WO2002058225A1

WO2002058225A1 - Method and an apparatus for the control of an electric motor and the use hereof

Info

Publication number: WO2002058225A1
Application number: PCT/DK2002/000002
Authority: WO
Inventors: Freddy Madsen; Claes Olsen; Carsten Simonsen
Original assignee: Aquadane Aps
Priority date: 2001-01-04
Filing date: 2002-01-03
Publication date: 2002-07-25
Also published as: WO2002058225A8

Abstract

The invention relates to a method and an apparatus for the control of a synchronous motor with a permanent magnet or optionally for the control of an asynchronous motor, in particular an asynchronous motor with a shortcircuit rotor. The motor is fed with voltage pulses, where the speed of the motor is controlled by controlling the frequency of the voltage pulses fed, and where the power fed to the motor is controlled in dependence on the speed. The voltage level of the individual voltage pulses is kept constant, while the power fed is controlled by varying the pulse duration, said pulse duration being determined as a function of the frequency, e.g. at a characteristic, a single voltage pulse being fed to the motor during each half-period, said pulse duration being changed by changing the width of this voltage pulse. It is ensured hereby that the number of revolutions of an electric motor may be controlled in a simple manner, thereby obviating the need for having an inverter with voltage control, and obviating the need for performing measurements of motor current and number of revolutions like in the prior art. The invention may particularly be used for operating a liquid pump, e.g. a pump for pumping water in an aquarium.

Description

METHOD AND AN APPARATUS FOR THE CONTROL OF AN ELECTRIC MOTOR AND TH E US E HEiZE OF

The prior art

The invention relates to a method for the control of a synchronous motor, in particular a synchronous motor having a permanent magnet, wherein the motor is fed with voltage pulses, wherein the speed of the motor is controlled by controlling the frequency of the voltage pulses fed, and wherein the power fed to the motor is controlled in dependence on the speed.

The invention also relates to an apparatus for the control of a synchronous motor, in particular a synchronous motor having a permanent magnet, said apparatus containing a voltage supply as well as a control circuit adapted to feed voltage pulses to the motor, said control circuit controlling the speed of the motor by controlling the frequency of the voltage pulses fed and controlling the power fed to the motor in dependence on the speed.

Small synchronous motors, which means motors having a nominal power of below about 200 W in this connection, where the rotor is formed by a permanent magnet, are generally known, particularly in one-phase embodiments, and are used e.g. for the operation of liquid pumps in household appliances, such as e.g. washers and dishwashers. These motors are especially suitable for use in wet or moist environments, or where there may be a risk of liquid leakages, as the rotor is formed by a permanent magnet and therefore does not require any electrical connection. The electrical parts of the stator, including the electrical wires, may be embedded completely in an insulating material, so that the electrical parts of the motor are insulated completely from the surroundings. Moreover, such a motor may be constructed such that it is integrated with a liquid pump, as the stator part is moulded together with or even constitutes the pump housing, and the permanent-magnetic motor is built together with the rotor parts of the pump.

In this type of application, the motor is usually fed from an AC voltage source, without any possibility of adjusting the number of revolutions. Control of the number of revolutions is possible by changing the frequency, as the number of revolutions is directly proportional to the frequency. This, however, involves the drawback that the power input of the motor will increase with a decreasing number of revolutions, as the impedance of the stator is predominantly inductive. Therefore, if no special measures are taken in order to control the power input at the same time, the stator is likely to get too hot and to be destroyed at a lower number of revolutions.

Further, asynchronous motors are known, having a short-circuit rotor for small powers, and these motors are used e.g. for liquid pumps, inter alia in structures having slot pipes. Normally, these small motors are not provided with a control of the speed of rotation, it being known, however, that controls of a conventional type may be used for these motors, but these controls are of complicated nature that requires feedback of measuring signals of the speed of rotation and motor current concerned, and are provided with a control of the voltage fed.

US 4,636,928 discloses a control for a small three-phase electric motor by means of an inverter, using pulse width modulation (PWM). This known control may be used without a feedback, where for a desired speed a period is calculated which determines the frequency and thereby the speed of the motor, and a period for the pulses is determined on the basis of a given frequency/voltage curve, followed by the generation of a desired PWM pattern which is then fed to the electric motor.

This prior art relates to a control where it is intended that the supply voltage is sinusoidal, which means that a PWM control is used with the consequent greater demands on apparatus as well as software in the control. Thus, in addition, during each half-period of the supply voltage there are several voltage pulses which, while having the same voltage level, are converted into a PWM pattern. This involves some drawbacks, cf. column 6, line 46 to column 7, line 28 of the document.

EP A 0 691 732 discloses a method and an apparatus for speed control of a synchronous motor having a permanent-magnetic rotor. The motor is fed from an inverter which generates voltage pulses, and which is connected to a DC voltage source at the input side. The number of revolutions is controlled in this prior art by controlling the frequency of the voltage pulses that are fed to the stator. The power fed to the motor is controlled by measuring the number of revolutions as well as the motor current, by converting the measured value of the number of revolutions into a rated value of the motor current, by comparing the measured value and the rated value of the motor current, and by using the result of this comparison for controlling, via a control circuit, the voltage level of the voltage pulses that are fed to the motor from the inverter. The conversion of the measured value of the num- ber of revolutions into a rated value of the motor current takes place by means of a static characteristic.

It is a drawback of this prior art that both the motor current and the number of revolutions have to be measured, as both of these quantities are incorpo- rated in the control circuit. Moreover, this solution requires that the voltage in the inverter can be adjusted. These circumstances cause this prior art solution to be relatively expensive and complicated. Finally, the static characteristic for the ratio of the number of revolutions to the motor current applies to the motor concerned, and if this known control is to be used for an- other motor type or size, a new characteristic has to be incorporated in the control. Finally, DE 30 00 058 A discloses an example of an aquarium pump control of the known, relatively simple type, which just involves control between two speeds and does not, like in the invention, allow control of a much more sophisticated nature such that wave effects, time rhythms, etc. may be generated with varying conditions of flow.

Object of the invention

The object of the invention is to provide a method and an apparatus of the type stated initially, which are not vitiated by the above-mentioned drawbacks, and which provide a control with surprisingly simple means, while achieving a good control of the synchronous motor in terms of quality.

This is achieved by the method of the invention in that the voltage level of the individual voltage pulses is constant, while the power fed is controlled by changing the pulse duration, and that the pulse duration is determined as a function of the frequency, e.g. at a characteristic, and that a single voltage pulse is fed to the motor during each half-period, and that the pulse duration is changed by changing the width of this voltage pulse.

The object is achieved by the apparatus of the invention in that the voltage from the voltage supply is constant, and that the control circuit is adapted to control the power fed by changing the width of the individual pulses, said control circuit being adapted to control the width of the individual pulses as a function of the frequency.

It is ensured hereby that the number of revolutions of a synchronous motor, in particular a synchronous motor having a permanent magnet, may be controlled in a simple manner, while the power is controlled in dependence on the number of revolutions. Since the voltage level of the pulses is maintained while the duration of the voltage pulses is controlled, the need for having an inverter with voltage control is obviated, and since the pulse duration is determined as a function of the frequency, the necessity of performing measurements of motor current and number of revolutions like in the prior art is obviated. In the invention, a given speed will mean that the frequency of the pulses is determined in dependence thereon, and the power fed is given in that the width of the individual voltage pulse is determined as a function of the frequency, e.g. on the basis of the given characteristic.

The invention also relates to a use of the method and/or the apparatus of the invention for operating a liquid pump, preferably a pump for pumping water in an aquarium.

This use has the advantage that it is easy to provide a control of the num- ber of revolutions in connection with the small liquid pumps which are used traditionally.

The dependent claims define advantageous embodiments of the invention.

The drawing

The invention will be described more fully below with reference to the drawings, in which

Fig. 1 shows a block diagram of a circuit which performs the method according to the invention,

fig. 2 shows a simplified circuit for performing the method according to the invention,

fig. 3 shows a characteristic curve course for the circuit of fig. 2 with a relatively high operational frequency,

fig. 4 shows a curve course corresponding to fig. 3, but at a lower operational frequency,

fig. 5 shows the input power of a synchronous motor as a function of the frequency, partly for a motor where the power is not controlled, and partly for a motor where the power is controlled according to the invention,

fig. 6 shows a control circuit according to the invention, and

fig. 7 shows an alternative embodiment of a control circuit according to the invention.

Description of embodiments

Fig. 1 shows a block diagram that illustrates a circuit for performing the invention. An electric motor 1 , which may be a single-phase synchronous motor, e.g. having a permanent magnet rotor, is fed with current from a motor control circuit 3 containing inter alia an inverter circuit, said motor control circuit being computer-controlled from a microprocessor, a PC or the like 2.

Fig. 2 basically shows the most important components of a circuit for performing the invention. The circuit includes an electric motor 1 which may be a synchronous motor e.g. having a permanent-magnetic rotor. The voltage across the stator winding of the motor is called U_M. The motor forms part of an H-bridge, in which four controlled switches, Si - S₄ are arranged in the four branches of the H. These controlled switches in the form of controlled semiconductor components may e.g. be transistors or IGTBs (Insulated- Gate Bipolar Transistor). A voltage Uf is applied across this H-bridge, said voltage being supplied by a voltage supply 4 which may e.g. be a rectifier coupled to an AC voltage U. However, the circuit may equally well be fed directly from a DC voltage source which has the voltage U_f.

The controlled switches Si - S₄ are controlled by a control circuit 2, as will be explained more fully below with reference to figs. 3 and 4. The two switches Si and S₄ are controlled jointly, so that they always open and close at the same time. Also S₂ and S₃ are controlled jointly, so that they, too, open and close at the same time.

In fig. 3, the mode of operation of the circuit of fig. 2 is illustrated in five curve courses at a specific frequency and thereby speed of rotation of the electric motor. The upper four curve courses show the states of the switches Si - S , viz. the logic states 1 (closed) and 0 (open). The lowermost curve course shows the resulting voltage across the motor, U_M. At the time ti, the two switches Si and S close, while the two other switches are open. This means that with the shown signs of the voltage UM the voltage +U_f is applied across the motor. At the time t₂, the two switches Si and S₄ open, while the two others are still open. This means that the voltage across the motor drops to 0. At the time t₃, the two switches S₂ and S₃ close, which means that the voltage across the motor is -U_f, as is shown in the lowermost curve course in fig. 2. At the time , S₂ and S₃ close again, so that the voltage across the motor is again 0. At the time t₅, Si and S₄ close again, and then the entire sequence repeats itself in a periodic sequence with the period t₅ - ti. During this period, the rotor in the motor, if it is unipolar, has performed a single rotation. As will be seen, there is no voltage across the motor in the whole period, the reason being that the power is adapted to the current speed of rotation.

If the speed of rotation is to be lower, the period t₅ - ti must be increased. The power fed must be reduced at the same time, as the impedance of the motor is predominantly inductive, and therefore more power will be fed to the motor at a lower number of revolutions, which may cause superheating and possibly breakdown. The power is reduced by shortening the time dur- iήg which voltage is applied to the stator in each half-period.

This is exemplified in fig. 4, which shows a curve course corresponding to the one which is mentioned above in connection with fig. 3. The two switches Si and S₄ will thus close at the times ti and t₅ and open at the times t₂ and tβ, while the switches S₂ and S₃ will close at the times t₃ and t₇ and open at the times and ts, as is shown in the four upper curve courses. The lowermost curve course shows the resulting voltage across the motor. The period t₅ - ti is longer in fig. 4 than the corresponding period in fig. 3, while the time during which voltage is applied across the terminals of the motor, e.g. t₂ - t-i, is shorter in the example shown in fig. 4 than in the example shown in fig. 3. Thus, less power will be fed to the motor in the example shown in fig. 4 than in the example shown in fig. 3, and the number of revolutions is reduced at the same time.

Instead of changing the width of the individual pulse in each half-period, pulses of a smaller width may be used, thereby allowing more pulses in each half-period. When changes are to be made in the power, then changes may be made in the number of pulses in each half-period, optionally combined with a change in the width of the individual pulse. The essen- tial thing in this connection is that the voltage of the pulses is unchanged, and that the sum of the widths of the individual pulses in each half-period is changed on the basis of the given function or characteristic.

Fig. 5 shows a curve course of the power input to a synchronous motor as a function of the frequency, partly for a motor where the power is not controlled (Pureg), and partly for a motor where the power is controlled accord- ing to the invention (P_reg)- At the mains frequency F_net. the two curves are at the same value, of course, viz. the rated power of the motor. For motors which are not controlled, the power input will decrease when the frequency increases, and increase when the frequency decreases, since, as men- tioned above, the motor constitutes a predominantly inductive load. Thus, at frequencies above the mains frequency, the load will frequently exceed the power of the motor, and the motor will therefore stop. At frequencies below the mains frequency, on the other hand, too much power will be fed to the motor, which will cause the motor to be superheated, which, at operation for an extended period of time, will result in destruction of the motor.

When the motor is controlled, the power input will increase at frequencies above the mains frequency F_net, while, at frequencies below the mains frequency, it will drop initially and then flatten out and eventually perhaps in- crease slightly again, as shown in fig. 5. The power increases above the mains frequency, since the load will typically increase at an increasing number of revolutions. Below the mains frequency the input power drops, since the load will typically drop at a decreasing number of revolutions. The shown course for the controlled motor represents an example of a chosen characteristic which is adapted to a concrete load with an associated specific characteristic of moment/number of revolutions.

When using motors intended for ordinary mains use, i.e. at frequencies of 50 Hz (or 60 Hz), the upper frequency F_øv will normally be limited by the power that can be fed to the motor. The lower frequency, F_nβd, will be limited by the circumstance that, normally, the motor cannot drive the load without either coming to a halt, or, if additional power is fed, suffering from overloading. A typical value of the lower frequency for a standard motor will be about 22 - 25 Hz.

Fig. 6 shows a circuit according to the invention for controlling an electric motor 1 in greater detail. In the same way as is shown in fig. 2, the motor 1 is incorporated in an H-bridge having the four controlled switches Si - S₄, which are transistors in this example. The supply voltage U_f is applied across the bridge coupling. The four switches are controlled by two driver circuits 5 and 6, which are preferably constructed as integrated circuits. The driver circuit 5 controls the switches Si and S₂, where the output HO (high side gate driver output) controls Si, and the output LO (low side gate driver output) controls S₂. The driver circuit 6 correspondingly controls the switches S₃ and S₄, with the output HO controlling S₃ and the output LO controlling S₄. Each driver circuit has an input HIN (logic input for high side gate driver output) and an input LIN (logic input for low side gate driver output) which receive signals from an output 9 of the microprocessor 2, optionally via optocouplers 11 and 12, as illustrated. The output 9 of the microprocessor has two signal outputs 9a and 9b, of which 9a is connected via the optocoupler 11 to the input HIN of the driver circuit 5 and the input

LIN of the driver circuit 6. Correspondingly, the output 9b is connected via the optocoupler 12 to the input LIN of the driver circuit 5 and the input HIN of the driver circuit 6.

When there is a high output signal on the output 9a of the microprocessor and a low output signal on the output 9b, this will mean that there will be high output signals on the output HO of the driver circuit 5 and on the output LO of the driver circuit 6, while there will be low signals on the output LO of the driver circuit 5 and on the output HO of the driver circuit 6. Hereby, the switches Si and S will be closed, and the two others will be open.

The same applies when there is a high output signal on the output 9b of the microprocessor and a low output signal on the output 9a, and this will mean that there will be high output signals on the output HO of the driver circuit 6 and on the output LO of the driver circuit 5, while there will be low signals on the output LO of the driver circuit 6 and on the output HO of the driver circuit 5. Hereby, the switches S₂ and S₃ will be closed, and the two others will be open.

When both outputs 9a and 9b have low output signals, none of the outputs of the driver circuits will have a high output signal, and therefore none of the switches Si - S₄ will be closed.

The microprocessor 2 controls the outputs 9a and 9b on the basis of a characteristic incorporated in the processor and on the basis of the desired speed of rotation of the motor 1 or the desired course of the speed of rotation. The characteristic for the motor may be selected by the user through an input module 7 of the processor 2, just as it may be changed by the user as needed. Frequently, one and the same characteristic may be used for motors of different types and different sizes, as a characteristic determined for a relatively small motor may also be used for a larger motor. The larger motor will have a smaller impedance, so that the power suitable for the small motor will also be suitable for the larger motor over a broad range, as the smaller impedance will cause feeding of more power to the larger motor with an unchanged characteristic.

If, in spite of this, the chosen characteristic is not suitable for the given motor, this can readily be observed by the user, as the motor will then e.g. come to a halt or get out of synchronism. The user may then change the characteristic or choose another characteristic until a suitable motor speed has been found. Selection of characteristic can normally be made by the user, since, normally, with the motor types and sizes concerned no damage to the motor can occur, as these types can usually tolerate being locked, provided that no power beyond the rated value is fed.

Moreover, the input module 7 enables selection between various courses or various values of the number of revolutions of the motor, it being e.g. possible to select or state a specific value, or to select one from several temporal courses of the number of revolutions.

In addition to other generally used elements in connection with processors such as storage units, etc., the processor furthermore has a display 8, which can display various parameters concerning e.g. selected number of revolutions, selected temporal course of the number of revolutions or selected characteristic.

Finally, the processor may have several outputs corresponding to the output 9 with the output signals 9a and 9b, e.g. the output 10 with the output signals 10a and 10b, as shown. These outputs, several of which may be provided, may control corresponding motor control circuits, so that the same processor may control several motors, optionally according to the same characteristic or to various characteristics, just as the courses of the numbers of revolutions may be the same or different.

Fig. 7, too, shows a control circuit according to the invention, similar to the one shown in fig. 6, but in which a different type of driver circuit 5' and 6' is used. Also these driver circuits are preferably constructed as integrated circuits. The driver circuits likewise each have outputs HO and LO which are coupled to the switches Si - S in the same manner as described in connection with fig. 6, as well as inputs IN and SD (Shut Down - Not). The inputs IN are coupled to the output 9a of the microprocessor 2 via an optocoupler 11 , there being coupled between the optocoupler 11 and the input IN of the driver circuit 6' a circuit 13 (NOT circuit) which has a low signal on the output when there is a high signal on the input, and a high signal on the output when there is a low signal on the input. The inputs SD are coupled to the output 9b of the processor via an optocoupler 12. The circuit operates in the way that when there is a high signal on the output 9a, there will also be a high signal on the input IN of the driver circuit 5', while there will be a low signal on the input IN of the driver circuit 6'. If, at the same time, there is a high signal on the output 9b and thereby on the inputs SD of the driver circuits, this will mean that there are high signals on the output HO of the driver circuit 5' and on LO of the driver circuit 6', while the other two outputs of the driver circuits will be low. This means that the switches Si and S₄ will be closed, while the two other switches will be open.

Conversely, when there is a low signal on the output 9a, there will likewise be a low signal on the input IN of the driver circuit 5', while there will be a high signal on the input IN of the driver circuit 6'. If, at the same time, there is a high signal on the output 9a and thereby on the inputs SD of the driver circuits, this will mean that there are high signals on the output LO of the driver circuit 5' and on HO of the driver circuit 6', while the other two outputs of the driver circuits will be low. This means that the switches S₂ and S₃ will be closed, while the two other switches will be open.

If the output signal 9b is low, the inputs SD of the driver circuits will corre- spondingly be low, which means that all the outputs of the driver circuits change to low signal, and therefore all the switches will assume an open state.

The microprocessor 2 operates in a manner similar to the one described in connection with fig. 6 and likewise has an input module 7 for inputting various parameters as well as a display 8 for displaying various parameters. Moreover, the processor may also have several outputs 9 and 10 for controlling motor control circuits.

Only the most essential elements are shown in connection with the circuits illustrated in figs. 6 and 7, as power supplies, adapter components, etc. are not shown. It should also be mentioned that when using switches Si - S which do not automatically open when the control signal disappears, special closing arrangements have to be provided in connection with these.

The control circuits according to the invention may e.g. be used for controlling motors which are coupled to liquid pumps, fans and the like in a power range of up to about 200 W. It should be mentioned here that various parameters, such as e.g. measured sizes of temperature, liquid flow or liquid amount, may be used as input signals for the processor 2 via the input module 7, and that the control of the connected motor or motors may then take place such that these parameters are maintained at a given value or follow a given course. Thus, the circuit may e.g. be used for controlling small circulation pumps, drainage pumps and the like, e.g. in the range up to about 200 W and preferably in a range about 10 - 70 W.

Particularly, the invention may be used in connection with aquarium pumps, where a motor with a connected pump, optionally where the pump is formed by the rotor of the motor, is encapsulated water-tight and placed in the aquarium. In certain aquaria it is important that the water is in constant flow, e.g. owing to the need of the fish for flows in the water and/or owing to necessity of the water always having the same temperature and the same concentration of substances everywhere in the aquarium. This is especially important in connection with salt water aquaria, and a homogeneous mixture of the water and optionally generation of varying flow conditions may be achieved by placing a motor pump having a control according to the invention in the aquarium, e.g. at a corner where, otherwise, the water will tend to be stagnant. Then, e.g., a program may be introduced into the processor, causing the pump to temporally operate at different speeds.

Further, several pumps may be arranged in the same aquarium, which pumps are controlled by the same processor. It is hereby possible to use various control programs for each pump, or the same programs may be used. If various programs are used, these may be e.g. be designed in the way that when one pump operates at a low speed, another operates at a high speed, and vice versa. It is ensured hereby that the flow in the aquar- ium is not uniform, but changes constantly, which may be desirable in many cases. In this manner, various desired flow conditions in the aquarium may be established with several pumps, and these flow conditions may e.g. follow a specific time schedule or a twenty-four hour rhythm. Also, when several push-pull operating pumps are used, it is possible to achieve a wave effect in the aquarium.

The invention may also be used in connection with fountains, where pumps driven by electric motors of the type described are frequently used. The fountain may then be controlled to operate according to given programs and time controls, and, when using several pumps, be controlled to generate water jets in different patterns and variations.

As mentioned before, the invention may be used for driving small circulation pumps, e.g. in small heating systems, such as in single-family houses and the like, where the number of revolutions of the pump may be controlled in dependence on various parameters, such as e.g. flow and temperature. Thus, with the invention, it is possible to establish control of the circulation in the system on the basis of the current need in a relatively simple manner, which will result in a saving of energy relative to known systems with con- stant on/off operation of circulation pumps. Also, in connection with small pumps, the invention may be used instead of control valves, such as e.g. thermostatic valves, in connection with heating systems. Thus, a small pump may be used instead of a thermostatic valve for controlling the admission of hot water to a radiator, the control being then performed in de- pendence on a measured temperature, e.g. the room temperature, and this measured temperature is then applied to the control circuit as an input pa- rameter for the microprocessor.

The invention may moreover be used in connection with pumps for dosing, as with a pump driven by a relatively small electric motor and with a control according to the invention it is possible to establish a very accurate dosing of small amounts or small flows of e.g. a liquid, a solution of a substance or the like relatively simply, which is otherwise relatively costly.

Finally, the invention may be used for small blowers and fans which are to be controlled. Such fans may be used in connection with cooling systems for the cooling of cooling ribs and other cooling faces, e.g. for the cooling of the heat-emitting faces in connection with cooling/freezing systems. Here, a control may expediently be established with the invention, so that cooling takes place on the basis of the current need for cooling instead of constant cooling or on/off control of the cooling.

Basically, the invention may be used in connection with small systems where previously it has not been practically possible or profitable to establish control with a view to reducing the consumption of energy. With the in- vention, it is possible to control the consumption of energy also in small systems in a relatively inexpensive and simple way, i.e. systems where the energy consumption of an electric motor is below about 200 W, and control may optionally be performed in dependence on various parameters, such as temperature, flow, amount, etc., said parameters being used as input signals for the processor in the control circuit.

Owing to the use of electric motors which may be encapsulated and electrically insulated completely against the surroundings, such as e.g. synchronous motors with a permanent magnet rotor or asynchronous motors with a short-circuit rotor, and because no electrical connections to transducers have to be set up for measuring e.g. number of revolutions, the invention is particularly expedient in connection with the use in aggressive environments and in environments involving a danger of explosion, fire and ignition of petrol vapours or other combustible liquids or gases, e.g. in connection with chemical processes.

Claims

PATENT CLAIMS

1. A method for the control of a synchronous motor, in particular a synchronous motor having a permanent magnet, wherein the motor is fed with voltage pulses, wherein the speed of the motor is controlled by controlling the frequency of the voltage pulses fed, and wherein the power fed to the motor is controlled in dependence on the speed, characterized in that the voltage level of the individual voltage pulses is constant, while the power fed is controlled by changing the pulse duration, and that the pulse duration is determined as a function of the frequency, e.g. at a characteristic, and that a single voltage pulse is fed to the motor during each half-period, and that the pulse duration is changed by changing the width of this voltage pulse.

2. An apparatus for the control of an electric motor (1), such as a synchronous motor, in particular a synchronous motor having a permanent magnet, said apparatus containing a voltage supply (4) as well as a control circuit (3) adapted to feed voltage pulses to the motor, said control circuit controlling the speed of the motor by controlling the frequency of the voltage pulses fed and controlling the power fed to the motor in dependence on the speed, characterized in that the voltage (Uf) from the voltage supply (4) is constant, and that the control circuit (3) is adapted to control the power fed by changing the width of the individual pulses, said control circuit being adapted to control the width of the individual pulses as a function of the fre- quency.

3. An apparatus according to claim 2, characterized in that the control circuit comprises a μ-processor (2), and that a characteristic for the function between frequency and pulse width is incorporated in it.

4. An apparatus according to claim 2 or 3, characterized in that the circuit comprises a plurality of controllable switches (Si - S ), e.g. power transistors, IGTBs or similar controlled semiconductor components, which are controlled by the control circuit for feeding the voltage pulses to the motor.

5. An apparatus according to claim 4, characterized in that the controllable switches are coupled to the motor in an H-bridge.

6. Use of a method according to claim 1 and/or an apparatus according to one or more of claims 2-5 for operating a liquid pump, e.g. a pump for pumping water in an aquarium.

7. Use according to claim 6, wherein the electric motor and thereby the pump are controlled according to a predetermined time control program.

8. Use according to claim 7, wherein several pumps are used together, preferably for pumping water in an aquarium optionally using a common control circuit for several of the pumps, at least one of the pumps being controlled according to a time control program which is different from the other programs.