Power frequency machines and high-frequency machines are distinguished based on the operating frequency of the UPS design circuit. The power frequency machine is designed based on traditional analog circuit principles and consists of a thyristor SCR rectifier, IGBT inverter, bypass, and power frequency boost isolation transformer. As its rectifier and transformer operate at a frequency of 50Hz, it is called a power frequency UPS as the name suggests. High frequency machines are usually composed of IGBT high-frequency rectifiers, battery converters, inverters, and bypasses. IGBT can be controlled to turn on and off by controlling the drive applied to the gate. The switching frequency of IGBT rectifier is usually several K to tens of KHz, even up to hundreds of KHz, far higher than that of power frequency machines, hence it is called high-frequency UPS.
In the power frequency UPS circuit, the main three-phase AC input is connected to a rectifier consisting of three SCR bridge arms through a commutation inductor and converted into DC voltage. Adjust the output DC voltage value by controlling the conduction angle of the rectifier bridge SCR. Due to the fact that SCR is a semi controlled device, the control system can only control the turn-on point. Once SCR is turned on, even if the gate drive is cancelled, it cannot be turned off. It can only be naturally turned off after its current is zero. Therefore, its turn-on and turn off are based on one power frequency cycle, and there is no high-frequency turn-on and turn off control. Due to the fact that SCR rectifier belongs to step-down rectification, the AC voltage output by the inverter of the DC bus voltage is lower than the input voltage. In order to obtain a constant 220V voltage for the output phase voltage, it is necessary to add a step-up isolation transformer to the inverter output
