn-off time of the thyristors 
of two bridge branches from the current and/or voltage on the load circuit 
via an integration operation. To obtain the given minimum turn-off time as 
a required value, the actual turn-off time is determined in one of the 
diagonals either separately from half period to half period, or averaged 
over a number of half periods, and adjusted accordingly via the turn-off 
time control device. 
However, it is a disadvantage of this prior art circuit that sudden changes 
in the impedance of the laod circuit, such as may occur, for example, due 
to bridge formations in inductively heated workpieces, the specific 
frequency of the parallel oscillatory circuit also changes suddenly in a 
way which cannot be compensated by the turn-off control device. As a 
result there is a risk that the current will not be switched from one 
bridge branch to the other, but be taken with unchanged polarity over more 
than one half period. This may lead to an increased voltage on the load 
capacitor which may destroy the inverse rectifier thyristors of the prior 
art frequency converter comprising a load circuit which has in addition to 
a load capacitor also a compensating capacitor in the connecting branch 
between the load inductance and the load capacitor. 
On the other hand German Patent Specification 27 22 814 discloses an 
arrangement for a detector circuit in which measuring transducers each 
associated with one bridge branch detect whether current is flowing 
simultaneously or there is zero current simultaneously in both bridge 
branches outside the interval determined by the turn-off time. As a result 
the direct current supply of the frequency of the inverse rectifier is 
switched off in case of interferences with individual thyristors of the 
bridge circuit. There is the risk that undesirable voltage peaks may 
destroy the thyristors of the inverse rectifier. The prior art detector 
circuit also has the following disadvantage: if the currents are not 
flowing simultaneously in the two bridge branches outside the turn-off 
time, but nevertheless they are not flowing in the phase position 
determined by the firing pulses, the prior art detector circuit is 
incapable of detecting such incorrect switching. 
It is therefore an object of the invention so to further develop a 
frequency converter of the kind specified as to prevent any impermissible 
increase in voltage at the thyristors due to sudden changes in the 
impedance in case of any incorrect switching. 
This problem is solved according to the invention by a frequency inverter 
of the kind specified, by the feature that a safety firing circuit 
controlled by a detector circuit to detect incorrect switching delivers 
firing pulses without delay to all the thyristors at least until all the 
thyristors are current-carrying. 
The immediate simultaneous constrained firing of all the bridge thyristors 
if the detector circuit has not detected correct switching results in a 
flow of current in the inverse rectifier not via the diagonally adjacent 
bridge branches, but via the thyristors in each of the directly adjacent 
branches. The method according to the invention results in symmetry of the 
voltages at the load points on one hand and at the feed points of the 
bridge circuit on the other, thus effectively preventing overvoltage at 
the thyristors. The detector circuit ensures that such constrained firing 
always takes place if the current does not flow in the bridge branches in 
the manner required by the firing pulses.

The invention will now be explained with reference to a drawing 
illustrating an embodiment thereof. 
A parallel oscillatory circuit frequency converter comprises a rectifier 1 
which is supplied by rotary current mains and with which a current and 
voltage regulator 2 is associated, a smoothing inductor 3 and an inverse 
rectifier 4. Both the rectifier 1 and the inverse rectifier 4 are 
thyristorized. 
A load circuit 5 constructed as a parallel oscillatory circuit comprises a 
resistor R, an inductance L, and a series circuit of two capacitors C, 
C.sub.K connected parallel to the inductance L, whereby the capacitor C is 
disposed in the connecting branch to the load points of the bridge circuit 
of the inverse rectifier 4. The parallel oscillatory circuit is so 
connected to load points of the bridge circuit of the inverse rectifier 4 
that the load circuit 5 is supplied with current alternately from the 
diagonally adjacent bridge branches of the inverse rectifier 4 formed by 
the thyristors a, a.sup.1 and b, b.sup.1. The firing pulses supplied to 
the firing electrodes of the thyristors of the inverse rectifier 4 and 
initiating switching are adjusted by a turn-off time control device 6. The 
trapezoidal current flowing into the load circuit 5 is supplied via a 
current transducer 8 to a discriminator 9 which supplies a control pulse 
to a gate-control turn-off time measuring device 7 when the current 
reaches crossover. By means of a further discriminator 10 connected in 
parallel with the capacitor C of the load circuit 5 the voltage crossover 
of the sinusoidal voltage is determined, and a control signal is delivered 
to the turn-off time measuring device 7 at crossover. The turn-off time 
measuring device 7 also receives a starting signal at the same time as the 
firing pulse delivered to the thyristors of the two diagonally adjacent 
bridge branches. The turn-off time determined by the turn-off time 
measuring device 7 is supplied as an actual time to the turn-off time 
control device 6, which after comparison with a supplied required value 
delivers firing pulses with a cycle frequency which is only slightly above 
the specific frequency of the parallel oscillatory circuit 5. This 
measuring device disclosed in European Patent 109 522 enables the turn-off 
time to be determined with high precision and the inverse rectifier to be 
operated with a reliable minimum turn-off time. 
The primary winding of respective transducers 11 and 12 is disposed in each 
branch of the two diagonally adjacent branches of the bridge circuit 4. 
The terminals of the secondary windings of the two transducers, which are 
bridged by rectifier diodes of opposite polarity (not shown), are 
connected to the inputs of two amplifiers 13, 14 which each carry on their 
output lines 15, 16 a d.c. signal when the measured value recorders 
constituted by the elements 11, 13 and 12, 14 detect current flowing via 
the associated branch of the bridge. The lines 15 and 16 are connected to 
the two inputs of an exclusive OR gate 17. The gate 17 always generates a 
pulse on its output line 21 when a current flows simultaneously or the 
current is zero simultaneously in the two diagonally adjacent branches of 
the bridge circuit of the inverse rectifier 4. To prevent the gate 17 from 
responding during switching, while a current flows in the two diagonally 
adjacent branches of the bridge circuit, during switching d.c. pulses are 
generated on the lines 19 and 20 from the firing pulses of the thyristors 
in the particular branches of the bridge circuit via a control apparatus 
18, so that when pulses are present on the lines 19 and 20, pulses on the 
output line 21 are suppressed. 
Up to this point the detector circuit is known from German Patent 
Specification 27 22 814. 
Output line 21 is connected via a further logic circuit 22 to the input of 
a safety firing circuit 24. 
The further logic circuit 22 has at its input two exclusive NOR gates. 
While one of the inputs of the first exclusive NOR gate is connected to 
the line 15, which delivers a signal when the transducer 11 carries 
current (thyristor a.sup.1) and the other input is connected to the line 
20, which is connected to the firing pulse line of the thyristor b, the 
signals of the line 16 (current signal in thyristor b.sup.1) and line 19 
(firing pulse signal for thyristor a) are applied to the inputs of the 
second exclusive NOR gate in the logic circuit 22. The connected gates 
ensure that a pulse is delivered on the line 23 to operate the safety 
firing circuit 24 even if the transducers 11, 12 do not simultaneously 
detect a flow of current--i.e., the detector circuit formed by gate 17 
does not respond--but the thyristors a.sup.1 and b.sup.1 are current in a 
phase position which does not correspond to the phase position given by 
the thyristor firing pulse. 
If therefore due to sudden changes in load circuit impedance and a 
consequent increase in specific frequency the polarity of the current is 
prematurely reversed, before the next switching phase is initiated, a 
difference is detected between the phase position given by the firing 
pulses and the actual phase position detected by the current transducers 
11; 12. The detector circuit completed by the aforedescribed gate 17 
therefore ensures that the safety firing circuit is operated in case of 
any incorrect switching. The safety firing circuit 24 operates an 
auxiliary firing pulse generator 25 whose output is connected to all the 
control signal connections of the thyristors of the bridge 4. If then the 
detector circuit 11 to 23 detect incorrect switching in the manner 
described hereinbefore, via its output lines 26a-d the auxilliary firing 
pulse generator 25 delivers firing pulses simultaneously to all the 
control lines of the thyristors. This takes place without delay, the 
result being symmetry in the distribution to the inverse rectifier 
thyristors of the voltage of the load capacitor C, when charged to an 
impermissibly high voltage due to incorrect switching. This prevents 
overvoltage at the thyristors. At the same time, the current peaks 
occurring as a result of the simultaneous firing of all the thyristors are 
below the values permissible for their dynamic operation. 
Preferably continuous firing pulses are used to bring the thyristors into a 
stable conductive condition when the safety firing circuit, 24 and the 
auxiliary firing pulse generator 25 respond. However, it is also possible 
to switch on the thyristors reliably by operating them with a sequence of 
firing pulses whose cycle frequency is higher than the cycle frequency of 
the turn-off time control device by at least a factor of 5.