CONTROL CARD FOR THE BRUSHLESS DIRECT CURRENT MOTOR

[0001] The present invention relates to a control card that controls a brushless direct current motor used in household appliances.

[0002] Particularly in the pumps used for the main circulation or water discharge operations in household appliances like dishwashers and washing machines, brushless direct current motors are preferred in terms of their high efficiency and silent operation. The brushless direct current motors are generally of three phase structure and are driven by the 120 degree trapezoidal wave form drive method. In the trapezoidal drive method, the electronic circuit structure providing speed control of the motor is of simple configuration and lower cost, however in the transition moments referred to as commutation wherein the motor current is transferred from one phase to the other phase, oscillations occur in the electric current supplied to the motor. The oscillations in the electric current causes the moment applied by the motor to also oscillate and thus results in the increase of noise. Moreover, in the state of the art, a microprocessor disposed in the motor control card compares the predetermined reference speed with the actual speed of the motor and generates PWM (pulse width modulation) signals having the suitable duty cycle in order to bring the motor speed to the reference speed, however the PWM signals cannot respond to the current oscillation during the commutation (Figure 1 ). Decreasing the current oscillation during commutation is also possible by controlling the phase currents directly with control of the current however the current detection resistor and analog signal amplifier circuit are required in order to maintain current control. The current control furthermore brings a considerable load to the microprocessor.

[0003] In the state of the art International Patent Application No. WO0227912, a motor control system is explained which applies the rounding effect on square wave phase currents in order to reduce the acoustic noise generated during the commutation passages. While the electric current that drives the motor is transferred from one phase to the other, the two phases overlap by means of the phase switches which are in transmission at the same time and the oscillation of the current is decreased by providing sudden increase at the moments of commutation.

[0004] The aim of the present invention is the realization of a control card that provides speed control of the brushless direct current motors used in household appliances like the dishwasher and the washing machine and decreases the motor noise.

[0005] The control card realized in order to attain the aim of the present invention and explicated in the claims, comprises a converter that converts AC voltage to DC voltage, a three-phase inverter which transforms the direct current received from the converter to three-phase current by means of the transistors and a microprocessor having a speed controller providing speed control of the motor by actuating the transistors in the three-phase inverter. The microprocessor compares the actual speed of the motor with the predetermined reference speed depending on the data received from the sensing unit which detects the rotor position and hence the rotational speed of the rotor by means of the hall sensors or the back EMF

measurement unit, controls the operation of the transistors in the three-phase inverter and changes the speed of the motor to the reference speed by generating duty cycle pulse width modulation (PWM) signals by means of the PI algorithm included in its software.

[0006] In the embodiment of the present invention, the microprocessor applies at least one generated PWM by increasing the duty cycle in the commutation duration wherein the current supplied to the motor is transferred from one phase to the other in the three-phase inverter.

[0007] During the commutation, the microprocessor calculates the duty cycle of at least one PWM generated thereby by multiplying with a coefficient that is greater than one.

[0008] In an embodiment of the present invention, during the commutation, the microprocessor applies more than one consecutive PWM signals by increasing the duty cycle.

[0009] In another embodiment of the present invention, the microprocessor

applies the PWM signals generated thereby during the entire commutation by increasing the duty cycle. [0010] In another embodiment of the present invention, the microprocessor applies one or more PWM signals by increasing the duty cycle only at the beginning of the commutation.

[001 1] In another embodiment of the present invention, the number of PWM

signals generated by the microprocessor by increasing their duty cycles is directly proportional to the switching frequency in the three-phase inverter.

[0012] In another embodiment of the present invention, the coefficient that is

greater than one to be multiplied for increasing the duty cycle of the PWM signals generated by the microprocessor is directly proportional with the electrical time constant of the motor.

[0013] The control card realized in order to attain the aim of the present invention is illustrated in the attached figures, where:

[0014] Figure 1 - is the schematic view of a control card.

[0015] Figure 2 - is the graph of the current applied to a electric motor and the

PWM signals in the state of the art.

[0016] Figure 3 - is the graph of the current applied to an electric motor and the

PWM signals in the embodiment of the present invention.

[0017] The elements illustrated in the figures are numbered as follows:

[0018] 1 - Control card

[0019] 2 - Converter

[0020] 3 - Three-phase inverter

[0021] 4 - Sensing unit

[0022] 5 - Microprocessor

[0023] The control card (1 ) user for controlling the brushless direct current

(BLDC) motor (M) comprises an AC-DC converter (2) for converting the AC mains voltage to DC voltage, a three-phase inverter (3) which transforms the direct current received from the AC-DC converter (2) to three phase (A, B, C) current by means of the transistors, a sensing unit (4) that detects the rotor position and hence the speed by hall sensors or by back EMF measurement method and a microprocessor (5) that provides speed control of the motor (M) by actuating the transistors in the three-phase inverter (3) with the signals received from the sensing unit (4).

[0024] The microprocessor (5) compares the actual speed of the motor (M) with the predetermined reference speed depending on the data received from the sensing unit (4), controls the operation of the transistors in the three-phase inverter (3) by generating PWM (pulse width modulation) signals having the suitable duty cycle by means of the PI (proportional integral) algorithm included in its software and changes the speed of the motor (M) to the reference speed.

[0025] In the embodiment of the present invention, within the commutation (X) duration wherein the current applied to the motor (M) is transferred from one phase (A, B, C) to the other phase (AB, AC, BC, BA, CA, CB) in the three phase inverter (3), the microprocessor (5) applies at least one PWM signal generated thereby by increasing the duty cycle.

[0026] The microprocessor (5) calculates the duty cycle of the PWM signals

generated thereby within the commutation (X) duration by multiplying with a coefficient that is greater than one (K>1).

[0027] The microprocessor (5) generates PWM signals having suitable duty cycle by comparing the actual speed of the motor (M) with the reference speed during operation of the motor (M) and adjusts the phase currents triggered by the PWM signals to a level that will bring the motor (M) to the reference speed and the PWM duty cycle determined by the PI control algorithm is increased during the commutation (X) providing to raise the phase current rapidly while passing from one phase to the other. Consequently, the microprocessor (5) decreases the current oscillations observed during the commutation (X), the moment oscillations and hence the acoustic noise during the operation of the motor (M) are decreased as a result of decreasing the current oscillations.

[0028] The coefficient (K) to be multiplied with the duty cycles of the PWM signals generated by the PI algorithm in the software of the microprocessor (5) is determined according to the relation between the pole number of the motor (M), the switching frequency and the electrical time constant of the motor (M). If the operating conditions of the motor (M) changes, modifications in the software of the microprocessor (5) can be made easily, for example only the value of the K coefficient can be changed, and the noise of the motor (M) is decreased by accommodating to different operating conditions.

[0029] In an embodiment of the present invention, during the commutation (X), the microprocessor (5) applies more than one consecutive PWM signals by increasing the duty cycle.

[0030] In another embodiment of the present invention, the microprocessor (5) applies the PWM signals generated thereby during the entire commutation (X) by increasing the duty cycle.

[0031] In another embodiment of the present invention, the microprocessor (5) applies one or more PWM signals by increasing their duty cycles only at the beginning of the commutation (X) and applies the PWM signals with duty cycles determined by the PI control algorithm in the remaining duration of the commutation (X).

[0032] In another embodiment of the present invention, the number of PWM

signals generated by the microprocessor (5) with increased duty cycle is directly proportional to the switching frequency in the three-phase inverter (3). For example, in the case wherein the switching frequency is 5 kHz, if the number of PWM signals with its duty cycle multiplied by the coefficient K is S1 , when the switching frequency is 10 kHz, then the number of PWM signals with its duty cycle multiplied by the coefficient K becomes S2 (S2>S1).

[0033] In another embodiment of the present invention, the coefficient K to be multiplied for increasing the duty cycle of the PWM signals generated by the microprocessor (5) is directly proportional to the electrical time constant (L/R) of the motor (M). In a motor (M) wherein the electrical time constant (L/R) is big, rapidly increasing the current with the PWM signal wherein for example the duty cycle is multiplied by a coefficient between 1 and 2 is difficult, therefore, the duty cycle of the PWM signal is applied by multiplying with a coefficient K that is for example greater than 2.

[0034] In the control card (1 ) of the present invention, by protecting the known speed feedback structure, a low current oscillation and hence a low moment oscillation is attained during the commutation (X) and the noise of the motor (M) is decreased by means of only the increase made in the duty cycle of the PWM signals applied by the microprocessor (5). An additional current control element is not required for decreasing the current oscillation.

It is to be understood that the present invention is not linnited by the embodiments disclosed above and a person skilled in the art can easily introduce different embodiments. These should be considered within the scope of the protection postulated by the claims of the present invention.