DE29814762U1 - Ballast for the control of DC-operated electromagnets - Google Patents

Description

PATENT ATTORNEY KALDENKIRCHENER STR. 35a

DIPL.-ING. M. BONSMANN D-41063 Mönchengladbach

EUROPEAN PATENT ATTORNEY TEL: (02161) 12114 · FAX: 16296

Rudolf Freiwald

Grotstraat 12, B-2400 Mol, Belgium

Ballast for controlling
DC-operated electromagnet

File: 9 8 164

The present invention relates to a ballast
for controlling DC-operated electromagnets.

Electromagnets require a relatively high current during attraction, which is no longer needed at this level in the attracted state to maintain the required holding force and would also lead to the destruction of the insulation of the electromagnet coil through overheating.

When operating on alternating voltage, the relatively high inrush current gradually decreases by itself when the air gap is reduced due to the increase in reactance. It stabilizes at a lower value when the coil is pulled in. To maintain the required holding force, an air gap is usually used. In these cases, however, humming noises occur, which are perceived as annoying and cause wear due to the vibrations that occur on the iron core and on any mechanical coupling elements that may be present. To avoid humming noises, a "short-circuit ring" is pressed into the coil core. This causes a fairly high current loss.

consumption, which mainly manifests itself in the form of heat. Vibrations and the associated wear can be avoided by operating the electromagnets with direct current.

However, with direct current, the automatic reduction in the operating current after the air gap is closed is no longer necessary. To ensure that the coil insulation is not destroyed by the high operating current, a limiting resistor is connected in series with the coil on the contactors operated with direct current using an auxiliary contact. In particular, with high switching frequency, a considerable amount of maintenance is required to maintain reliability, combined with longer downtimes. Maintenance work on lifting magnets or solenoid valves is difficult in places that are difficult to access, for example in compact devices. Frequent operational malfunctions can be the result.

It is known that PTC thermistors (PTCs) that are thermally coupled to the coils can be used to reduce the current as the temperature increases in order to protect the coil from overheating. However, this is only of interest in applications in which the coils remain switched on for a very long time and can also remain switched off long enough after being switched off, since such circuits do not allow sufficient current to flow through the coil again shortly after being switched off. It should also be remembered that the coil has a very high heat capacity and a PTC effect itself. Quite apart from the harmful effect of the high temperature on the service life of the coil, the heating up of a medium located near the coil is also a disadvantage.

In contrast, the object of the present invention is to provide a ballast for controlling DC-operated electromagnets,

which enables continuous operation with a high switching frequency with very little maintenance effort.

According to the invention, to solve this problem, it is proposed to provide an RC element in a ballast of the generic type, which has a PTC thermistor with a series-connected matching resistor and a capacitor connected in parallel thereto and is connected in series with the magnetic coil of the electromagnet.

When using a ballast according to the invention, there are hardly any signs of wear on the electromagnets, even in continuous operation. It is extremely reliable even with high switching frequencies and is virtually maintenance-free and very cost-effective.

The RC element forms a "single-step" direct current signal in the circuit of an electromagnet in such a way that a high excitation current flows briefly while the core is being attracted and - after the air gap has closed - a low excitation current that is just high enough to hold the core securely (see Fig. 2 and 3). In this way, the energy conversion in the entire system of the electromagnet and thus the temperature increase in it can be kept as low as possible.

The capacitor connected in parallel to the PTC thermistor initially acts like a low-resistance shunt across this PTC thermistor. A strong current flows briefly through the magnetic coil, which strongly attracts the magnetic core (see Fig. 2). After the charging current surge of the capacitor has subsided, only a current flows - reduced by the PTC thermistor, which has now heated up - but which is completely sufficient to keep the core of the electromagnet in the attracted state (see Fig. 2).

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When the operating voltage is switched off, the iron core drops again immediately. The energy stored in the electromagnet, which is also low due to the low holding current, is dissipated relatively slowly, so that no high induction voltage is generated in the coil. Since the resistance of the PTC thermistor quickly decreases again after the supply voltage is switched off, the capacitor is quickly discharged to such an extent that it can be switched on again shortly after switching off if necessary. The PTC thermistor effect has three functions in this circuit: Its low cold resistance ensures a relatively high pull-in current, which is also increased for a short time by the capacitor connected in parallel. After pulling in, its high hot resistance limits the current during the holding phase, and after switching off, it quickly drops to a low value, which ensures that the capacitor is quickly discharged and can thus provide a high pull-in current again in the next cycle. In applications with a particularly high switching frequency, a series circuit consisting of another light bulb and a diode should be inserted in parallel to the RC combination described above to further accelerate the discharge of the capacitor.

Since a ballast according to the invention results in a low continuous operating temperature of the magnet system and does not generate a high induction voltage in the electromagnet coil, it is particularly suitable for use in firedamp and explosion-prone environments. Since the ballast does not necessarily have to be mounted close to the electromagnet, it can be installed in an easily accessible location outside the hazardous zone. In addition, the coil including the cable connection can be cast into the housing of the electromagnet, since the low temperature occurring at the coil has a

very long service life of the coil can be expected. In the nuclear or chemical sector, and here in particular for fluids with a low boiling point or low decomposition temperature, ballasts according to the invention can preferably be used to control solenoid valves. Temperature-sensitive media are not heated, so that evaporation, coagulation or disintegration effects and viscosity changes are avoided and the occurrence of corrosion is minimized.

When using a ballast according to the invention, there is hardly any wear on the electromagnets - even with high switching frequency. The ballast is therefore also very well suited to controlling solenoid valves in analysis devices and in the food and pharmaceutical industries, as contamination of the respective medium, such as that caused by abrasion of the core material or the gradual decomposition of the "short-circuit winding" in solenoid valves operated with alternating current, is avoided. If it is to be expected that media that have an aggressive effect on copper or aluminum can get into the solenoid valve, the "short-circuit ring" should be removed from the solenoid valve before it is installed, or solenoid valves without a "short-circuit ring" should be used from the outset.

The service life of electromagnets, which may remain switched on for a particularly long time in certain applications, is significantly increased by a ballast according to the invention, since the coil temperature only rises a few degrees above the ambient temperature even in continuous operation. This property is also advantageous in air conditioning and refrigeration technology. Due to the largely maintenance-free nature of the ballasts operated by the invention,
Electromagnets, these can also be used in hard-to-reach places without any problems. The ballasts

themselves can be grouped together in an easily accessible and central location and the light bulbs - which are already present in the ballasts - can be used to monitor the function of entire control groups, e.g. in automatic sequence controls.

In a preferred embodiment of the invention, a bridge rectifier is provided which can be connected to an AC voltage source and passes the rectified voltage on to the RC element. At the end of the "single-step" direct current signal, the energy stored in the electromagnet, which is already low as described above, is dissipated in the remaining circuit slowly enough via the bridge rectifier, which works as a freewheeling diode in this phase, so that on the one hand no high kickback voltage is generated in the excitation coil, but on the other hand the capacitor is discharged quickly enough and as completely as possible so that the electromagnet can be switched on again safely - possibly shortly afterwards.

In a further advantageous development of the invention, an adjustment resistor is connected in series with the PTC thermistor. After the charging current surge of the capacitor has subsided, the adjustment resistor can be used to set the current that is sufficient to keep the core of the electromagnet in the attracted state, for example 10 to 20 mA. A second PTC thermistor in series with a reversely polarized diode can be connected in parallel to the capacitor if necessary to further optimize the function with regard to particularly high switching frequency, which can be particularly advantageous with electrically operated impact tools such as hammer drills, staplers, etc.

Preferably, the PTC thermistor is formed from one or more light bulbs connected in series or parallel. These can, for example, have a cold resistance of 2 kiloohms and

have a warm resistance of 12 kiloohms. From this relatively low warm/cold resistance ratio of 6:1 it is clear that the light bulbs used in this circuit are not loaded up to their nominal voltage for reasons of operational safety (the usual ratio is 10:1). Light bulbs have a relatively low heat capacity and can release their stored energy very quickly through radiation. A particularly rapid release of energy through radiation can occur with light bulbs that have a filament designed as a single coil.

Preferably, each bulb has a filament made of tungsten. Tungsten bulbs are the fastest PTC thermistors currently known with a high hot/cold resistance ratio.

However, it is conceivable that the PTC thermistor is designed as an iron-hydrogen resistor with a possibly higher hot/cold resistance ratio and an even shorter time constant than tungsten light bulbs.

The invention is explained in more detail below using the drawings as an example. They show:

Fig. 1 shows the circuit of an embodiment of the present invention and

Fig. 2 and 3 show various diagrams to explain the operation of the invention.

Fig. 1 shows a circuit in which the present invention is embodied. The output of the clock generator T has two connections T1 and T2, via which an alternating voltage (normally 230 volts) is delivered to the input terminals 1 and 2 of the ballast V according to the invention, which - depending on the application - is a "single-step signal" of any

Duration (see Fig. 2), or a pulse pattern that is variable in terms of the frequency of the switching frequency and the duty cycle (see Fig. 3). The input stage of the ballast V according to the invention consists of a bridge rectifier 3, which in the case of direct control with direct voltage is replaced by an inversely polarized diode at the input terminals 4 (anode) and 5 (cathode) if the clock generator used does not already contain one. The direct current signal reaches the RC combination according to the invention from terminal 5. The resistance chain in the form of the matching resistor 6 in series with the light bulb 7 is parallel to a series circuit consisting of a light bulb 8 and a diode 9. Finally, the capacitor 10 is parallel to the network just described. The circuit is then closed via terminal 12, a conductor of the supply cable (to the electromagnet), the magnetic coil M, the second conductor of the supply cable via terminal 11, the input terminal 4 to the negative pole of the bridge rectifier 3 (or the anode of the freewheeling diode) and via this to the output terminal of the clock generator T.

The PTC thermistor 7 (in the form of a light bulb or similar) has a cold resistance of, for example, 2 kiloohms and a hot resistance of, for example, 12 kiloohms. After switching on, a strong current flows briefly through the magnetic coil M, which attracts the magnetic core, whereby the capacitor 10 connected in parallel to the resistance chain - consisting of matching resistor 6 and PTC thermistor 7 - initially acts like a low-resistance shunt across the aforementioned resistance chain. The voltage drop across the RC combination is therefore briefly almost zero, since the capacitor "swallows" the entire available current to charge it. Therefore, the full supply voltage is initially present at the output terminals 11 and 12 and thus at the magnetic coil M.

(see Fig. 2). The magnetic core is attracted from its rest position to the end position. During this time, the charging current of the capacitor decreases to zero and the resistance of the PTC thermistor increases until the current in the entire circuit reaches a stationary value which is proportional to the applied DC voltage and the sum of all resistances: adaptation resistance, heat resistance of the PTC thermistor and coil resistance of the magnetic coil. The level of the stationary current can therefore be adapted to the given situation (counterforce on the magnet, pressure in the solenoid valve, etc.) by selecting the adaptation resistance, so that the electromagnet remains safely in its end position until the input signal is interrupted. The adaptation resistance can be designed as a potentiometer for this purpose. When switched off, the iron core immediately drops again. In this case, no high induction voltage is generated in the coil of the electromagnet, since only a small amount of energy is stored in the electromagnet due to the low holding current and this is also dissipated relatively slowly.

In contrast, the capacitor 10 is discharged quickly and completely, since the resistance of the light bulb 7 decreases rapidly, because it can release its stored energy through radiation. This is a prerequisite for being able to switch on again - which may be necessary shortly after switching off. This is particularly important if the pulsating mode of operation shown in Fig. 3 is present. If the repetition frequency is particularly high, it may even be necessary to include the series circuit shown in Fig. 1, consisting of the light bulb 8 and diode 9, in the circuit, so that the capacitor can be discharged even faster if necessary. This part of the circuit has no influence on the way it works when switched on, since the diode 9 blocks in the inrush current direction.

Claims (7)

Protection claims 1. Ballast for controlling DC-operated electromagnets, with an RC element (6, 7, 10) connected in series to the coil of the electromagnet (M), which has a PTC thermistor (7) connected in series with a matching resistor (6) and a capacitor (10) connected in parallel to the PTC thermistor (7) and the matching resistor (6). 2. Ballast according to claim 1, characterized in that a second PTC thermistor (8) is connected in series with a diode (9) blocking in the inrush current direction and parallel to the RC element (6, 7, 10). 3. Ballast according to claim 1 or 2, characterized in that a bridge rectifier (3) is provided which can be connected to an alternating voltage source and passes the rectified voltage on to the RC element (6 to 10). 4. Ballast according to one of claims 1 to 3, characterized in that the PTC thermistor (7) is formed from one or more incandescent lamps (7) connected in series or in parallel. 5. Ballast according to claim 4, characterized in that each incandescent lamp (7) preferably has a filament designed as a single coil. 6. Ballast according to claim 4 or 5, characterized in that each bulb has a filament made of tungsten. 7. Ballast according to one of claims 1 to 3, characterized in that the PTC thermistor (7) is designed as an iron-hydrogen resistor with a suitable hot/cold resistance ratio and a suitable time constant.