Current Sources & Diode Bridges /w inductive load 1

[n1mr0d]

Below is a circuit consisting of a current limiting reactor (L1), and a current transformer (T1) whose outputs are connected to two 3-phase diode bridges (R1 & R1). The bridges are series connected to a RL load.

What is the purpose of the CT? It seems to shift the current 180 degrees (antiphase). How do diode bridges deal with current sources and how does a big reactive load influence the behaviour of the circuit?

 

[Skogsgurra]

Looks like the CT plus rectifier boosts output voltage when current increases. Possibly to keep output DC voltage constant when load varies.

The phasing of the CT is not important since the output is rectified, i.e. "dephased".

Gunnar Englund

http://www.gke.org

 

[n1mr0d]

The problem is we are dealing here with current sources. They want current flowing, and develop voltages until that happens (or the core saturates). For the diodes to conduct they need to be forward biased with a voltage right?

I simulated this circuit in Matlab Simulink and found a smooth current (ie. very low ripple, due to the big inductance load), but the output voltages of the rectifiers are very erratic.

Do the reactors also act as a voltage source, as they tend to resist changes is current?

 

[n1mr0d]

The last line i meant current source

 

[Skogsgurra]

Your thinking about current sources is sound and correct. But the fact that you have a current source is not a problem. You can divide and conquer. Look at the CT/rectifier and load separately. The CT will force current through the load impedance (pure resistive in steady state and RMS-wise). The load will react with some voltage (IxR), which is expected.

Now, add the main voltage from the rectifier and you get another voltage, about 500 V.

Both these circuits are linear in the RMS sense (but not if you look at instantaneous values - that might be confusing). Being linear, it is legal to superimpose the two and - presto! - you have a voltage from the main circuit that has a droop (reactor impedance times current) plus a smaller voltage that increases when current increases. The result is that the voltage is either relatively constant as load current changes or it may be some other funtion of the load current.

What happens at high currents is dependent on when the CTs saturate. If the cores are designed to saturate at a certian current, you will have a (probably) constant voltage up to that point and then a decreasing voltage. As some kind of a crude current limit function.

Gunnar Englund

http://www.gke.org

 

[n1mr0d]

Just a quick thought. Due to the reactor and big inductive load the pf must be very bad (0, phi +90 deg). Has this any effect on the commutation of the diodes?