Method for easily starting a DC motor apparatus

Abstract: A DC motor apparatus which can be started with a power supply having a small current capacity and which provides a large driving force. The DC motor apparatus comprises a permanent magnet in a rotor of the DC motor, a plurality of coils provided at a stator of the DC motor, a distributor for controlling the switching of current paths to the coils and a position detector for detecting a rotational position of the rotor of the motor to output switching signal to the distributor. A pulsating unit is further provided for pulsating currents supplied to the coils whereby a current flowing from the distributor to any one of the coils can be pulsated when the DC motor is activated, resulting in easy starting the DC motor.

Description: The present invention relates to a method for starting up a DC motor apparatus with a small starting current. In a conventional DC motor which requires a large driving force, a magnetic path is formed by a magnetic material section to enhance a magnetic flux density generated by a coil. A magnetic flux from a permanent magnet incorporated in a movable part of a motor converges in such a magnetic material section having high permeability. Therefore, since an intensive magnetic attracting force is generated between the permanent magnet and the magnetic material section it is necessary to supply a large exciting current from a power supply to such a DC motor. Fig. 1 is a cross-sectional view indicating magnetic pole portions of a rotor and a stator of an ordinary DC motor. A permanent magnet 1 of the rotor has magnetic poles arranged alternately and coils 2 to 5 of the stator are formed around iron core 6. As explained above, since a magnetic flux generated from permanent magnet 1 converges in iron core 6 consisting of a magnetic material having a high permeability, an intensive magnetic attracting force is generated between iron core 6 and permanent magnet 1 and the rotor is restricted to stay in a constant direction. Therefore, it is necessary to generate a magnetic flux sufficient to overcome such restricting forces generated by other coils (coils 2 and 4 in Fig. 1), in order to rotate the rotor. Fig. 2 illustrates an example of an ordinary structure of a conventional DC motor apparatus. An output signal of position detector 7 for detecting a position of the permanent magnet of the rotor is supplied to distributor 8. Distributor 8 switches a current path to selectively supply a current from a DC power supply to any one of coils 2 to 5. When permanent magnet 1 begins to revolve, the current paths which supply currents to coils 2 to 5 are sequentially switched and the rotor of the motor comprising permanent magnet 1 rotates. Fig. 3 illustrates waveforms of the output of position detector 7 in the DC apparatus shown in Fig. 2 and waveforms of currents flowing through coils 2 to 5. In the structure shown in Fig. 2, in the case where permanent magnet 1 of the rotor is rotating, outputs a and b of position detector 7 alternately go to a positive level periodically due to change in position of the magnetic poles of permanent magnet 1. When output a at a positive level and output b at a zero level are input to distributor 8, positive currents flow through coils 2 and 4 and negative currents flow through coils 3 and 5. Next, when the position of the magnetic poles change and outputs a and b supply signals at positive and zero levels, respectively, to distributor 8, negative currents flow through coils 2 and 4 and positive currents flow through coils 3 and 5. Thus, the switching of coils currents is repeated due to the switching of signals output from position detector 7. In the conventional DC motor structured as illustrated in Fig. 1 and Fig. 2, an intensive magnetic attracting force is generated between iron core 6 and permanent magnet 1, and thus the rotor of the motor is restricted to stay in constant direction. Therefore, a large current is required to start such a DC motor and DC power supply source 9 and distributor 8 are both required to have large current capacities. Fig. 4 illustrates waveforms of the output of position detector 7 and waveforms of currents flowing through coils 2 to 5 for start. In the structure shown in Fig. 2, when permanent magnet 1 of the rotor stops, if output a and b of position detector 7 are at positive and zero levels, respectively, distributor 8 receiving outputs a and b causes positive continuous currents to flow through coils 2 and 4 and negative continuous currents to flow through coils 3 and 5. Regarding the prior art attention is also drawn to EP-A-316077 which relates to brushless DC motors. The present invention has been proposed considering the problems explained above. It is an object of the present invention to provide a method for starting up a DC motor apparatus which can solve the problems described above, namely, that can start a DC motor apparatus by means of a power supply source having a small current capacity and which provides a large driving force. The method of the invention is disclosed in claim 1 and prefered embodiments are disclosed in claims 2 to 5. Fig. 1 is a cross-sectional view illustrating magnetic pole portions of illustrating a rotor and a stator of an ordinary DC motor; Fig. 2 illustrates an example of an ordinary structure of a conventional DC motor apparatus; Fig. 3 illustrates waveforms of the output of a position detector and of coil currents after a DC motor apparatus is started; Fig. 4 illustrates waveforms of the output of the position detector and of coil currents when the DC motor apparatus shown in Fig. 2 is started; Fig. 5 is a diagram for explaining an operational principle of a DC motor according to the present invention; Fig. 6 illustrates an example of the structure of a DC motor apparatus according to the present invention; Fig. 7 illustrates another example of the structure of a DC motor apparatus according to the present invention; Fig. 8 illustrates waveforms of the output of a position detector and of coil currents when a DC motor apparatus of the present invention is started; and Fig. 9 illustrates still another example of the structure of a DC motor apparatus according to the present invention. Embodiments of the present invention will be described with reference to the accompanying drawings. Elements the same as those shown in Fig. 1 and Fig. 2 are denoted by like reference numbers. The principle of the present invention will be described first. Fig. 5 is illustrates an operating principle of a DC motor according to the present invention. In Fig. 5, when a current flows through coil 2 while permanent magnet 1 of the rotor is restricted, a magnetic attracting force is generated and permanent magnet 1 stays at a position where the above magnetic attracting force is balanced by a magnetic attracting force generated by the restricting magnetic field. Thereafter, when the current flowing through coil 2 is cut off, permanent magnet 1 begins to return to the original restricted position taken before the current was supplied, but a repeated swing is generated due to inertial force of the rotor. While inertial force is produced in the rotational direction, when a current is supplied again to coil 2, the rotor swings more widely than previously. A rotating force T has a relationship with the inertial force expressed in the following using an equation to express motion: T=J(d 2 /dt 2 )+D(d/dt)k Where, J: inertial force; D: control factor; k: spring constant; : rotating angle. When a current is initially supplied to a coil, no inertial force is generated because of the stationary condition. However, when a current is supplied again after the current has been cut off, rotating force T increases when force J is of a positive value resulting in an increase in rotating angle . Therefore, the rotating force can be caused to increase by controlling a coil current flow when the inertial force is applied in rotating direction, and thus a DC motor can be started with only a small current. An example of the structure of a DC motor according to the present invention utilizing such an operational principle as described above will be explained hereunder. Fig. 6 and Fig. 7 respectively illustrate structures of different embodiments of a DC motor apparatus according to the present invention. In Fig. 6, a pulsating means 10 for pulsating currents flowing through coils 2 to 5 is disposed between position detector 7 and distributor 8. The reference numeral 11 designates a start detector that detects the completion of starting of the DC motor and outputs a signal for stopping or bypassing the operation of pulsating means 10. In Fig. 7, a current supplied from DC power supply 9 is supplied to coils 2 to 5 through pulsating means 10. In a DC motor apparatus having the structure as shown in Fig. 6 and Fig. 7, position detector 7 detects the position of permanent magnet 1 and distributor 8 flows a current through a desired one of coils 2 to 5. This current is controlled by pulsating means 10 and caused to flow intermittently. As a result, the inertial force works on the rotor incorporating permanent magnet 1 resulting in the excitation of the DC motor with a small current, as described above. When detector 11 detects the completion of starting of the motor, detector 11 stops or bypasses operation of pulsating means 10. Fig. 8 illustrates waveforms of the output of position detector 7 and of currents flowing through coils 2 to 5 when the DC motor apparatus having the structure as shown in Fig. 6 and Fig. 7 is started. In the structure shown in Fig. 6, when permanent magnet 1 of the rotor stops, if outputs a and b of position detector 7 are at positive and zero levels, respectively, distributor 8 controlled by pulsating means 10 cause positive continuous currents to flow through coils 2 and 4 and negative continuous currents to flow through coils 3 and 5 at the same timing. As illustrated in the figures, the currents flowing through coils 2 to 5 are pulsated by pulsating means 10 at the time of starting the DC motor. After the motor has been started, currents similar to those in the prior art shown in Fig. 3 flow through coils 2 to 5. There is no substantial difference in effect of pulsating currents flowing through coils 2 to 5 in the example (refer to Fig. 6) where pulsating means 10 is disposed between position detector 7 and distributor 8 and in the example (refer to Fig. 7) where pulsating means 10 is disposed between DC power supply 9 and coils 2 to 5. Fig. 9 illustrates another embodiment where a DC motor apparatus according to the present invention is driven by a solar cell array 12 as a power supply. As indicated in the figure, start detector 11 is connected to an output terminal of solar cell array 12 so that a voltage appearing at this terminal drives pulsating means 10. After only limited exposure to sunlight, solar cell array 12 accumulates a small current capacity. Therefore, a conventional DC motor apparatus as shown in Fig. 2 may not be able to be started. In order to avoid such a problem, start detector 11 is connected to an output terminal of solar cell array 12. When the voltage at the output terminal is lower than a predetermined value, pulsating means 10 is activated to pulsate the currents fed from distributor 8 to the coils. As a result, the DC motor can be started with a small current. When an amount of sunlight increases, the output voltage of solar cell array 12 increased and pulsating means 10, automatically stops operation, resulting in a return to the stationary state. Particularly, when it is requested to start a pump with a solar cell array, a DC motor for driving a pump may frequently start and stop depending on an amount of sunlight. Such a problem can effectively be solved by using a method according to the present invention. As described above, the present invention can reduce a starting current by utilizing a structural characteristic of a DC motor enabling the provision of a DC motor apparatus with a simplified structure at low cost. The present invention is particularly suited for a DC motor apparatus in which a DC motor requiring a large starting current in comparison with a current in the stationary state is driven with a power supply having a small current capacity. While the present invention has been described in detail with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention claimed.

Claims: A method for starting-up a DC motor apparatus, said DC motor apparatus comprising: a permanent magnet (1) included in a rotor of said DC motor apparatus, a plurality of coils (2,3,4,5) provided at a stator of said DC motor apparatus, a distributing means (8) for controlling the switching of current paths of currents to said coils (2,3,4,5), a position detecting means (7) for detecting a rotational position of said rotor to output a switching signal to said distributing means (8), and a pulsating means (10) for pulsating said currents flowing through said coils; and wherein, during start-up of said DC motor apparatus, said pulsed current is pulsed such that a back-and-forth swinging of the rotor is generated due to (i) a magnetic attracting force between an iron core (6) of said stator and the permanent magnet (1), (ii) an intermittent magnetic attracting force generated by said pulsed current and (iii) an inertial force of said rotor, said pulsed current being supplied when said inertial force is in a particular rotational direction such that said rotor swings more widely and said DC motor apparatus can be easily started with small current. A method according to claim 1 wherein said DC motor apparatus comprises pulsating means (10) disposed between said position detecting means (7) and said distributing means (8). A method according to claim 2 wherein said DC motor apparatus further comprises an excitation detector (11) for detecting the completion of starting of said motor to stop or bypass the operation of said pulsating means (10). A method according to claim 1 wherein said DC motor apparatus comprises pulsating means (10) disposed between said plurality of coils (2,3,4,5) and a power supply which supplies currents to these coils. A method according to claim 4 wherein said DC motor apparatus further comprises an excitation detector (11) for detecting the completion of starting of said motor to stop or bypass the operation of said pulsating means (10).

Inventors: AKASAKA TOSHIYA, CHINO HIROYUKI, IWATA MINORU, AKASAKA, TOSHIYA, CHINO, HIROYUKI, IWATA, MINORU
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1992
Main classification: H02P6
IPC codes: H02P6