METHOD AND ARRANGEMENT FOR CONTROLLING AN INDUCTION MOTOR

Abstract: A device for controlling a three-phase induction motor that is driven by an inverter connection. The inverter is furnished to be fed with full-wave rectified grid alternating voltage (U) and supply the motor's various phases (R, S, T) with the drive voltages phase-displaced in the normal way and which consist of mainly square, in relation to a virtual zero reference level, positive and negative half-periods which alternate at a fixed or adjustable drive frequency corresponding to the required speed. The full-wave rectified grid alternating voltage (U) supplied to the inverter is arranged to be supplied to respective motor phase (R, S, T) via an electronic switch connection (TR1H, TR1L; TR2H, TR2L; TR3H, TR3L) whose respective on and off periods are determined by the drive frequency which is generated by an oscillator device (15; 25). The last mentioned is arranged to be supplied with a voltage (Um) which directly or indirectly represents the rectified grid alternating voltage (U) for the purpose of modulating the drive frequency with a signal that represents the variations in amplitude of the rectified grid alternating voltage. The invention also comprises a method of controlling an induction motor of the type specified.

Description: METHOD AND ARRANGEMENT FOR CONTROLLING AN INDUCTION MOTOR This invention refers to induction motors and concerns more exactly a method and arrangement for controlling a three-phase induction motor driven by an inverter connection to be supplied with full-wave rectified grid alternating voltage and to provide the motor's various phases with drive voltage consisting mainly of square shaped, in relation to a virtual zero reference level, positive and negative half periods that alternate with a fixed or adjustable drive frequency corresponding to the required speed. The speed of induction motors is determined in the first instance by the frequency of the feed alternating voltage supplied. To drive such a motor at a higher speed than that which can be achieved using the ordinary grid frequency it is usual to employ an inverter connection with the help of which the motor can be frequency-controlled so that it can be run at the required speed. To achieve a large degree of simplicity in the motor's control device it is desirable to drive the motor using square voltage pulses. At power levels exceeding about 300 W, this results in inconvenience as the motor losses will be large and the motor sound level will be high. In inverter operation of induction motors for power levels higher than about 300 W, the inverter is therefore generally designed so that it gives off a voltage that is shaped like a sine wave. The frequency of the alternating voltage supplied is only changed if the speed is to be changed or to compensate for greater or lesser slippage. It is also common that the amplitude of the alternating voltage supplied is changed so that it increases with increasing frequency. Inverters of the type described often become complicated and expensive to make, which results in inverter-controlled induction motors not being used in conjunction with domestic machines of various types even if they would provide purely technical advantages. Another problem with inverter-driven induction motors for powers above about 300 W is that they can give rise to considerable grid distortion. The reason for this is that a typical inverter connection is fed with rectified and smoothed grid alternating voltage, which implies that, seen from the grid, current flows to the inverter only during a short period in the region round the grid alternating voltage peak value, when a reservoir condenser included in the connection is charged. As the energy fed to the motor load is to be taken from the grid by current that flows only during brief periods, current transients occur that give rise to the grid distortion mentioned above. At higher powers this distortion reaches unacceptable levels and various types of filter must therefore be provided, which further increases the expense of the device. The invention is in the first instance envisaged for applications in conjunction with induction motors of small sizes and for very high speeds, exceeding 50 000 revolutions a minute, preferably in the speed range around 100 000 revolutions a minute. At these high speeds, the leakage inductance in the motor assumes a higher value due to the motor's small physical dimensions. This leak inductance is in the first instance concentrated to the air gap between the rotor and stator. If the motor is now driven using square voltage, the leak inductance will affect the current so that it will more or less adopt a sine-wave form. The desired curve form of current can thus here be achieved without the supply voltage being sine shaped. If, to obtain an inexpensive and simple inverter and also avoid the occurrence of grid distortion, such a motor is supplied with full-wave rectified unsmoothed grid alternating voltage, the higher drive frequency will be modulated by a frequency that is twice the grid alternating voltage frequency. Such a low-frequency modulation of the drive frequency results in the rotor, if it is braked by a load, changing its speed up and down with a frequency of 100 Hz (at a grid frequency of 50 Hz). If one has, for example, a rotor that rotates at say 90 000 revolutions per minute, it will result in the rotor, because of its small mass, reducing speed relatively greatly when the drive voltage is low. This fluctuation in the speed will increase the slippage of the rotor, which will increase the motor losses. An object of the invention is to provide a method of controlling inverter-driven induction motors and a device for controlling such motors by which the control device design will be simple and the motor be imparted acceptable efficiency so that such a motor can be used in price-sensitive products such as household machines of the vacuum cleaner and washing machine type, for example. Another object of the invention is to supply a control device which obviates the need to smoothen the full-wave rectified grid alternating voltage supplied to the inverter connection, so that current can flow from the grid during the whole period of the grid alternating voltage and not only during short parts of it. A further object of the invention is that an inverter-driven induction motor of small size fed with full-wave rectified unsmoothed grid alternating voltage and driven at very high speed, exceeding 50 000 revolutions a minute, preferably round 100 000 revolutions a minute, provides a control device that provides the motor with more constant and smaller slippage despite the variations in amplitude of the supplied unsmoothed grid alternating voltage. The stated objects will be achieved according to the invention by a method in accordance with patent claims 1 to 3 and by means of a device in accordance with patent claims 4 to 12. Other objects and advantages of the invention will be shown by the subsequent detailed description of a design example, with reference to the following drawings on which: Fig. 1 is a diagrammatic wiring diagram of the control device in accordance with the invention; Fig. 2 is a detailed wiring diagram for a practical control device; Fig. 3 is a diagram which shows the pulse forms, for certain drive pulses in the coupling in accordance with Fig. 2, necessary to control the motor; Fig. 4 is a diagram which shows the pulse forms for drive pulses that complement the drive pulses shown in Fig. 3, and Fig. 5 is a diagram which shows the curve form for a drive voltage supplied to a motor phase Fig. 1 shows the basic construction of a control device in accordance with the invention, with the necessary components specified in block form. A three-phase induction motor is shown to the right in the figure and is represented by three stator windings, LR, L § and D . The windings are coupled together at one end and the free ends are connected to connections R, S and T. These connections are supplied with drive voltages that are displaced in time through 120 degrees in relation to one another in the normal way. The connections R, S and T are connected to respective pairs of transistors TR 1H ,TR 1L ; TR 2H ,TR 2L ; TR 3H' TR 3L- The transistor in each pair are connected in series with each other and respective connections R, S and T are connected to the connection point between the two transistors in each pair. The transistors have in this case been shown as bipolar transistors but can obviously be of another type, such as the MOSFET or IGBT type. The collectors on transistors TR1H, TR2H and TR3H are coupled together and connected to unsmoothed full- wave rectified grid alternating voltage (U) via a conductor (10). The conductor is supplied from a full-wave rectifier (11) of the usual type, whose opposite poles are connected to a conductor (12) which constitutes a common reference potential for the device (neutral, supply, or earth). The full- wave rectifier (11) has two poles (13, 14) connected to the grid alternating voltage which, for example, has an effective value of 230 V and frequency 50 Hz. With these values, the full-wave rectified voltage (U) obtains an amplitude of 325 N. A condenser (C) is connected between the conductors (10, 12) but this does not constitute a reservoir condenser, but is only intended to permit high-frequency interference signals to pass. To drive transistors TR 1H , TR 1L ; TR 2H , R L ; TR 3H , TR 3L , it is necessary to have three drive pulses or, in all, six square shaped pulse series which are in a suitable way mutually displaced in time. To generate these pulse series, an oscillator (15) is arranged in the form of a voltage-controlled oscillator whose output frequency can be set at a value corresponding to the required speed of the motor. For this purpose a potentiometer (16) is used. The output signal from the voltage- controlled oscillator (15) is, via a conductor (17), transmitted to a drive circuit (33) which may be of the usual type (made as an integrated standard circuit) and generates six pulse series which, via conductors (18, 19, 20, 21, 22 and 23) are transmitted to the bases of respective transistors TRj H TRIL " ' rR 3H will be dealt with in greater detail in conjunction with the practical coupling shown in Fig. 2, a further voltage (U M ) is supplied to the voltage-controlled oscillator (15) and this voltage is derived from the full-wave rectified grid alternating voltage and is led via a resistor (R j ) to an input on the oscillator. The voltage (U M ) is designed to provide a modulation of the oscillator frequency which is dependent on the amplitude variations in the full-wave rectified grid alternating voltage. Fig. 2 shows a wiring diagram for a practical control device in accordance with the invention. The control device is an example of an inverter connection that generates a fixed or adjustable drive frequency that is supplied to a number of switch transistors for controlling their on and off times. The task of the switch transistors is, via connections R, S and T, to supply to the motor's stator windings a square shaped drive voltage that is derived from the full-wave rectified grid alternating voltage. The motor is not shown in Fig. 2, but consists of a three-phase induction motor with squirrel-cage rotor that is of small size and intended to be driven at high speed, in excess of 50 000 revolutions a minute, preferably in the speed range of 100 000 revolutions a minute. To generate the motor drive frequency, an oscillator (24) is arranged, which is of the ordinary 555 type. The oscillator's frequency (F osc ) is determined by a combination of resistors R 2 , R3 and R4, together with a condenser (C Q ). The frequency is to a certain extent adjustable using resistor R4, which is a potentiometer. The output frequency (F osc) from the oscillator (24) is lead to a frequency divider (25) of type 74175, which is made with six outputs on which six pulse series with a suitable time displacement occur. The outputs from the frequency divider (25) are connected in pairs to respective timer circuits (26, 27 and 28) of type 556. The timer circuits are made so that when one input receives a high potential they provide a corresponding high level on a corresponding output and this high level lasts for a certain fixed time which is determined by the timer itself. But the time between subsequent high level states at the output can vary depending on the oscillator frequency. In the connection shown the timer circuit (26) has two outputs for pulse series H^ and H^ nj and, finally, timer circuit (28) two outputs for pulse series L jn2 and ^. These six pulse series, H^ - L^, are conducted to corresponding inputs on a drive circuit (29) of type 2130, which is arranged to reinforce the input signals and in a corresponding way, six outputs provide the control pulses H j , H 2 , H3; L j , 1^, L for controlling corresponding switch transistors TR 1H , TR 2H , TR 3H ; TR 1L , TR 2L , TR 3L . The pulse series or input signals to the drive circuit (29) - L n 3) are not shown in the diagram, while the output signals from the drive circuit (29) are shown in Fig. 3 and Fig. 4 in which the oscillator output frequency (F osc ) is also shown. The switch transistors - TR31 are connected in pairs in series with one another between one conductor (30), connected to the full-wave rectified grid alternating voltage (U), and a conductor (31) connected to the common reference potential (neutral, live or earth). Fig. 5 shows the drive voltage at connection R and the corresponding case applies to the drive voltages at connections S and T. These drive voltages are displaced in time in relation to one another so that the connected induction motor is driven in a three-phase way. The drive voltage shown in Fig. 5 at output R varies positively and negatively around an imaginary reference potential (TJ/2) which is midway between the feed voltage (U) and the reference potential (0 V). As shown in the diagrams in Fig. 3 and 4, the high levels in the pulse series last a time, tg, which is fixed and determined by respective timer circuits (26, 27 and 28). But the low level periods that separate the high level periods, on the other hand, last a time, t j , which can vary. The variation has two components, one of which is determined by the oscillator frequency (F osc ) set using potentiometer (R 4 ). The second component is a variation in oscillator frequency that is required to be obtained by modulation using a voltage derived from the full- wave rectified grid alternating voltage. The aim of this modulation is to take into account the fact that the motor is fed with unsmoothed voltage, which implies that the amplitude is not constant but varies. By performing modulation so that the oscillator frequency increases when the amplitude of the full-wave rectified grid alternating voltage increases it is possibly to reduce the tendency of the motor to change speed in step with the variations in grid alternating voltage. With reference to the diagrams in Fig. 3 and 4, this may be expressed by the time (t j ) decreasing when the amplitude of the voltage mentioned increases. In this way it is possible to achieve less and more constant slippage of the motor despite the variations mentioned in the supply voltage. With reduced slippage, the motor losses drop and thereby improve the efficiency of the motor. As shown in Fig. 2, the oscillator (24) is made with a special input (32) which, via a resistor (R j ) can be connected to a point where the full-wave rectified grid alternating voltage occurs, such as a point corresponding to the point M in Fig.l. The effect of the modulation of the oscillator frequency on the method described is also shown in Fig. 5, where it is possible to see that the voltage at the connection R consists of a positive and a negative square-shaped half period. Each half period consists of a first part lasting a time t Q , which is fixed. Then follows a time, delta t, which is determined by the oscillator frequency, and which is also dependent on the modulation performed in the oscillator (24). It should be emphasized that the drive circuit is designed so that it prevents the two transistors in each pair, e.g. RJJJ, T R 1L' being able to conduct at the same time and thereby cause short-circuiting, with consequential damage to the transistors and other components. Certain supply voltages have been specified in Fig. 2, such as +5 V and +15 V. These voltages are generated in the normal way using a power supply unit, such as that indicated in Fig. 1. Even if the invention provides special advantages in conjunction with high-speed induction motors of small size it is not limited to such motors, but can to advantage be applied in conjunction with induction motors for other purposes, such as washing machines, in which the speed is in a lower range and the load varies widely and is heavy. The invention has been described in conjunction with an example in which a control device was built up using logical circuits of standard type. Obviously the control device can within the framework of the invention also be realized using a microprocessor with associated memory circuits, clock-pulse generators and suchlike. The way in which the control device works is determined in this case by the software developed for the microprocessor.

Claims: Patent claims 1. A method of controlling a three-phase induction motor driven by an inverter connection which is furnished to be fed with full-wave rectified grid alternating voltage and to provide the motor's respective phases (R, S, T) with drive voltage consisting mainly of square shaped, in relation to a virtual zero reference level, positive and negative half periods which alternate at a fixed or adjustable drive frequency corresponding to the required speed, characterized by the instantaneous amplitude of the drive voltage being determined by the instantaneous amplitude of full-wave rectified current (U) fed to the inverter and the drive frequency being modulated by a signal (U j ^), which represents the variations in amplitude in the rectified alternating voltage. 2. A method in accordance with patent claim 1, characterized by the drive frequency being generated by a voltage-controlled oscillator (15; 25) which is controlled by a voltage (U j ^ j ) which directly or indirectly represents the full-wave rectified grid voltage (U). 3. A method in accordance with patent claim 2, characterized by the drive voltage's positive and negative half periods being composed of a first part with a fixed pulse duration (tø) and a subsequent second part whose duration (delta t) varies so that it diminishes in step with the instantaneous amplitude of the full-wave rectified grid voltage (U) increasing, and vice versa. 4. A device for controlling a three-phase induction motor driven by an inverter connection furnished to be fed with full-wave rectified grid alternating voltage (U) and to provide the motor's respective phases (R, S, T) with drive voltages phase displaced in the normal way, consisting of mainly square-shaped, in relation to a virtual zero reference, positive and negative half periods which alternate at a fixed or variable drive frequency corresponding to the required speed, characterized by the full-wave rectified grid alternating voltage (U) supplied to the inverter being furnished to be supplied to respective motor phase (R, S, T) via an electronic switch connection (TR 1H , TR 1L ; TR 2H , TR 2L ; TR 3H , TR 3L ) whose on and off periods respectively are determined by the drive frequency which is generated by an oscillator device (15; 25) furnished to be supplied a voltage (U j ^) which directly or indirectly represents the rectified grid alternating voltage (U) for the purpose of modulating the drive frequency with a signal that represents the variations in amplitude of the rectified grid alternating voltage. 5. A device in accordance with patent claim 4, characterized by the drive voltage's positive and negative half periods being composed of a first part with a fixed pulse duration (tø) and a subsequent second part (delta t) whose duration varies so that it diminishes in step with the increase in the instantaneous amplitude of the full-wave rectified grid voltage (U) and vice versa. 6. A device in accordance with patent claim 5, characterized by the pulse duration for the positive and negative half periods being determined by a timer circuit (26, 27, 28, 29) furnished to, together with the drive frequency determined by the oscillator device (24), provide positive square shaped pulses (H^, H 2 , H3, L j , 1_, L3) of duration (tø) determined by the timer and separated by time periods (t j ) whose duration varies with the instantaneous amplitude of the full-wave rectified grid alternating voltage (U). 7. A device in accordance with patent claim 6, characterized by the motor speed being adjustable by adjustment of the oscillator output frequency (F osc ), at which the duration of the time periods (t j ) that separate the positive square shaped pulses is also dependent on the oscillator frequency (F osc ) thus adjusted. 8. A device in accordance with any of patent claims 4 - 7, characterized by the oscillator device consisting of a voltage controlled oscillator (24) which has one input (32) connected to the full- wave rectified grid alternating voltage (U). 9. A device in accordance with patent claim 8, characterized by the oscillator (24) having an output connected to a frequency divider (25) furnished to generate six square shaped signals with a period time corresponding to the drive frequency. 10. A device in accordance with any of patent claims 4 - 9, characterized by the switch connections for respective motor phase (R), consisting of two series- connected transistors (TRJJJ, RJJ with the connection point between the transistors connected to respective motor phase and arranged so as, during the positive half period, to create a cunent path from a connection (30) for the rectified grid alternating voltage (U) to the motor phase (R) and during the negative half period create a cunent path from the motor phase (R) to connection (31) with a common reference potential. 11. A device in accordance with patent claim 9 or 10, characterized by the frequency divider (25) having six outputs connected to three timer circuits (26, 27 and 28) furnished to generate signal pairs (H nj , ^; H jn2 , n2 ; H^, L^) which, via a drive circuit (29) is furnished to be supplied to respective pair of transistors (TR 1H , TR 1L ; TR 2H , TR 2L ; TR 3H , TR 3L ) for controlling the supply of cunent to respective motor phase (R, S, T). 12. A device in accordance with patent claim 11, characterized by the drive circuit (29) being designed so that simultaneous connection of the two transistors in each pair of transistors (TR 1H , TR 1L ; TR 2H , TR 2L ; TR 3H , TR 3L ) is prevented.

Inventors: LINDMARK MAGNUS CARL WILHELM, LINDMARK, MAGNUS, CARL, WILHELM
Applicants: ELECTROLUX AB, LINDMARK MAGNUS C W, AKTIEBOLAGET ELECTROLUX, LINDMARK, MAGNUS, CARL, WILHELM
Application year: 1995
Main classification: H02P7
IPC codes: H02M7, H02P27