48 Frequency Converters Compared to uncontrolled rectifi ers, fully-controlled rectifi ers result in major losses and disturbances in the supply network, because they draw a high reactive current when the thyristors conduct for short periods. This is one of the reasons why thyristors are mainly used in the inverter section of the FC. However, the advantage of fully- controlled rectifi ers is that they enable regenerative braking power in the intermediate circuit to be fed back to the supply network. 2.3.4 Active Front-End / Active Infeed For certain FC applications the motor sometimes works as a generator.
In these cases the energy balance can be improved by returning energy to the supply grid. Such FC’s require a controlled (active) rectifi er, which allows energy to fl ow backwards. Therefore these devices are called Active Front End (AFE) or Active Infeed Converters (AIC). Precondition for feeding back energy to the supply grid is that the voltage level in the intermediate circuit is higher than the grid voltage. This higher voltage must be maintained in all operating conditions. Various strategies are available to reduce the losses during standby and motor operation but none can completely eliminate losses.
Further additional fi ltering is required in generative mode as the generated voltage does not fi t the sine wave shape of the supply grid without. 2.4 Intermediate Circuit Depending on the design, the functions performed by the intermediate circuit include: • Acting as an energy buff er so that the motor can draw energy from/return energy to the grid via the inverter and as a means of accommodating intermittent load surges • Decoupling the rectifi er from the inverter • Reducing mains interference The intermediate circuit is based on one of four diff erent basic circuits, shown in Fig. 2.3 Main component topologies.
The type of intermediate circuit used is determined by the nature of the rectifi er and inverter with which it is to be combined. The basic diff erences between the various types of intermediate circuit are explained in the following sections.