Abstract:
The present invention relates to a novel process for coating objects by means of direct current, in which process an adjustable DC voltage is pulse-modulated with an adjustable AC voltage. The process is useful for electrochemical coating of objects with resinous coating material. Preferably, the pulse modulation of the DC voltage is limited to certain time intervals during the coating process and the pulse modulation is connected and disconnected with an adjustable duty ratio.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process and an apparatus for coating objects by means of direct current. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Processes for depositing layers on objects by means of a voltage which pulsates to a greater or lesser extent are known from the prior art. For example, unregulated voltage spikes in the microsecond range are produced by means of thyristor-controlled rectifiers. These voltage spikes are pure interference pulses and are not used as a reproducible method for influencing the deposition result. Furthermore, the following disadvantages are symptomatic of working with poorly smoothed thyristor rectifiers. 
     1. Spark formation even under the coating surface on the sheet-metal surface to be coated. 
     2. Severe electrolysis. 
     3. Film thickness reduction. 
     4. Formation of flakes in the foam layer and on the sheet-metal edges. 
     5. After production of a breakdown, a greater reduction in voltage is required in order to reliably avoid this phenomenon with the next part to be coated. 
     From Brown, William B. (Journal of Paint Technology Vol. 47, No. 605, June 1975), it is known for a square pulse shape in the region of seconds to be produced by interrupting (disconnecting) the deposition current. This procedure has a number of disadvantages. For example, the specified pulse durations are in the region of seconds, preferably up to 3-20 seconds. In these relatively long pauses, on the one hand, the heat is dissipated and, in consequence, the layer resistance is increased. On the other hand, a redissolving effect also occurs, and in addition a softening of the deposited film and removal of gas bubbles as a result of the coating flow. This results in a reduction in the film resistance. 
     The reduction in the heat developed and in the peak current must in this case take place by slowly raising the voltage. Specifically, if one starts with a pulsed square-wave voltage at the full coating voltage immediately, then the rating of the rectifier must be more than doubled. This increases, in particular, the costs for the rectifier. 
     Furthermore, the currently available rectifier generators have considerable disadvantages. Specifically, depending on the type, they have a residual ripple which depends on the nature and quality of the rectification and smoothing of the input AC voltage (cf. Vincent, Journal of Coatings Technology Vol. 62, No. 785, June 1990). In addition, this residual ripple is load-dependent, that is to say feedback takes place via the coating process itself. This residual ripple is then also evident only as interference. 
     From T. Ito and K. Shibuya, Metal Finishing, April 1967, pages 48-57, “Anodic Behavior in Electrophoretic Coating of Aluminum Alloys”, it is known for pulsed signals to be produced by alternating current that has been smoothed more or less poorly. Furthermore, processes using alternating-current deposition are known from the German Laid Open Specification 1646130 and the British Patent Application 1376761. In this case, anode plates are used as rectifiers. The anode plates pass current in only one direction, because of special coating. 
     However, to date, all the described processes have considerable defects. In particular, the breakdown behavior, throwing power, film thickness and film defects are, for example, dependent, inter alia, on the magnitude of the voltage in electro-dipping. In practice, this voltage is normally chosen such that an adequate level of cavity coating is achieved, with the minimum necessary external film thickness, in an acceptable coating time. In order to save coating material, and thus cost, when coating, efforts are made, inter alia, to achieve adequate throwing power with reduced external film thicknesses. With present products and the present technique described above, this development is subject to limits. 
     The present invention is accordingly based on the object of providing an apparatus for electrochemical coating of objects, by means of which the coating film characteristics and the application characteristics can be influenced systematically in order to obtain, for example, adequate throwing power with reduced external film thicknesses, or in order to achieve preliminary cross-linking during application. 
     This object is achieved in that an adjustable DC voltage is pulse-modulated by superimposing adjustable AC voltage components on it. 
     The adjustable AC voltage components are in this case preferably produced from cyclic signals, in particular harmonic oscillations (sinusoidal oscillations), which are easily available. 
     According to the invention, it is in this case possible by means of suitable circuits to subject the cyclic signals to preprocessing, preferably blocking of the negative voltage elements or rectification. 
     The invention furthermore provides for the capability to connect and disconnect the superimposition of the AC voltage components on the DC voltage with an adjustable duty ratio. In this way, the pulse modulation, as a variation of the conventional coating process using pure direct current, can be limited to specific time intervals during coating, for example at the start or at the end. 
     The ranges between 10:1 and 1:10 are known as preferred on:off duty ratios. The duration of the “on” period, in which pulse modulation takes place, is in this case between 10 ms and 100 s. 
     The DC voltages used according to the invention are in the range from 0 to 500 V. The AC voltage components used for superimposition are likewise between 0 and 500 V. In this case, the superimposition is carried out such that the resultant voltage does not change its direction, that is to say said voltage is a pulse-modulated DC voltage. The apparatus according to the invention is, however, not limited to this, so that it is invariably also possible to operate with a resultant AC voltage, if this provides advantages. 
     The cycle duration of the cyclic AC voltage components used for superimposition is, according to the invention, between 1 and 500 ms. This corresponds to a frequency of 1000 to 2 Hz. A frequency is preferably used which is obtained from the mains voltage, that is to say, for example, 50 Hz or a multiple of it. 
     There are various possibilities for producing a pulse-modulated DC voltage according to the invention. 
     One variant is to connect an AC (variable) transformer in series with a DC generator. 
     It is likewise possible to couple the AC (variable) transformer via a rectifier, so that a rectified AC voltage is introduced. If a diode is in this case connected between the alternating-current source and the input of the rectifier, further modulation of the voltage is achieved in such a way that only the positive or only the negative half-cycles reach the rectifier. 
     The optional use of pulse modulation can be carried out such that the AC voltage components are introduced via a mechanical or electronic relay. The latter may be driven via a function generator (that is to say with low current) in order to achieve a defined duty ratio. 
     A further variant for producing a pulse-modulated DC voltage according to the invention is obtained by connecting a function generator to the phase-gating controller of a three-phase rectifier. This saves the cost and space requirement for an additional AC generator. The function generator may be a commercially available electronic device. It is preferably a programmable microprocessor system, in particular preferably a computer having appropriate software, having an analog/digital converter for receiving the control voltage, and having an output unit for the trigger pulses. 
     One preferred application of the apparatus according to the invention is for electro-dipping. In this case, the amount of coating deposited in the processing time is directly dependent on the amount of charge which flows—and thus indirectly on the immersion voltage. It must be noticed that a gas layer, which can break down the current flow, occurs at the so-called breakdown voltage, as a result of heating and boiling processes. It is furthermore important to obtain a uniform and adequate film thickness of the coating even at inaccessible points, that is to say an adequate throwing power with reduced external film thicknesses. The process according to the invention surprisingly achieves an optimized result with respect to these requirements, some of which are contradictory. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in the following text with reference to the figures wherein: 
     FIG. 1 is a schematic view of an apparatus for coating objects according to a first embodiment; 
     FIG. 2 is a schematic view of an apparatus for coating objects according to a second embodiment; 
     FIG. 3 is a schematic view of an apparatus for coating objects according to a third embodiment; 
     FIG. 4 is a schematic view of an apparatus for coating objects according to a fourth embodiment; 
     FIG. 5 is a histogram (with a breakdown voltage plotted against a pulse proportion voltage) illustrating the results of a first example test; 
     FIG. 6 is a histogram (with a breakdown voltage plotted against a pulse proportion voltage) illustrating the results of a second example test; 
     FIG. 7 is a histogram (with a breakdown voltage plotted against a pulse proportion voltage) illustrating the results of a third example test; 
     FIG. 8 is a histogram (with a breakdown voltage plotted against a pulse proportion voltage) illustrating the results of a sixth example test; and 
     FIG. 9 illustrates the pulse modulation utilized in each of the first through fifth examples. 
    
    
     DESCRIPTION 
     FIG. 1 shows the DC generator  2  and the DC-decoupled AC variable transformer  1 . According to FIG. 1, the coupling, which can optionally be switched on and off via a switch c, takes place via the rectifier  3 . Depending on whether the diode b is or is not bridging the switch a, all the half-cycles or only the positive half-cycles are rectified by the rectifier. The respectively resultant pulse-modulated voltage is illustrated in FIG. 1 in Diagram a) (switch a open) and b) (switch a closed, diode bridged). The instantaneous values of the current and voltage can be detected and monitored by a measuring system  6 . The electro-dipping bath is denoted by the number  7 . 
     FIG. 2 shows a variant of the circuit from FIG. 1, in which, instead of the elements a, b and c, there is a semiconductor relay  4  between the variable transformer  1  and the rectifier  3 . This semiconductor relay  4  is controlled by a function generator  5 . The pulse modulation is in this way switched on and off with a defined duty ratio. Diagram a) at the lower edge of FIG. 2 shows schematically the resultant pulse-modulated voltage U tot  as a function of the signal U St  of the function generator. 
     FIG. 3 shows a circuit in which the function generator  8  acts on the phase-gating controller  9  of a thyristor bridge rectifier  10  for a three-phase source  11 . This results in cyclic switching between two phase angles F 1  and F 2 , which correspond to two output voltages U 1  and U 2 . The pulses then have the shape shown in Diagram  3   a  of smoothed three-phase pulses with two voltage levels. The residual ripple on the signals can be varied by the design of the smoothing device  12 . This circuit arrangement also makes it possible, of course, to switch over, via the function generator, between more than two voltage levels. 
     FIG. 4 shows a further variant of the apparatus according to the invention having a series circuit comprising a DC generator and an AC generator, in which series circuit the diode  13  has been added. 
     The rectifier circuit according to FIG. 1 has been used in the examples described in the following text. The maximum current level which can be achieved with the test layout was limited on average to  6  A by the variable transformer. The required current density was then reached by reducing the size of the active surface of the metal sheets to be coated. 
     Test Program for Examples 1 to 5 
     Coating of metal materials with various coatings (commercial products from BASF Lacke und Farben AG) 
     Qualities: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 FT 85-7042 
                 CATHODIP ® 
               
               
                   
                 FT 82-7627 
                 CATHOGUARD ® 
               
               
                   
                 FT 82-7640 
                 CATHOGUARD 350 ® 
               
               
                   
                 FT 25-7225 
                 CATHOGUARD 100B ® 
               
               
                   
                   
               
             
          
         
       
     
     Deposition conditions: 
     DC voltage: range of voltages up to breakdown in 20 V steps 
     Voltage pulses: 
     Example 1: Two 10 ms pulse half-cycles at 20 ms (equivalent to 100 Hz) 
     Example 2: One 10 ms pulse half-cycle at 20 ms (equivalent to 50 Kz) Switch positions a)+b) at 0, 30, 60, 150, 250 V 
     Example 3: One pulse half-cycle; 10 s pulsed voltage, 110 s DC voltage (Pulses: 60, 150, 250 V) 
     Example 4: One pulse half-cycle; 10 s DC voltage, 110 s pulsed voltage (Pulses: 60, 150, 250 V) 
     Example 5: One pulse half-cycle; 60 s DC voltage, 60 s pulsed voltage (Pulses: 60, 150, 250 V) 
     Evaluation: breakdown voltage, film thickness SD 
     Test results: 
     EXAMPLE 1 
     Pulse modulation with two pulse half-cycles is set (frequency equivalent to 100 Hz, cf. Diagram a) in FIG.  9 ). The results are shown in FIG.  5  and Tables 1 and 2 (Column 1). Up to a level of 60 V, the breakdown voltage is governed by the peak voltage reached. In some cases, the pulsed element was increased to 250 V. This allowed peak voltages to be achieved, some of which were 40-50 V above those of pure DC deposition. 
     EXAMPLE 2 
     Pulse modulation with one pulse half-cycle was set (frequency equivalent to 50 Hz, cf. Diagram b) in FIG.  9 ). The results are shown in FIG.  6  and Tables 1 and 2 (Column 2). Considerably higher peak voltages were possible with all products by reducing the pulse repetition rate. This effect started even with voltage pulses of 30 V, and increased as the pulse level rose. With voltage pulses of 150-250 V, the difference between the breakdown voltage of DC deposition and the possible voltage peaks rose to values of 70-80 V. The film thickness at 20 V below the breakdown voltage decreased as the pulse proportion increased. 
     EXAMPLE 3 
     Coating operations were carried out with a 10 s pulse-modulated DC voltage (equivalent to 50 Hz), followed by 110 s of pure DC voltage (Diagram C) in FIG.  9 ). The results are shown in FIG.  7  and Tables 1 and 2 (Column 3) and are similar to those from Example 2, in which the DC voltage had voltage pulses superimposed on it throughout the entire coating process. 
     EXAMPLE 4 
     Coating was carried out with 10 s DC voltage and then 110 s DC voltage with a superimposed pulsed voltage (equivalent to 50 Hz) (Diagram d) in FIG.  9 ). The corresponding results can be found in Tables 1 and 2 (Column 4). In contrast to Example 3, the voltage pulses in this case were therefore not applied until after a coating time of 10 s. This variation allowed a further increase in the peak voltage to be achieved. With FT 82-7627, this effect resulted in improvements of a maximum of 20 V; with FT 82-7640, 20-40 V higher voltage peaks occurred. The most significant change was with FT 25-7225, with voltage increases of up to 60 V. 
     EXAMPLE 5 
     60 s DC voltage and 60 s DC voltage with superimposed pulse voltage were set (Diagram d) in FIG.  9 ). The results were identical to Example 4 (cf. Column 5 in Tables 1 and 2). 
     EXAMPLE 6 
     A bias resistor was integrated in the test layout. The results are shown in FIG.  8 . When the bias resistor was used, the reduction in the film thickness which was otherwise observed as the pulsed voltage amplitude was increased up to 150 V was no longer evident. Tables 3 and 4 show the data associated with FIG.  8 . 
     Result of Examples 1 to 6 
     The film thicknesses achieved at 20 V below the breakdown voltage are noted on the respective bars in all the graphs. It can be seen from this that, with the exception of the test conditions for Example 6, the achievable film thickness is reduced as the pulse level increases. This effect amounts to a few μm up to a pulse level of 150 V. The relevant film thicknesses are summarized in Table 2. 
     On the basis of the results shown above, the novel process is distinguished by the following advantages: 
     1. The sum voltage can be increased considerably above the breakdown voltage of conventional processes before any breakdown occurs. 
     2. The voltage which must be applied to achieve a specific film thickness can be varied over a wide range by the process according to the invention, by setting the ratio of the pulsed voltage element and the DC voltage element. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Influence of the AC voltage element 
               
               
                 on the breakdown voltage 
               
             
          
           
               
                   
                   
                   
                 50 Hz 
                 50 Hz 
                 50 Hz 
               
               
                   
                   
                   
                 10 s pulse + 
                 10 s DC + 
                 60 s DC + 
               
               
                   
                 100 Hz 
                 50 Hz 
                 110 s DC 
                 110 s pulse 
                 60 s pulse 
               
               
                   
                   
               
             
          
           
               
                 FT 85-7082 
                   
                   
                   
                   
                   
               
               
                 DC voltage 
                 400 volts 
                 380 V 
                 380 V 
                 380 V 
                 380 V 
               
               
                 DC + 30 V AC 
                 360-390 V 
                 380-410 V 
               
               
                 DC + 60 V AC 
                 340-400 V 
                 360-420 V 
                 360-420 V 
                 380-440 V 
                 380-440 V 
               
               
                 DC + 150 V AC 
                   
                 300-450 V 
                 300-450 V 
                 300-450 V 
                 320-470 V 
               
               
                 DC + 250 V AC 
               
               
                 FT 82-7627 
               
               
                 DC voltage 
                 360 V 
                 360 V 
                 360 V 
                 360 V 
                 360 V 
               
               
                 DC + 30 V AC 
                 340-370 V 
                 350-380 V 
               
               
                 DC + 60 V AC 
                 320-380 V 
                 320-400 V 
                 340-400 V 
                 360-420 V 
                 360-420 V 
               
               
                 DC + 150 V AC 
                 260-410 V 
                 280-430 V 
                 260-410 V 
                 300-450 V 
                 300-450 V 
               
               
                 DC + 250 V AC 
                 160-410 V 
                 200-450 V 
                 200-450 V 
                 200-450 V 
                 200-450 V 
               
               
                 FT 82-7640 
               
               
                 DC voltage 
                 350 V 
                 350 V 
                 350 V 
                 350 V 
                 350 V 
               
               
                 DC + 30 V AC 
                 330-360 V 
                 340-370 V 
               
               
                 DC + 60 V AC 
                 300-360 V 
                 320-380 V 
                 310-370 V 
                 360-420 V 
                 350-410 V 
               
               
                 DC + 150 V AC 
                 240-390 V 
                 260-410 V 
                 240-390 V 
                 300-450 V 
                 300-450 V 
               
               
                 DC + 250 V AC 
                 120-370 V 
                 160-410 V 
                   
                 180-430 V 
                 180-430 V 
               
               
                 FT 25-7225 
               
               
                 DC 
                 340 V 
                 320 V 
                 320 V 
                 320 V 
                 320 V 
               
               
                 DC + 30 V AC 
                 300-330 V 
                 300-330 V 
               
               
                 DC + 60 V AC 
                 280-340 V 
                 280-340 V 
                 280-340 V 
                 300-360 V 
                 300-360 V 
               
               
                 DC + 150 V AC 
                   
                 240-390 V 
                 260-410 V 
                 300-450 V 
                 300-450 V 
               
               
                 DC + 250 V AC 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Film thickness SD which is achieved 20 V 
               
               
                 below the breakdown voltage 
               
               
                 (Variation of the DC voltage and AC voltage element) 
               
             
          
           
               
                   
                   
                   
                 50 Hz 
                 50 Hz 
                 50 Hz 
               
               
                   
                   
                   
                 10 s pulse + 
                 10 s DC + 
                 60 s DC + 
               
               
                   
                 100 Hz 
                 50 Hz 
                 110 s DC 
                 110 s pulse 
                 60 s pulse 
               
               
                   
                   
               
             
          
           
               
                 FT 85-7042 
                   
                   
                   
                   
                   
               
               
                 DC voltage 
                 22 μm 
                 22 μm 
                 22 μm 
                 22 μm 
                 22 μm 
               
               
                 DC + 30 V AC 
                 20 μm 
                 22 μm 
               
               
                 DC + 60 V AC 
                 19 μm 
                 20 μm 
                 19 μm 
                 22 μm 
                 22 μm 
               
               
                 DC + 150 V AC 
                   
                 18 μm 
                 16 μm 
                 22 μm 
                 19 μm 
               
               
                 DC + 250 V AC 
               
               
                 FT 82-7627 
               
               
                 DC voltage 
                 26 μm 
                 26 μm 
                 26 μm 
                 26 μm 
                 26 μm 
               
               
                 DC + 30 V AC 
                 25 μm 
                 24 μm 
               
               
                 DC + 60 V AC 
                 25 μm 
                 23 μm 
                 25 μm 
                 25 μm 
                 25 μm 
               
               
                 DC + 150 V AC 
                 24 μm 
                 22 μm 
                 23 μm 
                 27 μm 
                 25 μm 
               
               
                 DC + 250 V AC 
                 23 μm 
                 21 μm 
                 16 μm 
                 21 μm 
                 17 μm 
               
               
                 FT 82-7640 
               
               
                 DC voltage 
                 33 μm 
                 33 μm 
                 33 μm 
                 33 μm 
                 33 μm 
               
               
                 DC + 30 V AC 
                 33 μm 
                 33 μm 
               
               
                 DC + 60 V AC 
                 30 μm 
                 31 μm 
                 28 μm 
                 34 μm 
                 33 μm 
               
               
                 DC + 150 V AC 
                 31 μm 
                 27 μm 
                 22 μm 
                 34 μm 
                 27 μm 
               
               
                 DC + 250 V AC 
                 17 μm 
                 22 μm 
                   
                 22 μm 
                 19 μm 
               
               
                 FT 25-7225 
               
               
                 DC 
                 17 μm 
                 15 μm 
                 15 μm 
                 15 μm 
                 15 μm 
               
               
                 DC + 30 V AC 
                 16 μm 
                 13 μm 
               
               
                 DC + 60 V AC 
                 16 μm 
                 13 μm 
                 13 μm 
                 14 μm 
                 13 μm 
               
               
                 DC + 150 V AC 
                   
                 12 μm 
                 11 μm 
                 15 μm 
                 13 μm 
               
               
                 DC + 250 V AC 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 FP 224/93 Residual ripple FT-25-7225 without R v   
               
               
                 Table entries = film thickness in μm 
               
             
          
           
               
                   
                 Extension of frequency 
                 10 s Start 
                 After 10 s 
                 After 60 s 
               
             
          
           
               
                   
                 — 
                 30 V 
                 60 V 
                 150 V 
                 60 V 
                 150 V 
                 60 V 
                 150 V 
                 60 V 
                 150 [lacuna] 
               
               
                   
                   
               
             
          
           
               
                 200 V 
                   
                   
                   
                 10.7 ± 0.4 
                   
                   
                   
                   
                   
                   
               
               
                 220 V 
                   
                   
                   
                 12.4 ± 0.8 
                   
                 9.9 ± 0.8 
               
               
                 240 V 
                 10.3 ± 0.4 
                   
                   
                   35 ± 19.8 
                   
                 10.6 ± 0.6  
                   
                 13.0 ± 0.4 
               
               
                 260 V 
                 11.4 ± 0.2 
                 10.1 ± 0.3 
                 12.5 ± 0.6 
                   
                   
                   
                 11.9 ± 0.4 
                 14.4 ± 0.6 
                 10.9 ± 0.2 
                 11.9 ± 0.5 
               
               
                 280 V 
                 14.3 ± 0.4 
                 12.8 ± 1.1 
                 27.6 ± 1-3.2 
                   
                 26.6 ± 10.8 
                   
                 14.0 ± 0.6 
                 15.2 ± 0.7 
                 12.5 ± 0.6 
                 13.6 ± 0.9 
               
               
                 300 V 
                 14.0 ± 0.4 
                 40.7 ± 2.5 
                   
                   
                   
                   
                 51.7 ± 11  
                   
                  28.1 ± 10.4 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 FP 224/93 Residual ripple FT-25-7225 R v  = 150 Ω 
               
               
                 Table entries = film thickness in μm 
               
             
          
           
               
                   
                 Extension of frequency 
                 10 s Start 
                 After 10 s 
                 After 60 s 
               
             
          
           
               
                   
                 — 
                 30 V 
                 60 V 
                 150 V 
                 60 V 
                 150 V 
                 60 V 
                 150 V 
                 60 V 
                 150 V 
               
               
                   
                   
               
             
          
           
               
                 Voltage 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (breakdown 
               
               
                 voltage - 
               
               
                 20) 
               
               
                 240 V 
                   
                   
                   
                 14.8 ± 0.2 
               
               
                 260 V 
                   
                   
                   
                 16.7 ± 0.7 
                   
                 13.0 ± 0.5 
                   
                 15.3 ± 0.5 
                   
                 13.1 ± 0.4 
               
               
                 280 V 
                   
                   
                   
                 18.0 ± 0.7 
                   
                 14.4 ± 0.7 
                   
                 16.4 ± 0.7 
                   
                 14.5 ± 0.5 
               
               
                 300 V 
                 15.5 ± 0.5 
                 16.5 ± 0.6 
                 16.8 ± 1.1 
                 19.1 ± 0.7 
                 15.3 ± 0.5 
                 17.1 ± 0.7 
                 17.7 ± 0.7 
                 18.4 ± 1.0 
                 15.6 ± 0.5 
                 17.0 ± 0.6 
               
               
                 320 V 
                 16.9 ± 0.7 
                 17.5 ± 0.7 
                 18.8 ± 0.9 
                   
                 17.3 ± 0.5 
                 22.5 ± 6.6 
                 17.4 ± 0.6 
                   
                 16.8 ± 0.4 
                 18.7 ± 0.7 
               
               
                 340 V 
                 18.5 ± 2.0 
                 31.4 ± 4.6 
                 19.8 ± 1.6 
                   
                 20.6 ± 5.7 
                   
                 19.3 ± 2.0 
                   
                 18.4 ± 1.1 
               
               
                 360 V 
                 26.4 ± 1-1.2 
               
               
                   
                 break- 
               
               
                   
                 down