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.

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 a energy saver and in many applications also noise limiter.

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 than inverter circuit with thyristors/ IGCTs / IGBTs converts dc to ac with desired frequency. The changing or shifting of frequency happens in the converter stage.

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.

frequency converter basic

Frequency converter block diagram

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 which 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

Frequency converter block diagram

frequency converter circuit 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

Typical IGBT – Insulated gate Bi-polar Transistor