METHOD AND DEVICE FOR DRIVING AN ASYNCHRONOUS MOTOR WITH SPEED CONTROL BY MEANS OF A CONTROLLED INVERTER

Abstract: In a method for driving an asynchronous motor with speed control by means of a controlled inverter, the desired value of the phase difference between the motor voltage and the motor current is determined as a function of the frequency of the voltage supplied by the inverter. The amplitude of the voltage supplied by the inverter is controlled to make the actual value of the said phase difference equal to the desired value at the frequency at issue. The desired value function is determined on the basis of the desired value given for the individual motor at nominal frequency and by means of the equivalent diagram of the motor. A device for carrying the method into effect comprises a detector (7) for determining the actual value of the phase difference between the motor voltage and the motor current, a comparator (8) for determining the difference between said actual value and a desired value of said phase difference, said desired value being a function of the frequency of the voltage supplied by the inverter, and a control unit (10) actuated by the difference between said actual value and said desired value, for controlling the amplitude of the alternating voltage supplied by the inverter toward equality between said actual value and said desired value at the frequency at issue.

Description: METHOD AND DEVICE FOR DRIVING AN ASYNCHRONOUS MOTOR WITH SPEED CONTROL BY MEANS OF A CONTROLLED INVERTER The present invention relates a method and a device for driving an asynchronous motor with speed control by means of a controlled inverter. Today, the most frequently used electric motor in machines and processes is the asynchronous motor, largely because it is a standard product which is produced in long series and therefore has a low price and also because it is simple and robust. However, one disadvantage of the asynchronous motor is that speed control is difficult. Asynchronous motor. speed control has previously necessitated ' the use of, for example, mechanical varia- tors, hydraulic couplings or series resistances. How¬ ever, these speed control devices have considerably re- duced the efficiency of the asynchronous motor. A much improved speed control of asynchronous motors is achieved by controlling the motor speed by means of a motor voltage of variable frequency. For this purpose, a frequency converter is usually employed which consists of a rectifier supplied by the mains voltage, followed by a frequency-controlled inverter. By such speed control, the power requirement can frequently be reduced, whereby energy is saved. Speed control by varying the frequency of the motor voltage requires that the supply voltage is changed at the same time, if the torque is to remain unchanged. Adaptation to the load on the motor, which usually varies with varying speed, may then also be ef- fected. Up to now, the voltage change has been achieved by some form of preprogramming, i.e. such that each motor voltage frequency is associated with a predeter¬ mined value of the voltage applied to the motor. It will be appreciated that such preprogramming enables the motor to operate at maximum efficiency over the entire speed range only on condition that the depen¬ dency of the load on the speed is not changed, and that the preprogramming has been correctly adapted to the dependency on the speed. If, however, the depen¬ dency of the load on the speed is changed in time, which is normal in many applications, maximum effi¬ ciency will not be achieved because the preprogrammed supply voltage is incorrect. In consequence of the different characteristics of asynchronous motors, i.e. the dependency of their torques on the magnetising current, and varying losses in the inverter, the requisite voltage for obtaining a desired output torque. is difficult to determine. Speed controlled asynchronous motors therefore operate but rarely with maximum efficiency. It is the object of the present invention to provide a method and a device for such driving of an asynchronous motor with speed control by means of a controlled inverter that maximum efficiency is achieved independently of the speed and the load. In the method according to the invention, this object is achieved in that the desired value of the phase difference between the motor voltage and the motor current is determined as a function of the fre¬ quency of the voltage supplied by the inverter, and that the amplitude of the voltage supplied by the inverter is controlled to make the actual value of said phase difference equal to the desired value at the frequency at issue. In the device according to the invention, this object is achieved in that the device is characterised by a detector for determining the actual value of the phase difference between the motor voltage and the motor current, a comparator for determining the difference between said actual value and a desired value of said phase difference, said desired value being a function of the frequency of the voltage supplied by the inverter, and a control unit actuated by the diffe¬ rence between said actual value and said desired value, for controlling the amplitude of the alternating voltage supplied by the inverter toward equality between said actual value and said desired value at the frequency at issue. The invention utilises the fact that, upon correct magnetisation of the motor, i.e. at the ideal power factor or phase shift between the motor voltage and the motor current and, thus, the correct magnetisation current, the output torque is proportional to the product of the peak value of the magnetisation current and the cosine of the phase angle between the voltage applied and the current supplied to the motor. Further¬ more, use is made of the fact that the optimal phase angle as a function of the motor speed can be deter¬ mined in advance on the basis of the maximum motor phase angle at normal speed, i.e. when the frequency of the mains voltage is 50 or 60 Hz. The determination of the desired value made in the method according to the invention is repeated at suitable intervals in order to maintain a correct actual value independently of changes in the frequency and the motor load. The desired value function is determined on the basis of the desired value given for each individual motor at nominal frequency. The desired value function is preferably stored permanent- ly, and the desired value at issue is produced from the stored desired value function. Equivalent to the utilisation of the phase difference is the utilisation of a function thereof, such as the power factor, which is the cosine of the phase difference. The device according to the invention preferably comprises a memory for permanently storing the desired value function as determined on the basis of the de- sired value given for each individual motor at nominal frequency, said memory being addressable by means of the frequency of the voltage supplied by the inverter. The invention thus makes it possible to achieve correct magnetisation of the motor independently of the motor speed and load. This means that the motor operates the whole time at an ideal power factor (cosφ) or an almost ideal power factor. The torque supplied can then also be readily calculated and, furthermore, may be utilised for control purposes. In view hereof, the device according to the in¬ vention may preferably be provided with a calculating unit which gives the motor torque as a function of the said actual value and the amplitude of the motor current. Furthermore, the device then preferably has a second control unit to change the frequency of the voltage supplied by the inverter in response to the motor torque. Moreover, the device may be provided with an indicator to show the motor torque. The invention'will be described in more detail below, reference being had to the accompanying drawings. Fig. 1 is a diagram showing the motor torque as a function of the speed. Fig. 2 is a block diagram illu¬ strating an embodiment of the device according to the invention. Fig. 3 illustrates the relationship between the power factor and the motor speed. Fig. 4 is a block diagram illustrating a possibility of expanding the device shown in Fig. 2. The diagram in Fig. 1 shows the torque of an asynchronous motor as a function of the speed at two different supply voltage frequencies and for different motor currents. Curves A and B are examples of the speed dependency of a load. If a curve I in Fig. 1 is assumed to represent the torque curve at issue of an asynchronous motor, and the motor load has the appearance shown by the curve A, the motor operates at a speed n, , i.e. with a lag s = 1 - n /n, wherein n is the synchronous speed. If, at a constant load dependency according to curve A, the synchronous speed is increased from n to, for example, n_,, the supply voltage must be in¬ creased such that the motor torque curve will agree with curve II in Fig. 1. In this manner, the motor speed will be equal to n.,, and for correct magnetisa¬ tion the lag must be essentially unchanged. if, at an unchanged motor voltage frequency, the motor load is changed from a value on the curve A to a value on the curve B, also the magnetisation current to the motor must be changed in order to main¬ tain maximum motor efficiency. if, for example, the motor operates at the point of intersection between curves I and A in Fig. 1, and if the motor load decreases to curve B in Fig. 1, the motor voltage, and thus the motor current, must be changed in order to obtain a torque according to curve III in Fig. -1, whereby the lag is maintained unchanged and the efficiency at its maximum. If, instead, the motor has an operating point corresponding to the intersection between curves II and A, and if the motor load is decreased to the load according to curve B, the motor torque must be reduced in accordance with the curve IV. It will thus be evident that a fixed preprogramming of the amplitude of the voltage applied to the motor can give maximum efficiency of the motor only in excep- tional cases. Every asynchronous motor has an individual power factor (cosφ) which is indicated by the manufacturer for the nominal frequency f , i.e. usually 50 or 60 Hz. When this power factor value is known, it is possible to determine the desired value of the power factor or the phase difference between motor voltage and motor current for each frequency f within the entire frequency range from direct current up to maximum motor speed. The appearance of this desired value function as regards cosφ will appear from Fig. 3. The appearance of the desired value function is pro- duced more particularly by means of the frequency dependency of the basic equivalent diagram of the asynchronous motor at nominal load, and the given power factor at nominal frequency and the said nominal load. It will thus be of the type e -k«f wherein k is given by the individual power factor at f . The present invention utilises the fact that the desired value of the power factor or, generally, the desired value of the phase difference between motor voltage and motor current can be determined for each frequency or speed. By producing the corre¬ sponding actual value, and by controlling the amplitude of the supply voltage in response to the difference between the desired value and the actual value, the actual value thus is made equal to the desired value at the frequency a ; t issue. This control is carried out by means of the embodi¬ ment of the device according to the invention shown in Fig. 2. The device comprises a rectifier 1 connect¬ ed to the A.C. mains and supplying direct voltage to a controlled inverter 2 which in turn drives an asynchronous motor 3. The voltage supplied by the inverter 2 is determined by means of a voltage con¬ trol unit 4 which supplies the requisite control pulses to the inverter 2 in response to two input parameters, viz. a desired value f, of the motor voltage frequen- cy and a value U of the motor voltage. The desired value f, can be set manually or automatically in response to a parameter in the process or machine in which the motor 3 is operating. To produce the input parameter 0, the following units are utilised. A voltage detector 5 is connected to the output of the unit 4 to produce at its output a signal whose phase corresponds to the phase of the voltage applied to the motor. Alternatively, the voltage detector 5 may be connected to the lines between the inverter 2 and the motor 3. A current detector 6 is connected to one of the lines between the inverter 2 and the motor 3 to produce a second A.C. voltage signal whose phase corresponds to the phase of the motor current. The outputs from the detectors 5 and 6 are connected to two inputs to a phase detector 7 which is adapted to supply at its output a signal <■?__ + . representing the actual value of the phase dif- ference between motor voltage and motor current. This signal is supplied to an input to a comparator 8 whose other input is supplied with the desired value of the said phase difference. This desired value is ob¬ tained from a memory unit 9 in which the desired value function is permanently stored. The desired value at issue is obtained at the output of the memory unit 9 by addressing by means of the desired value f, of the frequency at the input of the memory unit. At the output of the comparator 8 there is thus obtained the difference between φ Q,6S • and φC-Ctt as Δφ. This signal Δφ is supplied to a calculating unit 10 which determines a change in the amplitude of the motor voltage on the basis of the magnitude and sign of the phase difference Δφ. It will be appreciated that the device according to the invention will control the amplitude of the voltage supplied by the inverter in such a manner that the actual value of the phase difference between motor voltage and motor current is made equal to the desired value of said phase difference at the frequency at issue. In this manner, there is obtained automatical¬ ly the correct magnitisation current in the motor, independently of the speed and load. If the load or the speed is changed, the device according to the invention will thus automatically adjust the amplitude of the A.C. voltage from the inverter 2 until the output signal of the comparator 8 is zero. The block diagram in Fig. 4 illustrates a possibi- lity of expanding the control in the device according to Fig. 2. A calculating unit 11 is adapted to cal¬ culate the motor output torque T which, at maximum power factor, is proportional to the peak value I σf the motor current, obtained for example from the current detector 6 in Fig. 2, and the power factor. The output signal T of the unit 11 can be utilised for display in an indicator 12 and can also be supplied to a second comparator 13 for comparison with a refe¬ rence value T - of the torque. In dependency upon the result of this comparison, the voltage control unit 4 can be supplied, by means of a correcting unit 14, with a corrected value fcorr. of the freq^uenc J y des For the interrelation of the above-mentioned units and the device in Fig. 2, Fig. 4 also shows the units 4, 8 and 10. The reference value T - of the motor torque may represent, for example, a maximum value of this torque. If the maximum value is exceeded, the correct- ing unit 14 may produce, for example, a torque reduc¬ tion by reducing the frequency f in relation Alternatively, the correcting unit 14 may be desig 3 ned to control fcorr. in such a manner that ΔT is made equal to zero, which means that the motor is driven at constant torque. The above-mentioned embodiments of the device according to the invention can be modified in many ways within the scope of the invention, and the inven- tion thus is not restricted to the embodiment exem¬ plified, but may be realised for example by means of a microprocessor.

Claims: CLAIMS 1. A method for driving an asynchronous motor with speed control by means of a controlled inverter, c h a r a c t e r i s e d in that the desired value of the phase difference between the motor voltage and the motor current is determined as a function of the frequency of the voltage supplied by the in¬ verter, and that the amplitude of the voltage supplied by the inverter is controlled to make the actual value of said phase difference equal to the desired value at the frequency at issue. 2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the desired value function is determined on the basis of the desired value given for the individual motor at nominal frequency and by means of the equivalent diagram of the motor. 3. A method as claimed in claim 2, c h a r a c ¬ t e r i s e d in that the desired value function is stored permanently, and that the actual value at issue is produced from the stored desired value func- tion. 4. Method as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the power factor is utilised as a measure of the phase difference. 5. A method as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the desired value determination is repeated in order to maintain the correct actual value independently of changes in fre¬ quency and motor load. 6. A device for driving an asynchronous motor (3) with speed control by means of a controlled inverter (2), c h a r a c t e r i s e d by a detector (7) for determining the actual value of the phase difference between the motor voltage and the motor current, a comparator (8) for determining the difference between said actual value and a desired value of said phase difference, said desired value being a function of the frequency of the voltage supplied by the inverter, and a control unit (10) actuated by the difference between said actual value and said desired value, for controlling the amplitude of the alternating voltage supplied by the inverter toward eqaulity between said actual value and said desired value at the frequency at issue. 7. A device as claimed in claim 6, c h a r a c ¬ t e r i s e d by a memory (9) for permanent storing of the desired value function as determined on the basis of the desired value given for the individual motor at nominal frequency and by means of the equiva- lent diagram of the motor, said memory being address¬ able by means of the frequency of the voltage supplied by the inverter. 8. A device as claimed in claim 6 or 7, c h a ¬ r a c t e r i s e d by a unit (11) for calculating the motor torque as a function of the said actual value and the amplitude of the motor current. 9. A device as claimed in claim 8, c h a r a c ¬ t e r i s e d by a second control unit (14) for chang¬ ing, in response to the motor torque, the frequency of the voltage supplied by the inverter. 10. A device as claimed in claim 8 or 9, c h a ¬ r a c t e r i s e d by an indicator (12) for display¬ ing the motor torque.

Inventors: ELFNER BO AOKE, LINDVALL PER, ELFNER, BO, AOKE, LINDVALL, PER
Applicants: ELFI INNOVATIONER, EL-FI INNOVATIONER AB
Application year: 1987
Main classification: H02P7
IPC codes: H02J3, H02P23, H02P27