# Investigation of Regenerative Braking Performance of Brushless Direct Current Machine Drive System
## 4 Experimental Study and Results  
The practical features and feasibility of the regenerative braking circuit model of a BLDC machine drive system are evaluated by using an experimental setup which is presented in Figure 4. Equations (4)-(6) and the braking control scheme are implemented via TMS320F28335 digital signal processor. Using the known parameters and the measured internal resistance value of Equation (4), the required duty ratio for the maximum regeneration operation is calculated according to the regeneration command. In the meantime, instead of a PI controller, a hysteresis controller is activated and the PWM reference ratio is assigned as the calculated critical value. The converter has a maximum DC-link voltage of 13.5 V \(\times\) 4, i.e., 54 V, and a rated RMS (root mean square) current of 5.3 A. The experimentsare verified using the MAXON EC60 slotless type BLDC machine which is coupled to a specially designed flywheel, as shown in Figure 4. The system specifications, BLDC machine, and flywheel parameters are given in Table 1. The system specifications and machine parameters such as the terminal resistance, terminal inductance, on-resistance of MOSFET, on-resistance of diode, flywheel inertia have the values of 1.12 \(\Omega\), 0.82 mH, 0.024 \(\Omega\), 0.05 \(\Omega\), 0.06089 kgm\({}^{2}\), respectively. In the experiments, the temperature changes are carried out by using an ambient temperature controller for the same SoC values. The surface temperatures of the lead-acid batteries are captured by means of specific temperature sensors. Internal resistance variations are measured at reference points of 25 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, and 35 \({}^{\circ}\)C. These values are obtained as 0.909 \(\Omega\), 0.837 \(\Omega\), and 0.716 \(\Omega\) at the temperatures \(T_{1}\), \(T_{2}\), and \(T_{3}\), respectively as shown in Figure 5 In addition, the initial operation parameters are presented in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Rated power (W) & 400 \\ Nominal voltage (V) & 48 \\ No-load speed (min\({}^{-1}\)) & 3100 \\ Max. continuous current (A) & 5.3 \\ Max. efficiency (\%) & 84 \\ Terminal resistance (\(\Omega\)) & 1.12 \\ Terminal inductance (mH) & 0.82 \\ Torque constant (mNm/A) & 147 \\ Rotor inertia (gcm\({}^{2}\)) & 831 \\ Mass density (kg/m\({}^{3}\)) & 7858 \\ Mass (kg) & 10 \\ Surface area (m\({}^{2}\)) & 0.16 \\ Flywheel inertia (kgm\({}^{2}\)) & 0.06089 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Maxon EC60, the brushless direct current (BLDC) machine and flywheel parameters. Figure 4: Experimental test bed: Maxon EC60 slotless type BLDC machine coupled to the special flywheel. Figures 6-8 show the results of the experiments for calculated specific duty ratio (0.78). According to Figure 6, at 2.2 s, the regenerative braking has started, causing the voltage to jump from 48.8 V to 56 V. The output current value of the boosting converter at the starting of regenerative braking is measured as \(-\)6.6 A. Note that a reversal of current occurs when the driver shifts from the inverter to boost converter operation. In this case, the total deceleration time is recorded as 120 s. The specific regenerative braking time is measured as 14.2 s. In Case-2, the temperature of the battery increases to 30 \({}^{\circ}\)C by the heating controller. The internal resistance is measured as 0.837 \(\Omega\). The voltage increases to 55.7 V for the specific duty ratio (0.78) in Figure 7. In Figure 8, the voltage of the battery increases to 55.5 V for the regenerative braking at 35 \({}^{\circ}\)C. As can be seen from the experimental results, it is determined that the regenerative braking performance increases for the certain values of the duty ratio. The experimental results for different temperatures and duty ratios are presented in Table 3.