Abstract:
A drive unit, in particular for a fan in motor vehicles, is indicated, comprising an electric motor ( 12 ) operated on a DC line (U Bat ), and comprising a device ( 13 ) for controlling the speed of the motor ( 12 ) by changing the operating voltage applied at the motor ( 12 ). In order to cost-effectively realize a stepwise adjustment of the motor speed, the speed control device ( 13 ) comprises a plurality of resistors (R 1 -R 3 ) and a plurality of semiconductor switches (T 1 -T 3 ) for selectively connecting the motor ( 12 ) in series with at least one of the resistors (R 1 -R 3 ) (FIG.  1 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention is based on a drive unit, in particular for a fan in motor vehicles, according to the preamble of claim  1 .  
           [0002]    Such drive units are used in motor vehicles as fan motors to air condition or heat the passenger compartment, and to cool the coolant of the internal combustion engine. In the case of air-conditioning or heater fans, it is known, for example, that a step switch can be used to select various fan levels at which the DC motor connected to the vehicle&#39;s 12-volt electrical system is set at different speeds. In the case of air-conditioner fans for the coolant of the internal combustion engine, it is known that the speed of the DC motor driving the fan for the radiator can be adjusted using an electrical control unit as a function of engine management data.  
           [0003]    A known control unit for controlling a DC motor (DE 197 56 461 A1) comprises a semiconductor switch developed as MOSFET that is connected in series with the DC motor, and the control electrode of which is acted upon by a pulse width modulated control signal. The DC motor is therefore provided-by means of the semiconductor switch-with a mean voltage, the level of which depends on the modulation depth of the control signal. The speed of the DC motor decreases as the mean voltage decreases. The control signal is generated as a function of a setpoint value-actual value comparison of the power of the DC motor. It is also possible to generate the control signal from a set speed that is specified manually, for example, and eliminate a speed closed control loop.  
         ADVANTAGES OF THE INVENTION  
         [0004]    The drive unit according to the invention having the features of claim  1  has the advantage that a stepwise adjustment of the speed of the motor can be realized in very cost-effective fashion. Since the semiconductor switch is not operated in cyclical fashion as with the known control unit, but is instead operated in such a fashion that one semiconductor switch at a time is fully connected, and the remaining semiconductor switches are fully blocked, the electromagnetic compatibility (EMC) of the drive unit is not critical. When low-resistance semiconductor switches are used, the only power losses that occur in them are negligibly small, so that large-volume heat sinks can be eliminated. The device for controlling speed is compact and is capable of being integrated in a connector of the drive unit according to an advantageous embodiment of the invention, for example.  
           [0005]    Advantageous further developments and improvements of the drive unit indicated in claim  1  are possible due to the measures described in the further claims.  
           [0006]    According to an advantageous exemplary embodiment of the invention, for n+1 speed levels, the device comprises n series circuits, each having one resistor and one semiconductor switch, as well as a further semiconductor switch connected in series with the motor to the DC power supply. Each of the n series circuits is connected in parallel with the semiconductor switch of the series circuit for the subsequent speed level, and the last series circuit is connected in parallel with the further semiconductor switch. Resistors having identical ratings can be used for speed levels that differ by identical amounts in terms of speed, and the semiconductor switches can be dimensioned differently in accordance with their different current loads in order to make use of cost advantages. When the further semiconductor switch is closed, the maximum speed is set.  
           [0007]    According to an alternative exemplary embodiment of the invention, for n+1 speed levels, the device has n series circuits, each comprising one resistor and one semiconductor switch, and one further semiconductor switch connected in series with the motor to the DC power supply, whereby the n series circuits are connected in parallel with each other and with the further semiconductor switch. The resistors are to be rated differently in accordance with the selected speed level. In this case as well, however, the semiconductor switches can be dimensioned differently due to the fact that their current load decreases as the speed decreases. When the further semiconductor switch is closed, the maximum speed is set. 
       
    
    
     SUMMARY OF THE DRAWINGS  
       [0008]    The invention is described in greater detail in the following description with reference to the exemplary embodiments presented in the drawings.  
         [0009]    [0009]FIG. 1 shows a circuit diagram of a fan that can be operated at four fan levels,  
         [0010]    [0010]FIG. 2 shows the same representation of the fan in FIG. 1 according to a further exemplary embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]    The fan shown in the circuit diagram in FIG. 1 is intended for use in motor vehicles and is operated on the 12-volt DC power supply of the motor vehicle. It has a blower or fan wheel  11  that is coupled with a drive unit. The fan can be operated at four fan levels having different speeds, for the purpose of which the drive unit comprises an electric motor  12  developed as a DC motor and driving the fan wheel  11 , as well as a device for controlling the speed of the motor  12 , referred to hereinbelow as the speed control device  13 .  
         [0012]    As illustrated schematically in FIG. 1, the speed control device  13  is integrated in a connector  14  of the drive unit or the fan, which said connector comprises two plug contacts  15 ,  16  for connection to the DC power supply U Bat  of the motor vehicle, and one plug contact  17  for inputting a data signal for the fan level setting.  
         [0013]    The speed control device  13  comprises a plurality of resistors R 1 -R 3  and a plurality of semiconductor switches in the form of power transistors T 1 -T 3 , which are developed as low-side-power MOSFET switches in this case. For the four fan or speed levels realized in the exemplary embodiment, the speed control device  13  comprises three series circuits, each having one resistor R 1 -R 3  and one transistor T 1 -T 3 , as well as a further transistor T 4  developed as a power MOSFET switch. Each of the three series circuits is connected in parallel with the transistor of the series circuit for the subsequent speed level. The series circuit comprising resistor R 1  and transistor T 1  is therefore connected in parallel with transistor T 2 , and the series circuit comprising resistor R 2  and transistor T 2  is connected in parallel with transistor T 3 . The last series circuit comprising resistor R 3  and transistor T 3  is connected in parallel with the further transistor T 4 , which is connected in series with the motor  12  to the DC power supply U Bat . If n+1 speed levels are provided, n series circuits each comprising one resistor R and one transistor T are available, whereby-as described-each of the n series circuits is connected in parallel with the transistor of the series circuit for the subsequent speed level, and the last series circuit comprising resistor and transistor is connected in parallel with the further transistor T which—as described—is connected in series with the motor  12  to the DC power supply U Bat . The control electrodes of the transistors T 1 -T 4 —their gates in the case of MOSFETs—are connected to an electronic control unit  18  with its input situated at the plug contact  17 , which said electronic control unit actuates the transistors T 1 -T 4  in selectively conductive fashion. Only one of the transistors T 1 -T 4  is fully connected at a time, while the remaining ones are blocked. In accordance with the data signal provided at the input of the electronic control unit  18  via the plug contact  17 , the electronic control unit  18  generates a switch-point signal for one of the transistors T 1 -T 4  that causes the transistor to become connected; said switch-point signal is applied until the data signal is changed. To switch on the lowest fan level, at which the motor  12  runs at the lowest speed, the switch-point signal is applied to transistor T 1 . The operating voltage applied to the motor  12  is reduced by the amount of the voltage drop produced by the resistors R 1 -R 3  as compared with the DC power supply U Bat . As a result, the speed is reduced by a maximum amount as compared with the nominal speed of the motor  12 . At the highest fan level with the maximum speed of the motor  12 , the control signal is forwarded to the further transistor T 4 . The motor  12  is connected to the DC power supply U Bat  and runs at nominal speed. Each of the remaining transistors is kept blocked.  
         [0014]    The power loss ocurring in the individual transistors T 1 -T 4  depends on the forward DC resistance of the transistors R DS(on)  and the motor current  1 , and equals I 2·R   DS(on)  in each case. This power loss can be kept low if low-resistance transistors T 1 -T 4  are used. Additionally, cost advantages can be achieved by dimensioning the transistors T 1 -T 4  differently in accordance with their different loads, whereby the lowest load is applied to transistor T 1 , and the highest load is applied to transistor T 4 . Due to the minimal power loss of transistors T 1 -T 4 , the cooling surfaces required for transistors T 1 -T 4  can be kept relatively small, and all transistors T 1 -T 4  can be combined on one common heat sink  19 , which is indicated using dashed lines in FIG. 1.  
         [0015]    The speed control device  13  shown in the circuit diagram in FIG. 2 for the motor  12  driving the fan wheel  11  is modified to the extent that all three series circuits, each comprising one resistor R 1 -R 3  and one transistor T 1 -T 3 , are connected in parallel with each other and with the further transistor T 4 , which, in turn, is connected in series with the motor  12  to the DC power supply U Bat . The mode of operation of the speed control device  13  is unchanged. For identical speed gradations, it is only necessary to dimension the resistors R 1 -R 3  differently. While resistors R 1 -R 3  can have identical ratings for identical speed intervals between the individual speed levels in the case of the speed control device  13  according to FIG. 1, the resistors must be multiplied in this case as the speed levels become slower. Advantageously, however, further speed levels can be obtained by simultaneously triggering a plurality of transistors T 1 -T 3 , e.g., T 1  and T 3  or T 1  and T 2 , or T 2  and T 3 .