Induction cooking hob including a cooking area with three or more induction coils and a method for controlling a cooking area

Abstract

The present invention relates to an induction cooking hob (10) including at least one cooking area (12), wherein the cooking area (12) comprises at least three induction coils (14). The induction coils (14) of at least one cooking area (12) are arranged side-by-side and in series. Each induction coil (14) of at least one cooking area (12) has an elongated shape. The longitudinal axes of the induction coils (14) within one cooking area (12) are arranged in parallel. Each induction coil (14) of the cooking area (12) is associated with a dedicated induction generator (16). The induction generators (16) are connected or connectable to at least one current line (18). The induction generators (16) are connected to and controlled or controllable by at least one control unit (20). Requested powers (rP) for each used induction generator (16) are adjusted or adjustable independent from each other by a user interface (22). Instant powers (iP) of the induction generators (16) within a cycle pattern (T1, T2, . . . , T11) are controlled or controllable independent from each other by the control unit (20). Further, the present invention relates to a method for controlling a cooking area.

Claims

1. An induction cooking hob including at least one cooking area, wherein: the at least one cooking area comprises at least three induction coils, each induction coil of the at least one cooking area is associated with an induction generator, the induction generators are connected to at least one current line, the induction generators are connected to and controlled by a control unit, requested powers for each used induction generator are adjusted by a user interface, the control unit is configured to control instant powers of the induction generators within each cycle pattern of a plurality of cycle patterns, wherein each of the cycle patterns defines whether each of the induction coils are simultaneously activated or deactivated, and the control unit is configured to select at least one of the cycle patterns of the plurality of cycle patterns, and wherein a sum of the instant powers of activated induction coils within each selected cycle pattern is equal to a sum of the requested powers for each activated induction coil.

2. The induction cooking hob according to claim 1, wherein the induction coils of the at least one cooking area have an oval and/or elliptical shape.

3. The induction cooking hob according to claim 1, wherein the induction generators are connected to the same current line.

4. The induction cooking hob according to claim 1, wherein the induction generators are connected to at least two different current lines, wherein said current lines have different phases.

5. The induction cooking hob according to claim 1, wherein the control unit is provided for performing at least one cooking mode, wherein the activated induction coils work with one single setting of the requested power.

6. The induction cooking hob according to claim 1, wherein the control unit is provided for performing at least one cooking mode with at least two different settings of requested powers, wherein at least one activated induction coil works with the setting of one requested power and at least one other activated induction coil works with the setting of another requested power.

7. The induction cooking hob according to claim 1, wherein the at least one cooking area comprises four induction coils.

8. The induction cooking hob according to claim 1, wherein the induction cooking hob comprises a number of pot detection devices, wherein each said induction coil is associated to at least one pot detection device.

9. The induction cooking hob according to claim 1, wherein the plurality of cycle patterns is stored in a memory of the control unit.

10. A method for controlling a cooking area on an induction cooking hob, wherein the cooking area comprises at least three induction coils and said method comprises the steps of: setting a requested power for each used induction coil by a user interface, selecting a number of subsequent plurality of cycle patterns from a table stored in a memory of a control unit, defining activated and deactivated induction coils based on each of the selected cycle patterns, determining a cycle time for each selected cycle pattern and a power balance between the activated induction coils, so that a desired average power for each induction coil is obtained over a period of one or more selected cycle patterns, and the sum of instant powers of the activated induction coils within each selected cycle pattern is equal to the sum of the requested powers for each used induction coil.

11. The method according to claim 10, characterized in that the difference between the instant powers of adjacent activated induction coils is small enough, so that the difference between frequencies associated to the instant powers avoids the generation of audible interference.

12. The method according to claim 11, characterized in that a predetermined number of the induction coils are activated within one cycle pattern.

13. The method according to claim 10, characterized in that the desired average power for each induction coil over the period of one or more selected cycle patterns is equal to the requested power for said induction coil.

14. The method according to claim 10, characterized in that a variation of the total instant power of the activated induction coils is a value as allowed by a given electromagnetic compatibility (EMC) standard.

15. The method according to claim 10, characterized in that a variation of total instant power of the activated induction coils within one cycle pattern is uniform.

16. The method according to claim 10, characterized in that the method is provided for an induction cooking hob including at least one cooking area, wherein: the at least one cooking area comprises at least three induction coils, each induction coil of the at least one cooking area is associated with an induction generator, the induction generators are connected to at least one current line, the induction generators are connected to and controlled by a control unit, requested powers for each used induction generator are adjusted by a user interface, the control unit is configured to control instant powers of the induction generators within each cycle pattern of the plurality of cycle patterns, wherein each of the cycle patterns defines whether each of the induction coils are simultaneously activated or deactivated, and the control unit is configured to select at least one of the cycle patterns of the plurality of cycle patterns, and wherein a sum of the instant powers of activated induction coils within each selected cycle pattern is equal to a sum of the requested powers for each activated induction coil.

17. An induction cooking hob comprising: at least one first cooking zone having a first induction coil and a first pot detection device for detecting when a pot is placed above the first induction coil in the at least one first cooking zone; at least one second cooking zone having a second induction coil and a second pot detection device for detecting when a pot is placed above the second induction coil in the at least one second cooking zone; and at least one control unit operatively coupled to each of the first and second induction coils and the first and second pot detection devices, said at least one control unit being configured in a predetermined cooking mode to apply a first predetermined power to the first induction coil, and to apply a second predetermined power to the second induction coil, the second predetermined power that is applied to the second induction coil is different from the first predetermined power that is applied to the first induction coil.

18. The induction cooking hob according to claim 17, said predetermined cooking mode being user-selectable via a dedicated key of a user interface of said hob.

19. The induction cooking hob according to claim 18, said first predetermined power and said second predetermined power also being user-selectable via said user interface.

20. An induction cooking hob comprising: a first cooking zone and a second cooking zone having respective first and second induction coils; a pot detection device configured to detect when a pot is located in each of the aforementioned cooking zones; and a controller configured to activate each of the coils only when a pot is located in the respective cooking zone, the first induction coil being activated at a first power level and the second induction coil being activated at a second power level higher than the first power level, wherein cooking power to the pot is automatically adjustable between the first and second power levels by sliding the pot across the cooking hob from the first cooking zone to the second cooking zone and from the second cooking zone to the first cooking zone, respectively.

Description

(1) The present invention will be described in further detail with reference to the accompanied drawings, in which

(2) FIG. 1 illustrates a schematic circuit diagram of a cooking area for an induction cooking hob according to a preferred embodiment of the present invention,

(3) FIG. 2 illustrates a schematic top view of the induction cooking hob according to the preferred embodiment of the present invention, and

(4) FIG. 3 illustrates a further schematic top view of the induction cooking hob according to the preferred embodiment of the present invention.

(5) FIG. 1 illustrates a schematic circuit diagram of a cooking area 12 for an induction cooking hob 10 according to a preferred embodiment of the present invention.

(6) The cooking area 12 comprises four induction coils 14 arranged side-by side. In this example, the four induction coils 14 form a line. A first, second, third and fourth induction coil 14 is denoted by the letter A, B, C and D, respectively. Further, the cooking area 12 comprises four induction generators 16, a current line 18, a control unit 20 and a user interface 22. The current line 18 is provided for supplying rectified mains voltage. The current line 18 is connected to power input terminals of the four induction generators 16. Each induction generator 16 corresponds with one induction coil 14. An output terminal of each induction generator 16 is connected to the associated induction coil 14. The user interface 22 is connected to an input terminal of the control unit 20. Four output terminals of the control unit 20 are connected to corresponding control input terminals of the induction generators 16. For example, the induction generator 16 is realized by a half-bridge inverter. Each induction coil 14 is associated to at least one pot detection device 15.

(7) By operating the user interface 22 different cooking modes can be selected by a user. For example, the user interface 22 may comprise dedicated touch keys for said cooking modes. In a preferred embodiment the following four cooking modes are provided. According to a first cooking mode, the four induction coils A, B, C and D work with one single power setting. According to a second cooking mode, the four induction coils A, B, C and D work with two different power settings, wherein the first and second induction coils A and B work with one power setting and the third and fourth induction coils C and D work with another power setting. According to a third cooking mode, the four induction coils A, B, C and D work with two different power settings, wherein the first induction coil A works with one power setting and the second, third and fourth induction coils B, C and D work with another power setting. According to a fourth cooking mode, the four induction coils A, B, C and D work with two different power settings, wherein the first, second and third induction coils A, B and C work with one power setting and the fourth induction coil D works with another power setting. The third and fourth cooking modes are the same in view of a functional aspect.

(8) In the first cooking mode, the induction coils 14 covered by cookware are activated at the same working frequency in order to cancel acoustic interference noise. However, in the second, third and fourth cooking modes, the induction coils 14 are affected by different power settings and therefore by different frequencies, so that acoustic interference noise has to be avoided. The acoustic interference noise occurs, if the frequency difference between adjacent induction coils 14 is within the audible range of the human ear. Since the power is set by the user, the frequency depends on the power setting, so that often the frequency difference may be within the audible range.

(9) In order to avoid the acoustic interference noise, the induction coils 14 are activated and deactivated according to a number of subsequent cycle patterns T1 to T11, in which not all of the induction coils 14 are activated during the same time. The sum of the instant powers iP of the activated induction coils 14 is kept in such a way that the differences of the instant powers iP between the cycle patterns T1 to T11 are small. In general, the variance of the instant powers iP must be small enough in order to comply with existing norms for flickering on the current line 18. The used cycle patterns T1 to T11 are structured in such a way that adjacent activated induction coils 14 have a small or no frequency difference. In contrast, the activated induction coils 14, which are not adjacent, may have different frequencies and powers.

(10) The induction coils 14 activated at a certain time should have a total instant power iP, which is equal to the sum of all requested powers rP. However, a variation of the total instant power iP between the cycle patterns T1 to T11 may be allowed with the scope of the EMC norms.

(11) The following table illustrates the possible combinations of activated and deactivated induction coils A, B, C and D, in which two, three or four of the induction coils A, B, C and D are activated at the same time. The first induction coil A is adjacent to the second induction coil B, in turn the second induction coil B is adjacent to the third induction coil C, and the third induction coil C is adjacent to the fourth induction coil D, as shown in FIG. 1. The second line to the fifth line of said table indicate the activated and deactivated states of the induction coils A, B, C and D, respectively. The eleven different cycle patterns are denoted by T1 to T11 in the first line. The last line of the table indicates the number N of the simultaneously activated induction coils A, B, C and D.

(12) TABLE-US-00001 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 A X X X X X X X B X X X X X X X C X X X X X X X D X X X X X X X N 4 3 3 3 3 2 2 2 2 2 2

(13) A number of the cycle patterns T1 to T11 is selected from the above table. A relative cycle time t for each selected cycle pattern T1 to T11 and a power balance between the induction coils A, B, C and D is set in such a way, that the desired average power for each induction coil A, B, C and D is achieved over one or more cycle patterns T1 to T11. The instant power iP of the individual induction coils A, B, C and D depends on the number of activated induction coils A, B, C and D, the selected power balance and the total requested power rP. It is preferred, that as many induction coils A, B, C and D as possible are activated within the given cycle pattern T1 to T11, so that the variation of the instant powers iP of the induction coils A, B, C and D are minimized, and that the power is uniform.

(14) The following table illustrates the default individual duty settings of the induction coils A, B, C and D for each cycle pattern T1 to T11. The numerical values in the second line to the fifth line of said table indicate the percentages of the power of the induction coils A, B, C and D, respectively. The last line of the table indicates the number N of the simultaneously activated induction coils A, B, C and D.

(15) TABLE-US-00002 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 A 0.25 0.33 0.33 0.33 0.5 0.5 0.5 B 0.25 0.33 0.33 0.33 0.5 0.5 0.5 C 0.25 0.33 0.33 0.33 0.5 0.5 0.5 D 0.25 0.33 0.33 0.33 0.5 0.5 0.5 N 4 3 3 3 3 2 2 2 2 2 2

(16) In particular, the cycle patterns T6, T7 and T11 can be selected as the preferred last cycle patterns, wherein the power balance between two activated induction coils A, B, C and/or D can be adjusted in order to achieve the desired power distributions. The activated induction coils A, B, C and/or D of the cycle patterns T6, T7 and T11 are not adjacent. Thus, the activated induction coils A, B, C and/or D of the cycle patterns T6, T7 and T11 may have arbitrary frequencies without generating acoustic interference noise.

(17) According to a first example, the requested power rP for the first induction coil A is rP=100 W, for the second induction coil B is rP=150 W, for the third induction coil C is rP=350 W, and for the fourth induction coil D is rP=400 W.

(18) In said first example, the method of selecting the cycle patterns and setting the duty are performed as follows. A cycle pattern with three activated induction coils B, C and D is selected, wherein the induction coil A is omitted, which has a requested power rP closest to the difference between the highest and second highest requested power rP. A further cycle pattern is selected with two activated induction coils C and D having the highest and second highest requested power rP until the power is reach for the second highest power. Another cycle pattern is selected with two activated induction coils A and D having the highest requested power iP and the biggest distance from each other. The power balance of the activated induction coils A and D is adjusted in order to reach the requested power rP. Thus, the cycle patterns T5, T9 and T11 are selected.

(19) The relative cycle time t of the cycle pattern T5 is calculated in such a way, that the lowest requested power rP of the activated induction coils B, C or D is reached. This is the requested power rP=150 W for the second induction coil B. Further, the sum of the instant powers iP of the activated induction coils B, C and D is equal to the sum of the requested powers rP for the induction coils A, B, C and D, which is rP=1000 W. The relative cycle time t of the cycle pattern T5 is given by:
t(T5)=iP(B)/(rP(A,B,C,D)/3)=150 W/(1000 W/3)=0.45

(20) The relative cycle time t of the cycle pattern T9 is calculated in such a way, that the instant power iP of the third induction coil C during the cycle pattern T5 is reached. Said instant power iP of the third induction coil C during the cycle pattern T5 is given by:
iP(C;T5)=(rP(A,B,C,D)/3)*t(T5)=(1000 W/3)*0.45=150 W

(21) In the cycle pattern T9 there are two activated induction coils C and D, so that the instant power iP of each activated induction coil C and D is given by:
iP(C)=iP(D)=rP(A,B,C,D)/2=1000 W/2=500 W.

(22) The remaining power of the third induction coil C during the cycle pattern T9 is given by:
iP(C;T9)=rP(C)−iP(C;T5)*t(T5)=200 W

(23) The relative cycle time t of the cycle pattern T9 is given by:
t(T9)=iP(C;T9)/(rP(A,B,C,D)/2)=200 W/(1000 W/2)=0.4

(24) The relative cycle time t of the cycle pattern T11 is given as the remaining time.
t(T11)=1−t(T5)−t(T9)=1−0.45−0.40=0.15

(25) Since the two remaining activated induction coils A and D are not adjacent, the power balance of said two induction coils A and D can be arbitrarily adjusted in order to obtain the desired power for both induction coils A and D. The instant powers iP of the activated induction coil A and D are given by
iP(A;T11)=rP(A)/t(T11)=100 W/0.15=666.7 W
iP(D;T11)=1−iP(A;T11)=333.3 W

(26) The actual power aP of the fourth induction coil can be verified by:
aP(D)=1000 W*(0.45/3+0.4/2)+333.3 W*0.15=400 W

(27) The following table illustrates the relative cycle times t, the instant powers iP, the actual powers aP and the requested powers rP of the cycle patterns T1 to T11 according to the first example.

(28) TABLE-US-00003 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 aP rP t 0 0 0 0 0.45 0 0 0 0.40 0 0.15 iP (A) 0 0 666.7  100  100 iP (B) 333 0 0  150  150 iP (C) 333 500 0  350  350 iP (D) 333 500 333.3  400  400 Sum 1000 1000 1000 1000 1000

(29) According to a second example, the cooking zones associated to the second, third and fourth induction coils B, C and D are always linked. In the second example, other combinations of cycle patterns are used.

(30) FIG. 2 illustrates a schematic top view of the induction cooking hob 10 according to the preferred embodiment of the present invention. A small cooking vessel 28 and a big cooking vessel 30 are arranged on the induction cooking hob 10. FIG. 2 relates to the second example.

(31) The induction cooking hob 10 comprises a cooking area 12 including the four induction coils 14 arranged in series. Moreover, the induction cooking hob 10 comprises two further induction coils 24 and 26. The four induction coils 14 are elliptical, while the further induction coils 24 and 26 are circular. The longitudinal axes of the four induction coils 14 are arranged in parallel. The small cooking vessel 28 is arranged above the first induction coil A, while the big cooking vessel 30 is arranged above the second, third and fourth induction coils B, C and D. The positions of the small cooking vessel 28 and the big cooking vessel 30 relates to the second example.

(32) The second example differs between two cases. In a first case the power setting of the first induction coil A is lower than the individual requested powers rP of the other induction coils B, C and D, while in a second case the power setting of the first induction coil A is higher than the individual requested powers rP of the other induction coils B, C and D.

(33) In the first case the cycle patterns T1 and T5 are applied. The cycle pattern T1 is applied until the requested power for the first induction coil A is reached, while the cycle pattern T5 is applied during the rest of the time.

(34) The sum of the instant powers iP of all activated induction coils A, B, C and/or D is always equal to the sum of the requested powers rP. In the cycle pattern T1 there are four activated induction coils A, B, C and D, so that the instant powers of each induction coil A, B, C and D is a quarter of the sum of the requested powers. The sum of the requested powers rP is:
rP(A,B,C,D)=100 W+300 W+300 W+300 W=1000 W

(35) The relative cycle time t of the cycle pattern T1 is given by:
t(T1)=iP(A)/(rP(A,B,C,D)/4)=100 W/(1000 W/4)=0.4

(36) The remaining relative cycle time t of the cycle pattern T5 is given by:
t(T5)=1−t(T1)=1−0.4=0.6

(37) The following table illustrates the relative cycle times t, the instant powers iP, the actual power aP and the requested powers rP of the cycle patterns T1 to T11 according to the first case of the second example.

(38) TABLE-US-00004 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 aP rP t 0.4 0 0 0 0.6 0 0 0 0 0 0    1 iP (A) 250  100  100 iP (B) 250 333.3  300  300 iP (C) 250 333.3  300  300 iP (D) 250 333.3  300  300 Sum 1000 1000 1000 1000

(39) In the second case the power setting of the first induction coil A is higher than the individual requested powers rP of the other induction coils B, C and D.

(40) In the second case the cycle patterns T2 and T11 are applied. The cycle pattern T2 is applied until the requested powers for the second and third induction coils B and C are reached. In the cycle pattern T11 the instant powers of the first and fourth induction coils A and D are matched in order to obtain the requested powers for said first and fourth induction coils A and D.

(41) The sum of the instant powers iP is always equal to the sum of the requested powers rP. In the cycle pattern T2 there are three activated induction coils A, B and C, so that the instant power of each induction coil A, B and C is a third of the sum of the requested powers. The sum of the requested powers rP is:
rP(A,B,C,D)=300 W+100 W+100 W+100 W=600 W

(42) The instant power iP of each induction coil A, B and C during the cycle pattern T2 is
iP(A)=iP(B)=iP(C)=rP(A,B,C,D)/3=200 W

(43) The relative cycle time t of the cycle pattern T2 is given by:
t(T2)=rP(B)/(rP(A,B,C,D)/3)=100 W/(600 W/3)=0.5

(44) The remaining relative cycle time t of the cycle pattern T11 is given by:
t(T11)=1−t(T2)=1-0.5=0.5

(45) Since the two remaining activated induction coils A and D are not adjacent, the power balance of said two induction coils A and D can be arbitrarily adjusted in order to obtain the desired power for both induction coils A and D.

(46) The instant powers iP of the activated induction coil A and D are given by
iP(D;T11)=rP(D)/t(T11)=100 W/0.5=200 W
iP(A;T11)=rP(A,B,C,D)−iP(D;T11)=600 W−200 W=400 W

(47) The actual power aP of the first induction coil A can be verified by:
aP(A)=(600 W*0.5)/3+(400 W*0.5)=300 W

(48) The following table illustrates the relative cycle times t, the instant powers iP, the actual powers aP and the requested powers rP of the cycle patterns T1 to T11 according to the second case of the second example.

(49) TABLE-US-00005 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 aP rP t 0 0.5 0 0 0 0 0 0 0 0 0.5   1 iP (A) 200 400 300 300 iP (B) 200 100 100 iP (C) 200 100 100 iP (D) 200 100 100 Sum 600 600 600 600

(50) FIG. 3 illustrates a further schematic top view of the induction cooking hob according to the preferred embodiment of the present invention. Two medium cooking vessels 32 and 34 are arranged on the induction cooking hob 10. FIG. 3 relates to an example with two current lines on different phases.

(51) The induction cooking hob 10 comprises the cooking area 12 including the four induction coils 14 arranged in series. Additionally, the induction cooking hob 10 comprises the two further induction coils 24 and 26. The four induction coils 14 are elliptical, while the further induction coils 24 and 26 are circular. The longitudinal axes of the four induction coils 14 are arranged in parallel. A first medium cooking vessel 32 is arranged above the first induction coil A and the second induction coil B, while a second medium cooking vessel 34 is arranged above the third induction coil C and the fourth induction coil D. The first induction coil A and the second induction coil B are supplied by a first current line, while the third induction coil C and the fourth induction coil D are supplied by a second current line, wherein the first and second current lines are on different phases.

(52) In order to avoid the acoustic interference noise, the adjacent induction coils A, B, C and/or D cannot be activated at the same time with a frequency difference within the audible range. The sum of the instant powers iP of the first and second induction coils A and B should be constant. In a similar way, the sum of the instant powers iP of the third and fourth induction coils C and D should also be constant.

(53) The following table illustrates the possible cycle patterns T1 to T3.

(54) TABLE-US-00006 T1 T2 T3 A x x B x x C x x D x x

(55) The following table illustrates the relative cycle times t, the instant powers iP, the actual powers aP and the requested powers rP of the cycle patterns T1 to T3 according to an example, in which the first induction coil A and the second induction coil B are supplied by the first current line, while the third induction coil C and the fourth induction coil D are supplied by the second current line, wherein the first and second current lines are on different phases.

(56) TABLE-US-00007 T1 T2 T3 aP rP t 0 0.5 0.5 iP (A) 350 980 0 490  500 iP (B) 350 0 980 490  500 iP (C) 350 420 0 210  200 iP (D) 350 0 420 210  200 Sum 1400 1400 1400 1400

(57) If the sum of the requested powers rP of the first induction coil A and the second induction coil B is equal or about equal to the sum of the requested powers rP of the third induction coil C and the fourth induction coil D, then the cycle pattern T1 is applied the full time. However, if the above requested powers are different, then the cycle patterns T2 and T3 are applied, wherein the relative cycle time t is 0.5 or 50%. The sum of the instant powers iP of the first induction coil A and the second induction coil B is equal to the sum of the corresponding requested powers rP. In a similar way, the sum of the instant powers iP of the third induction coil C and the fourth induction coil D is equal to the sum of the corresponding requested powers rP.

(58) Another application of the present invention is the activation of a further cooking mode, wherein the requested power (rP) changes automatically according to the position of the cooking vessel on the cooking area. The system performs a pot detection on all coils in the cooking area. Depending on which coil or coils) that is (are) covered by the cooking vessel, power is applied to the coil (coils) according to a preset pattern. The requested power (rP) can for instance be low, for example about 400 W, if the cooking vessel is placed on one of the extreme parts of the cooking area. In contrast, the requested power (rP) can be high, for example about 3000 W, if the cooking vessel is placed on the other (opposite) extreme part of the cooking area. At last, the requested power (rP) can have an average value, if the cooking vessel is placed on a central portion of the cooking area, between the extreme parts. A user could be allowed to change the preset pattern from the user interface to obtain the best pattern for the cooking needs at every instance. Applied on the embodiment in FIG. 1, pair of coils could be utilised as the defining regions of preset power, e.g. if a vessel is placed on coils A+B a high power is applied, if placed on coils B+C a medium power I applied and if placed on coils C+D a low power is applied. Naturally, other combinations are possible. If a cooking vessel is moved or removed, a new pot detection can be performed to ensure that only the relevant coil or coils are active.

LIST OF REFERENCE NUMERALS

(59) 10 induction cooking hob 12 cooking area 14 induction coil 16 induction generator 18 current line 20 control unit 22 user interface 24 further induction coil 26 further induction coil 28 small cooking vessel 30 big cooking vessel 32 medium cooking vessel 34 medium cooking vessel A first induction coil B second induction coil C third induction coil D fourth induction coil N number of activated inductions coils Tn cycle pattern t relative cycle time iP instant power aP actual power rP requested power