## 2 Description of the Measurement System
### Motor Test Bench  
Figures 2 and 3 show a schematic diagram and a photo of the motor test bench. Figure 1: Photo of the measurement system. Figure 2: The schematic diagram of the test bench. 2.1.1. The Bench, the Analyzed and Load/Drive Motor  
The rigid bench is bolted to a welded base frame attached to the floor of the building. The analyzed motor, with its controller and \"accelerator pedal\", the load/drive motor, the optical LED and rotary torque sensors are fixed to the bench. The analyzed motor can be any kind of electric motor, in certain cases equipped with its controller and \"accelerator pedal\". It must be emphasized that the controller is not a part of the measurement system; thus, a description of it is not given here. Foot and flanged motors can also be fixed to the bench by applying custom-made adapters. The fixing of the optical LED and rotary torque sensors is solved by magnetic base stand holders. The load/drive motor can be freely moved on the rigid bench in directions x and slightly y, while the analyzed motor is in direction z. Therefore, the proper alignment of the shafts can be adjusted by applying a dial gauge or a laser shaft alignment tool. The load/drive motor is a three-phase induction motor, and as its name suggests, it has a dual function:  
1. It can be supplied by a variable DC voltage (0-400 V) and used as a \"load motor\" for braking the analyzed motor. 2. It can be supplied by a three-phase AC voltage with a variable voltage (0-400V) or frequency (0-50 Hz) and can be used as a \"drive motor\" for driving the analyzed motor. 2.1.2. Data Acquisition System  
In Figure 2, the different parts of the applied data acquisition system are also presented. These parts are:  
1. Optical LED (ROS-P, Monarch Instrument) and rotary torque (HBM T22/200) sensors for angular speed and torque measurement. 2. Shunt resistor for the measurement of the intensity of the electric current flowing through the windings of the motor. 3. The voltage input module (NI 9239) and compact DAQ chassis (NI USB 9162) together with a PC. 4. Self-developed software component for data collection, processing and storing. The measured torque, angular speed and current intensity are converted to voltage signals by the sensors and the shunt resistor and fed into the NI voltage input module, which is connected to a PC through the Compact DAQ Chassis. The optical LED sensor operates from a DC power supply (6 V). It provides a \(U_{OLS}=6\) or 0 V output voltage signal depending on whether light is reflected from the rotating object to the sensor. To do this, reflective strips are fixed to the circumference of the rotating object (e.g., to the shaft of the motor). The maximum angular speed which can be measured by the sensor is 25,0000 RPM, but applying more than one strip, this maximum value is reduced. To check the accuracy of the value measured by the optical led sensor, we measured the  
Figure 3: Photo of the test bench. speed of the drive motor in the 0-3000 RPM range and compared it with the value shown by the frequency converter. The measured values showed good agreement. The optical LED sensor and its output voltage signals can be seen in Figure 4.  
The output voltage signals are counted by applying our self-developed NI LabVIEW program [25], and the angular speed is calculated from the number of strips (N\({}_{\text{strip}}\)) and the number of detected voltage signals (N\({}_{\text{signal}}\)) in a short \(\Delta\)t time using the following formula:  
\[\omega=\frac{2\pi}{\Delta\text{t}}\cdot\frac{\text{N}_{\text{signal}}}{\text {N}_{\text{strip}}} \tag{1}\]  
The rotary torque sensor is connected to the analyzed and load/drive motors through clutches. The size of the applied clutches depends on the maximum possible torque delivered by the motors to the sensor. The applied torque sensor has a nominal (rated) torque of 200 Nm, and its accuracy class is 0.5%. (It must be emphasized that the T22 torque meter is available with rated torques varied in the range of 0.5-1000 Nm.)