RU143346U1 - DIAGRAM FOR DETERMINING THE ELECTRIC POWER CONSUMPED BY ASYNCHRONOUS MOTORS WHEN TESTED BY THEIR MUTUAL LOAD TEST - Google Patents

Abstract

A circuit for determining the electric power consumed by induction motors when tested using the mutual load method, characterized in that common industrial electrical measuring instruments are used as measuring instruments for measuring AC power at a frequency of 50 Hz supplied to the inputs of both frequency converters and one of the asynchronous motors, and DC power transmitted via a common DC bus; the use of two contactors, allowing to de-energize the input of the rectifier of one of the frequency converters.

Description

The utility model relates to the field of electrical engineering and can be used when testing induction motors by the method of their mutual load as a circuit for determining the power supplied to the motors [1, 2, 3].

In the considered scheme of tests of induction motors by the method of mutual load, frequency converters are used, which are a source of non-sinusoidal voltage and variable frequency. Therefore, to assess the power loss in the frequency converters in such a circuit, it is necessary to determine the AC power in a three-phase non-sinusoidal circuit with a current frequency that can significantly differ from 50 Hz. The direct measurement of this power requires the use of special instruments and circuits for measuring electric power.

Currently, there are many methods for measuring electrical power in non-sinusoidal AC circuits.

One of the methods is reduced to the simultaneous formation of integrals from the product of voltage and current signals in the load and from the product of voltage and current signals in the load delayed by a fixed time interval. The measurement result is determined by the equal values of the cumulative integrals in the moments of equality of the values of the integration results even from the beginning of the measurements [4].

Another method is based on converting the instantaneous power consumed by the load into an alternating voltage proportional to this power. From the generated electrical voltage, pulsating voltages of various polarities are isolated. Smoothing the pulsations of both voltages, they are converted into constant bipolar voltages equal in magnitude to the peak values of the pulsating voltages. Constant bipolar voltages are summed, and the result of the summation is measured with a DC voltmeter, calibrated in units of power [5].

To measure AC power in non-sinusoidal circuits, Hall effect sensors are often used. So in the prototype circuit for testing asynchronous motors using the mutual load method, a method for measuring current and voltage signals by means of shunts, Hall sensors, an adder and a software product that provides mathematical processing of fixed signals is proposed [6].

The above methods are characterized by the use of special sensors, microcircuits, the presence of complex software. This leads to a complication and increase in the cost of the measuring circuit of the stand.

The purpose of the proposed utility model is to reduce the cost and simplify the measuring scheme of the stand.

This goal is achieved through the rejection of the use of special sensors, microcircuits, sophisticated software for signal processing; the use of general industrial electrical measuring instruments as means of measuring the power of alternating current with a frequency of 50 Hz and electrical measuring instruments of direct current; use of two contactors, providing phased switching of the test circuit during the measurement process. Thus, the power supplied to the engines is determined indirectly.

In FIG. Figure 1 shows the test circuit of induction motors by the method of their mutual load and the measuring devices connected to the circuit. In FIG. 1 the following elements are indicated: 1, 2 - tested induction motors; 3, 4 - frequency converters; 3.1, 4.1 - uncontrolled rectifiers of the first and second frequency converters; 3.2, 4.2 - DC links of the first and second frequency converters; 3.3, 4.3 - controlled inverters of the first and second frequency converters; 5, 6 - contactors; 7, 8, 9 - wattmeters; 10 - DC ammeter; 11 - DC voltmeter; 12 - common DC bus; 13 - mechanical connection of the shafts of engines 1 and 2, rotating with the same angular frequency.

The circuit consists of three common industrial electrical measuring instruments for measuring AC power with a frequency of 50 Hz supplied to the inputs of both frequency converters and one of the asynchronous motors; from common industrial electrical measuring instruments for measuring DC power transmitted over a common DC bus; two contactors, allowing to de-energize the input of the rectifier of one of the frequency converters.

The determination of the electric power consumed by induction motors when testing them using the mutual load method for the adopted stand scheme (see Fig. 1) is carried out in three stages.

At the first stage, the circuit shown in FIG. 2, and the following actions are carried out:

- power is supplied from the network to the rectifier of the first converter 3.1: both machines 1, 2 are started idling at the rated frequency of the voltage supplying the motors;

- at a constant frequency of the voltage supplying engine 2, the frequency of the voltage supplying engine 1 decreases, due to which 1 is loaded in generator mode, and 2 in engine mode; measurements are made of the power transmitted through the DC bus 12, and the power supplied to the motor 2 from the controlled inverter 4.3, the difference of which determines the dependence of losses in the controlled inverter 4.3 from the power consumed by motor 2:

- at a constant frequency of the voltage supplying engine 2, the frequency of the voltage supplying engine 1 increases, due to which 2 is loaded in generator mode, and 1 is in engine mode; measurements are made of the power transmitted through the DC bus 12 and the power generated by the generator 2, the difference of which determines the dependence of the losses in the controlled inverter 4.3 on the power generated by the generator 2 and transmitted from it to the DC link:

- we determine the dependence of the power supplied to the inverter 4.3 of the second frequency converter 4, on the power consumed by the motor 2:

- we determine the dependence of the power supplied to the inverter 4.3 of the second frequency converter 4, on the power generated by the generator 2:

Using dependencies (3) and (4) we obtain the dependencies:

At the second stage, losses in an uncontrolled rectifier are determined. The measurements in the second stage are carried out according to the circuit shown in FIG. 1, the difference of which is that electric energy from the network to the DC links is transmitted through both uncontrolled rectifiers simultaneously. Measurements are carried out under the same operating conditions of asynchronous machines as in the first stage.

As a result of measurements, the following dependences are obtained:

- power at the output of the rectifier 4.1 of the second frequency converter 4 from the power consumed by the motor 2:

where P _inv (W _d ) is determined by the expression (3);

- power at the output of the rectifier 4.1 of the second frequency converter 4 from the power generated by the generator 2:

where P _inv (W _g ) is determined by the expression (4);

- power losses in the rectifier 4.1 of the second frequency converter 4 from the power transmitted by it to the DC link when the machine 2 is operating in both motor and generator modes:

where W _with - the power consumed by the second frequency Converter 4 from the network.

The second stage ends with the determination of the dependence of the power loss in the uncontrolled rectifier 4.1 on the power supplied to it from the network:

in accordance with expressions (9) and (10).

It is necessary to perform actions in accordance with formulas (1) - (10) for both the first and second frequency inverters.

The constituents W _c , W _g , W _d , P _{pc are} obtained from the measurement results. The first two stages are carried out only one. The purpose of the first two stages is to obtain dependencies (1) - (10) for their further reuse.

The third stage is carried out as follows. A power meter 9 is not required at this stage. At a certain steady state, the values W _c1 , W _c2 , P _{pc are measured} , after which the following quantities are expressed and found from the following available dependencies:

- from (11), (12) is determined ΔP _vyp1 , ΔP _vyp2 ;

- from (9) and (10), _Ppr1 , _{Ppr2 is determined} ;

- from (7) and (8), P _inv1 , P _{inv2 is determined} ;

- from (5), (6), ΔP _inv1 , ΔP _{inv2 is determined} ;

- from (3) and (4) is determined by W _d and W _g .

The practical value of the presented scheme for determining the power lies in the fact that at a nominal frequency of the voltage of the tested motors equal to 50 Hz, all measurements require the use of measuring instruments designed for the same voltage frequency, which greatly simplifies and cheapens the measuring circuit.

Information sources:

1. Patent for the invention of R.F. No. 2433419 G01R 31/34, 2010.

2. Avilov, V.D. A mathematical model of the test process of induction motors by the method of their mutual load [Text] / V.D. Avilov, D.I. Popov, A.V. Litvinov // Bulletin of SibADI. 2013. No5 (33). S. 75-81.

3. Avilov, V.D. Assessment of energy efficiency of the application of the method of mutual load when testing asynchronous traction motors [Text] / V.D. Avilov, D.I. Popov, A.V. Litvinov // Proceedings of the Trans-Siberian Railway. - Omsk, 2013. - No. 3 (1). - S. 2-7.

4. Patent for the invention of R.F. No. 2229723 G01R 21/06, G01R 21/133, 2004.

5. Patent for the invention of R.F. No. 2296338 G01R 21/06, 2007.

6. Patent for utility model R.F. No. 80018, IPC G01R 31/04, 2008.

Claims (1)

A circuit for determining the electric power consumed by induction motors when tested using the mutual load method, characterized in that common industrial electrical measuring instruments are used as measuring instruments for measuring AC power at a frequency of 50 Hz supplied to the inputs of both frequency converters and one of the asynchronous motors, and DC power transmitted via a common DC bus; the use of two contactors, allowing to de-energize the input of the rectifier of one of the frequency converters.