JPS5863002A - Dynamic brake automatic controller for independent transport device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the control of electric traction motors, and more particularly to an automatic control system for dynamic braking in self-supporting vehicles.

INDUSTRIAL APPLICATION This invention is used for the electric (transportation) locomotive which can obtain the desired deceleration characteristic with respect to the dynamic braking of the traction electric motor.

Independent transport devices using mechanical, hydraulic and hydromechanical transmissions have recently become available all over the world! They are being replaced by DC, DC/AC and AC traction drives industrially developed in If countries.

Wrinkle 1 [[[[[[[]] DC/AC drives are used in automatic control systems for dynamic braking in various self-contained transport devices.

The above-mentioned known automatic control device for dynamic braking in transport equipment is known from Mu.D., published in the USSR.
jlCf14 Noah and N, A, d Galski, r
! ! y) The output is increasing. Universal Trams for Air Rotating Vehicles
ionsof Pn@umatic Wheel@d
V@hlalworks Hav1mgIncreas@d
'Unlt Pow@r) J% Maahlnostro@ny* Magazine, Moscow 11976) is equipped with a DC traction generator, and the output of the DC traction generator is controlled by a braking contact. Through the device, a plurality of traction motors with current limiting resistors are coupled by a plurality of field windings connected in parallel and a coil of an overload protection relay of a charger provided in series. The braking resistor is connected to the armature of the traction motor through the aforementioned braking contactor.

The field winding of the traction generator is connected to an exciter.

One conductor of the field winding of the exciter is connected through a contactor to the anode path of the rectifier and a first resistor, and the other conductor of the field winding is connected through the contactor to the negative terminal of the battery. connected to. The first resistor is connected to the cathode of the diode and the second resistor. The second resistor is connected to the cathode bus of the bridge circuit. The bridge circuit is operated by a synchronous generator). The anode of the diode is connected to the third resistor, ! w! to the positive & side terminal of the storage battery through a circuit consisting of a resistor and a controller of s4.
The first resistor is connected by the braking contactor of the overload protection relay of the generator.

The traction generator, exciter and synchronous generator are mechanically coupled to the shaft of the dial engine.

The controller is mechanically coupled to the brake.

The control position of the controller is changed as the rotation angle of the brake pedal position increases. The individual control positions of the controller are determined corresponding to the excitation currents of the exciter, the traction generator and the traction motor, that is, the states are determined based on the individual braking characteristics.

When the diesel engine rotates at high speed, the synchronous generator voltage R9J is activated to limit the voltage applied across the terminals of the armature winding of the traction motor, and the voltage return , the excitation state remains essentially constant.

As the rotational speed of the dial engine increases, the voltage of the synchronous generator increases. The voltage is 100%.

It is rectified by the voltage circuit and influences the excitation circuit of the exciter, so that as the voltage increases, the excitation current of the exciter decreases. As a result, the voltage applied across the terminals of the armature winding of the traction motor does not exceed the maximum permissible value.

In the device described above, the generator overload protection relay is used to limit the excitation current of the traction motor to within a maximum permissible value. When the excitation current reaches the maximum permissible value, the overload protection relay of the generator is actuated and the contacts of the relay interrogate a fourth resistor in the exciter circuit of the exciter. The excitation current flowing in the excitation circuit for the exciter decreases in accordance with the decrease in the excitation current of the traction motor.

The above-mentioned automatic control devices have the disadvantages that the setting of the braking characteristics is discontinuous, that the adjustment method is complex, and that it is difficult to limit the excitation current and voltage of the traction motor.

Furthermore, in the known device there is no means for limiting the active emf of the traction motor.

Furthermore, as a prior art, an automatic control device for dynamo braking in an independent transport device equipped with a traction synchronous generator having a field winding is known (published in the Soviet Union,
T.F., Kuznetsov, V.I.

Su4Fuka, other arrival, ``AC/DCDrive+es of Diesel
Locomotleves) J, transport knot,
Moscow, 1978).

The automatic control device is provided with a rectifying bridge circuit on the output shaft of the 111m traction generator, and the rectifying bridge circuit connects the field winding and excitation of the traction motor connected in series through a current limiting resistor. It has a current converter. The first conductor of the armature SMA of the traction motor is connected to a braking resistor arranged in series, and the last conductor of the armature winding is coupled to a common point of the commutating bridge N-path. A converter of the armature current and its sum is inserted between the common point of the armature windings and the connection point of the two braking resistors. The field winding of the traction generator is connected to a thyristor excitation regulator actuated by an exciter. The thyristor type excitation switching device is
The excitation control is controlled by an excitation control having an input terminal for receiving a signal from a control device. The controller receives signals from a traction motor excitation current converter, a traction motor armature current and sum converter, and a brake controller. The traction generator and exciter are diesel
drive the engine.

The above-described control device has the advantage that the excitation current of the traction motor is only one current and that an essentially constant braking force is obtained, and indeed a constant braking force is obtained. The reactive emf is limited by a return method that prevents and reduces the armature current of the traction motor when the vehicle is at high speed. Partial braking characteristics are set continuously by rotating the controller handle.

- The above-mentioned control device has a discontinuity in that the braking characteristics are partially set, f,i! Limitations include the complexity of one method and the incomplete functionality of the reactive emf limiter.

Also known from the prior art is a device for dynamic braking of a DC motor, which device comprises an excitation current adjustment device for a DC traction motor, the excitation current adjustment device having an output for traction motors. Rotational speed converter connected to input of motor active emf limiter, output is O1'1''
- a voltage converter coupled to the 10,000 inputs of the OR dart, the other input of the OR dart being connected to the input of an adder, the inputs of the adder respectively being connected to the excitation current converter of the traction motor; It is connected to the braking force strength setting device, and one input is 1rOR? -), the other input is connected to the setting device, and the output is coupled to the input of the excitation current node device (USSR Inventor's Certificate No. 647, No. 321, Class HO2P 3/
12. Registered on February 15, 1979).

It is known that the above-mentioned devices have a low reliability, which is a disadvantage of using contact-based switching methods for switching from one control mode to another. The limitations of the prior art devices described above include the use of contacts 1118 to perform the function of the braking force intensity setter and the complexity of the reactive emf setter.

It is an object of the present invention to overcome the disadvantages of the various conventional devices mentioned above and to improve the operational reliability of an automatic control device for dynamic braking in a DC motor.

The present invention makes it possible to change the logic from one control form to another using a non-contact system, since the logic is changed during the operation of the circuit components, and the braking force is increased using a non-contact system. Dynaty in DC motor using setter
An object of the present invention is to provide an automatic braking control device.

The object of the foregoing is to provide an automatic control device for dynamic braking in a self-supporting transport device, the automatic control device comprising: a rotational speed limiter, the output of which is connected to the input of a traction motor active emf limiter; an excitation current adjustment device for controlling the excitation current of the traction motor, the traction motor having a voltage converter coupled to the excitation current of the traction motor and a first input of a braking force limiter; and a second input of the braking force limiter is connected to the excitation current converter;
In an automatic control device for dynamic braking in an independent transportation device in which an input of is coupled to a braking force strength setting device, based on the MK of the present invention, the excitation electric current m> of the traction motor and the braking force limiter are interconnected. three rectifier bridge circuits) are formed, the outputs of two of the three rectifier bridge circuits are connected correspondingly, and the outputs of the third rectifier bridge circuit of the three rectifier bridge circuits are connected correspondingly. is coupled to the output of the rectifier bridge circuit and one input of the comparison ≧2y), and the second input of the ratio soft anyy) is connected to the traction motor active emf limiter through a diode. It will be achieved.

Preferably, the wrinkle traction motor active emf limiter uses a diode bridge circuit consisting of two parallel circuits, each of the two parallel groups having two oppositely connected circuits.
Nine resistors connected in series are provided in parallel with the group of diode bridge circuits, and the connection tag in the center of the resistor is connected to each of the diode bridge circuits. connected to the common point of the group.

The above-described circuit arrangement improves the reliability of the device, since it makes the excitation current of the traction motor constant above the rotational speed and then the braking force constant. , and it is possible to limit the reactive electromotive force by switching multiple circuits using a non-contact switching method at the initial stage of a preset rotational speed.

Preferably, the automatic control of dynamics and braking in the independent transport device includes a speed stabilizer, the speed stabilizer comprising a contactless speed potentiometer and a correspondingly connected three
Two rectifier bridges and circuits of the three rectifier bridge circuits are connected to a rotation speed converter, and the third rectifier bridge of the three rectifier bridge circuits and the circuit are connected to The bridge circuit is coupled to the non-contact speed potentiometer, a resistor is provided in the output circuit of the rectifier bridge circuit, and the center tag of the resistor and the common point of the rectifier bridge circuit are coupled to the rectifier bridge circuit through a diode. Comparison 22 is connected to the input of G.

The circuit arrangement described above significantly improves the reliability of the overall device and facilitates the operation of the device, making it possible to obtain a predetermined running speed when the independent transport device travels downhill for long periods of time. It has advantages based on the sumi-ji.

The invention will now be described in detail (with reference to specific embodiments) in connection with the accompanying drawings, in which: FIG.

The automatic control device for dynamic braking in an independent transportation device of the present invention comprises one excitation current 11ii1tIJ device 1
([Figure 1]), and the excitation current adjustment device has a field winding of 1t! 3 includes a synchronous generator 2 for traction.

The excitation current regulating device 1 also includes its own generator 4, which has a control 1! on its output side. It has a rectifier circuit 5, a control unit, and a controller capable of fJ. The field winding 7 of the own generator 4, which is arranged in series with the field winding 3 of the traction synchronous generator 2, uses a self-excitation circuit and is connected to the storage battery through a current limiting resistor 8 and a cut-off diode 9. is combined with A rectifier bridge circuit 10 is provided at the output of the traction synchronous generator 2, and an amplifier 11 is connected to the input of the control unit 22 and the toro. The traction synchronous generator 2 and the own generator 4 have a technical connection to the diesel engine 12 . The comparison unit 13 has an excitation current l
The output of the rectifier circuit 1o is connected to the input of the amplifier 110 of the traction device 1, and the output of the rectifier circuit 1o is connected to the field windings 14 and 15 of the traction motor connected to the thorax. The armatures 16 and 17 of the braking resistors 1-8 and 1
Connected to 9. Traction motor's excitation current converter 20
is connected to a rectifier bridge circuit 21, while the braking force intensity setting device 22Iri is coupled to a rectifier bridge circuit 23, which rectifier bridge circuits 21 and 23 are connected correspondingly and excite the traction motor. It is provided as an element constituting the current and braking force limiter 24.

The third rectifier circuit 25 that constitutes the excitation current and braking force limiter 24 of the traction motor includes a voltage converter 26,
rectifier circuits 21 and 23 connected in series;
and is coupled to one input of the comparison unit y) 13.

The rotational speed converters 27 and 28 of the traction motor are connected to parallel-connected diode bridge circuits 29 and 30, respectively, forming a reactive emf limiter 31; A dynamic resistor 33 is coupled to the other input of comparison unit 13 through a diode 34 .

1 consisting of diode bridges 35 and 36 connected in parallel forming a reactive emf limiter 31;
The pair g20 is coupled to a voltage U, K regulated power supply, a braking force intensity setter 22, a resistor 37, and a second input of a comparator 13.

The resistors 32 and 37 are connected in series, and the common m connection of the resistors is connected to the oppositely connected diode bridges 29 and 30. and 35 and 36
connected at a common point.

The device for automatic control of dynamic braking in a self-contained transport device, which is the object of the present invention, also includes a diesel engine control pedal 38 and a brake pedal 39.

The automatic control system for dynamic braking in the independent transport device illustrated in FIG. 1 operates as follows.

A signal U, which is proportional to the excitation current of the traction motor, is applied from a current converter 20 to an Ii current bridge circuit 21 connected in series with a rectifier bridge circuit 23.

Applicable! ! The i-stream pre- and zo-circuits 23 are operated by the braking force strength setting device 22. The braking force intensity setter 22 is shown as a synchronous device whose rotor is mechanically coupled to the brake pedals 3, 9 of the teasel engine.

The sum of the signals U, +U,! U4 is (where ty is a signal proportional to the excitation current of the traction motor, U8 is a signal extracted from the braking force strength setting device 22, and U
4 is a control signal), rectifier bridge circuits 21 and 2
3 and applied to the input of the comparison unit, )13. l&U, is compared with a drive signal U having a constant value in a comparison unit 13.

The deviation signal U from the comparison unit is amplified by an amplifier 11 and rectified by a rectifier coupled to the series connected field windings m3 and 7 of the synchronous generator 2 and the own generator 4, respectively. Bridge circuit 50 is applied to control unit 6.

When braking the transport device (due to the lever 39 being pushed to the stop position of the transport device), the signal from the braking force intensity setting device 22 does not reach zero, and the input of the comparison unit 130 is as follows. A control signal proportional to the maximum excitation current signal U!+OKυ4 is received, and the maximum excitation current signal is the drive signal U!
That is, it is compared with U-kura -U.

In the above case, the automatic control device according to the invention maintains the maximum excitation current 4. The voltage applied to the braking resistor (section OA in FIG. 2) corresponds to the aforementioned maximum excitation current. At point A in FIG.
The signal from the excitation current converter 20 is equal to the O signal. When the travel speed of the independent transport device exceeds vI, the signal from the voltage converter 26 becomes larger (as compared to the signal from the excitation current converter 20). This closes the rectifying bridge circuit 21 and a signal proportional to the voltage U1 is applied to the comparison unit 13.

The automatic control device proposed above includes braking resistors 518 and 1
Maintain the maximum voltage applied to 9.

As long as the value of the voltage applied to the resistors 18 and 19 remains constant, the automatic control device
Limit the braking force in state B.

As the running speed increases from the running speed v1 as a starting point, the exciting current decreases monotonically.

At point B in FIG. 2, the large electric motor circuits 29 and 30 are
1. Since the maximum value of the signal extracted from the bridge circuit 35 exceeds the signal from the bridge circuit 35 operating at a constant voltage U. At the point B, the reactive electromotive force limiter 31 operates. Therefore, the diode 34 remains conductive and the second input of the comparator 34 receives the applied signal for comparison with the signal U.

When the running speed increases from V to V (Fig. 182), the deviation voltage signal 4U compared at comparison unit 13 becomes larger), so that the excitation current of synchronous generator 2 (Fig. 1) increases. decreases, and the output voltage of the rectifier circuit 10 and the excitation current of the traction motor also decrease. Output voltage U of the traction motor.

decreases (@if!ABC in Figure 2).

Therefore, when the brake pedal 39 is pressed to the stop position, the automatic control device of the present invention controls the section OA that limits the excitation current, the section AB that limits the maximum braking force, and the traction electric motor. Classification Be that limits the active electromotive force
The braking operation is performed based on the ideal braking characteristic curve @0ABC. .

In addition to the above-mentioned ideal braking characteristic curve 0ABC, the driver of the independent transport device may obtain partial braking characteristics within the braking characteristic range shown in the ideal braking characteristic curve (FIG. 2). The brake pedal 39 can be pressed so that the This is achieved by setting the brake pedal 39 in an intermediate position. In the above case, the braking force intensity setting device 22 (FIG. 1) outputs a signal with a large zero error. The signal is rectified by the bridge circuit 23,
A signal proportional to the excitation current is then applied. The added signal Uj +tr, is applied to the comparison unit 13, and the output signal is applied within the comparison unit y) to the drive signal Us,
That is, it is compared with U, +U, =U.

By reference to the above equation, it is clear that the drive signal is reduced by an amount equal to the braking force intensity signal. Therefore, the automatic control device of the present invention can maintain the excitation current constant at a level much smaller than the maximum allowable flow. In other words, the line segments OA, OB, Q in the braking characteristic curve
C and OD (FIG. 2) are formed.

In the area where the braking characteristic OAB is formed, the signal of the braking force intensity setting device 22 (FIG. 1) taken out from the brake circuit 36 is smaller than the setting signal U1. Therefore, this signal does not affect the above-mentioned braking characteristics j11. In the area where the braking characteristic 0ABC is formed, the signal taken from the bridge circuit 36 (FIG. 1) exceeds the setpoint signal U. Therefore, diode 34 conducts quickly. The reactive electromotive force is the a-line nc (Figure 2)
The limit is immediately reached when the limit is reached.

However, in some cases, an independent transport device may travel on a 1/CM, Fj down slope for an extended period of time, but the transport device is maintained at a moderate speed without driver intervention. It is essential that

To achieve the above objectives, the automatic control system illustrated in FIG. 1 is provided with a speed stabilizer 40 (FIG. 3).

#Speed□Stabilizer [40 includes a handle 41 for setting a predetermined speed, and the handle is part of a non-contact speed setter 42. The output of the speed setter 42 is
It is connected to the current flow circuit 43. The output of the rectifying bridge circuit 43 is connected to two terminals connected in series through a resistor 44.
The inputs of the two bridge circuits 45 and 46 are coupled to the rotational speed converters 27 and 27 of the traction motor.

A first input of comparator unit 13 is connected to the center lug of resistor 47 (coupled through diode 47) and to the common connection point of rectifier bridge circuits 43 and 46.

The automatic control system for dynamic braking in a self-contained transport device, illustrated in Figure 3, operates as follows. do. The predetermined running speed is
It is set by the hand handle 41 of the armless speed setting device 4-2. The driver of the transportation device sets a predetermined running speed, e.g.
Rotate the handle 41 so that it is set to l. From that K, the non-contact speed setter 42 outputs a signal U! corresponding to a predetermined traveling speed. Output. The # signal is transmitted from the bridge 1. circuit 43
The mosquito rectifier circuits 45 and 46 operate based on signals from the rotational speed converters 27 and 28 of the traction motor. Rectifier i current bridge circuit 43.45 and 46Fi, resistor 46
is provided at its output as a load. A control signal is applied through a diode 47 into the resistor 46 and from their common connection point to the input of the comparator unit 1s. The control signal is equal to the difference signal shown in the equation below.

ty, "U, - (U, , +U, 1), where U4 is a control signal, Uv is a rich signal generated by the speed setting device 42, and U and U are traction signals. Rotational speed of electric motor
This is the signal generated by the fertility converters 27 and 28).

From the above description it is clear that the automatic control device proposed here is able to maintain the independent transport device at a moderately constant speed. If the rotational speed of the traction motor increases,
The signal (U, +U, , ) also increases, the control signal U4 decreases, and the deviation signal U■U, -0
4 will also become smaller. The deviation signal is amplified by an amplifier 11, and then passed through a controller 22 to a controllable rectifier.

circuit 5 so that the excitation current of the traction motor increases until the control signal is equal to a preset signal. In other words, the braking force increases until the traveling speed of the vehicle reaches a preset speed, and the same applies when the braking force decreases. It is therefore clear that the running speed of the feeder is maintained at a constant level with an accuracy similar to that of a static control device.

By operating the speed setter 42 to vary the speed stability level (K), the operator can maintain the transportation device at a predetermined travel speed, such as V, , V, , V, etc.

Therefore, the driver can set the speed of the transport device to any predetermined travel speed, which depends on the slope condition and the magnitude of the slope.

The preferred embodiment, the circuit configuration of which has been described above, substantially simplifies the automatic control circuit for dynamometer braking in conventional self-contained transportation devices, and improves the reliability of the electric drive. and shorten the time for Komadushi. Furthermore, by providing the automatic control device with a speed stabilizing device, the driver's fatigue and stress are reduced and the effort required when the transportation device travels on a downward slope for a long period of time.

[Brief explanation of the drawing]

FIG. 1 is a schematic circuit diagram of an automatic control device for dynamo braking in an independent transport device according to the invention vUK, and FIG. 2 is an electric drive for traction implemented by the automatic control device according to the invention. FIG. 3 is a schematic circuit diagram illustrating another embodiment of an automatic control device for dynamic and braking for an independent transport device equipped with a speed stabilizer according to the present invention. Figure. (Explanation of symbols) 13 - Comparison unit, 21 - Rectifier bridge circuit, 23
--- Rectifier circuit, dicircuit, 24--exciting current and braking force limiter, 25--rectifier circuit, dicircuit, 26--voltage converter,
27.28-Rotational speed converter, 29°30-Diode bridge circuit, 31-Reactive electromotive force limiter, 32-Resistor, 34-Diode, 3i5.36-Diode bridge circuit, 37...Resistor, 40 ...speed stabilizer,
42--Non-contact speed setter, 43--Rectifier bridge circuit, 44--Resistor, 45.46--Rectifier bridge circuit, 47--Diode. Patent applicant Danadi Aleksandrovichino F.F. (7 others) Patent attorney Akimizu Akira Patent attorney Kazuyuki Nishidate Patent attorney Matsushita Shuoyuki Yamaguchi Continued from page 1 0 Inventors Anatoly Petrovich Proligin Moscow, USSR Ulitsa Trokhimova 2/1 Kwarchila 30 0 Inventor Anatoli Danilovich Mashikhin Moscow, USSR Ulitsa Marciala Tymoshenko 28 Kwarchila 45 0 Inventor Mikhail Pavlovich Askinad Moscow, USSR・Trifonovskaya Ulitsa India Corpus 2 Kwarchila 73 0 Inventor Julie Mikhailovich Andrev Soviet Union Moscow Nagatinskaya Ulitsa 15 Corpus 1 Kwarchia 53 0 Inventor Samuil Isaevich Kagan Soviet Union Sodeino Minskoy Obrasti・Ulitsa 40 Lett・Oktoyavlaya 31 Kwarchyla 62 ■Applicant Vyacheslav Valerianovich Seliberstov United States Moscow 2 Dorozhny Proezyt 16 Korps 1 Kwartyk 67 ■Applicant Nikolai Antrevich Osipov Soviet Union Moscow Ulitsa Takakkaya 28/14 Kwarchila 11 0 Applicant Anatoli Petrovich Zoroligin Soviet Union Moscow Ulitsa - Trokhimova 2/1 Kwarchia 30 ■Applicant Anatoli Danilovich Mashkhin Soviet Union Moscow Ulitsa Marciala Tymoshenko 28 Kwarchia 45 ■Applicant Mikhail・Pavlovich Askinazi, Soviet Union, Moscow, Trifuonovskaya Ulitsa, 60 Corpus 2, Kwarchila 73 ■Applicant: Julie Mikhailovich Andrev, Soviet Union, Moscow, Nagatinskaya Ulitsa, 15 Corpus 1, Kwarchila 53 ■Applicant: Samuil Isaevich, Kagan Soviet Union Zodino Minskoy Obrastiuri □ Tour 40 Lett Oktoyablaya 31 Kwarchira

Claims (1)

[Scope of Claims] 1. An automatic control device for dynamometer braking in an independent transportation device, the automatic control device having an output connected to an input wall of a traction electric motor activating electromotive force limiter. a traction motor excitation current regulator, comprising a speed converter and a voltage converter having an output coupled to an excitation current of the traction motor and a first input of a braking force limiter; The excitation current of the traction motor and a third input of the braking force limiter are connected to an excitation current converter, and the excitation current of the traction motor and a third input of the braking force limiter are coupled to a braking force intensity setting device. In an automatic control system for dynamic braking in an independent transportation device, three rectifying circuits (21, 23 and 25) are connected to each other, and the excitation current and braking force limiter (24) of the traction motor are connected to each other.
#2 of the three rectifier circuits and dicircuits
The outputs of the two rectifier circuits (21 and 23) are the corresponding KII! ! connected to the output of the #3 rectifying bridge circuit O3 C)*R bridge circuit (25) and one input of the comparator unit (13), The input is passed through the diode (34) to limit the electromotive force activating the traction motor by 4!(31)K.
An automatic control device for dynamics and braking in an independent transportation device, characterized in that the devices are connected to each other. l ml Quoted Motor Glue Activating Electromotive Force Limiter (3
1) uses a diode bridge circuit (29, 30, 35 and 36) consisting of two parallel groups, #each of the two parallel groups consists of two diode bridge circuits (29°) connected oppositely. 30, and 35.36), and a series connected resistor (32Th and 37) connects the diode pre-ttZH path C29, 30, oh! 35
36) in parallel with El & sec, the central connection tap of the resistor being connected to a common point of each individual group of the diode bridge circuit.
Automatic braking control device for vertical transportation equipment as described in 2. 3. The automatic control device for Dyna-braking in the independent transportation device includes a speed stabilizer (40), which is connected to a non-contact speed setter (42) and correspondingly. Three rectifier circuits (43, 45 and 4
6) - to l), two of the three rectifier bridge circuits
The rotational speed converters (27 and 28) are connected to the two rectifier circuits (45 and 46), and the third rectifier circuit (43) of the three rectifier circuits is connected to the non-contact type coupled to the speed setter (42), a resistor (44) is provided in the output circuit of the rectifier bridge circuit (43, 45 and 46), the center tap of the resistor and the rectifier bridge circuit (43) 45 and 46) is connected to the input of the comparison unit (13) through diode P (47).