RU2566394C2 - Measuring method of magnetisation current of asynchronous motor with phase rotor, which operates under load - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention relates to electromechanics. In order to measure magnetisation current of an asynchronous motor with a phase rotor, which operates under load, the motor is connected through a shaft to a similar asynchronous motor; rotor winding of the first motor is connected to rotor winding of the second motor, and stator winding of the second motor is short-circuited. Measurements of an instantaneous value of magnetisation current of the first motor are made from two similar shunts, one of which is connected to primary winding (stator winding) of the first motor, and the second one is connected to stator winding of the second motor, with that, measuring terminals of shunts are in-series connected in an opposite manner.

EFFECT: provision of possible measurement of magnetisation current of an asynchronous motor at operation under load.

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Description

The method relates to electrical engineering, can be used for demonstration or research purposes.

The known method according to the patent of the Russian Federation No. 1017083, G01R 31/34, "A method of measuring the magnetizing current of an induction motor", authors: Ushakov A.V., Mladentsev V.I. - [1], which involves the measurement of current parameters only in the primary winding of the transformer, in the absence of load, that is, in idle mode. The disadvantage of this method [1] is the impossibility of measuring the magnetizing current when operating under load. When an asynchronous motor operates under load, the magnetizing current is part of the primary current and there is no section of the electric circuit where the magnetizing current flows without a load component and could be measured. However, it is known that the magnetizing current with the appearance of the load current does not remain constant, the same as in idle mode, but decreases. The reason for the decrease in the magnetizing current is the voltage drop on the scattering inductance of the magnetic flux of the primary winding (stator winding) and the active resistance of the stator winding.

Closest to the proposed is the method according to the patent of the Russian Federation No. 2323749, G01R 19/00, "Method for measuring the magnetizing current of a transformer operating under load", authors: Gukov D.V., Gukov A.D. and others - [2], which involves the use of two shunts, one of which is included in the primary and the second in the secondary winding of the transformer, the measuring terminals of the shunts being connected in series in the opposite direction, and the shunts are calibrated so that the ratio of the voltage drops on the shunts during flow the same current is equal to the transformation ratio of the transformer. The disadvantage of this method [2] is the impossibility of measuring the magnetizing current of an induction motor, since the frequency of the current in the primary winding (stator winding) is equal to the mains frequency - 50 Hz, and the frequency of the secondary winding current (rotor winding) is equal to the slip frequency. Adding or subtracting signals of different frequencies cannot isolate a signal proportional to the magnetizing current.

The proposed method for measuring the magnetizing current of an asynchronous phase rotor motor operating under load is that the asynchronous motor with the phase rotor is connected by a shaft to the exact same asynchronous motor, the stator winding of the first motor is connected to the network, the rotor winding of the first motor is connected to the rotor winding the second motor, and the stator winding of the second motor is short-circuited. Measurements of the instantaneous value of the magnetizing current of the first motor are made from two identical shunts, one of which is included in the primary winding (stator winding) of the first motor, and the second in the stator winding of the second motor, and the measuring terminals of the shunts are connected in series.

During operation of the first induction motor with a phase rotor, an EMF is induced in the rotor, the magnitude of which is determined by the transformation coefficient (the ratio of the number of turns of the stator winding to the number of turns of the rotor winding), the EMF frequency is the slip frequency f _s = f ₁ (n ₀ -n) / n ₀ ; where: f _s - slip frequency; f ₁ - frequency of the supply network (50 Hz); n ₀ - synchronous speed of rotation of an induction motor; n is the actual rotation speed of the induction motor; Under the influence of EMF, a current with a slip frequency flows in the rotor winding. This current flows through the rotor winding of the second induction motor and creates there a rotating magnetic field with a rotation speed n _s = 60 * f _s / p, where: p is the number of pole pairs. In this case, the rotor of the second engine rotates at a speed n (it is rotated by the first engine). The rotation speed of the magnetic field of the rotor of the second motor relative to the stator is: n + n _s = n ₀ and does not change when the rotation speed of the motor n is changed (provided that the phasing is correct). Accordingly, the frequency of the current in the stator winding of the second motor will be constant and equal to the frequency of the supply network (50 Hz); In this case, the voltage in the stator winding of the second motor will be equal to:

U ₁ * K _Tr * 1 / K _Tr = U ₁ , that is, equal to the voltage of the supply network. Thus, the current flowing through the short-circuited stator winding of the second motor can be considered the load component of the first asynchronous motor, and the second motor in this design is used as a frequency and voltage converter for winding the rotor of the first motor. The second motor will also consume magnetizing current, which can distort the measurement result, but since the second motor is used in short circuit mode, the magnetic flux and magnetizing current will be negligible.

The inventive device for welding meets the requirement of "novelty", as it has new features:

1) an asynchronous motor with a phase rotor is connected by a shaft with exactly the same asynchronous motor;

2) the rotor winding of the first engine is connected to the rotor winding of the second engine, and the stator winding of the second engine is short-circuited;

3) measurements of the instantaneous value of the magnetizing current of the first motor are made from two identical shunts, one of which is included in the primary winding (stator winding) of the first motor, and the second in the stator winding of the second motor, and the measuring terminals of the shunts are connected in series.

From the current level of technology and technical literature known electrical machine voltage and frequency converters with asynchronous motors having a phase rotor. But it is the application of such a combination of features that allows to obtain a new quality to the method of measuring the magnetizing current of an induction motor with a phase rotor - the ability to measure under load. This allows us to conclude that the criterion of "inventive step".

In FIG. 1 shows a diagram for implementing a method for measuring the magnetizing current of an asynchronous motor with a phase rotor. The diagram shows: ADP1 - an asynchronous motor with a phase rotor; ADF2 is an auxiliary engine for making measurements. Sh1 - a shunt in the stator winding circuit ADF1; Sh2 - a shunt in the stator winding circuit ADF2; The device consists of a primary winding 1, a secondary winding 2 and a magnetic circuit 3. The measuring terminals from the shunts Ш1 and Ш2 are connected counter-in-series.

вычиталось из падения напряжения на шунте Ш1 от тока i₁(t). Осциллограф в такой схеме должен показать намагничивающий ток i_нам(t), поскольку $$i_{нам}\left(t\right)=i_1\left(t\right)=i_1\left(t\right)-i_2^{\text{'}}\left(t\right)$$

.Measurement by the proposed method presented in FIG. 1 is produced as follows. The stator circuits of ADF1 and ADF2 motors include shunts Ш1 and Ш2, the shunts are chosen the same. The terminals of the shunts Ш1 and Ш2 are connected so that the voltage drop on the shunt Ш2 from the current of the stator winding ADF2 $$i_2^{\text{'}\text{'}}\left(t\right)$$

subtracted from the voltage drop on the shunt Ш1 from the current i ₁ (t). The oscilloscope in such a circuit should show the magnetizing current i to _us (t), since $$i_{n\mathrm{but}m}\left(t\right)=i_{\mathrm{one}}\left(t\right)=i_{\mathrm{one}}\left(t\right)-i_2^{\text{'}\text{'}}\left(t\right)$$

.

The tests were carried out in a laboratory setup with two shafts connected by MTF-012-6 crane motors with a rated power of 2.2 kW, rated rotation speed of 890 rpm, rated voltage of 380 V, rated stator current of 7.6 A, rated current of rotor 11, 5 A. Shunts Sh1, Sh2 used standard 10 A; 75 mV; Oscilloscope S1-65. During the experiment, it is necessary to perform phasing, that is, to select the phase in which you want to include the Ш2 shunt and to ensure the on-board inclusion of the shunts. After tuning the circuit, the magnetizing current is reflected on the oscilloscope. With an increase in the supply voltage, it increases, its non-sinusoidality increases, with an increase in the load on the motor (using the working mechanism), the magnetizing current decreases.

The tests carried out allow us to conclude that the claimed invention meets the criterion of "industrial applicability".

Information sources

1. RF patent No. 1017083, G01R 31/34, “Method for measuring the magnetizing current of an induction motor”, authors: Ushakov A.V., Mladentsev V.I.

2. RF patent №2328749, G01R 19/00, "Method for measuring the magnetizing current of a transformer operating under load", authors: Gukov DV, Gukov AD and etc.

Claims (1)

A method of measuring the magnetizing current of an asynchronous motor with a phase rotor operating under load, characterized in that the induction motor with a phase rotor is connected by a shaft to the exact same asynchronous motor, the stator winding of the first motor is connected to the network, the rotor winding of the first motor is connected to the rotor winding of the second motor and the stator winding of the second motor is short-circuited, the instantaneous values of the magnetizing current of the first motor are measured with two identical shunts, one of which include in the primary winding (stator winding) of the first motor, and the second in the stator winding of the second motor, and the measuring terminals of the shunts are connected in series in the opposite direction.