# Smoothing choke

In the load system of each rectifying system the output voltage is the sum of two components: constant and varying. In order to reduce the pulsation, which are usually unfavourable for the receiver, a rectifying filter is connected between the rectifier output and the load.
ELHAND company produces ED1W smoothing chokes which are applied in the rectifying filters.

Smoothing filters – they serve to correct the shape of time runs of rectifier’s voltage and current. The filter system has a small impact on the value of constant component but it limits the varying component, and thus the pulsation factor.
The properties and efficiency of rectifying filter operation are determined by smoothing factor:

 where: kt1 and kt2 pulsation factors (for voltage and current) at the rectifier output and input, respectively.

The function of filter is often fulfilled by ED1W smoothing choke connected in series with the load (Fig. 1a). The inductance of smoothing choke operating in the output circuit of m-pulse rectifier which supplies the receiver of R resistance, with set bs smoothing factor of output voltage and current is expressed by the following equation:

 where: R- receiver resistance; r- internal resistance of rectifier circuit; m coefficient depending on rectifier type; bs-smoothing factor; f –frequency of rectifier supply voltage.

 a) b) c) Fig. 1 The most popular systems of smoothing filters a) inductive , b) inductive-capacitive of G type, c) inductive-capacitive of P type.

In 1-phase 1-pulse rectifiers with inductive filter it is very difficult to obtain a continuous character of current in the load circuit because the current pulses occur only every second half-period. Therefore the induction filters rather do not cooperate with 1-pulse rectifiers. 1-phase 2-pulse rectifiers with filter in the form of induction choke are used more often (Fig. 2). In this system the continuous flow of current without big pulsation is achieved already at relatively small load currents.

 a) b) Fig .2 2-pulse bridge rectifier with induction filter. a) diagram of system, b) time runs of currents.

If the choke reactance ωL>>R then the filtration of current pulsation in the circuit is very good. Additional advantage of this system consists in that the average current value does not depend on inductance.
The limitation of current pulsation through the increase of choke inductance does not cause the voltage losses.
Rectifying filter in the form of ED1W smoothing choke is much more effective if it cooperates with rectifier, in which the varying components has several times higher frequency (e.g. in pulse converters).
In practice the induction filters are applied mostly in 3-phase system of higher power values.
The structure of LC filter (Fig. 1b, 1c) with parameters favourable both at low and high load currents is obtained by connecting a smoothing choke with capacitor. In this system the choke constitute a serial impedance and the capacitor additionally shunts the load for varying components. The chokes of ED2W type are often used version of smoothing chokes.
They have two independent windings located on the core, in shape of UI .
They are used in the systems cooperating with high-power multi-pulse converters.

Fig .3 Diagram of ED2W smoothing choke.

If the efficiency of a single filter is too small, then it is possible to obtain a further reduction of varying component by the construction of multistage filter consisted of several links connected in cascade. A resultant smoothing factor is as follows:

 where: b – smoothing factor of multistage filter;  b1 ,b2 – smoothing factor for successive filter stages.

The application of smoothing filter has a significant impact on the output characteristics of the entire rectifying system. Considerable oscillation of current of voltage caused by resonant character of LC and its significant quality factor may occur in the circuit in non-fixed states arising during switching the rectifier on and off.

ELHAND smoothing choke, type ED1WHX-40mH / 600A; 6kV

ELHAND smoothing choke, type ED2W-2x0,033mH / 3500A