EP1706939A1

EP1706939A1 - An induction motor control system

Info

Publication number: EP1706939A1
Application number: EP04797150A
Authority: EP
Inventors: Marcos Guilherme Schwarz; Ronaldo Ribeiro Duarte
Original assignee: Empresa Brasileira de Compressores SA; Whirlpool SA
Current assignee: Whirlpool SA
Priority date: 2003-11-28
Filing date: 2004-11-24
Publication date: 2006-10-04
Also published as: CN1886886A; KR101074557B1; JP4598776B2; CN100539386C; JP2007535283A; MXPA06005527A; KR20060121135A; BR0305338A; BRPI0305338B1; WO2005053147A1; ECSP066680A; AR046667A1

Abstract

An induction motor (10) control-system (1) / induction motor (10) assembly, an induction motor control system (1), a method of controlling an induction motor (10), and a compressor controlled with an induction motor according to the teachings of the present invention are described. The objective is to provide an assembly, a system and a method with which it will be possible to operate a motor (10) in a broad range of values of the network feed voltage (VAC), as well as a form of adjusting the controlled voltage (VC) that enables the correction of the voltage applied as a function of the network frequency (fAc).

Description

AN INDUCTION MOTOR CONTROL SYSTEM

This application claims the priority of Brazilian patent case N°.

PI0305338-5 filed on November 28, 2003 which is hereby incorporated by reference. Field of Invention

The present invention relates to an induction-motor-control- system/induction-motor assembly, a system of controlling an induction motor, a method of controlling an induction motor, in addition to a compressor controlled with an induction motor according to the teachings of the present invention. Description of the Prior Art Single-phase induction motors are widely used due to their simplicity, robustness and high performance. They are applied in household appliances in general, such as refrigerators, freezers, air conditioners, hermetic compressors, washing machines, motor-pumps, fans and some industrial applications. The known induction motors are generally provided with a cage- type rotor and a coiled stator, build from two windings, one of them being a running winding and the other a starting winding. During normal operation of the compressor, the running winding is fed by an alternate voltage, the starting winding being temporarily fed at the beginning of the start operation, creating a turning magnetic field in the stator gap, a condition that is necessary for accelerating the rotor and starting it.

The turning magnetic field may be achieved by feeding the start coil with a time-lag current with respect to the current circulating through the main winding, preferably at an angle close to 90 degrees. This time lag between the current circulating in the windings is achieved either by constructive characteristics of the windings or by installing an external series impedance with one of the winding, but generally in series with the starting winding. This value of the current circulating through the starting windings, during the process of starting the engine is generally high, requiring the use of some kind of switch for interrupting this current after passage of the time necessary to promote acceleration of the motor. Once the motor has been set to work, the magnetic field created by the running winding interacts with the field induced in the rotor and maintains the turning field necessary for the functioning of the motor.

Further with respect to the starting of this type of motor, since the rotor does not turn with the mere application of a voltage to the stator, means should be provided for causing the motor to turn, so that the machine can operate.

Examples of usual ways of starting an induction motor are described in the book by FITZGERALD, A. E., KINGSLEY, C. & KUSKO, A. - Electric Machines, Ed. McGraw-Hill do Brasil,1975. Due to constructive characteristics, induction motors should work in limited voltage and frequency ranges. A feed voltage much higher than that of the project increases the exciting winding too much, thus increasing the magnetic field in the gap and, consequently, saturating the ferromagnetic material of the rotor and stator. If the voltage is raised above this point, the current rises very rapidly due to the low reluctance of the magnetic field. On the other hand, a feed voltage much lower than that voltage of- the project significantly reduces the exciting current of the motor, thus reducing the torque available on the shaft, so that, in this case, the motor cannot support the normal load applied to its shaft, which gets into blocking state. A similar situation is caused by the variation in frequency. Drawbacks of the Prior Art

As far as the starting forms of single-phase induction motors are concerned, although present-day techniques make the starting of a motor feasible, such solutions result from more complicated constructions, since in the case of the use of a capacitor in series with the starting winding there is a good result with regard to the start of the motor, but this raises the costs of the final solution of the equipment. As far as the problem of variability of the level of voltage in the network of determined regions is concerned, one of the ways used for solving this problem has been the employment of supersized motors that work in a broad voltage range. These motors work below their maximum load, so that, even if there is a reduction in the feed, the motor still has traction force to bear the load. Also, due to the robustness of the motor, the latter may undergo a greater overvoltage than smaller motors, without problems of overcurrent and heating. This solves the problem of variation in the feed voltage, but evidently results in a need for the construction of large-size and heavy motors and, consequently, with a high cost.

With regard to the problems of variability of frequency in different regions of application of the motor, the solution has been the employment of different motors for different frequencies of the feed voltage, generating greater complexity in the management of production and stock and making it difficult or even preventing products from being used in regions having different feed frequencies.

A further solution employed at present is the use of motors with "taps" for selecting various voltage values of the feed network. This solution solves the problem of variation in voltage, but requires the employment of various power switches for commuting the taps of multivoltage motors.

Another option used for solving this problem is the employment of voltage stabilizers associated to the motor. This solution also proves to be little practical and further has a high cost. Objectives of the Invention The objectives of the present invention are an induction-motor control/induction-motor assembly, an induction-control system, a method of controlling an induction motor, in addition to a compressor controlled with an induction motor according to the teachings of the present invention, wherein it is possible to operate with the motor in a broad range of voltage values of the feed network.

It is also an objective of the present invention to provide such an assembly, system, method and compressor with the possibility of correcting the voltage applied as a function of the frequency of the feed network.

Further, according to the teachings of the present invention, one the possibility of starting the motor assembled in isolation or in conjunction with a compressor is foreseen, dispending the use of a starting capacitor. Further, according to the teachings of the present invention, another objective is to avoid oversizing of the motor and, consequently, the rise in cost of material, size and weight of the equipment.

A further objective of the present invention is to prevent overheating of the motor when the network voltage is high, improving its performance and expected life. Brief Description of the Invention

In order to solve the problems existing in the prior art and to achieve the objectives of the present invention, the assembly, system, method and compressor of the present invention are for use on a single- phase induction motor that is designed for operating below the usual value of the network, so that it will always be possible to feed the motor, and for this purpose it is sufficient if the value of the controlled voltage is lower than the value of the minimum network value, that is to say, the value of the network voltage should be a reduced one. The sizing of the motor is effected in such a way that the minimum torque required to -start the load is achieved with a voltage lower than the minimum voltage expected in the feed network. Thus, the torque supplied by the motor will always be greater than the minimum required, in any voltage condition supplied by the network. In order to conform the operation of the frequency fluctuation in the network, or even to adjust the motor when the latter is installed in a region where the network frequency is different from the nominal frequency of the motor, the establishment of an altered nominal voltage of the motor is foreseen, so that an adjustment of the level of voltage required for the motor to operate with a nominal current is used, thus preventing the problems of low torque or heating and burning the motor.

The objectives of the present invention are achieved by means of an induction-motor-control-system / induction-motor assembly, the motor and induction having a^; nominal operation voltage, the control system comprising a central processing unit, the central processing unit modulating the level of a controlled network voltage, the controlled voltage being applied to the induction motor, the network voltage fluctuating from a minimum network voltage, the network voltage having a network frequency, and the induction motor having, a nominal operation voltage and a nominal operation frequency, the induction motor having a nominal operation voltage lower than the value of the minimum network voltage, the processing unit measuring the network frequency and, when there is a difference between the network frequency and the nominal operation frequency, the processing unit establishes a value of an altered nominal voltage, the altered nominal voltage being applied to the induction motor and, when the network frequency is equal to the network frequency, the processing unit alters the value of the controlled voltage to the level of the nominal voltage.

The objectives of the present invention are also achieved by means of an induction-motor control system comprising a central processing unit associated to a network voltage, the processing unit being associable to the induction motor, the induction motor having a nominal operation voltage, the network voltage fluctuating from a minimum network voltage, the induction motor being fed by a controlled voltage obtained from the network voltage, the controlled voltage being adjusted by the central processing unit, the induction motor having a nominal operation voltage, the value of the minimum network voltage being a value higher than the value of the nominal operation voltage of the induction motor, the central processing unit adjusting the controlled voltage as a function of variations in the network voltage and, in starting, applies the network voltage to the induction motor.

The objectives of the present invention are further achieved by means of a method of controlling an induction motor, the induction motor having a nominal operation voltage and a nominal operation frequency, the induction motor being fed by a controlled voltage that is obtained from the modulation of a network voltage, the network voltage having a network frequency, the method comprising the steps of: (a) measuring the network voltage, measuring the controlled voltage and measuring the network frequency, b) comparing the value of the network frequency measured with the nominal operation frequency and, when there is a different between the network frequency and the nominal operation frequency, establishing a value of an altered nominal voltage and applying the controlled voltage in the value of the altered nominal voltage to the induction motor and, when the network frequency is equal to the nominal operation frequency, adjusting the value of the controlled voltage to the level of the nominal voltage. Brief Description of the Drawings

The present invention will now be described in greater detail with reference to an embodiment represented in the accompanying figures.

- Figure 1 represents a block diagram of the system of the present invention; and - Figure 2 represents a time diagram of the fluctuation of the network voltage. Detailed Description of the Figures

As can be seen in figure 1 , an electric induction motor 10 is fed by a controlled voltage Vc, which is obtained by modulating a network voltage VAC-

In order to carry out this modulating, a central processing unit 8 controls a set of switches 6 and 7, which are selectively actuated, controlling the level of the controlled voltage Vc.

The set of switches 6, 7 essentially comprises running switches 6 electrically interconnected to the running winding of the induction motor 10, and starting switches 7, which are electrically interconnected to the starting winding of the motor 10, which allows these windings to be selected by the processing central 8.

In order to monitor the level of the network voltage VAC and the level of the controlled voltage Vc, a first and a second voltage measuring devices 4, 11 are foreseen, respectively, both of them being associated to the central processing unit 8. This voltage sensors may be, for example, resistive dividers applied to an analog input of the central processing unit 8.

Further, a frequency sensor 5 is foreseen, which is associated to the central processing unit 8 and the network voltage VAC- This frequency sensor 9 may be a digital counter of cycles per second effected by the network voltage, may be an analog frequency converter for voltage or any circuit that determines the cycles effected by the network voltage in a certain interval of time.

According to the teachings of the present invention, the system 1 will measure the voltage and the current applied to the motor 10 and adjust these magnitudes by means of the central processing unit 8, the level of the controlled voltage V_c that is effectively applied to the motor 10, besides the measurement of the network frequency f_Ac so that the value of the controlled voltage Vc can be adequated when the network frequency f_Ac is different from the nominal operation frequency fN_M of the motor 10, in addition to the form of starting the motor 10 according to the teachings of the present invention. Control of the Level of the Controlled Voltage Vc

As can be seen in figure 2, the network voltage V_Ac will fluctuate within a foreseeable range of maximum network voltage V_AC- AX and a minimum network voltage V_AC-MIN-

In accordance with the teachings of- the present invention, a motor 10 having a nominal operation voltage VNM lower than the minimum network voltage V_AC-MIN should be used.

The motor 10 should be designed to function with a nominal operation voltage VN or an effective voltage value lower than the minimum value expected in the network feed voltage V_Ac that is to say, lower than the minimum network voltage V_AC-MIN, according to figure 2.

With this embodiment, since the nominal operation voltage VNM always remains below the value of the minimum network voltage V_AC-MIN, it will always be possible to feed the motor 10, and for this purpose it is enough for the value of the controlled voltage Vc to be lower than the value of the minimum network voltage V_AC-MIN. that is to say, the value of the network voltage V_AC should be reduced by modulation of the running switch 6. With this control system 1 , it is possible to operate the motor in a broad range of values of the network feed voltage V_AC. Starting the Motor 10 An advantage resulting from this form of application of the motor operating in conjunction with the control system 1 of the present invention, or even being controlled by means of a control system 1 having the operation voltage VNM designed to be below the minimum network voltage V_AC-MIN, lies in the fact that it is not necessary to use the starting capacitor for increasing the starting torque, since this increase is achieved by applying a voltage higher than the nominal voltage during the starting period.

This advantage occurs because, at the instant of starting the motor 10, the running switches 6 and starting switches 7 are commanded to apply the controlled voltage Vc to the motor 10, for instance, at the same value of the network voltage V_Ac. With the value of the network voltage V_Ac being higher than the value of the nominal operation voltage V_NM, a greater current i circulating in the motor 10 is guaranteed, to provide the torque required for the motor 10 rotor to start turning, even without the use of a starting capacitor. In addition, the assembly of the present invention further prevents the oversizing of the motor 10, resulting in a reduction of the material cost, size and weight-of these pieces of equipment.

Since in determined conditions the network voltage V_Ac may have very high values to proceed with the application of the controlled voltage Vc at the same value of the network voltage V_Ac, as described above, it may be necessary that, during the starting, the controlled voltage V_c applied to the motor 10 should only be adjusted to a value higher than the nominal operation voltage VNM of the motor.

Anyway, it should be observed that the value of nominal operation voltage V_NM should be designed so as to guarantee that the central processing unit 8 can always impose a controlled voltage value Vc higher than the value necessary to obtain the nominal torque of the motor 10.

Operationally, the starting of the motor 10 is effected by simultaneously energizing the starting and running windings, until the motor rotor has reached the nominal rotation or, at least, has a rotation substantially close to the nominal rotation, so that its normal functioning can be guaranteed, even after turning off the starting switch 7, which occurs after a pre-defined time for starting the motor.

Adjustment of the Controlled Voltage Vc as a Function of the Network Freouencv f_An

In order for the motor 10 to be always in an optimum condition, the central processing unit 8 should adjust the controlled voltage Vc as a function of variations in the network voltage V_Ac, to keep the value of the controlled voltage V_c constant. This is effected by measuring the frequency of the network voltage f_Ac by means of a frequency sensor 5, which, from the central processing unit 8, adjusts the controlled voltage Vc to keep it suitable to the normal conditions of the motor 10. Thus, if there is a variation in the frequency of the network voltage V_Ac, the controlled voltage Vc should be increased or decreased to prevent decrease or increase in the current i of the motor 10 stator, caused by the variation of the respective impedance for the new value of the network frequency f_Ac. The variations in current caused by variation in the load on the shaft will not affect the output voltage value imposed by the controller, which will only be-adjusted for variations in the frequency.

In order to implement this, the motor-control system 1 / motor 10 assembly should be designed, as well as the control system 1 taken in isolation, so that the processing unit 8 can measure the network frequency f_AC and compare the value of this measurement with the value of the motor operation frequency f M, the latter being previously established as a function of the type of motor 10 which is intended to be used. When the processing unit detects that there is a difference between the network frequency f_Ac and the nominal operation frequency fN . the processing unit 8 should establish a new value of nominal voltage VN that will be an altered nominal voltage VNM- _A. The value of the altered nominal voltage VNM-A is corrected as a function of the network and the correction will be proportional to the difference between the value of the network frequency f_Ac and the nominal operation frequency f_NM of the motor 10 and the controlled voltage V_c applied to the motor 10 will have the altered nominal voltage V_NM-A-

With the value of the altered nominal voltage V_NM-_A, the system can apply a corrected controlled voltage V_c for the new value of the nominal voltage to be applied to the motor 10, which will operate with a differentiated level of voltage as compared with the level of nominal voltage of the motor VNM and, in this way, it will prevent the current I of the motor 10 stator from being raised or lowered as a function of the variability of the network voltage f_AC.

In addition to making the adequate control over the motor 10 feasible in situations in which the network frequency f_AC is not constant, problems of use of a motor designed for a determined network frequency f_Ac of a determined region, which is employed in another region that has a different network frequency f_Ac can further be prevented.

The system of the present invention prevents problems of burning the motor 10 or low torque thereon. This fact can be understood based on the following example: if a motor 10 that is designed for operation with a network frequency f_Ac of 50 Hz is fed with a network frequency f_AC of 60 Hz, that is, higher than that foreseen in design, the motor impedance will increase and the current i of the motor 10 will decrease, resulting in low torque of the motor. In an inversed situation, if a motor designed for a network frequency f_Ac of 60 Hz is fed with a network frequency of 50 Hz, it will have an overcurrent i, which will result in excessive heating and even burning the motor 10.

Method of Controlling a Motor 10

In order to control the motor-control system 1 of the present invention, a method is foreseen which has the steps of initially measuring the network voltage V_AC, the controlled voltage Vc and the network frequency f_AC. The measured value of the network frequency f_Ac should be compared with the nominal operation frequency f_NM, the latter being already previously known due to the constructive characteristics of the motor 10. if a difference between the measured value of the network frequency f_Ac and the nominal operation' frequency fNM is detected, it can be concluded that the motor 10 is operating out of the ideal conditions, and so the controlled voltage Vc applied to the motor 10 should be raised or lowered, as already described. This raising or lowering the level of the controlled voltage Vc is carried out by a step in which the processing unit 8 establishes a new value for the controlled voltage Vc, designated altered nominal voltage VNM-A and, from this moment on, it can operate under this new condition of network frequency f_Ac without the problems of low torque or excessive heating of the motor 10.

When the processing unit 8 concludes that there is no difference between the value of the network frequency f_Ac and the nominal operation frequency f_N . the value of the controlled voltage V_c should be reduced to the level of the nominal voltage VNM- As far as the steps of the method of the present invention at the time of starting the motor 10 are concerned, the controlled voltage Vc of value equal to the network voltage V_Ac to the motor-starting winding should be applied and such voltage applied should be maintained for a starting time long enough for the motor to be substantially close to the nominal rotation and be able to operate normally.

As described above, considering that in determined- conditions the network voltage V_Ac may have too high values for proceeding with the application of the controlled voltage Vc at the same level of the network voltage V_Ac, it may be necessary that, during the starting, the controlled voltage Vc applied to the motor 10 should be adjusted only to a value higher than the nominal operation voltage V_NM motor, in this case being lower than that of the network voltage V_Ac.

Further, according to the teachings of the present invention, one should provide a compressor that is driven by a motor 10 provided with a control system 1 as described above should be foreseen, and this compressor may be employed either in conjunction with or separate from the compressor. A preferred embodiment having been described, one should understand that the scope of the present invention embraces other possible variations, being limited only by the contents of the accompanying claims, which include the possible equivalents.

Claims

1. An induction motor (10) control system (1 ) / induction motor (10) assembly having a nominal operation voltage (V_NM), the control system (1 ) comprising a central processing unit (8), the central processing unit (8) modulating the level of a network voltage (V_AC) to a level of controlled voltage (V_c), the controlled voltage (V_c) being applied to the induction motor (10), the network voltage (V_Ac) fluctuating from a minimum network voltage (V_AC-MIN), the network voltage (V_Ac) having a network frequency (f_Ac), and the induction motor (10) being designed for a nominal operation voltage (VNM) and a nominal operation frequency (f M), the assembly being characterized in that the induction motor (10) has a nominal operation voltage (V_NM) lower than the minimum network voltage (V_AC-MIN), the processing unit (8) measures the network frequency (f_Ac), and when there is a difference between the network frequency (f_Ac) and the nominal operation frequency (f_NM), the processing unit (8) establishes a value of an altered nominal voltage (VNM-A), the altered nominal voltage (VNM-A) being applied to the induction motor (10), and when the network frequency (f_AC) is equal to the network frequency (f_Ac), the processing unit alters the value of the controlled voltage (V_c) to the level of the nominal voltage (V_NM)-

2. An assembly according to claim 1 , characterized in that the value of the altered nominal voltage (V_NM-A) is corrected as a function of the network frequency and the correction is proportional to the difference between the value of the network frequency (f_Ac) and the nominal operation frequency (f_NM) of the induction motor (10).

3. An assembly according to claim 1 or 2, characterized in that the processing unit (8) reduces the value of the network voltage (V_AC) to the value of controlled voltage (Vc), the value of the controlled voltage (Vc) being lower than the value ot the minimum network voltage (V_AC-_MIN)-

4. An assembly according to claim 1 , 2 or 3, characterized in that the processing unit (8) comprises first and second voltage measuring devices

(4, 11), the first voltage measuring device (4) measuring the network voltage (V_Ac) and the second voltage measuring device (11) measuring the controlled^' voltage (Vc), the processing unit (8) controlling the level of the controlled voltage (Vc) from the measurements of the first and second voltage measuring devices (4, 11).

5. An assembly according to claim 4, characterized in that the control system (1 ) comprises a set of switches (6, 7) controlled by the central processing unit (8), the set of switches (6, 7) comprising:

- a running switch (6) electrically interconnected to a running winding of the induction motor (10); and

- a starting switch (7) electrically interconnected to a starting winding of the induction motor (10), the starting winding being selectively actuated by the processing central (8) when the induction motor (10) is starting, the processing circuit (8) applying the network voltage (V_AC) to the starting winding and to the running winding.

6. An assembly according to claim 5, characterized in that the processing unit (8) commands the turning-off of the switch (7) of the starting winding of the induction motor (10), when a previously established time for starting the motor has finished.

7. A control system (1) of an induction motor (10), comprising a central processing unit (8) associated to a network voltage (V_AC), the processing unit being associable with the induction motor (10), the induction motor (10) having a nominal operation voltage (V_N), the network voltage (V_AC) fluctuating from a minimum network voltage (V_AC-MIN), the induction motor (10) being fed by a controlled voltage (V_c) obtained from the network voltage (V_Ac), the controlled voltage (V_c) being adjusted by the central processing unit (8), the induction motor (10) having a nominal operation voltage

(VNM), the system being characterized in that the value of the minimum network voltage (V_AC-MIN) is higher than the value of the nominal operation voltage (VNM) of the induction motor (10), the central processing unit (8) adjusts the controlled voltage (V_c) as a function of variations in the network voltage (V_AC) and, upon starting, applies the network voltage (V_AC) to the induction motor (10).

8. A system according to claim 7, characterized by comprising:

- first and second voltage measuring devices (4, 11);

- a frequency sensor (5) measuring a network frequency (f_Ac) of the network voltage (V_AC), and the induction motor (10) has a nominal operation voltage (VNM) lower than the value of the minimum network voltage (V_AC-MIN), the processing unit (8) measures the network frequency (f_Ac), and when there is a difference between the network frequency (f_Ac) and the nominal operation frequency (f_NM) the processing unit (8) establishes a value of an altered nominal voltage (VNM-A), the value of the altered nominal (VNM-A) is corrected as a function of the network frequency and the correction is proportional to the difference between the value of the network frequency (f_AC) and the nominal operation frequency (f_NM) of the induction motor (10), the altered nominal voltage (V_NM-A) being applied to the induction motor (10), and when the network frequency (f_Ac) is equal to the network frequency (f_Ac), the processing unit alters the value of the controlled voltage (Vc) to the level of the nominal voltage (VNM).

9. A system according to claim 8, characterized by comprising a starting switch (7) being electrically interconnectable to a starting winding of the induction motor (10), the starting winding is selectively actuated by the processing central (8) from the starting switch (7) when the induction motor (10) is starting, the processing circuit (8) applies the network voltage (V_AC) to the starting winding.

10. A system according to claim 9, characterized in that the processing circuit (8) applies the network voltage (V_AC) for a starting time, the starting time corresponding a time necessary for the induction motor (10) to be substantially in nominal rotation.

11. A method of controlling an induction motor (10), the induction motor (10) having a nominal operation voltage (VNM) and a nominal operation frequency (f_N ), the induction motor (10) being fed by a controlled voltage (Vc) that is obtained by modulating a network voltage (V_Ac), the network voltage (V_Ac) having a network frequency (f_AC), the method being characterized by comprising the steps of:

(a) measuring the network voltage (V_AC), the controlled voltage (Vc) and the network frequency (f_AC),

(b) comparing the value of the network frequency (f_AC) measured with the nominal operation frequency (fNM) and, when there is a difference between the network frequency (f_AC) and the nominal operation frequency

(fN ), establishing a value of an altered nominal voltage (VNM-A) and applying the controlled voltage (Vc) at the altered nominal voltage (V_NM-A) to the induction motor (10), and * when the network frequency (f_Ac) is equal to the nominal operation frequency (fN_M), adjusting the controlled voltage (Vc) to the level of the nominal voltage (V_NM).

12. A method according to claim 11 , characterized in that, prior to step (a), in starting the motor (10), there is a step of applying the controlled voltage (Vc) equal to the network voltage (V_AC) to a starting and a running winding of the induction motor (10).

13. A method according to claim 11 , characterized in that, prior to step (a), in starting the motor (10), there is a step of applying the controlled voltage (V_c) lower than the network voltage (V_AC) to a starting and a running winding of the induction motor (10).

14. A method according to claim 12 or 13, characterized in that the application of the network voltage (V_AC) to the starting winding of the induction motor (10) is maintained for a starting time, the starting time corresponding a time necessary for the induction motor to be substantially in nominal rotation.

15. A compressor characterized by being actuated by an induction motor (10), the motor comprising a control system (1) as defined in claims 7 to 10.