The bench is designed for conducting labs in “Electrical machines”.
Structurally the bench consists of two parts:
- housing, in which part of electrical equipment, electronic boards, front panel, power module and tabletop of integrated desktop are installed;
- machine assembly, which includes DC motor, asynchronous motor with wound rotor, one asynchronous motor with squirrel-cage rotor, as well as an optical speed sensor with the definition of the direction of rotation.
The bench can be supplemented with electrical machinery unit on the basis of electric motors of small (90 W) or large (0.55 kW) power.
The bench housing contains:
- Frequency converter for generating variable frequency three-phase AC network and voltage of asynchronous motor and three-phase transformers power supply. The converter is based on a microcontroller MB90F562 (Fujitsu) and power intelligent module PS11033 (Mitsubishi). The controller is used for calculating input data (specifying voltage and frequency) and output (current, voltage) signals, for data exchange with PC (RS-485), and displaying the measured values on the bench front panel. The power module includes power circuits of three-phase bridge rectifier, three-phase bridge inverter on IGBT transistors, as well as drivers and protection circuits (short circuit, insufficient supply voltage drivers, improper control signals input). The frequency converter allows the user to explore the asynchronous motor in all the four quadrants of the mechanical characteristics.
- Pulse-width converter for the armature circuit and the DC motor excitation winding power supply, as well as power supply of rotor circuit of three-phase asynchronous motor with wound rotor in the mode of synchronous motor and generator. The pulse-width converter is implemented on the basis of the power element of the frequency converter. Two of its arms are used for obtaining reversible symmetric PWC, and the third arm is used as an irreversible PWC for three-phase asynchronous motor rotor. Winding power supply is implemented on a single International Rectifier MOSFET transistor. The control system is based on a microcontroller AT Mega163 (Atmel) and implements the calculation of input (specifies voltage, frequency, and current for dynamic braking) and output (currents of anchor, excitation, rotor) signals, provides data exchange with PC (RS-485), the displaying of measured values on the bench front panel. The pulse-width converter of DC motor armature circuit is supplemented with a closed system mode (current or speed control), as well as generator mode.
- Measurement unit is based on digital measuring devices. In addition to direct current and voltage measurements, each channel can calculate:
- effective value of alternating current and voltage;
- shift angle between current and voltage, as well as calculate cos(φ);
- active power.
Measured values can be displayed on the front panel or transmitted to a PC.
- Relay-contactor control, which allows the user to:
- switch the circuit of asynchronous motor with squirrel-cage rotor (star / delta);
- introduce the inductance in the circuit of the stator of asynchronous motor with squirrel-cage rotor, in the parallel (second) three-phase transformer circuit;
- change the value of the load resistor in three-phase circuit;
- connect asynchronous motors to 3 ~ 380/220 V 50 Hz network or frequency converter;
- Resistors in excitation winding circuit (two stages);
- Load resistors in three-phase circuit (three stages);
- Overvoltage dump resistors on intelligent modules;
- Incandescent lamps for three-phase generator synchronization with network.
Frequency converter and pulse-width converter are switched on for internal network operation (recovery mode) in order to reduce power consumption from the network.
The wiring diagrams of the studied objects are depicted on the front panel. All the diagrams are divided into groups in accordance with the lab theme. The panel contains switching sockets, indicators of digital devices, switchgear, and controls that allow the user to change parameters of the elements during the laboratory work.
Controls on the bench front panel:
- setpoint potentiometer to control the reverse pulse-width converter, the reference signal of the closed system;
- setpoint potentiometers of pulse-width converters of power supply for DC motor excitation windings and asynchronous motor wound rotor in synchronous machine mode;
- setpoint potentiometers of frequency converter, which allow for the smooth change of output frequency (0 ÷ 163 Hz) and the output voltage settings (0 ÷ 220 V);
- relay subsystem controls.
To carry out the lab it’s necessary to assemble the circuit of the studied object, using standardized jumpers, which allow the user to assemble the circuit without loss of clarity.
Laboratory work is possible in manual mode as well as in PC mode. Software and a set of methodological and technical documentation intended for academic staff are supplemented to the laboratory bench.
Laboratory work with the use of “EM” software implies data collection in the background mode, the automatic completion of required data tables, and construction of real-time graphs from the data obtained during the conduction of the laboratory work.
Using this software, it is also possible to conduct complex laboratory work, in which the student is supposed to pass the following stages:
- theoretical survey of the material related to the work performed;
- assemblage, validation and correction of the experiment circuit on the bench virtual panel;
- laboratory work conduction on the bench.
In case of failing the theoretical test or circuit assemblage, the student is not allowed to go to the next stage.
For some labs the automatic laboratory work under PC control is envisaged. It is only necessary to assemble the experimental circuit and set the specific parameters of the experiment, after which there is a fully automatic characterization of the machines studied.
In all the versions of laboratory work conduction the experimental results are obtained in the form of tables of values and graphs. The tables can be exported to the clipboard or Office application MS Excel, the graphs can be copied to the clipboard. For the graphs scale changing (in manual or automatic mode), rendering parameters (color, thickness, line type) setting, data smoothing, graph axes setting and other options are envisaged.
The program also provides the possibility of manual remote control of all bench nodes, which allows the user to check their performance and conduct laboratory work in a completely remote mode.
During the bench operation the software allows for the duplication of the bench panel on the PC screen. In this case, all the measured values are displayed in real time mode.