DE202011050264U1 - Device for inductive transmission of electrical energy - Google Patents

Abstract

Device for the inductive transmission of electrical energy from a stationary unit having a power supply unit (6) and a primary coil (2) to a mobile unit with a secondary coil (4) and an electrical load (5), characterized in that the device for feeding electrical power from the power supply unit (6) in the primary coil (2) and subsequent interruption of this feed, for measuring the time course of the resulting decay of an electrical operating variable of the primary coil (2), for comparing at least one parameter of the measured course with a corresponding parameter of a reference curve , which was determined at an interruption of the power supply to the primary coil (2) without the presence of an electrically conductive foreign body (11) in the region of the magnetic field generated by the primary coil (2), and to determine from this comparison, whether an electrically leiti Foreign object (11) in the range of the primary coil (11) generated magnetic field is provided is set up.

Description

The invention relates to a device for inductive transmission of electrical energy, in which an electrically conductive foreign body can be detected, according to the preamble of claim 1. Devices of this type are used for the inductive charging of a built-in electric vehicle, rechargeable battery. During the energy transmission, a magnetic field of high field strength and flux density is established between a stationary primary coil and a vehicle-side secondary coil. This is necessary in order to induce a sufficiently high current in the secondary coil for the desired transmission power.

If objects made of metallic materials are introduced into the area of such a field, eddy currents are induced in them which lead to a heating dependent on the material, the duration of the introduction and the magnitude of the field strength. In the presence of appropriate conditions, such an article may reach a temperature which may cause damage, e.g. for melting in plastic surfaces, or can lead to hazards to persons. The latter occur in particular when the secondary side has been removed and heated metal objects are freely accessible and can be touched by persons.

Due to the characteristic of previous applications for inductive energy transmission systems, a corresponding risk from metallic foreign bodies was assessed as irrelevant or, for example, in the case of industrial trucks (AGV) by brushes mounted in front of the secondary-side users, attempts to remove such objects from critical field areas. In the case of vehicles with driver, it may be pointed out in the course of a training to pay attention to such objects during operation and to remove them before commissioning of the inductive transmission or to not put the inductive transmission into operation in case of doubt. For a largely automatic operation or for higher security requirements, which can be assumed in particular when using such systems in publicly accessible areas, the previous security measures appear to be unsuitable or at least inadequate.

From the WO 2009/081115 A1 For example, a method for detecting an electrically conductive foreign body on an apparatus for inductive transmission of electrical energy is known in which temporarily increases the primary voltage and the reaction of the system is observed. Normally, in this case, due to a secondary-side power control, which keeps the transmitted power constant, a corresponding reduction of the primary current. However, a conductive foreign body on the primary coil ensures in this case by its power loss absorption for an increase in primary power and can be detected by this. If a foreign object is detected, the primary voltage is switched off to prevent heating of the foreign body. As problematic appears in this prior art that a small foreign body is difficult to detect, since the primary voltage can not be increased significantly compared to their nominal value.

The invention is based on the object to improve the reliability of an inductive energy transmission system to show a new solution for the detection of an electrically conductive foreign body, which is characterized by high accuracy and reliability.

This object is achieved by a device having the features of claim 1. Advantageous embodiments are specified in the subclaims.

According to the invention, first of all electrical power is fed from a power supply unit into the primary coil, this feed is interrupted and the time profile of the thereby caused decay of an electrical operating variable of the primary coil is measured. At least one parameter of the measured course is compared with a corresponding parameter of a reference course, which was previously determined without the presence of an electrically conductive foreign body. On the basis of this comparison it is determined whether an electrically conductive foreign body is present in the region of the magnetic field generated by the primary coil. In this way, the presence of a conductive foreign body can be detected with high accuracy, since such Abklingvorgang without conductive foreign body is essentially determined only by the low losses of the primary coil and therefore already significantly changed by the presence of a small foreign body.

It is expedient if the parameter used as the decision criterion is a relative change of the measured electrical operating variable within a predetermined time interval since the interruption of the power supply or the length of the time interval since the interruption of the power supply until reaching a predetermined relative change of the measured electrical operating variable. By considering relative changes, the dependence on absolute values of electrical operating variables, which are influenced by many factors and can drift in the long term as a result of aging processes, is avoided.

Advantageously, the device according to the invention makes it possible not only to determine whether a foreign body is present at all or not, but also to determine from the time course of the decay of the measured electrical operating variable a measure of the power produced by the foreign body during normal operation of the device is recorded. On the basis of this performance measure, it can be decided whether the power supply must remain switched off permanently, or whether an emergency operation is still possible with such reduced power that the power consumption of the foreign body is no danger and can be tolerated.

It is also very expedient to minimize the influence of the secondary side on the foreign body detection at the primary coil, since this in particular allows a periodic detection of foreign objects with high accuracy during the ongoing operation of the energy transmission to the secondary side. For this purpose, the power output from the secondary coil to the consumer is interrupted before the interruption of the power supply to the primary coil on the secondary side and thus eliminates an influence of the consumer on the decay process of a primary-side operating size.

Further particularly advantageous measures can be taken from the remaining subclaims. Hereinafter, embodiments of the invention will be described with reference to the drawings. In these shows

1 a schematic representation of a device for inductive energy transfer from a charging station to an electric vehicle,

2 a block diagram of the primary side of an apparatus for inductive energy transmission,

3 a more detailed circuit diagram of some components of the block diagram of 1 .

4 an exemplary course of the decay of the primary current at interruption of the power supply without the presence of a conductive foreign body,

5 an exemplary course of the decay of the primary current at interruption of the power supply in the presence of a conductive foreign body,

6 a partial circuit diagram of the secondary side of an apparatus for inductive energy transmission,

7 a summary of the advantageous method in the form of a program flowchart.

1 shows an electric vehicle 1 which is used to charge its battery over the primary coil 2 a charging station is, in a schematic sectional view. At the bottom of the vehicle 1 is located in a housing 3 a secondary coil 4 that come with a charging electronics 5 connected as a consumer. This converts the parameters of inductively into the secondary coil 4 transferred electrical power in to charge the battery of the vehicle 1 suitable values. The primary coil 2 is from a power supply unit 6 fed the charging station and is in a housing 8th housed, which is stationarily mounted on a vehicle parking space. The power supply unit 6 is from a control unit 7 the charging station controlled.

Dashed are in 1 some field lines 9 of the primary coil 2 indicated during operation magnetic alternating field. Its main direction corresponds to the direction of the coil axis of the primary coil 2 and thus is the vertical direction. In the gap 10 immediately above the housing 8th the primary coil 2 During operation, there is a high magnetic field strength and flux density.

On the case 8th the primary coil 2 lies a metallic foreign body 11 , This may be, for example, from another vehicle, which is in front of the vehicle 1 at the charging station, have solved. It could also be a commodity lost by a person or an empty beverage can. Not least could be the foreign body 11 also intentionally deposited there by a person in sabotage intent. As already stated, the foreign body would 11 at a current supply of the primary coil 2 heat as a result of the eddy currents induced in it and thereby become a source of danger. Moreover, through him would be the efficiency of energy transfer to the secondary coil 4 impaired.

2 shows a block diagram of the power supply unit 6 from 1 with attached primary coil 2 , The power supply unit 6 contains a rectifier 12 , a DC link 13 , an inverter 14 and a matching circuit 15 , which are connected in series in the order named. The rectifier 12 is connected to a power supply network and converts its AC voltage into a DC voltage in the DC link 13 smoothed and buffered. The DC link voltage is provided by a converter 14 converted into an alternating voltage of predetermined frequency, via the balancing circuit 15 to the primary coil 2 is directed. The adjustment circuit 15 forms together with the primary coil 2 a resonant circuit and is dimensioned so that this resonant circuit when passing through the inverter 14 predetermined frequency is in resonance.

A somewhat more detailed representation of a part of the components of the primary side, namely the inverter 14 , the adjustment circuit 15 and the primary coil 2 shows 3 , The inverter 14 is constructed as H-bridge with four switches S ₁ to S ₄ , which are each connected to a free-wheeling diode, not shown. The adjustment circuit 15 exists in 3 only from a single capacitor C _T , which is parallel to the output of the inverter 14 is switched. In fact, the balancing circuit 15 be much more complex and contain a variety of components including inductors, but here it depends only on the formation of a resonant circuit, for which the individual capacitor C _{T is} basically sufficient, the example here as a balancing circuit 15 Is accepted.

The primary coil 2 contains a primary inductance L _P and in series thereto an ohmic winding resistance R _P , which without the presence of a foreign body 11 the dominant contribution to the total power dissipation of the primary coil 2 and the matching circuit 15 formed resonant circuit provides. Neglected are magnetization and eddy current losses, also called iron losses, in field-guiding elements which can be provided on both the primary side and the secondary side. The frequency with which the switches S ₁ to S _{4 are} actuated, is tuned to the resonant frequency of the resonant circuit so that it is operated in resonance.

The further resistor R _F in series with the primary coil 2 provides a simple model for the conductive foreign body 11 that is, when energizing the primary coil 2 absorbs electrical power and converts it into heat. Without presence of the foreign body 11 is the value of R _F in 3 equals zero. To the foreign body 11 For example, it would be possible to measure the primary power and the secondary power of the entire system and, by subtraction, the foreign body 11 determine the power loss caused. However, when charging an electric vehicle 1 In contrast to the charging of small devices such as mobile phones or electric toothbrushes a relatively large power must be transmitted, such a solution would be subject to a great inaccuracy, since two relatively large power values would have to be measured and subtracted from each other.

In order to achieve high accuracy in foreign body detection, according to the invention, the power supply to the primary coil 2 through the inverter 14 completely interrupted and the resulting decay of the primary current I _P or the primary voltage U _P measured. It is understood that this is the beginning of energization of the primary coil 2 must happen very soon, that is, before a foreign body 11 can heat up so much that it becomes a danger. The results of such measurements of the primary current I _P are in the 4 and 5 for two different cases, namely in 4 without the presence of a foreign body 11 , ie for R _F = 0, and in 4 in the presence of a foreign body 11 , ie represented for R _F ≠ 0. The primary current I _P at the time t _{0 of} the interruption of the power supply from the inverter 14 in the first case the value I ₀ , in the second case the value I _0F .

As both figures show, the primary current I _P has the course of a decaying oscillation. Its envelope I _PH (t) is known to be due to a decaying exponential function I _PH (t) = I ₀ · exp [- (t - t ₀ ) / τ] or I _PH (t) = I _0F · exp [- (t - t ₀ ) / τ] whose time constant τ = (2L _P ) / (R _P + R _F ). In the presence of a foreign body 11 , ie for R _F ≠ 0, the decay time constant is smaller, ie the primary current I _P sounds faster than without foreign matter 11 , The in 4 illustrated course of the primary current I _P is a reference curve, the one time without the presence of a foreign body 11 , ie for R _F = 0 is measured and stored, not the entire time course, but only characteristic parameters of the same as I ₀ , I _ref and Δt = t ₁ - to be stored t ₀ , where I _{0 is} the start value of the envelope of I _P at the time t ₀ of interruption of the power supply, and I _{ref is} the value of the envelope of I _P at time t ₁ .

It is understood that despite interruption of the primary power supply, the primary current I _{P must} continue to flow, so that by the additional power loss of any foreign body 11 influenced decay of I _P can take place. Dependent on the internal topology of the adjustment circuit 15 This may mean that through the switches S ₁ to S _{4 of} the inverter 14 a closed path must be created for the decay process. For example, if the trimming capacitor C _{T is} not parallel, but in series with the primary coil 2 would be, then by a closed state, the switch S ₂ and S ₄ in the open state, the switch S ₁ and S ₃ despite interruption of the power supply from the inverter 14 a closed circuit for the out of the primary coil 2 , the serial balancing capacitor C _T and possibly the foreign body resistance R _F existing resonant circuit can be provided.

To the presence of a foreign body 11 determine the value I _{1 of} the primary current I _P after the expiration of a predetermined time interval .DELTA.t = t ₁ - t ₀ since interruption of the power supply from the inverter 14 with that without the presence of a foreign body 11 measured Reference value I _{ref are} compared, where I ₁ <I _ref the presence of a foreign body 11 displays. Alternatively, the length of the time interval between the interruption of the power supply at time t ₀ and the reaching of the reference value I _ref by the primary current I _P with the reference time interval .DELTA.t = t ₁ - t _{0 are} compared, wherein a shorter time interval than .DELTA.t the presence of a foreign body 11 displays. This procedure is permissible if the output value I _{0F of} the primary current is interrupted when the power _{supply is} interrupted in 5 exactly with the corresponding value I _{0 of} the reference curve in 4 matches.

In practice, however, it may happen that the output value I _{0F of} the primary current I _P when the power _{supply is} interrupted in 5 something of the corresponding value I _{0 of} the reference curve in 4 deviates, ie that the prevailing at the reference measurement operating state of the system is no longer completely reproducible later. Therefore, it is preferable to compare the relative change of the primary current I _P between the values I _0F and I ₁ with the relative change between the values I ₀ and I _ref in the reference _measurement , ie instead of the absolute values I ₁ and I _ref respectively Conditions I _0F / I ₁ and I ₀ / I _ref to consider.

The output values I ₀ and I _{0F of} the primary current I _P need not necessarily be the values prevailing prior to the interruption of the power _{supply, as shown in FIGS} 4 and 5 but one could also each use a measured shortly after the interruption of the power supply value of I _P as the output value, ie the beginning of the time interval .DELTA.t for the measurement of the decay of the primary current I _P need not be the time of interruption of the power supply, but he could also be shortly afterwards. Like from the 4 and 5 can be seen, could in this case, the change in the decay behavior by the presence of a foreign body 11 be determined.

Upon detection of a foreign body 11 In the simplest case, the power can be fed into the primary coil 2 , which was interrupted anyway for the measurement, remain interrupted and an indication activated and / or a message to a higher-level unit and / or the user of the electric vehicle 1 be dropped off; be discontinued; be deducted; be dismissed. However, it is also possible that a foreign body 11 has a size that would pose a hazard to operating the system at rated power but would be tolerable in reduced power operation. In this case, it is of interest to allow such reduced power operation, especially if rapid removal of the foreign matter 11 not as possible.

To operate at a reduced capacity despite the presence of a foreign matter 11 To enable, the size of the foreign body needs to be 11 , ie its power consumption during normal operation with rated power, can be quantified, thus that of the inverter 14 delivered power can be reduced to a value at which the power consumption of the foreign body 11 represents no danger. This quantitative determination of a foreign body 11 absorbed power loss can be based on the circuit model of 3 and the measured time courses according to 4 and 5 respectively.

Due to the formula for the exponential curve of the envelope of the primary current I _P (t) applies to the reference current profile of 4 : I _ref = I ₀ exp [- (t ₁ -t ₀ ) / τ _ref ] with the time constant τ _ref = (2L _P ) / R _P

From this the value of R _P can be calculated: R _P = [2L _P / (t ₁ -t ₀ )] · [-ln (I _ref / I ₀ )]

Analog applies to the current flow with existing foreign body 11 from 5 : I ₁ = I _0F exp [- (t ₁ -t ₀ ) / τ ₁ ] with the time constant τ ₁ = (2L _P ) / (R _P + R _F )

From this the sum R _P + R _F can be calculated: R _P + R _F = [2L _P / (t ₁ -t ₀ )] · [-ln (I ₁ / I _0F )]

This results in R _F being subtracted from R _P : R _F = [2L _P / (t ₁ -t ₀ )] ln [(I _ref * I _0F ) / (I ₁ * I ₀ )]

The in the time constants τ _ref and τ ₁ incoming inductance L _{P of} the primary coil 2 is either known or, assuming that ωL _P >> R _P + R _F , can be approximated as L _P = U _P / (ω I _P ), to which in addition to the primary current I _P and the primary voltage U _P measured must become.

The by a primary current I _P with the effective value I _Peff in a foreign body 11 with the effective resistance R _F converted power loss P _F is: P _F = R _F * I _Peff ²

When the maximum tolerable power loss of a foreign body 11 has the value P _Fmax , then the maximum permissible effective value I _{Pmax of} the primary current I _{P is} as follows:

Insertion of R _F yields:

The RMS value of the primary current I _P can be derived from the converter 14 be set by the control of the switches S ₁ to S ₄ to a desired value. In this way, even in the presence of a conductive foreign body 11 still be reduced compared to the rated power of the system reduced power. When charging an electric vehicle 1 In any event, incomplete charging as a result of reduced transmission power is preferable to abandoning charging in the presence of a foreign body, at least to allow for limited mobility.

If an electric vehicle 1 already accordingly 1 in charging position, then can the secondary coil 4 and the charging electronics 5 because of the inductive coupling of the secondary coil 4 with the primary coil 2 the course of the primary current I _P at an interruption of the primary-side power supply from the inverter 14 influence. This influence must be taken into account in the foreign body detection, ie eliminated or at least minimized.

As 6 shows, this is the secondary coil 4 , which is analogous to the primary coil 2 can be represented by a secondary inductance L _S and in a series to this switched winding resistance R _S , with the charging electronics 5 who have an in 6 not shown compensation circuit for forming a resonant circuit includes, connected via two switches S ₅ and S ₆ , by a short circuit of the secondary coil 4 can be made or the circuit between the secondary coil 4 and the charging electronics 5 can be interrupted.

When carrying out the foreign body detection according to the invention, the in 6 shown switch position in which both switches S ₅ and S _{6 are} open, advantageous, since in this case no secondary current I _S in the secondary coil 4 can flow. As a result, additional losses are avoided on the secondary side and the preceding embodiments with reference to the 3 to 5 apply without restriction. It is understood that for controlling the secondary-side switches S ₅ and S _6, a suitable communication connection between the control unit 7 the charging station and a corresponding control unit on board the vehicle 1 must exist.

Alternatively, the secondary coil 4 be short-circuited by the switch S ₅ . This also makes the power flow to the charging electronics 5 interrupted, the secondary current I _S , however, can continue to flow through the secondary inductance L _S and the winding resistance R _S. The latter in this case absorbs a power dissipation P _S = R _S * I _Seff ² , where I _{Seff is} the effective value of the secondary current I _S. Since I _{Seff is} proportional to I _Peff , the secondary-side winding resistance R _{S on the} primary side acts as an additional resistor k · R _S in series with the primary-side winding resistance R _P , where k = I _Seff / I _{Peff is} a proportionality factor. In the preceding formulas, R _P must then be replaced by R _P '= R _p + k · R _s . Otherwise, the foreign body detection and the determination of the despite a foreign body 11 still permissible primary current I _Pmax as described _above .

A coherent overview of a preferred embodiment of the advantageous method for performing on a device according to the invention is 7 in the form of a program schedule. The procedure begins in step 16 with the shutdown of the consumer, ie that on the secondary side either both switches S ₅ and S _{6 are} opened, or first parallel to the secondary coil 4 lying short-circuit switch S ₅ closed and then in the to the charging electronics 5 leading circuit breaker S _{6 is} opened. It is understood that step 16 only necessary if a secondary page is present.

Subsequently, in step 17 the power supply from the inverter 14 in the from the adjustment circuit 15 and the primary coil 2 existing resonant circuit interrupted. In step 19 then the decay of the primary current I _{P is} measured. In step 20 is determined by comparing the deviation of the ratio I _0F / I ₁ of the ratio I ₀ / I _ref with a threshold, whether an electrically conductive foreign body 11 in the range of the magnetic field of the primary coil 2 exists or not.

If the comparison in step 20 positive turns out to be in step 21 a measure of the power loss of the foreign body 11 calculated in terms of its equivalent resistance R _F. Based on the result of this calculation is in the subsequent step 22 Checked whether an emergency operation with reduced transmission power makes sense, ie whether a significant power can still be transmitted. If the comparison in step 22 positive turns out to be in step 23 the primary power through the inverter 14 set to a reduced value compared to the nominal value at which the power loss of the foreign body is still tolerable .. Thereafter, in step 24 the power supply from the inverter 14 in the primary coil 2 resumed and the consumer, ie the charging electronics 5 switched on again by the circuit breaker S _{6 is} closed and the short-circuit switch S _{5 is} opened. The foreign body detection and treatment is finished.

The continuation of the operation in step 24 is done bypassing the steps 21 to 23 then immediately after the step 20 when the comparison in step 20 has a negative result, so no foreign body is detected. When in step 22 is decided that because of the size of the foreign body no meaningful emergency operation with reduced power is possible, then in step 25 the supply of primary power switched off permanently. Alternatively to calculating the power loss of the foreign body 11 in step 21 can step in a positive comparison result 20 in a simplified advantageous embodiment also immediately to step 25 passed over and the primary power basically switched off permanently. This variant is in 7 shown in dashed lines.

In 7 not shown is the activation of a display and / or the posting of a message to a parent unit and / or the user of the electric vehicle. In principle, such measures always take place when the comparison in step 20 positive, since the presence of a foreign body is always a disturbance that must be remedied sooner or later, depending on its extent. In the case of a very small foreign object, which results in only a slight reduction of the transmission power, an indication may be sufficient to alert the vehicle user to the fault when returning to the vehicle. In the case of a large foreign body, which also makes emergency operation impossible, it seems necessary to immediately alert the vehicle user that the electric vehicle 1 can not be charged, for example, by a horn signal or by sending a text message to a mobile phone.

From the foregoing description, variations in the implementation of the invention will become apparent to those skilled in the art. For example, the circuit topology of the matching network 15 modified while maintaining the characteristics of a resonant circuit in combination with the primary coil and extended compared to the example shown here. Furthermore, iron losses in field-guiding elements can be taken into account in the calculation of the power loss by using in the power-loss calculation one of two deviating, ie slightly larger exponents of the effective value I _{Peff of} the current. Furthermore, the envelope or the effective value I _{Peff of} the primary current I _P can be approximated by metrologically easily detectable quantities, for example the envelope by the difference in magnitude of two successive peak values or the rms value by the rectification value, which is known to be proportional to the effective value. Such and comparable modifications are at the discretion of the person skilled in the art and are intended to be encompassed by protection of the claims.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• WO 2009/081115 A1 [0004]

Claims (13)

Device for the inductive transmission of electrical energy from a stationary unit to a power supply unit ( 6 ) and a primary coil ( 2 ) to a mobile unit with a secondary coil ( 4 ) and an electrical consumer ( 5 ), characterized in that the device for feeding electrical power from the power supply unit ( 6 ) in the primary coil ( 2 ) and subsequent interruption of this feed, for measuring the time course of the thereby caused fading of an electrical operating variable of the primary coil ( 2 ), for comparison of at least one parameter of the measured curve with a corresponding parameter of a reference curve, which in case of an interruption of the power supply to the primary coil ( 2 ) without the presence of an electrically conductive foreign body ( 11 ) in the region of the primary coil ( 2 ) was determined, and to determine from this comparison, whether an electrically conductive foreign body ( 11 ) in the region of the primary coil ( 11 ) generated magnetic field is present, is set up. Device according to claim 1, characterized in that the device for detecting the presence of an electrically conductive foreign body ( 11 ) is established when the deviation between the parameter of the measured course and the corresponding parameter of the reference course exceeds a predetermined threshold. Apparatus according to claim 1 or 2, characterized in that the parameter is the relative change of the envelope of the time course or the rms value of the measured electrical operating variable within a predetermined time interval, which begins with or shortly after the interruption of the power supply, or that the parameter Length of a time interval beginning with the interruption of the power supply or shortly thereafter, within which the envelope of the time course or the rms value of the measured electrical operating variable reaches a predetermined relative change. Device according to one of claims 1 to 3, characterized in that the electrical operating variable of the current I _P through the primary coil ( 2 ) or the voltage U _P across the primary coil ( 2 ). Device according to one of claims 1 to 4, characterized in that the device is set up in the presence of a foreign body ( 11 ) the feeding of electrical power into the primary coil ( 2 ) and to activate a display unit and / or issue an error message to a higher-level unit and / or a user of the device. Device according to one of claims 1 to 4, characterized in that the device is set up in the presence of a foreign body ( 11 ) from the time course of the decay of the measured electrical operating variable to determine a measure of the performance by the foreign body ( 11 ) is received during normal operation of the device. Apparatus according to claim 6, characterized in that the device is arranged, the feeding of electrical power into the primary coil ( 2 ) to be interrupted only when the measure of the foreign body ( 11 ) power consumed during normal operation of the device exceeds a predetermined threshold. Apparatus according to claim 6 or 7, characterized in that the device is set up on the basis of the determined measure of the foreign body ( 11 ) in the normal operation of the device recorded power in the primary coil ( 2 ) so far that the foreign body ( 11 ) absorbed power remains below a predetermined threshold. Apparatus according to claim 8, characterized in that the current fed into the primary _inductance is adjustable to a value which is less than or at most equal to a value I _Pmax , which according to the formula

in which P _{Fmax is} the maximum permissible value of the foreign body ( 11 ) absorbed power, L _{P is} the value of the inductance of the primary coil ( 2 ), .DELTA.t the length of a predetermined time interval, I ₀ and I _0F, the amplitude or the RMS value of the primary current I _P before or shortly after the interruption of the power _supply without presence of a foreign body ( 11 ) or in the presence of the foreign body ( 11 ) and I _ref and I _{1 are} the values to which the envelope or the effective value of the primary current I _P within the time interval Δt after interruption of the power supply without the presence of a foreign body or in the presence of the foreign body ( 11 ) decays. Device according to one of claims 1 to 9, characterized in that the device is arranged, in the presence of a mobile unit with a secondary coil ( 4 ) and an electrical consumer ( 5 ) before the interruption of the Power supply to the primary coil ( 2 ) the power output from the secondary coil ( 4 ) to the consumer ( 5 ) to interrupt. Device according to claim 10, characterized in that the secondary coil ( 4 ) short circuitable and / or the circuit between the secondary coil ( 4 ) and the consumer ( 5 ) is interruptible. Device according to one of claims 1 to 11, characterized in that by the interruption of the power supply to the primary coil ( 2 ) an electrical resonant circuit without external excitation can be produced, the primary coil ( 2 ), and that the course of the measured electrical operating variable has the form of a decaying oscillation of this resonant circuit. Device according to one of claims 1 to 12, characterized in that at least one adjacent to the primary coil ( 2 ) arranged measuring coil for measuring the time course of the interruption of the supply of electrical power in the primary coil ( 2 ) caused decay of an electrical operating variable of the primary coil ( 2 ) is provided.

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