WO2004006421A1

WO2004006421A1 - Drive circuit for an electric motor

Info

Publication number: WO2004006421A1
Application number: PCT/NL2003/000494
Authority: WO
Inventors: André Veltman
Original assignee: Electrische Apparatenfabriek Capax B.V.
Priority date: 2002-07-03
Filing date: 2003-07-03
Publication date: 2004-01-15
Also published as: EP1518317A1; NL1020987C2; AU2003252616A1

Abstract

A drive circuit for an electric motor, comprising a switchable DC/AC frequency converter, control means for controlling said frequency converter to deliver a pulse width-modulated excitation signal to the motor, setting means for providing a setting signal for setting a desired motor operation, and comparator means for comparing the current motor operation with the setting signal for driving the control means. The simulator means are arranged for analog simulation of the electrical behaviour of the motor for the purpose of comparing the current motor operation with the setting signal. In an embodiment, a first-order low-pass filter is used for simulating the motor behaviour for the purpose of providing an essentially constant relation between the effective stator voltage and the stator frequency so as to retain a sufficiently large motor torque in the case of an increasing frequency.

Description

Drive circuit for an electric motor.

The invention relates to a drive circuit for an electric motor, comprising a switchable DC/AC frequency converter, control means for controlling the frequency converter to deliver a pulse width- modulated excitation signal to the motor, setting means for providing a setting signal for setting a desired motor operation, and comparator means for comparing the current motor operation with the setting signal for driving the control means. The use of pulse width modulation (PWM) is known per se in the prior art for controlling electric motors used for various fields of application and having small to medium and larger power levels of up to a few W.

For example, in order to retain a sufficiently large motor torque when the frequency increases, the effective stator voltage (expressed in Volt) of the motor must likewise increase, preferably directly proportional to the stator frequency (expressed in Hz), i.e. the Volt/Hz value should be constant as much as possible.

In the prior art this is generally effected by generating a setting signal whose amplitude varies along with the frequency in order to obtain the desired motor operation. In practice, however, the generation of such a setting signal is fairly complicated, and the technical implementation thereof is relatively costly, due for example to the use of special microprocessors, memories comprising embedded software or other programmable devices, partially because the stator voltage must not equal zero during standstill of the motor because of the ohmic resistance of the stator winding(s).

In particular when three-phase motors are to be driven from a DC supply source, the known drive circuits for producing a three-phase excitation signal, i.e. three setting signals mutually shifted 120° in phase and their respective varying amplitudes, for controlling the motor are very complicated.

Furthermore, the DC power supply of the frequency converter must be as smooth or constant as possible, which means that relatively large smoothing capacitors are required when rectifiers are used, for example for feeding the frequency converter from the DC supply mains, which capacitors make it impossible or more difficult to use such drive circuits in small -sized apparatuses.

Since the drive circuits that are known in practice use a fixed switching frequency, practically at all times within the audible range, for controlling the DC/AC frequency converter, the known drive circuits produce a constant audible sound, which the users of devices provided with such a drive circuit experience as very objectionable and i ritating.

Consequently it is an object of the invention to provide a drive circuit of the kind referred to in the introduction whose cost price has been considerably reduced, and by means of which the above problems can be solved in part or in whole, or be alleviated to a considerable extent, and by means of which a sufficiently precise control of the motor operation is possible. According to the invention, the aforesaid problems are solved in that simulator means are provided for analog simulation of the electrical behaviour of the motor for the purpose of comparing the current motor operation with the setting signal or the desired motor operation. The invention advantageously utilises the insight that the electrical characteristics of an electric motor, such as an induction motor, can be simulated by analog means in a relatively simple manner without complicated and costly digital solutions being required, which has a positive effect not only on the simplicity of design and the cost price of the drive circuit but also on the physical dimensions thereof, wherein it is no longer necessary to use a setting signal whose amplitude varies with the frequency in order to obtain an intended motor operation. Simulator means having a first-order low-pass filter characteristic can be used advantageously in particular for providing a sufficiently large motor torque, to which end the effective stator voltage V (Volt) of the motor must be directly proportional to the stator frequency f (Hz), i.e. the proportion V/f must be constant.

A motor excitation signal having an inverted low-pass filter characteristic is obtained by applying a setting signal having a constant amplitude in a feedback loop from the excitation signal delivered by the frequency converter to the comparator means, such that the simulator means having a low-pass filter characteristic are connected for providing an output signal equal to the setting signal.

That is, the DC/AC frequency converter produces an excitation signal which initially has a constant amplitude up to the cut- off frequency of the low-pass filter function, and which subsequently has a constant Volt/Hz development, as intended.

In a practical application, an RC or RL low-pass filter having a cutoff frequency that is related to the turnover frequency of the motor may be used. In practice this value will typically be a few Hz, for example 5 Hz. It will be appreciated by those skilled in the art that this is a very favourable solution from the viewpoint of costs. This in contrast to prior art drive circuits, which use a setting signal whose amplitude must vary with the frequency, as explained above, which can only be realised in practice by means of relatively complex circuitry and correspondingly higher costs.

The drive circuit according to the invention is in particular suitable for controlling a three-phase motor, to which end a DC-AC frequency converter, control means, comparator means and simulator means are provided for each phase in an embodiment of the invention, with the setting means comprising a generator arranged for generating three 120° phase-shifted setting signals, one for each respective motor phase. Such a generator can be realised in a fairly simple manner in the form of an analog generator or, preferably, in the form of a generator comprising a simple digital microcontroller which, in accordance with the embodiment of the invention as discussed above, only needs to generate three 120° phase-shifted sinusoidal signals in that case, independently of the frequency, which can be programmed in a relatively simple manner.

It will be understood that the intended motor operation is determined in the drive circuit according to the invention by the selection of the simulator means in combination with the setting signal. Although simulator means having a low-pass filter characteristic have been discussed in greater detail in the foregoing, it will be understood that also other filter characteristics may be defined for realising a respective motor operation. In this connection, higher-order filters, filters for realising quadratic characteristics etc. may be considered.

To that end, the simulator means may consist of analog filters, either passive filters built up of coil(s) (L) and/or capacitor(s) (C) and/or resistor(s) (R) , or active filters built up of electronic components. In an embodiment of the invention, further filter means are connected in the path of the setting signal for further simulating the electrical characteristics of the electric motor.

A problem that is further encountered in the case of pulse width modulation by means of DC/AC frequency converters is that the frequency converter must generate relatively narrow pulses in order to deliver excitation signals having a relatively large amplitude. Due to the inherent dead time, i.e. the time required for extinghuishing or turning off the switches so as to prevent short-circuit paths via the switches, and to tolerances, parasitical capacitances and inductances and other inaccuracies in the switches and the connecting wiring thereof, the pulse width that can be realised is in practice limited to a certain minimum.

Semiconductor switches, such as field effect transistors, will furthermore undergo inadmissible heating up in that case, because relatively much power is dissipated in the transistors, which not only means a loss of efficiency but also makes the motor control less accurate.

Since the control means for the DC/AC frequency converter in the drive circuit according to the invention may operate as a hysteresis control element, for controlling the switches of the frequency converter with an asynchronous frequency, i.e. without a fixed switching frequency, the problems associated with narrow pulses as described above will be avoided in the drive circuit according to the invention. Furthermore, no objectionable bleeps or whines will be produced in the audible range. The sound produced by the drive circuit and the motor according to the invention has the characteristics of a spread spectrum type of noise.

Although the modulation index of the frequency converter will saturate in the case of an increasing frequency if the amplitude of the excitation signal exceeds the value of the supply source, the analog simulation of the motor characteristics leads to a limiting process, which in itself takes place gradually, which process is also referred to as "clipping" in professional literature, in which the waveform of the excitation signal changes from a P M sine wave to a block wave, without undesirable narrow pulses being produced. This, too, significantly contributes to the reduction of undesirable electromagnetic radiation, as a result of which the drive circuit according to the invention produces less undesirable interference and is suitable for use in environments in which stringent requirements obtain as regards the Electromagnetic Compatibility (EMC). As already set forth in the introduction, the smoothing capacitor of the DC power supply is a costly and voluminous component (usually having a capacity of a few thousand μF with a relatively high voltage of 400 V or higher in the case of mains power supply). Since the drive circuit according to the invention is capable of suppressing relatively large supply voltage variations, as a result of the feedback via the simulator means and the setting signals having a constant value, the drive circuit according to the invention advantageously makes it possible to give the smoothing capacitor a value which may be a few orders smaller than with the prior art drive circuits having DC/AC frequency conversion, whilst retaining a sufficiently accurate control behaviour.

Those skilled in the art will appreciate that a smaller smoothing capacitor will provide significant advantages as regards the use of the invention in small -si zed apparatuses, and in particular will help to achieve the intended cost reduction. The comparator means may comprise an operational amplifier of simple design, which, in accordance with the invention, helps to provide a drive circuit which is simple and advantageous from the viewpoint of costs. The DC/AC frequency converter may be of a type which is known per se, comprising semiconductor switching means, such as field effect transistors. Because narrow signal pulses are avoided when using the drive circuit according to the invention, the semiconductor switches will need to dissipate less power than is the case with the prior art, and defects in the transistors will have hardly any influence on the motor control, as a result of the feedback that takes place via the simulator means.

The invention furthermore provides a method for controlling an electric motor by delivering to said motor, via a DC/AC frequency converter, an excitation signal that has been pulse width-modulated in dependence on a set motor operation and the current motor operation, which method is characterized in that the current motor operation is simulated by analog means. In particular, the motor operation is simulated by low-pass filtering, in which a setting signal having a constant amplitude is provided and in which the simulator means are controlled to provide an output signal equal to the setting signal. For driving a three-phase motor, the method comprises the step of simulating the electrical characteristics of the motor for each phase separately, with the setting means providing three 120° phase- shifted setting signals.

The invention furthermore comprises an apparatus having an electric motor and a drive circuit connected to the motor as discussed in the foregoing.

The invention will be explained in more detail hereinafter by means of a description of schematic diagrams as shown in the appended Figures. Figure 1 shows a block diagram of a first embodiment of the drive circuit according to the invention, to which an induction motor is connected.

Figure 2 graphically shows the trend of the desired magnetic field flux and of the realised magnetic field flux of an induction motor connected to the drive circuit that is shown in the block diagram of Figure 1.

Figure 3 graphically shows the trend of the input signal and of the output signal of the drive circuit that is shown in the block diagram of Figure 1 for the magnetic flux trend as shown in Figure 2 of the connected induction motor.

Figure 4 shows a block diagram of a second embodiment of the drive circuit according to the invention, to which an induction motor is connected.

Figure 5 shows a block diagram of an embodiment of the drive circuit according to the invention, to which a three-phase induction motor is connected. The operation of the drive circuit according to the invention will be explained in more detail hereinafter on the basis of block diagrams of embodiments, to which embodiments the invention is by no means limited, however. In Figure 1 the drive circuit according to the invention is indicated as a whole by reference numeral 1, in its most elementary form it comprises a DC/AC frequency converter 2 provided with switches 3, which are generally semiconductor switches such as field effect transistors, and control means for suitably switching the switches 3. The frequency converter 2 comprises a power connection 5, between which connection 5 and the signal earth 13 of the circuit a DC voltage (V_DC) is applied. An induction motor 8 is connected to an output 7 of the frequency converter. An output of the comparator means 9 is connected to a control input 6 of the control means 4. Setting means 10 for setting a desired motor operation are connected to a first input (+) of the comparator means 9, and simulator means 11 for simulating the electrical behaviour of the motor operation are connected between a second input (-) of the comparator means 9 and the output 7 of the frequency converter 2. The simulator means 11 and the comparator means 9 form a feedback loop for the frequency converter 2.

In the illustrated embodiment of the drive circuit 1, the comparator means 9 are shown as summing means having an inverting (-) input and a non-inverting (+) input. Consequently, the output signal of the comparator means 9 comprises the difference between the signals presented to the two inputs thereof.

For the explanation of the invention it is now assumed that the simulator means 11 operate as a first-order low-pass filter. That is, exhibiting an approximately unattenuated operation for signal frequencies of up to the cutoff frequency f₁₉ and a gradual, first-order increasing attenuation from f_x for the higher frequencies. The cutoff frequency f_j is related to the turnover frequency of the motor 8. The operation of the drive circuit is as follows. The setting means 10 provide a setting signal S_ref having a constant amplitude for setting the desired motor operation. Feedback information with regard to the motor excitation signal S_mot delivered on the output of frequency converter 2 is given to the comparator means 9 and the control means 4 via the simulator means 11, in such a manner that the frequency converter 2 produces an excitation signal for providing a feedback signal S_siIn having a constant amplitude on the output of the simulator means 11.

This means that the motor excitation signal S_mot produced by the frequency converter 2 will exhibit an amplitude shift which is the inverse of the low-pass filter characteristic of the simulator means 11. That is, the amplitude of S_mot will be substantially constant or level for low frequencies of up to f_ls and the amplitude will increase with the frequency for frequencies higher than f_x. It will be understood that this is precisely the intended excitation signal S_mot for retaining a sufficiently large motor torque, viz. a constant effective value (stator voltage) V for low frequencies, which does not become equal to zero, and a constant Volt/Hz proportion for the higher frequencies of the excitation signal S_mot. Figure 2 graphically shows the trend in time of the magnetic field flux φ^ to be realised, as represented by the setting signal S_ref which, in the present case, is a sinusoidal signal having a constant amplitude, and the realised magnetic field flux φ_mot of the motor 8. As the Figure shows, the realised magnetic field flux follows the desired magnetic field flux very precisely.

Since Figures 2 and 3 are only meant to illustrate the invention, no specific values are provided.

Those skilled in the art will appreciate that substantially any desired motor characteristic can be simulated for the purpose of having the frequency converter produce a desired excitation signal by suitably selecting the characteristics of the simulator means 11. Since the circuit according to the invention is capable of presenting a setting signal having a constant amplitude, the drive circuit according to the invention is very easy to realise from a technical point of view. The simulator means may consist of analog filters, either passive filters built up of coil(s) (L) and/or capacitor(s) (C) and/or resistor(s) (R), or active filters built up of electronic components, as is known to those skilled in the art. It will be understood that passive components are preferred for reasons of costs.

The comparator means 9 may comprise a simple operational amplifier, for example, and the control means 4 may be in the form of a hysteresis control element for controlling the switches 3 of the frequency converter with an asynchronous frequency. No objectionable sounds will be produced in the audible range. As a result, the drive circuit 1 according to the invention does not produce any objectionable bleeps or whines, but a frequency-spread sound which is hardly audible, if at all .

Said feedback via the simulator means 11 enables the drive circuit 1 according to the invention to cope with relatively large supply voltage variations without inadmissible and adverse effects as far as the intended motor control is concerned. As a result, a smoothing capacitor 12 (necessary at all times) for smoothing supply voltage variations in the DC power supply V_DC that is used in the circuit according to the invention may be a few orders smaller than the smoothing capacitors that are used in drive circuits that are known from the prior art. This leads not only to a significant saving in costs, but also to an enormous gain in space, so that the drive circuit according to the invention can also be implemented in relatively small-sized apparatuses.

A gradual transition from a sine-shaped excitation signal to a block-shaped excitation signal will take place upon clipping of the excitation signal against the supply voltage V_DC, which has a positive effect on the EMC characteristics of the drive circuit. A runaway will automatically result in a field attenuation.

Figure 4 is a block diagram of a second embodiment of the drive circuit according to the invention, indicated as a whole by reference numeral 15, with further filter means 16 being connected in the path of the setting signal between the setting means 10 and the comparator means 9 for providing a desired frequency-dependent motor operation, for example for further defining, e.g. reducing, the magnetic field flux in the low-frequency range.

Figure 5 illustrates in the form of a block diagram a drive circuit according to the invention, indicated as a whole by reference numeral 20, for driving a three-phase motor 21 from a single DC power supply V_DC.

The phases U, V, W of the three-phase motor 21 are each connected to a drive circuit 1 or 15 according to the invention, with the setting means 10 being combined into setting means 22, which produce three sine-shaped 120° phase-shifted setting signals S_{ref >w} having a constant amplitude.

The setting means 22 can be advantageously realised as a digital, asynchronous generator, for example a suitable programmed microcontroller. The constant amplitude makes it possible to use a relatively simple and inexpensive controller, with a modest number of programme coding lines. Also the use of an analog signal generator or a number of mutually phase-locked or synchronised analog generators is possible, of course. The motor 21 can be made to rotate in one direction or in the other, reverse direction by changing the phase order of the setting si gnal s S_{ref u>VιW}.

The drive circuit 20 according to the invention has been successfully used for driving three-phase motors having power levels of up to a few k from 0 to 200% of their nominal speed in both directions of rotation, being fed from a single-phase, rectified AC power supply source 230V/50 Hz, using standard commercially available components. The setting of the desired motor operation, for example of the desired motor speed and the like, takes place via the setting means 10, 22.

Reference numeral 23 in Figure 5 schematically represents an apparatus, illustrated in broken lines, comprising an electric motor and the drive circuit according to the invention.

The invention is not limited to a specific type of induction motor. The invention may also be used with different kinds of electric motors, among which a so-called brushless DC motor. Such motors, including the associated control electronics, are known per se to those skilled in the art and need not be explained in more detail as such herein.

It will be appreciated by those skilled in the art that the embodiments as described above may be modified and extended in many ways without departing from the scope of the invention as defined in the appended claims.

Thus it is possible, for example, to integrate the control means 4 and the comparator means 9 into a single unit, or to provide comparator means 9 in the form of a comparator exhibiting a hysteresis operation.

Claims

1. A drive circuit for an electric motor, comprising a switchable DC/AC frequency converter, control means for controlling said frequency converter to deliver a pulse width-modulated excitation signal to the motor, setting means for providing a setting signal for setting a desired motor operation, and comparator means for comparing the current motor operation with the setting signal for driving the control means, characterized by simulator means provided for analog simulation of the electrical behaviour of the motor for the purpose of comparing the current motor operation with the setting signal.

2. A drive circuit according to claim 1, characterized in that said simulator means are connected for providing a signal representing the current motor operation from the simulator means for comparison by the comparator means.

3. A drive circuit according to any of the preceding claims, characterized in that said simulator means have a low-pass filter characteristic.

4. A drive circuit according to claim 3, characterized in that said setting means are arranged for providing a setting signal having a constant amplitude, and in that said simulator means are connected for providing an output signal equal to the input signal.

5. A drive circuit according to any of the preceding claims, characterized in that said motor is a three-phase motor, with a DC-AC frequency converter, control means, comparator means and simulator means being provided for each phase and the setting means comprising a generator arranged for generating, by digital means, three 120° phase- shifted setting signals, one for each respective motor phase.

6. A drive circuit according to claim 5, characterized in that said generator is arranged for generating said setting signals having a constant amplitude independently of the frequency.

7. A drive circuit according to claim 5 or 6, characterized in that said generator comprises a microcontroller.

8. A drive circuit according to any of the preceding claims, characterized in that said control means comprise a hysteresis controller for controlling the switches of the DC/AC frequency converter with an asynchronous frequency.

9. A drive circuit according to any of the preceding claims, characterized in that said simulator means comprise a first-order low- pass filter, with a cut-off frequency related to the turnover frequency of a motor to be connected.

10. A drive circuit according to claim 9, characterized by further filter means for further simulating the electrical behaviour of the electric motor.

11. A drive circuit according to claim 10, characterized in that said further filter means are connected in the path of the setting signal .

12. A drive circuit according to any of the preceding claims, characterized in that said comparator means comprise an operational amplifier. 13. A drive circuit according to any of the preceding claims, characterized in that said comparator means have a hysteresis operation. 14. A drive circuit according to claim 13, characterized in that said switching means comprise semiconductor transistors, in particular field effect transistors. 15. A method for controlling an electric motor by delivering to said motor, via a DC/AC frequency converter, an excitation signal that has been pulse width-modulated in dependence on a set motor operation and the current motor operation, which method is characterized in that the current motor operation is simulated by analog means. 15. A method according to claim 15, characterized in that the current motor operation is simulated by low-pass filtering.

17. A method according to claim 15 or 16, characterized in that a setting signal having a constant amplitude is provided and in that said simulator means are controlled to provide an output signal having a constant amplitude. 18. A method according to claim 17, characterized in that said motor is a three-phase motor, and in that the electrical characteristics of the motor are simulated for each phase separately, with the setting means providing three 120° phase-shifted setting signals. 19. A method according to claim 15, 16, 17 or 18, characterized in that said simulator means are arranged for controlling the DC/AC frequency converter to deliver an excitation signal having an essentially constant amplitude for frequencies of up to a few Hz and exhibiting an essentially constant proportion between the effective excitation voltage and the frequency thereof for higher frequencies. 20. An apparatus comprising an electric motor, characterized by a drive circuit according to any of the preceding claims, which is connected to the motor.

21. An apparatus according to claim 20, characterized in that said electric motor is an induction motor. 22. An apparatus according to claim 20, characterized in that said electric motor is a brushless DC motor.