## 4 Experiment Results and Discussion
### Back EMF Measurement Experiment  
To confirm the back EMF component of the motor, the back EMF time data measured in the CW and CCW directions at 1500 RPM are shown in Figure 7a. In the given graph, the back EMF is not shown as a constant but appears as a sinusoidal wave that changes with time. In addition, if Equation (3) in Section 2.1 is applied at the same RPM, the back EMF should always be the same for CW and CCW, but the actual measurement results differed. Figure 6: Comparison of CW (clockwise) and CCW (counterclockwise) of the sample motor. The figure shows that even if the same voltage is applied, the noise and vibration of CW and CCW differ in magnitude and frequency. The direction of rotation is shown in (**a**), and in the graph, red is CCW and blue is CW: (**a**) Results of FFT of CCW and CW noise components; (**b**) results of FFT of CCW and CW vibration components. Figure 7: The result of measuring the back EMF of the sample motor with an oscilloscope. Red is CCW, blue is CW, and numbers in graphs (**b,c**) represent motor RPM. The contents of each Figure are as follows: (**a**) Back EMF time data measured with an oscilloscope at 1500 RPM; (**b**) magnitude of the eighth-order vibration component at 1500 RPM to 3500 RPM; (**c**) magnitude of the DC component at 20 RPS to 60 RPS. To accurately measure the back EMF according to the RPM, each RPM was measured three times from 1500 RPM to 3500 RPM, and the components of the back EMF were analyzed using FFT. Based on the back EMF FFT result, the magnitude of the eighth-order component, which is the main rotational vibration component, is shown in Figure 7b, and the magnitude of the DC component is shown in Figure 7c. Theoretically, the rotational speed of the motor at 1500 RPM is 25 [rev/s], and the eighth-order value is 200 [rev/s]. The first point in Figure 7b, which is the experimental result, was 200 [Hz]. It can be seen that the vibration frequencies of the back EMF and motor are the same. Similarly, it can be seen from Figure 7b that the eighth-order component is represented as the main vibration component for all 2000-3500 RPM. This indicates that both the CW and CCW vibration components of the back EMF coincide with the noise vibration components of the motor and that the back EMF value is not constant by \(\omega\), but by sinusoidal components that change with time. In addition, as the RPM increased, the magnitudes of both the DC component and the eighth-order component of the back EMF tended to increase. In addition, even at the same RPM, the magnitudes of the eighth-order component of CW and CCW were different, as were the DC component. In Figure 7b, it can be seen that the magnitude of the eighth-order component is that CCW is larger than CW, which makes it possible to see that the rotational vibration of the motor is larger in CCW. As shown in Figure 7c, the magnitude of the DC component is slightly larger than that of CW. However, compared to the eighth-order component, the magnitude of the DC component is overwhelmingly 10 times larger. This difference in the DC components causes a difference in the voltage applied to the motor, which in turn affects the rotation speed of the motor. Therefore, even if the same voltage is applied, the CW rotates more slowly than the CCW, and the frequency of the rotation component is lower. Based on these results, compared with Figure 6b in Section 4.1, it can be seen that the frequency of the rotational vibration is lower in CW than in CCW, and the magnitude is smaller. On the other hand, the eighth-order component of Figure 6b and its harmonic component can be expressed in a gradually decreasing form. However, in the CCW of Figure 6a, it increased and then decreased in the range of 1500 Hz to 5000 Hz. Judging from the results of Figure 7a, these peaks appear to be caused by sparks between the brush contacts, and it is expected that CW and CCW will differ depending on the wear condition of the brush. Sparking noises do not significantly affect vibration but seem to affect noise.