FI115860B - Method for automatic measurement of ohmic resistance of an asynchronous machine rotor - Google Patents

Description

115860

Method for the automatic measurement of the ohmic resistance of an asynchronous rotor - Förfarande för automatisk mätning av den ohmska hos rotorn i en asynchronous mask

The invention relates to a method for automatically measuring the ohmic resistance of an asynchronous machine rotor when the machine is controlled through an inverter while being actuated by a non-rotating field.

In an asynchronous machine whose speed and torque are controlled, in particular according to the control method applied to field 5, all resistances, ie ohmic and inductive resistances, must be known in order to make the control as accurate as possible. The resistances can be evaluated and / or measured.

Measurements are made either with a rotating, unloaded rotor or with a locked (braked) rotor. When the test current for resistance measurement is passed through a stator 10 with the rotor unloaded, most of the current passes through the main reactance, which is defined by the main inductance (counter-inductance), whereby the main inductance can be measured but ohmic resistance of the rotor cannot be measured. However, when measuring with a locked rotor, the test current also passes through the rotor so that its ohmic resistance can also be measured. However, both methods have drawbacks. Measurement with a rotating, unloaded rotor is often not possible if, for example, the motor is integral with the finished product and: its shaft is fixedly loaded. On the other hand, locking the motor imposes great demands on the mechanical brake device, especially when it is subjected to full torque, so that this method is substantially more expensive. Second is difficult '. In the case of measurements of the Utena locked rotor, there is a shift current in the rotor rods ... which occur at high frequencies in the range of 30 to 60 Hz, which causes too large a measurement value for the ohmic resistance of the rotor.

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In addition, when measuring ohmic resistance, its operating temperature is often disregarded. * ··. patient-dependent variations. Depending on the operating temperature, it may increase or decrease, · · ·. 25 jobs 20% - 30%. This means that the equivalent circuit of the asynchronous machine, which forms the basis of the measurement, * * "* does not apply in normal use.

US 5,689,169 discloses a method in which stator and rotor leakage inductances and rotor ohmic resistance are measured while the rotor is stationary by adjusting q and d components in a "field-directed" control process. Thus, * · '· 30 one of the stator phase windings receives a test signal having a frequency approximately 2 115860 equal to the operating frequency, and for example about 30 Hz. The current component Iq is set to zero to avoid the occurrence of a rotating torque, and at the same time the prevailing voltages Vq and Vd fed back to the regulator are measured. Given the frequency of the test signal and the previously measured stator ohmic resistance, the approximate value of the 5 rotor resistance can be calculated. The reason for this approximation is that the frequency of the test signal is selected to be relatively high so that relatively simple mathematical equations can be used in the calculation, which require little computing power from the microprocessor used in the controller. However, the relatively high frequency of the test signal, about 30 Hz, has the disadvantage of generating a shift current on the rods of the 10 rotors, which results in the measured values of the ohmic resistance of the rotor being too high. In extreme cases, the measured values may be 100% - 150% too high. Compared to an inverter with only current sensors, this method also has the disadvantage that it also has to use voltage sensors.

The method of the species mentioned above is known from Danfoss Drives A / S, Denmark, 15 in EPE'97, pages 3.370-3374. In this method, the following measurements and calculations are made, with reference to the conventional one-phase equivalent circuits of the asynchronous machine shown in Figures 1 and 2 in the accompanying drawings. Figure 1 illustrates in detail a continuous-state equivalent circuit, and Figure 2 shows a simplified equivalent circuit calculated (guided) on the stator side by the effective number of turns: 1. A stator, more particularly one phase winding of the stator, is fed to . ·. such a test voltage Usa in the form of a specified dc voltage, and the resulting /, ·, stator current Isa is measured. Since the stator leakage inductance and Los pääinduk- dance Lm (vastainduktanssin) inductive reactances (inductances) • also representatives. 25 dc, the ohmic resistance Rs of the stator can be calculated from the values of U] and Isa.

• · · • Ύ: 2. The sum of the leakage inductances Los and Lar, “transient inductance” L’s, converted to the stator side, is calculated as follows in Figure 2:, ···. A short limb pulse, comprising high frequency components, is introduced into the stator with a duration of a few milliseconds and an amplitude Usa so that, according to Figure 2, the inductance of the main inductance L'm at these high frequencies is so large that the current passing through the inductance L'm can be ignored. After that: the current generated by this pulse at the trailing edge of Isa's curve is sampled. Based on the sample values, the time constant L'S / (RS + R'r) and the differential quotient dlsa / dt equal- '! * 35s Usa = RsIsa + L's (dlsa / dt) are calculated.

• · * 3 115860 3. The voltage is then applied to the stator with a frequency so low that the current through the rotor Isy is negligible and the current in the stator Isa is practically equal to the excitation current Im through the main inductance. Knowing the stator resistance Rs and the current Isa, the stator inductance Ls = Lm 5 + Los can be determined. In addition, the dynamic main inductance L'Dm (also called differential master inductance) applied to the stator side can be determined and its value LDm calculated. The main dynamic inductance is determined by applying the test voltage of the DC voltage and the AC voltage superimposed on it to the stator, and the resulting AC current is measured (determined as DC in the workstation). 10 This measurement is made with different pre-magnetization DC currents (workstations).

4. Thus, in the equivalence circuits of Figures 1 and 2, all values except rotor ohm resistance Rr are known, and the rotor ohmic resistance Rr can in principle be calculated. However, the manner in which this is done is not explained in detail in the said article.

The object of the invention is to determine the rotor resistance of an asynchronous machine faster than before and to prevent measurement errors caused by shear current.

According to the invention, the solution to this problem comprises a method for automatically measuring the ohmic resistance of an asynchronous machine when the machine is controlled through a inverter while being actuated in a non-rotating field. The method comprises the steps of: 1. measuring ohmic resistance, leakage inductances and main inductance applying to a single phase winding of the asynchronous machine a test signal formed from a predetermined DC signal superimposed on the ac voltage signal, wherein the frequency of the ac voltage signal corresponds approximately to the nominal frequency of the asynchronous machine, '·': 3. Measure the amplitude and phase of the phase signal produced by the test signal, and 4. Calculate the ohmic resistance of the rotor based on the values measured in steps a) and c). basis.

•, .30 With this solution, one measurement of the test signal-dependent phase signal is sufficient.

Correspondingly, the measuring time is shortened. Since the frequency of the AC voltage signal corresponds to approx. · · ·. main asynchronous machine has a very low rated leaving frequency at which the asynchronous machine runs during the 'ί' operation, due to the known frequency of the rotating field and the asynchronous:.:: machine's nominal speed, and also due to the transfer current. '· · 35 measurement inaccuracies disappear.

4, 115860

Preferably, the ohmic resistance of the rotor applied to the stator side is first determined and the actual ohmic resistance of the rotor is calculated using the measurement values of steps a) and c).

The frequency of the ac voltage signal is preferably between 1 and 8 Hz.

Preferably, the DC voltage signal is a DC voltage selected such that the resulting direct current is less than the rated excitation current of the asynchronous machine.

Preferably, the DC amperage is selected such that the main dynamic inductance is approximately equal to the static main inductance of the asynchronous machine.

It may be arranged that the test signal is a phase voltage, the reference value of which is set based on a pre-measured characteristic stored in the memory showing the dependence of the phase current on the reference value.

The invention and its embodiments will now be described with reference to the examples in the accompanying drawings, in which: Figure 1 illustrates a conventional, more detailed equivalent circuit for an asynchronous machine, Figure 2 shows a reduced equivalent circuit for an asynchronous machine while the values are derived from the stator side. and the dependence of the dynamic main inductance LDm on the magnetizing direct current of the asynchronous machine, * · ·: Fig. 4 shows a phase voltage curve used as a test signal, comprising ·; * ·: DC voltage and superimposed saw voltage,; ' 5 shows a block diagram of a converter controlling an asynchronous machine whose resistances are automatically measured by a control device,. ''; Figure 6 shows a detailed block diagram of the converter control device of Figure 5. * · *, 25 charts.

i · · ·; * ·: Since the determination of the stator ohmic resistance Rs, the leakage inductances Los and LCTr and the main inductance Lm can normally be done by the process described in steps 1), 2) and 3), as well as the parameter conversion *; ·; 2, 115860, the following provides a detailed explanation of the fourth step required to determine the ohmic resistance Rr of the asynchronous machine.

Referring to the reduced equivalent circuit of Fig. 2, the following equation U '(1) 5 applies where R'r is the ohmic resistance of the rotor converted to the stator side, U'm is the voltage drop across the main inductance L'm and I′sy is the current through the rotor. The horizontal line above each quantity means that complex values are being processed.

In addition, for the stator-side modified rotor ohmic resistance the following holds: ϋ r 10 = (2)

Mr S

In this equation, Lr is the same as Lm + Los, and s is equal to that left by the asynchronous machine. Since s is 1 when the asynchronous machine is stationary, and the measurement is made using the main dynamic inductances, it can be shown that K = «', · (3) L Dm. The dynamic inductances LDm and L'Dm are known from step 3) described initially. Tun- • · ·. it is R'r. Assume that the LOT is approximately the same as half the transient inductance at L's. In this fourth step of the process to the stator phase windings'. conducting a test signal in the form of a phase voltage Usa consisting of a dc voltage signal and a superimposed saw voltage signal as shown in FIG. 4, and measuring the resulting stator voltage Isa **. The voltage loss U'm acting over the main inductance can be expressed as: V'm = Usa-RsIsu-jG> -L ', lsa (4)'. ·. ' In this case, when converted to the stator side, the rotor current holds for Γν = τ „cose (5): * ·. 25 In this equation, Θ is the phase difference between the voltage U'm and the current lsa. The arctan function of the ratio of the ima- ginary part of the equation (4) to the real part gives the phase difference a of the voltages 6 115860 ϋsa and U'.When the phase difference between the voltage Usa and the current Tm is denoted φ, θ = α + φ . The angle φ can be determined by a discrete Fourier transform, for this purpose the sampled values of the current Isa are multiplied by a complex e-function exponentially of the frequency ω of the current Tsa, which oscillates in the same phase as the test voltage. The sample values are numerically integrated into a complex number, and the angle φ is obtained by generating the arctan function of the relation between the real and imaginary parts of this figure.

By summing a and φ, we obtain Θ and hence, according to equation (5), the current I'sy. Since in the reduced equivalent circuit of Fig. 2 TJ'm and 7ϊν are in the same phase, R 'r is obtained from the ratio U'm / Fsy.

A small angular frequency ω is used to prevent shift current. However, too little angular frequency causes current to pass through the main inductance and not through the ohmic resistance of the rotor. It has been found that both requirements are met at a frequency within the range of the nominal leftover frequency fs, usually between 1 and 8 Hz.

In addition, the voltage of the test signal must be kept low at this frequency because the impedance of the asynchronous machine is low. However, the nonlinearities and dead times of the inverter switch elements mean that its output current, and thus also the stator current of the asynchronous machine, is not proportional to the inverter control voltage and the control voltage reference voltage when the control voltage is set in a preset value. Thus, the phase voltage cannot be derived by merely cutting current Isa without measuring the output voltage of the inverter or the phase voltage, respectively, with the aid of a voltage sensor, and without measuring the stator current or phase current Isa, respectively. Before starting the measurement of the ohmic resistances and inductances of an asynchronous machine; ··· 25 thus the inverter output voltage, that is, the asynchronous machine input voltage, equals. ·: *. dependence of the female curve on the reference voltage of the control voltage, and each stator current

For Isa, the deviation (error) from the direct characteristic curve representing the ideal case is recorded. This offset is used for the automatic correction of the reference value of the control voltage that determines the test signal.

: ": 30 The amplitude of the DC current in the stator current determined by the test signal Usa is conductive. by comparing the dependence of the characteristic curve of the dynamic main inductance LDm ... with the corresponding characteristic of the static main inductance Lm.

'·; · * These characteristics are shown in Figure 3, where the dotted dash represents a static:, · | the main inductance LDm as a function of the magnetization current Im. The static inductance Lm is defined by 35 as the angle of the straight slope drawn from the origin to the action point of the magnetization curve, i.e., by the ratio Om / Im, where Orn [Vs] is the magnetic flux. Dynamic inductance, also called differential inductance, corresponds to the slope of the magnetization curve at a predetermined point. Dynamic inductance itself can be expressed as: 5 + (6) m

In step 3) of the measurement process already mentioned, the transient dynamic inductance L'Dm was used to calculate the value LDm. Based on these values, all other values must also be expressed as dynamic values. However, given equation (3), the problem arises that R'r must be determined from the ratio U'm / Tsy. 10 According to equation (4), U'm depends on mm. magnify L's. However, L's is a static inductance, and since it is not known how this static inductance is distributed between the leakage ductances L0s and LOTs, neither can the transient dynamic inductance L'Ds be calculated. However, for a perfectly accurate calculation of the ohmic resistance of the rotor, transient dynamic inductance L'Ds 15 should be used instead of static inductance L's. To solve this problem, the measurement is made with a direct current at which the static main inductance Lm is equal to the dynamic main inductance LDm. The static transient inductance L's can be expressed as such: L \ = K + U, - TTT- (?) (Tr / * ". 20) and the dynamic transient inductance L'Ds as such: L'os = LDm + Las- ( 8) When LDm and Lm are equal, L's is equal to L'Ds, which means that when the excitation current of Fig. 3 is suitably selected, the determined value of inductance L 'can be used as the value of dynamic inductance L'Ds.

; '' '; 25 In Figure 3, the dotted dashed curve shows the dynamic main inductance LDm and the integer *. the dashed line represents the main static inductance Lm at different current amplitudes in an asynchronous machine with a nominal output power of 7.5 kW, an operating voltage of 380 V, and an operating frequency of 50 Hz. The curves intersect at a point approximately 40% of the rated: excitation current Imn, which is 14.64 A. At this point, the dynamic inductance and 30 static main inductance are equal. This means that for measuring the resistance of the rotor ohmic resistance, the DC current of Isa should be set to approximately 40% of the nominal excitation current Imn.

Referring now to Figures 5 and 6. The inverter of Fig. 5 controls the rotational speed of a three-phase asynchronous machine 1. To this end, it comprises a rectifier-5 frequency bridge 3 supplied from a three-phase mains current 2 and a DC voltage intermediate circuit 4 comprising a choke coil 5 and a smoothing capacitor 6. An ohmic voltage divider 7 is provided adjacent as a measurement value, the inverter having transistors having parallel parallel diodes connected in parallel, wherein said lower voltage is supplied to the comparator of the regulator 11 via an analog / digital converter 9 and a control unit 10, whereby the components 9, 10 and 11 together form The current sensor 12 is connected to each cable of the asynchronous machine 1. Each current sensor 12 transmits to the analog / digital converter 9 a measured value of phase-15 currents Ia, Ib and Ic passing through phase windings a, b and c of the asynchronous machine 1. Together with the control unit 10 and the current sensors 12, the analog / digital converter 9 forms the current measuring device 13 shown in Figure 6. With the asynchronous machine standing, the ohmic resistances and inductances of the coils a, b and c and the inductances respectively are phase like current Ia, but the amplitude of both is only half the amplitude of phase current Ia. Based on the three phase currents Ia-Ic, the current measuring device 13 calculates the stator current 7 (u, which corresponds to the phase current Ia, except for the proportional factor.) The stator current Tsa is φ For a voltage applied to the phase windings of 25 Vs, the stator current 7.fl is sampled.

v. The sample values are multiplied by a complex e-function whose exponent contains the frequency ω = 2π fs of the current "* lsa, and whose oscillation is in phase with the current 7,

The time 'where Uref determines this oscillation and fs is the test, or not,' 'of the asynchronous machine 1. The sample values are numerically integrated into a complex number. The phase difference φ is calculated by constructing the quotient between the real and the imaginary part and generating: ': the arctan function of this quotient. The ohmic resistance Rr of the rotor is then calculated in the function unit by the amplitude of the stator current 7sa, the phase difference φ, the left frequency '. fs and previously defined parameters Rs and L 's.

To pre-verify that the stator current Isa corresponding to the desired current Ia is determined; '·. 35 is determined by the correct phase voltage generating the test signal, irrespective of the nonlinearities and dead times of inverter 8, when the voltage is again determined by the corresponding reference voltage Uret supplied to the regulator 11, either the current voltage characteristic or the correction values are stored in tabular form in the function correction function unit 16 as a function of the stator current Isa. The DC signal component 5 (about 40% of the nominal excitation current) is set in the function unit 17 based on the nominal excitation current Imn and a predetermined stator ohmic resistance, and the test signal generator 18 superimposes the saw frequency, , and thereafter corrected on the basis of the stator current Isa 10 measured in the power correction function unit 16, so that the correct reference value Uref of the control voltage of the inverter 8 is obtained, and thus also the phase voltage Usa corresponding to the stator current Isa.

When the correct reference value Uref and hence the phase voltage Usa is determined, the control unit 10 or the function unit 15 contained in the control unit 10 calculates the voltage-15 loss U'm according to equation (4), and when angle Θ is determined by angle φ 7'vv according to equation (5), and the rotor ohmic resistance R'r is calculated from equation (1) by equations (4) and (5), and the rotor ohmic resistance Rr is calculated from predefined inductances according to equation (3).

The test signal shown in Figure 4 is represented as a saw voltage signal, but may also be in the form of a pulse pulse or a sine wave, and may be supplied until the resulting stator current is stabilized, i.e., until the phase difference φ and the stator current amp; 1 lith Father is stabilized. The period during which the test signal is applied is about 5 seconds • f · m · 1, but it depends on the size of the asynchronous machine.

In carrying out the measurement process of the invention with an asynchronous machine of 7.5 kW, a operating voltage of 380 V and an operating frequency of 50 Hz, the following values were obtained during the three steps described at the beginning: Rs = 0.65 Ω, L's = 8.3 mH, and L'Dm = 88.7 mH. Based on this, the dynamic inductance was calculated to be LDm = 92.7 mH. In order to determine the transformed ohmic resistance of the rotor, the test signal Usa was set to a frequency of · · · 30, with a nominal left frequency of fs = 2 Hz. In the error correction function unit, after a 16-factor correction, the magnitude of the test signal Usa was 21 V. After the calculation, the step: ··· he difference φ = -0.226 rad, and the stator current amplitude Isa = 20.4 A. Resistance R′r. For · 1, this resulted in a value of 0.39 Ω, and according to equation (3), a value of Rr = 0.44 Ω was obtained.

• Compared to the correct value of the rotor ohmic resistance of 0.45 Ω, the error was about 2.3:> '· ί 35%, which is a typical value by this method and is sufficiently accurate to exchange:. ·: For the inverter with field-oriented guidance, as in the present case.

10 1 1 5 8 6 0

The ohmic resistance RT1 of the rotor, determined at a predetermined temperature T1, can be recalculated to the ohmic resistance RT2 at another temperature T2 by equation (9).

+ CT

Rt1 = Rtx'1 \ ^ Tt (9) 5 In this equation, KT is a material constant (for example, copper KT = 235 when the operating temperatures T1 and T2 are measured in ° C).

However, equation (9) assumes that another temperature T2 is known, but this is not always the case. However, the method of the invention can determine the ohmic resistance of a rotor during a short shutdown of an asynchronous machine without knowing the temperature. 1 * · »115860 11

List of physical units

Usa Test signal, phase voltage, stator voltage

Father Phase current, stator current, phase signal

Ia, Ib, Ic Stator phase currents

Um Voltage over main inductance

Im magnetizing current

Imn Rated magnetisation current

Isy Rotor Power

Rs Ohmic resistance of the stator

Rr Ohmic resistance of the rotor

Uref Reference voltage

Lm Static main inductance

Lom Dynamic master inductance

Ls Stator Inductance (Lm + L0S) Lös Stator Leakage Inductance L'm Converted Main Inductance L'Dm Converted Dynamic Main Inductance L'Ds Converted Dynamic Transient Inductance L's Converted Transient Inductance (Las + Lor) U'm Voltage Overduct Current through transformed main inductance I'sy Converted rotor current, R'r Converted rotor resistance 0 Phase difference between U'm and Tm: * (X Phase difference between Um and U'm •: φ Usa and Isii phase difference ':' · ω Angular velocity 2π fs;. fs Test frequency or nominal miss frequency, respectively; f: s

Om Main Flow, ·. T i, T2 Asynchronous machine operating temperatures I * RT1, RT2 Ohmic resistance at various operating temperatures materiaal Material constant of rotor conductor material t · i> »

Claims (6)

A method for automatically measuring the ohmic rotor resistance (Rr) of an asynchronous machine (1) controlled by an inverter (8) while being actuated by a non-rotating field, characterized in that the method comprises steps in which ) measuring the ohmic stator resistance (Rs), leakage inductances (Los, Lor) and main inductance (Lm) in the asynchronous machine; of the asynchronous machine, the frequency of the AC voltage signal approximately equal to the nominal lag frequency (fs) of the asynchronous machine, (c) measuring the amplitude and phase (φ) of the phase signal generated by the test signal (Isa), and d) ) on the measured values according to steps a) and c). 15 Method according to claim 1, characterized in that first the ohmic rotor resistance (R'r) transformed to the stator side is defined, and the actual ohmic resistance (Rr) of the rotor is calculated on the measurement values according to steps a) and c). Method according to claim 1 or 2, characterized in that the frequency of the alternating voltage signal (fs) is in the range 1 - 8 Hz. * '20 Method according to any of the preceding claims 1-3, characterized in that 'the direct current signal is a direct current selected so that the direct current it generates is'. · Less than half of the nominal magnetization current (Imn) of the asynchronous machine (1). 5. A method according to claim 4, characterized in that the amperage current of the DC current; ·, Quantity is chosen so that the dynamic main inductance (LDm) is about the same as>> · 25 the static main inductance (Lm) of the asynchronous machine (1). 1 4 j Method according to any of the preceding claims 1-5, characterized in that: the test signal is a phase voltage (Usa), whose reference (Uref) is set on the basis of a previously measured, in a memory stored characteristic curve, wherein the characteristic the curve describes the relationship between the phase current (Isa) and the reference.