**4. Uninterruptible Induction Motor Drive Implementation**

The uninterruptible induction motor drive and the experimental setup are shown in Figure 9. Several voltage sources were connected to perform different measurements. The figure shows the measurement powered by the battery and the grid. The motor has a mechanical power of 370 W, which can be loaded dynamically with any torque with a magnetic brake pad. The brake pad controller instrument indicates the actual speed and torque. Furthermore, the instrument has analogue output, thus, with four channel datalogger the actual mechanical and electrical power can measure at same time. From these, the efficiency can be determined.

**Figure 9.** The implemented uninterruptible induction motor drive with mechanical load.

The three-phase rectifier is half-controlled converter. It contains three thyristors (SCR) and three diodes. This rectifier is designed for previous research (max. output power is 10.56 kW), but it can also be inserted into the low power uninterruptible induction motor drive. The firing circuit is made up of pulse transformers. Thyristors are turned on by a microcontroller-timer circuit as a function of the desired firing angle. The firing angle directly controls the average value of the DC voltage. By changing the firing angle, the induction motor draws different proportions of electrical power from the grid and the LLC converter.

The three-phase inverter has already been built for previous research purposes. The inverter is designed to be universal. It contains 1200 V, 100 A IGBT modules with the corresponding driver circuit. The output of the LLC converter can be connected to the inverter's DC link circuit. Potentiometers can be used to adjust the modulation index, which controls the motor voltage, the switching frequency, the first-order frequency, as well as ramp speed.

#### **5. Measurements**

In this section, the measurements of implemented circuits are described. The measurements confirm the correctness of the calculated values, examine the transients in time; thus, the stability of the PID control circuit can be examined. In addition, the efficiency of the LLC converter and the entire motor drive are determined.

#### *5.1. LLC Converter Energy-Free Turn on Transient*

Turning on the energy-free LLC tank and output filter capacitor results in a large inrush current, as can be seen in Figure 10. At the moment of switching on, the microcontroller switches on the inverter with 160 kHz and then controls it at a frequency of 100 kHz for a short time, this ensures the lowest possible inrush current. During this time, the PID controller algorithm also starts and takes over the control. At this measurement, the LLC converter was fed by a 3 × 12 V lead-acid battery.

**Figure 10.** Energy-free turn on transient when no load is applied at the output.

#### *5.2. LLC Converter Normal Operation*

The voltage and the current at the output of the inverter of the LLC converter are shown in Figure 11. In the figure on the left, there is a light load on the output, the inverter is controlled above the resonant frequency (>100 kHz). In the figure on the right, the inverter is controlled at a frequency close to the maximum gain when heavy load is applied at the output. The high oscillation in the voltage is due to the parasitic oscillation of the SiC MOSFETs. Oscillation occurs because the surge voltage resonates with the MOSFET's drain-source parasitic capacitor (CDS), with stray inductance (LS) of printed circuit board wires and with the input filter capacitor. This oscillation is reduced by a snubber capacitor connected close to the MOSFETs to the input DC link. At this measurement, the LLC converter was fed by a 3 × 12 V lead-acid battery.

**Figure 11.** LLC converter at normal operation: (**a**) at resonant frequency with light load when f = 119.3 kHz; (**b**) at heavy load when f = 48.17 kHz.

Figure 12 shows the output voltage at constant load. The voltage is kept stable by the PID controller. At this measurement, the LLC converter was fed by a 2 × 12 V lead-acid battery (23.9 V).

**Figure 12.** Input voltage, current and the output voltage at constant mechanical load.
