Controller of an electric motor

Abstract

A controller for an electric motor includes a protective circuit for limiting current or for polarity reversal protection, the protective circuit including a field effect transistor having a gate. The protective circuit further includes a control unit for providing a control voltage for the gate, a smoothing capacitor for charge storage being provided at the gate.

Claims

1. A controller for an electric motor, comprising: a protective circuit for at least one of limiting current and protecting against a polarity reversal, wherein the protective circuit includes a field effect transistor having a gate; a control unit to output a control voltage for the gate, wherein there is a smoothing capacitor for charge storage at the gate, wherein the control unit is configured to control the smoothing capacitor with the aid of pulse width modulation to hold a voltage at the gate above a predetermined value.

2. The controller of claim 1, wherein there is a forward diode between the control unit and the gate to prevent a discharge of the smoothing capacitor during a time interval of the pulse width modulation.

3. The controller of claim 1, wherein the protective circuit includes polarity reversal protection and the field effect transistor is inserted into a supply line of the electric motor to a supply voltage.

4. A controller for an electric motor, comprising: a protective circuit for at least one of limiting current and protecting against a polarity reversal, wherein the protective circuit includes a field effect transistor having a gate; a control unit to output a control voltage for the gate, wherein there is a smoothing capacitor for charge storage at the gate, wherein the protective circuit includes a switch-on current limitation, and wherein the field effect transistor is connected, in series with an intermediate circuit capacitor, to a supply network.

5. A controller for an electric motor, comprising: a protective circuit for at least one of limiting current and protecting against a polarity reversal, wherein the protective circuit includes a field effect transistor having a gate; a control unit to output a control voltage for the gate, wherein there is a smoothing capacitor for charge storage at the gate, wherein the electric motor is intended to run for only a fraction of the time during which the controller is supplied with a voltage.

6. A method for controlling a protective function in a controller, the method comprising: detecting that a fault condition does not exist, wherein the controller includes a protective circuit for at least one of limiting current and protecting against a polarity reversal, and wherein the protective circuit includes a field effect transistor having a gate; a control unit to output a control voltage for the gate, wherein there is a smoothing capacitor for charge storage at the gate; providing a pulse-width modulated voltage at the gate of the field effect transistor; and selecting a duty factor of the pulse-width modulated voltage so that the voltage at the gate is held above a predetermined value.

7. The method of claim 6, wherein the duty factor is selected as a function of a capacitance of the smoothing capacitor and of a leakage current of the field effect transistor.

8. The method of claim 6, wherein the pulse-width modulated voltage is reduced if a fault condition is detected.

9. A non-transitory computer readable medium having a computer program, which is executable by a processor, comprising: a program code arrangement having program code for controlling a protective function in a controller, by performing the following: detecting that a fault condition does not exist, wherein the controller includes a protective circuit for at least one of limiting current and protecting against a polarity reversal, and wherein the protective circuit includes a field effect transistor having a gate; a control unit to output a control voltage for the gate, wherein there is a smoothing capacitor for charge storage at the gate; providing a pulse-width modulated voltage at the gate of the field effect transistor; and selecting a duty factor of the pulse-width modulated voltage so that the voltage at the gate is held above a predetermined value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a wiring diagram of a controller for an electric motor.

(2) FIG. 2 shows curves over time of voltages at the controller from FIG. 1.

(3) FIG. 3 shows a flow chart of a method for controlling a protective function in the controller from FIG. 1.

DETAILED DESCRIPTION

(4) FIG. 1 shows a controller 100 for an electric motor 105. Controller 100 is configured for operating electric motor 105 on an electrical supply network 110 as a function of a control signal. An interface 115 may be provided for receiving the control signal and possibly for the return transmission of data.

(5) Controller 100 includes a control unit 120, which, in the specific embodiment shown here, includes what may be a digital processing unit 125, in particular in the form of a programmable microcomputer, and a power circuit 130. Processing unit 125 may include, in addition to interface 115, a clock generator 135 for providing a constant clock signal, or a programmable counter or timer 140. In the exemplary specific embodiment shown, processing unit 125 transmits a direction signal and a pulse-width modulated speed signal to power circuit 130 in order to effectuate a desired direction of rotation and rotational speed of electric motor 105. In one specific embodiment, diagnostic information may be transmitted from power circuit 130 back to processing unit 125.

(6) A bridge circuit 145, which is shown as a full bridge by way of example, connects electric motor 105, according to the inputs of power circuit 130, to a supply voltage, which is essentially drawn from supply network 110.

(7) In addition, an optional first and an optional second protective circuit 150 are provided. Protective circuit 150 shown at the top in FIG. 1 includes a field effect transistor 155, which here is an n-channel MOSFET. Field effect transistor 155 is installed in the forward direction, i.e., current also flows when field effect transistor 155 is switched off; when field effect transistor 155 is switched on, its installed free-wheeling diode is bridged. First protective circuit 150 therefore implements polarity reversal protection. A smoothing capacitor 165 is provided between a gate 160 of field effect transistor 155 and a terminal of supply network 110. An optional forward diode 170 is provided between control device 120 and gate 160 of field effect transistor 155 of protective circuit 150. In one specific embodiment, forward diode 170 is included in control unit 120. Forward diode 170 is polarized, as a function of the design of field effect transistor 155, in such a way that it permits charging of smoothing capacitor 165 when a voltage of control unit 120 has a corresponding polarity, but prevents a discharge when the voltage of control unit 120 has the reversed polarity.

(8) Control unit 120 is configured for controlling protective circuit 150 with the aid of a pulse-width modulated signal in such a way that a control voltage sets in at gate 160, the control voltage being above a predetermined value, a so-called plateau voltage, which is typically approximately 7 V.

(9) Smoothing capacitor 165 is periodically charged by way of the pulse-width modulated signal and is permanently discharged by way of a leakage current of field effect transistor 155. The modulation frequency and the duty factor of the pulse-width modulated signal are selected in such a way that the charging pulses essentially equalize the discharge over time, so that the control voltage at gate 160 is high enough to allow current to flow through field effect transistor 155.

(10) Protective circuit 150 shown on the right in FIG. 1 includes the same components as the other protective circuit 150 described above and implements a switch-on current limitation. The switch-on current limitation may be used to extend the service life of a relay in the vehicle, which supplies controller 100 with a voltage. Such a relay is used in a wide variety of vehicles under the designation KL15 relay. During every switch-on process of this relay, if a switch-on current limitation is not used, a high charging current flows into intermediate circuit capacitor 175, which greatly loads the relay.

(11) If the relay is closed, so that controller 100 is supplied with a voltage, Mosfet 155 is blocked and intermediate circuit capacitor 175 is connected to the vehicle electrical system only via a current limiting resistor 180. Only a low charging current flows into intermediate circuit capacitor 175, so that this is slowly charged. After a predetermined charging time after the relay is closed, controller 100 activates the voltage at gate 160 of field effect transistor 155 shown on the right in FIG. 1. Intermediate circuit capacitor 175 is connected to the vehicle electrical system with low resistance for as long as the relay is closed, without a high charging current flowing. After intermediate circuit capacitor 175 is charged, electric motor 105 may be switched on.

(12) FIG. 2 shows curves over time 200 of voltages at controller 100 from FIG. 1. Voltages are plotted in the vertical direction and times are plotted in the horizontal direction. A first curve 205 shows a pulse-width modulated signal, which controller 120 outputs at one of the protective circuits 150, and a second curve 210 shows the control voltage, which sets in at gate 160 of the corresponding field effect transistor 155.

(13) Using a commercially available field effect transistor 155 and a smoothing capacitor 165 having a capacitance of approximately 1 μF, second curve 210 may permanently be above a voltage of approximately 10 V if the first curve periodically delivers a positive voltage pulse of approximately 3 ms during a first time period T1 and subsequently remains at a low voltage level of approximately 100 ms during a second time period T2. The current uptake of an exemplary implementation of controller 100 from FIG. 1 is approximately 5.5 mA under these conditions. As compared to a specific embodiment in which a smoothing capacitor 165 is not used and field effect transistor 155 is controlled with a constant signal, this corresponds to energy savings of approximately 6.5 mA.

(14) FIG. 3 shows a flow chart of a method 300 for controlling a protective function in controller 100 from FIG. 1. Method 300 is configured, in particular, to be run on control unit 120. A first part of method 300 begins in a step 305, in which a current flowing through electric motor 105 is sampled. In a subsequent step 310, a check is carried out as to whether the sampled current is greater than a predetermined threshold value. If this is the case, it is determined that a fault condition exists and, in a step 315, a pulse width ratio of a signal, which controls protective circuit 150 shown on the right in FIG. 1, is modified in such a way that only a reduced current or even no current at all may flow through field effect transistor 155. If it is determined in step 310, however, that the determined current is lower than the predetermined threshold value, the pulse width ratio is set, in a step 320, in such a way that the current may flow freely through field effect transistor 155. Subsequently, the method returns from one of the steps 315 or 320 to step 305.

(15) A second part of method 300 begins in a step 325, in which it is determined whether the polarity with which controller 100 is connected to supply network 110 is reversed, i.e., whether reversed polarity exists. If this is the case, the method continues with step 315, otherwise with step 320, as described above. Method 300 then returns from step 315 or 320 to step 325.

(16) The first part of method 300 relates to protective circuit 150 shown on the right in FIG. 1, and the second part relates to protective circuit 150 shown at the top. If only one of the protective circuits 150 is provided, then only the part assigned to this part of method 300 may be omitted. The two parts of the method may also be carried out concurrently or integrated with one another, it being possible to carry out steps 305 and 325 in a fixed sequence or independently of one another. In yet another specific embodiment, the two parts of method 300 run fully independently of one another.