Method for starting a single-phase induction motor

Abstract

The invention relates to a method (19) for starting an electric single-phase induction motor (1), wherein during a start-up interval of the start-up cycle for starting said electric motor (1), the frequency (f.sub.ref) of the electric current for driving said electric motor (1) is set to a first frequency (f.sub.start), and later to the operating frequency (f.sub.run) of the electric motor (1), wherein the first frequency (f.sub.start) is higher than the operating frequency (f.sub.run).

Claims

1. A method for starting an electric motor, comprising: driving said electric motor at least in part and/or at least at times with two electric supply conductors; during a start-up interval of the start-up cycle for starting said electric motor from a shut-down state to an operational state, setting the frequency (f.sub.ref) of the electric current for driving said electric motor to at least one frequency (f.sub.start), then to a catch-up frequency (f.sub.catch) and later to the operating frequency (f.sub.run) of said electric motor; wherein said at least one frequency (f.sub.start) during the start-up interval and/or during the start-up cycle is at least in part and/or at least at times higher than said operating frequency (f.sub.run); wherein the catch-up frequency (f.sub.catch) is lower than the at least one frequency (f.sub.start) and the operating frequency (f.sub.run); measuring the electric current (I) consumed by said electric motor, and determining the success of the start-up interval and/or the start-up cycle by checking whether a decrease in the current drawn by the motor occurs in response to the change in frequency (f.sub.ref) from the frequency (f.sub.start) to the catch-up frequency (f.sub.catch) and to, the operating frequency (f.sub.run), wherein a decrease in the current drawn by the motor is indicative of the success of the start-up interval and/or start-up cycle to start the electric motor from the shut-down state to the operational state.

2. The method according to claim 1, wherein said electric motor is at least in part and/or at least at times operated as a single-phase induction motor, wherein said single-phase induction motor preferably comprises at least one main winding and/or at least one auxiliary winding and/or at least one capacitor device.

3. The method according to claim 1, wherein at least said first frequency (f.sub.start) is approximately twice the operating frequency (f.sub.run) of the electric motor and/or is chosen so that the electric motor essentially yields an increased, preferably a maximum output torque.

4. The method according to claim 1, wherein said electric motor is driven at least in part and/or at least at times in a current limiting mode.

5. The method according to claim 4, wherein said electric motor is driven at least in part and/or at least at times in a maximum tolerable current limiting mode (I.sub.max).

6. The method according to claim 1, wherein during the start-up interval and/or during the start-up cycle the frequency (f.sub.ref) is at least in part and/or at least at times lowered to essentially the actual motor rotation speed (f.sub.rotor) and/or to a frequency, being lower than the operating frequency (f.sub.run) of the electric motor.

7. The method according to claim 1, wherein another start-up interval and/or another start-up cycle is initiated, if the present start-up interval and/or the present start-up cycle was not successful.

8. The method according to claim 1, wherein the frequencies (f) used and/or the voltages (U) used and/or the time intervals used and/or the ramp times used during the start-up interval and/or during the start-up cycle are varied, in particular between different start-up intervals and/or between different start-up cycles.

9. A controller unit for an electric motor designed and arranged in a way that it performs at least in part and/or at least at times a method according to claim 1.

10. An electric motor device comprising at least one controller unit according to claim 9.

11. The method according to claim 1, wherein the at least one frequency (f.sub.start)=60 Hz.

12. The method according to claim 1, wherein the catch-up frequency (f.sub.catch)=15 Hz.

13. The method according to claim 1, wherein the operating frequency (f.sub.run)=30 Hz.

14. The method according to claim 1, wherein the at least one frequency (f.sub.start)=60 Hz, the catch-up frequency (f.sub.catch)=15 Hz, and the operating frequency (f.sub.run)=30 Hz.

15. A method for starting an electric motor, comprising: driving said electric motor at least in part and/or at least at times with two electric supply conductors; during a start-up interval of the start-up cycle for starting said electric motor from a shut-down state to an operational state, setting the frequency (f.sub.ref) of the electric current for driving said electric motor to at least one frequency (f.sub.start), and later to the operating frequency (f.sub.run) of said electric motor; wherein said at least one frequency (f.sub.start) during the start-up interval and/or during the start-up cycle is at least in part and/or at least at times higher than said operating frequency (f.sub.run); measuring the electric current (I) consumed by said electric motor, and determining the success of the start-up interval and/or the start-up cycle by checking whether a decrease in the current drawn by the motor occurs in response to the change in frequency (f.sub.ref) from the frequency (f.sub.start) to the operating frequency (f.sub.run), wherein a decrease in the current drawn by the motor is indicative of the success of the start-up interval and/or start-up cycle to start the electric motor from the shut-down state to the operational state, and wherein, when said electric current for driving said electric motor is set to said at least one frequency (f.sub.start), said electric motor is driven at least in part and/or at least at times in a maximum tolerable current limiting mode (I.sub.max).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention and its advantages will become more apparent, when looking at the following description of possible embodiments of the invention, which will be described with reference to the accompanying figures, which are showing:

(2) FIG. 1: a preferred embodiment of a controller for controlling the power supply to a single-phase induction motor in a schematic view;

(3) FIG. 2: a typical embodiment for a single-phase induction motor in a schematic view;

(4) FIG. 3: a first embodiment of a method for starting a single-phase induction motor;

(5) FIG. 4: a second embodiment of a method for starting a single-phase induction motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) In FIG. 2 a typical embodiment of a single-phase induction motor 1 is shown in a schematic drawing. The single-phase induction motor 1 comprises a rotor 2 that is rotatably arranged inside the housing of the single-phase induction motor 1. The rotation of the rotor 2 (which is indicated by the bent arrow inside the rotor 2) is caused by a rotating magnetic field that is generated by the main winding 3 in combination with an auxiliary winding 4. Both the main winding 3 as well as the auxiliary winding 4 are forming the main part of the stator of the single-phase induction motor 1. Since the single-phase induction motor 1 comprises only two electric wires 5, 6 for its power supply, a special device has to be introduced for generating a phase shift between the main winding 3 and the auxiliary winding 4, thus enabling the generation of a rotating magnetic field. This special device is typically realised with a capacitor 7—as it is done in the present example, as shown in FIG. 2. The capacity of the capacitor 7 is chosen in a way that the phase shift between main winding 3 and auxiliary winding 4 is sufficiently shifted apart from each other, in particular when the single-phase induction motor 1 is started from a halted position.

(7) In FIG. 1, the single-phase induction motor 1 is used as part of an arrangement 8, comprising the single-phase induction motor 1, a mechanical load 9 and a controller unit 10. In the present embodiment, the mechanical load is a pump 9. The pump 9 can be used for pumping water out of a well or out of a storage tank, for example. However, different types of mechanical loads can be equally employed.

(8) In FIG. 1 on the left side, the controller unit 10 for the single-phase induction motor 1 is depicted. The controller unit 10 consists of several subunits 12, 13, 14, 15 that are arranged in a common housing 11 (this is indicated by a dashed line in FIG. 1). Of course, it is also possible to provide separate housings for at least some of the subunits 12, 13, 14, 15 of the controller unit 10 and/or to arrange at least some of the subunits 12, 13, 14, 15 with a certain separation from each other.

(9) As can be seen from FIG. 1, the electric connection between the controller unit 10 and the single-phase induction motor 1 is made by only two electric wires 5, 6. This way, a single-phase induction motor 1 can be used for replacing a previously used constant speed electric motor in an already existing machinery, for example. In such a case, this replacement would introduce the possibility to drive the pump 9 at different speeds, which is obviously advantageous.

(10) In the presently shown embodiment of the controller unit 10, essentially four subunits 12, 13, 14, 15 are depicted: the electronic controller 12, the electric current controller 13, the inverter 14 and an electric current sensor 15. The different subunits are interconnected by electric signal lines 16, where appropriate. The signal lines 16 can be (in part) of an analogue type and/or (in part) of a digital type. Of course, it is also possible that at least some of the signal lines 16 are designed as a common data bus or the like.

(11) The electronic controller 12 performs the major controlling task. In the presently depicted embodiment, the electronic controller 12 is designed as a single printed board electronic computer unit. The electronic controller comprises several interfaces for receiving necessary data as well as for transmitting control signals. Apart from the already mentioned signal lines 16, leading to and coming from other components of the controller unit 10, the electronic controller 12 comprises an input line 17 through which command signals and the like can be inputted (and presumably status signals or the like can be outputted). The electronic controller 12 is programmed in a way to perform the start-up cycles, as shown in FIG. 3 and FIG. 4. It can be programmed in a way that it can perform only one type of start-up cycle. Also, it is possible that the electronic controller 12 is able to perform both (or other and/or additional start-up methods), as requested by the user through input line 17.

(12) A first output signal is transmitted via one of the signal lines 16 to the electric current controller 13. Electric current controller 13 compares the target value (as set forth by the electronic controller 12) with an actual value (as measured by the electric current sensors 15). Based on this comparison, an output signal is generated that is transmitted to the inverter 14 through a signal line 16.

(13) The inverter 14 not only receives a signal from the electric current controller 13, but also directly from the electronic controller 12. Essentially, the electronic controller 12 determines the frequency to be outputted by the inverter 14, while the signal, received from the electric current controller 13 essentially determines the voltage of the output signal and/or the duty cycle of the output signal. The output signal of the inverter 14 forms the electric power that is transmitted through the electric cables 5, 6 to the single-phase induction motor 1. In the presently shown embodiment, the amplifiers are included in the inverter 14. However, it is also possible to provide at least some of the amplifiers as separate components.

(14) The actual electric current through the electric wires 5, 6 is measured by the electronic current sensor 15 and fed back to the electric current controller 13. Furthermore, the measured value is transmitted to the electronic controller 12 itself. This way, the electronic controller 12 is able to determine whether the start-up of the single-phase induction motor 1 has been successful, for example.

(15) In FIG. 3 a first possible embodiment for a start-up sequence for starting a single-phase induction motor 1 is shown. In the figure, altogether three graphs are shown: the frequency graph 18 the voltage graph 19 and the electric current graph 20. In each of the graphs, on the abscissa 21 the evolving time is plotted, while on the ordinate 22 the value of the respective parameter is shown (i.e. frequency, voltage and current).

(16) At t.sub.1 the start-up sequence (the start-up interval) starts by setting f.sub.ref to f.sub.start. f.sub.ref denotes the reference value of the frequency, as requested by the electronic controller 12. In the present example, the frequency f.sub.ref is set to be twice the normal operating frequency f.sub.run of the single-phase induction motor 1. At the same time t.sub.1, the electronic controller 12 demands the electric current controller 13 to ramp up the electric current I to the maximum allowed electric current I.sub.max. To be able to use some feedback by the electric current sensor 15, the electric current controller 13 will slowly ramp up the voltage U.sub.control for increasing the electric current I to the single-phase induction motor 1. As soon as the actual current I through the single-phase induction motor 1 reaches the maximum at t.sub.2, a further increase of the electric current is inhibited by limiting the driving voltage U.sub.control.

(17) Parallel to this the rotor 2 of the single-phase induction motor 1 will start to rotate, which can be seen in the frequency graph 18. Here, the actual rotating frequency f.sub.rotor of the rotor 2 of the single-phase induction motor 1 is plotted. During this initial phase, starting with t.sub.1 (and ending with t.sub.3), the torque that can be produced by the single-phase induction motor 1 is relatively high, so that the single-phase induction motor 1 can be started even with the mechanical load (i.e. the pump 9) connected to the single-phase induction motor 1. However, due to the comparatively high frequency f.sub.ref, the actual rotating frequency f.sub.rotor that can be achieved by the rotor 2 is lower than the normal rotating frequency when the single-phase induction motor 1 is driven with the nominal running frequency f.sub.run.

(18) After a set time (which can be chosen with a sufficiently high safety margin, so that the probability of actually starting the single-phase induction motor 1 is sufficiently high), the frequency will be ramped down to the “catching frequency” f.sub.catch. This value is chosen to be close to the rotor frequency f.sub.rotor that the single-phase induction motor 1 will achieve during the initial start-up phase. After this ramp-down process, the rotor 2 of the single-phase induction motor 1 will be caught at capture time t.sub.4. Here, the rotor 2 begins to follow the frequency of the driving electric current. This can be detected by a significant decrease of the current I, drawn by the single-phase induction motor 1 (and measured by the electric current sensor 15). This behaviour can be seen in the current graph 20 of FIG. 3. The small peak that can be seen in the voltage graph 19 around t.sub.4 is an artefact of the controlling method.

(19) Once the rotor 2 of the single-phase induction motor has been caught, the start-up sequence continues with a “normal” start-up sequence according to the state of the art, where the frequency f is slowly ramped up from f.sub.catch to the normal running frequency f.sub.run motor 1 is driven during normal operation. In particular, this ramp-up can be done with a constant ratio of U/f.

(20) At t.sub.5 the reference value of the frequency f.sub.ref is finally reaching the normal operating frequency f.sub.run. The rotor 2 of the single-phase induction motor 1 follows with a slight delay. As it is normal for induction motors, the rotor 2 shows a slight slip as compared to the driving frequency. This behaviour is due to the design of the single-phase induction motor 1 and is normal.

(21) The frequencies in the above described example are set to be f.sub.run=30 Hz, f.sub.catch=15 Hz and f.sub.start=60 Hz.

(22) In FIG. 4 a modification of the embodiment of a start-up method for a single-phase induction motor 1, as shown in FIG. 3, is shown. Here, only the output frequency f.sub.ref of the electronic controller 12 is shown for elucidating the method. Initially, at t.sub.1, the reference frequency f.sub.ref is set to f.sub.start (for example 60 Hz). Having reached the starting frequency f.sub.start (where a quite high torque is generated in the single-phase induction motor 1), however, the frequency is not held at this value. Instead, practically instantaneously the controller unit 10 starts to lower the reference frequency f.sub.ref down to the catch-up frequency f.sub.catch. In the embodiment chosen, an S-shaped ramp is chosen. However, different shapes of ramps can be chosen as well. Furthermore, it is of course possible to introduce a short time delay between t.sub.1 and the beginning of the first ramp-down process.

(23) At t.sub.2 the catch-up frequency f.sub.catch of the single-phase induction motor 1 is reached by the reference frequency f.sub.ref. Now, it is checked whether the single-phase induction motor 1 has been started (and reached a sufficiently high turning speed). If this start-up has been verified, the initial start-up interval ends and a “normal” ramp-up of the frequency is initialised (see time interval t.sub.4 to t.sub.5 in FIG. 3).

(24) If, however, it has been detected that the single-phase induction motor 1 has not been started (and/or has not acquired a sufficiently high rotating speed), a second start-up interval is initiated at t.sub.2. Now, the reference frequency f.sub.ref, set by the electronic controller 12 is again set to the start-up frequency f.sub.start. Now, this start-up frequency f.sub.start is held for a certain time span, in the presently shown example 0.5 seconds. At the end of this holding interval, at t.sub.3, the reference frequency f.sub.ref is once again lowered to catch-up frequency f.sub.catch. As soon as this catch-up frequency has been reached at t.sub.4, it is once again checked, whether the single-phase induction motor 1 has actually been started. Once again, if the start-up of the single-phase induction motor 1 has been confirmed, the “normal” ramp-up scheme according to the time interval between t.sub.4 and t.sub.5 in FIG. 3 is initiated.

(25) If, however the start-up has been not successful again, another start-up interval is started at t.sub.4. Now, the starting frequency f.sub.start is held for one second till t.sub.5, when the reference frequency f.sub.ref is once again lowered.

(26) This start-up scheme is continued, until a stop condition is met. This stop condition can be derived from external parameters (for example a temperature sensor in the single-phase induction motor 1). Also, internal parameters can be used, for example a timeout condition or a minimum current I.sub.stop, where the start-up cycle stops if the measured current is below said value of I.sub.stop. The actual current can be determined by the (internal) electric current sensor 15.

(27) It should be mentioned that these stop conditions can also be applied to the embodiment, as shown in FIG. 3.

(28) While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present.