## 3 Characteristics Test of Electromagnetic Dampers
### Comparative Analysis of Test Results
Hence, it was confirmed that to reinforce the damping force using the eddy current, it is necessary to limit the degree of contribution  
Figure 6: Maximum damping forces of the dampers under different excitation conditions: (**a**) amplitude–maximum force relationship for the coil type; (**b**) amplitude–maximum force relationship for the eddy-current type; (**c**) amplitude–maximum force relationship for the composite type; (**d**) frequency–maximum force relationship for the coil type; (**e**) frequency–maximum force relationship for the eddy-current type; (**f**) frequency–maximum force relationship for the composite type. of the damping force due to the eddy current with consideration of the desired maximum damping force and the rate of increase in the damping force due to the strengthening of the excitation conditions. This is a major consideration in design, because the coil and oxygen-free copper tube must be placed together to ensure controllability and enhance the damping force. The comparison results for the maximum damping forces of the three electromagnetic dampers under the same excitation conditions (frequency and amplitude) are shown in Figure 8. For all three dampers, as the excitation amplitude increased, the maximum damping force increased linearly. In contrast, as the excitation frequency increased, the maximum damping force increased non-linearly. For the composite- and coil-type dampers, the maximum damping force varied but remained within the ranges of 78-88% and 58-60%, respectively, of that for the eddy current-type damper. The maximum damping forces of the three types of electromagnetic damper in the excitation test are presented in Table 3. Because the shaking table could not be excited under certain amplitude conditions (\(\geq\)5 Hz), the excitation range was set with consideration of the allowable capacity of the shaking table. The three electromagnetic dampers exhibited the maximum damping forces when the excitation amplitude was 15 mm and the excitation frequency was 4 Hz. The maximum damping forces of the composite and coil types were 88% and 60%, respectively, of that of the eddy-current type. Remarkably, the maximum damping force when the eddy current-type electromagnetic damper was excited with an amplitude of 3 mm and a frequency of 7 Hz was 1.83 N weaker than that when the damper was excited at a frequency of 6 Hz. Figure 8: Maximum damping forces of the dampers according to the excitation conditions: (**a**) changes in the excitation amplitude; (**b**) changes in the excitation frequency; (**c**) mesh plot for the excitation conditions. Figure 3:0 Comparison of hysteresis curves: (**a**) excitation at an amplitude of 3 mm and a frequency of 1 Hz; (**b**) excitation at an amplitude of 3 mm and a frequency of 4 Hz; (**c**) excitation at an amplitude of 15 mm and a frequency of 4 Hz. In contrast, the coil-type electromagnetic damper did not exhibit a reduction in the maximum damping force under the same excitation conditions. However, when the frequency is \(\geq\)8 Hz, the maximum damping force is expected to decrease. Therefore, it is necessary to prevent the magnetic saturation of the pole pieces and magnetic body, as well as magnetic imbalance. In the coil-type electromagnetic damper used in the test, the stator coils were arranged in a slotless structure; thus, there was no eddy current loss [21]. Analysis of the foregoing results revealed an effective design direction for applying electromagnetic dampers in the field of civil engineering. It is necessary to improve the damping performance using the eddy current while maintaining controllability using the coil. Additionally, it is necessary to determine the driving stroke with consideration of the frequency and amplitude to be countered and to arrange the conductors and magnetic bodies. However, when applying a slot or bridge to reinforce the damping force, it is necessary to consider cogging [22].