Frequency converter function and basic

Frequency converter converts the frequency of ac current, that is it converts a 50 or 60 hz ac current to ac current of any desired frequency. The device may also change the voltage if it is required.

                                                       Fig: Frequency converter Basic

Why we need frequency converter

Frequency converters are used in order to facilitate a accurate control of critical processes, such as: Cooling system (radiators, pumps), Fuel system (booster, heaters…), Ventilation (engine hall ventilation). The frequency converter is also an energy saver and in many applications noise limiter.

                                                     Fig: Frequency Converter Block Diagram

How frequency converter works

To change the frequency of ac current frequency converter goes through two-stage conversion. First, it converts ac to dc, then secondly dc to ac of desired frequency.

So frequency converter has two sets of working- first the rectifier circuit converts ac to dc and then inverter circuit with thyristors/ IGCTs / IGBTs converts dc to ac with desired frequency. The changing or shifting of frequency happens in the converter stage.

 

                                           Fig: Typical IGBT – Insulated gate Bi-polar Transistor

If voltage conversion is desired, a transformer will usually be included in either the ac input or output circuitry and this transformer may also provide galvanic isolation between the input and output ac circuits.

A battery may also be added to the dc circuitry to improve the converter’s ride-through of brief outages in the input power.                                                   

1.      AC Line voltage: 3 x 400 … 500 V AC, 50 / 60 Hz.

2.      Line filter: Limits electric interferences into the public power supply network and improves the electromagnetic compatibility of the device with regard to interferences from the power supply network.

3.      Rectifier: Converts the three-phase A.C. voltage into D.C. voltage.

4.      DC bus: D.C. voltage in idle mode = √2 x line voltage [V]

5.      DC bus capacitors: Smoothes the DC bus voltage

6.      Inverter: Converts the D.C. voltage into a variable A.C. voltage with variable frequency.

7.      Motor voltage: Variable A.C. voltage, 0 – 95% of supply voltage.

Variable output frequency: 0 – 700 Hz

8.      Control card: The control card controls the frequency converter. It generates the pulse pattern that is used to convert the D.C. voltage into a variable A.C. voltage with variable frequency.

Schematic diagram of the power section of a converter

                                                           Fig:  Frequency converter block diagram

AS Starting lockout; EW Ground fault transformer; GL Rectifier with Thyristor pre-charge; SE Current detection; SNT Switching power supply unit; WS Inverter control; WR Inverter; ZK DC bus Device variant; BC Braking chopper. Accessories BW Braking resistor; MF Motor filter; NF Line filter

 

Fig: Frequency converter circuit diagram