TESTING OF ELECTRICAL MACHINES USING A DATA ACQUISITION AND PROCESSING SYSTEM

Abstract –

The paper deals with electrical machines testing , including high power ones, using an Data Acquisition and Processing System (DAPS), based on a PC compatible microsystem. There are presented the architecture and the main measurement possibilities of DAPS in electrical

machines testing, in various functional conditions: constant frequency steady state (used in classical standard tests), variable frequency conditions (used in asynchronous motors testing by mixed frequency method) and finally, transient conditions. For every testing condition, there are considered measured quantities, their processing methods, and are illustrated with some practical examples, developed by authors.

INTRODUCTION

In the electrical machine design, the accuracy of the calculated values of the parameters and characteristics is limited by the saturation effect of magnetic coils, the skin current effect and various technological causes. In some circumstances, not all of the initial data are known at the designing time. There are specific situations when the parameters and characteristics of electrical machines can be determined fast and accurate only by These results are necessary to determining the performance of those electric machines, to modelling them and toexperimental me ans [1]. improving the design method. Modern testing methods (variable frequency testing, variable active power in transient regime, frequency response) are efficient but they cannot be accomplished with classic measuring apparatuses. Specialized measuring equipment is required to perform such tests on an industrial testing platform . To answer the above requirements,

an electric mac hine testing unit (DAPS) was devised by designing a data acquisition board and a specialized software package, both mounted on a PC compatible machine.

THE HARDWARE ARCHITECTURE

The Data Acquisition and Processing System (DAPS) was conceived for testing electric machines in a wide range of operating conditions. The block diagram of the Data Acquisition and Processing System (DAPS) is presented in Figure

1. The DAPS consists of three major parts: the Transducer and Analog Signal Adapter (TASA), the Data Acquisition Module (DAM) and a PC-microcomputer. The hardware structure and tasks of the TASA and DAM are given in [2, 3, 4], while a general description of the TAS

A is shown in Figure 1. This module translates the analog signals acquired from the Electrical Machine (EM) to standard values [-10V ¸ +10V] compatible with the DAM inputs. The TASA module has been devised such as the DAPS can be used to perform tests on electrical machines operating in a various functional regimes.

The latest version of TASA is provided with voltage and current galvanic insulated transducers, which assure a sufficient precision for all the input channels.

The TASA has the following inputs:

· four voltage inputs with various measurement domains between 110 V and 660 V (regularly being used for measuring the ME supply voltages);

· four current inputs for measuring currents between 5 A and 10 A

(regularly being used to measure the ME supply currents);

· four small voltage inputs, to acquire signals in a 10 V range from magnetic field transducers or rotation transducers;

· two large current inputs (500 A –1500 A) used to studying the field currents in transient conditions of synchronous machine. Since the DAM has 8 differential inputs,not all the TASA channels can be used at a time.

A circuit for the testing of the synchronous machine is given in Fig. 1.

The following entities are measured: 3 stator voltages, 3 stator currents, the rotation transducer signal and the rotor field current. Current and voltage measure transformers are interposed between the tested machine and TASA if the voltages on the three phase stator windings are higher then the upper limit of TASA inputs.

DAPS OPERATING MODES

The DAPS is provided with three data acquisition and primary processing software programs to be used specifically for the following operating modes: steady state periodical conditions, slowly dumped periodical conditions and transient aperiodical condition. Based on these programs for data acquisition and primal processing, further software was developed in a modular structure.

A. Steady-state periodic conditions at rated frequency.

This is the most common regime used in electric machine testing. The DAPS acquires blocks of momentary values of current and voltage on each phase of the tested machine. Consecutive series of measurements are started manually by the human operator, each time the supply voltage is set up to a prescribed value (conform to the test program) and it reaches steady -state periodic conditions.Based on the momentary acquired values of voltage and current, i.e., v(t) and i(t), the DAPS calculates the following: the maximum values Vm, Im, the rms values Vrms, I rms, the phase angle j, the active power P, and the frequency f. In the case of harmonic conditions, the micro-system computes cosj and the reactive power, as well. If the waveform is not pure sinusoidal, then cosj and the reactive power are expressed relatively to the fundamental harmonic. All the above mentioned computations are conducted for each block of momentary values acquired from each phase and the results are accordingly synthesized in the measurement table. In this table, the Media raw contains, respectively, the averages of the currents, voltages, frequencies and cosj, but the sum of powers (active and reactive).The human operator may start a computer procedure to do the harmonic a nalysis of the acquired signals, as well.

B. Steady-state conditions at variable frequency .

The induction motor parameters are obtained as frequency functions by testing it in a short circuit mode at variable frequency. The data acquisition and processing are conducted similarly to the above regime, but the starting of each series of acquisition is accomplished in an automatic manner . Previously the test, the operator prescribes the frequencies at which the DAPS has to acquire each block of momentary values. While the frequency of the power supply is continuously decreased, the DAPS calculates in real time the current frequency to starting an acquisition process as a prescribed value is reached.Table 1 contains the results obtained by processing a block of acquired values in testing an induction machine at steady-state conditions at variable frequency. The acquired block is displayed in a graphical form by the DAPS for the required phase, as shown in Fig. 2.

Fig. 2. Display of a data acquisition block in steadystate short-circuit conditions of an induction machine.

C. Slowly dumped periodical conditions

In this regime, the data acquisition is continuously carried out in an automatic manner, as long as the acquired entities still present significant changes. The same values like in the above case are calculated, but rather continuously, for each half-period of the acquired signals. A no-load starting of an induction machine (0,63 kW, star connection, 380 V, 2900 rpm) is next presented. To increase the starting time, the supply voltage was reduced to 88 V, while the inertial constant J was increased by an additional inertial rotating part. Since the number of the lines in the measurement table is quite high, a few samples have been picked-up to illustrate the experiment as given in Table 2. There was added some more information in this measurement table: the second column represents the acquisition time and the third column in each block of values contains the value of the voltage signal from the rotation transducer.

Table 2. Samples of data acquired from an induction motor in a slow dumped no-load starting conditions

An other example of graphical versatility of DAPS, the active power used up by a 1,1 kW, 380 V, 920 rpm motor to start, and the speed as voltage signal are displayed in a graphical forms as a function of time in Fig. 3.

Based on the active power components, the torque curve M(n) of induction motors can be determined. This method plays an extremely important role in determining the torque curve for the very high power machines, for which the load test can not be carried out.

Fig. 4 represents the torque M as a function of slip (M expressed in per units,) completed in testing a 2800 kW/6000 V asynchronous motor at slow dumped conditions. The overall errors for currents and voltages are less than 0.2%, while they are smaller than 0.5% for the active power.

D. Mixed Frequency Conditions

These conditions are established every time an asynchronous machine is artificially loaded by

connecting to mixed frequency power supplies [5]. To carry out this test, the power supply of

the tested machine is built of a couple of synchronous generators connected in series and

having different frequencies (e.g., 44 Hz and 50Hz). DAPS is used to find out the effective

values of the supply currents and voltages in these specific conditions. Current and voltage as time function in a test of an induction machine with the mixed frequency method are presented in Fig. 5.