*5.5. Uninterruptible Induction Motor Drive*

The measurement of the uninterruptible induction motor drive are shown in Figure 15. During the measurement, the motor is loaded with constant torque at a constant speed. Figure 15a shows the three operating states:


**Figure 15.** Uninterruptible induction motor drive measurement: (**a**) three operating states; (**b**) the rectifier's firing angle changes slowly.

At the beginning of the measurement, the rectifier supplies a DC voltage of 350 V to the DC link. The LLC converter was then set to a 320 V setpoint, so its output is turned off by the control circuit, so electricity only comes from the grid to the three-phase inverter. The three-phase inverter monitors the voltage of the DC link, so it is not a problem that the DC voltage rises above 330 V. The inverter uses pulse width modulation to reduce the motor voltage so that the iron core will not be saturated. Operating state 1 can only be achieved if the rectifier produces a higher voltage than the LLC converter. In contrast, diodes and thyristors do not open.

The measurement examines the sudden change from the first operating state to the second operating state. The rectifier will then turn off completely, simulating a grid power outage. The LLC converter immediately turns on and charges the output capacitor and maintains the voltage at 320 V.

The rectifier then turns on and the firing angle decreases slowly, causing the output voltage to increase. The rectifier and the LLC converter feed the three-phase inverter in different proportions. This ratio can be adjusted with the firing angle. If the voltage rises above 320 V, the LLC converter continuously increases the frequency of the LLC tank, thus decreasing the amount of voltage generated by the LLC converter. Figure 15b shows this process back and forth over a longer period (operating stages: 2 → 3 → 1 → 3 → 2).

#### *5.6. Efficiency Measurements*

The left side of Figure 16a shows the efficiency measurement of the standalone LLC converter. The figure on the right Figure 16b shows the efficiency between the input of the LLC converter and the mechanical power measured on the motor shaft. During measurement Figure 16a, the LLC converter was operated at approximately 505 W output power, when the input power is 580 W (at Vin = 40 V). The converter can be overloaded due to the large heat sink. There is a difference between the efficiencies of a voltage boost LLC converter and a voltage drop LLC converter with the same power. A higher efficiency can be achieved with a voltage reducing LLC converter [14–16]. This is because their input voltage is higher, but the input current is lower. At lower currents, the power loss on the MOSFETs is significantly lower. The efficiency of the presented voltage boost LLC converter is lower due to the relatively high current at the input and the high channel resistance of the MOSFETs. These result in a large static power loss.

**Figure 16.** Efficiency: (**a**) between LLC converter input power and output power; (**b**) between LLC converter input power and motor's mechanical power.

The efficiency of the 370 W induction motor used during the measurement was approx. 80.4% at the maximum load. As the load torque decreases, the efficiency also decreases. In the following, it can be seen in Figure 17 how the efficiency varies between the mechanical power of the motor and the input of the LLC converter at different input voltages. During the measurement, the rated torque was not applied to the motor, only 1.15 Nm. The speed varied from 0 to 2000 RPM, which was set by the variable frequency drive. As the input voltage decreases, the efficiency also decreases, as a higher input current is required to achieve the same mechanical power. Due to the higher current consumption, the static power loss increases, resulting in reduced efficiency.

**Figure 17.** The total efficiency for different input voltages when the torque constant 1.15 Nm (the RPM changes 0–2000).

#### **6. Discussion and Conclusions**

In summary, the uninterruptible induction motor drive is working properly and stable. The measurements were made under different conditions, as the LLC converter was also powered by a battery, solar panel and power supply. However, this does not affect the result of the demonstration that the uninterruptible induction motor drive can be used reliably at low power. During the measurements, no condition occurred when the LLC converter was powered by a solar panel and a battery at the same time. This is because an electronic converter that charges the battery with the DC voltage generated by the solar panel has not yet been completed. Research is currently underway to create an MPPT (Maximum Power Point Tracking) control circuit that can handle dynamic loads well. By implementing the MPPT circuit, it is possible to power the LLC converter from a solar panel and a battery at the same time. In terms of measurements, it seems that it is not worthwhile to power the LLC converter from a solar panel alone, as achieving continuous rated power is not guaranteed. The solar panel would be able to supply the battery and the LLC converter simultaneously if an MPPT control circuit was also installed. In the event of a sudden power consumption, the batteries will withstand this without any problems, so that the voltage of the solar panel will not drop greatly. Conventional MPPT controllers do not fit well with this system, as sudden power consumption can occur in this case, this control algorithm cannot handle it properly, so research is focused on developing a new type of MPPT controller in the future. In the previous research, where the LLC converter was designed in theory and the parameters were checked by simulation, an efficiency of 90.6% was obtained. The implemented converter achieved an efficiency of 88.3%. In the simulation, the capacitors, inductors, the transformer was ideal, so in reality it is therefore less efficient. Furthermore, the power supply of the LLC converter electronics is also covered from the input and this was not set in the simulation. The driver circuit of the power MOSFETs consume the most power from the auxiliary power supply.

The efficiency of the entire drive could otherwise be improved if the LLC converter was supplied from a higher input voltage. However, an additional cost would occur as more batteries and solar panels would be required. The efficiency would not increase significantly though, as the lowest efficiency in the system is the motor. Another possible way to increase the efficiency is replacing the voltage-hertz control algorithm in the three-phase inverter with Fuzzy-logic Field-Oriented Control [17,18].

**Author Contributions:** Conceptualization, R.R.B. and I.B.; software, R.R.B.; validation R.R.B. and I.B.; formal analysis, R.R.B.; investigation, R.R.B.; resources, R.R.B. and I.B.; data curation, R.R.B.; writing—original draft preparation, R.R.B.; writing—review and editing, R.R.B.; visualization, R.R.B.; supervision, I.B.; project administration, I.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

