AC Current and SCR Drive

For diode bridge full conduction:

IRMS ac=.816 x Idc ave

Rule of thumb, use 0.85 x Idc ave for phase control, continous conduction.

Load LR time constant, line impedance, etc all play a part.

You can do the integrals for the wave forms for various conduction angles. With continuous conduction you will conduct for 60 degrees with 6 scr's.

see: S.B.Dewan and A.Straughen, Power semiconductor circuits, John Wiley and Sons, ISBN 0-471-21180-x

Dear Forum members,

Further to the above discussion, I have a small problem to be solved. One of our customer is planning to set up a test bed for 750 V DC 900 KW DC motor. The scheme is as follows :

There will be 3 Nos of 625 KVA 415 V AC DG synchronising panel which will be used to power a test bed. The 3 DG's will be synchronised and fed to transformer rating of 1500 KVA 415 V AC ( primary- star and secondary - Delta) through a ACB of 415 V rating. This is step up transformer which steps up yhr voltage from 415 V AC to 800 V AC . The secondary of transformer is connected to Thyristor panel through a ACB. My doubt is regarding the specifications of ACB. in order to feed a constant voltage of 750 v DC to the DC motor whether 800 V AC is sufficient. Do we not have to account for the Voltage drop in the cable and the thyristor panel. I think as per the above reply given by Mr. GW, the AC voltage on the input side of Thyrisator panel should be

V ac= 0.816 x Vdc. Is my calculation correct. What are the other factors to be taken into consideration while selection of switchgear and transformer. Please advice .

The rms value of the line current is Idc/sqrt(3.0).

Derivations are available in a book on power electronics authored by M H Rashid PHI/Pearson

Sorry to disagree, but he asked for the AC line current, not the leg current in each of the six arms.

Thanks, it is three phase.

The DC current is rectangular correct, if sin (say with a harmoninc filter infront) the input to the filter is then ??? %?

No, the armature DC current wave form driving a motor load will have a possible DC component and then "camel humps" along the top. The longer the time constant the flatter the humps. If it is pure resistive load the waveform will trace rectified AC volt, saw tooth to "bumping along" the AC crest depending on phase angle. If the motor load is discontinuous the current waveform will look like a camel hump with a zero current flat spot between humps until the next scr fires. Under this condition the DC voltage waveform will have more of a saw tooth appearance, but when the current is zero it has a sharp rise to the CEMF and stays there until the next SCR is fired.

Wish I could post pictures, then I could give you some waveforms from a scope.

Now, with the field circuit where the time constant is often 1 to 2 seconds, the current is essentially flat, with square pulses in the ac lines.

Armature time constants are in milliseconds, not seconds. Eg, 100 hp 240vdc has te ~ 0.150 seconds.

For your information to confirm AC line amps vs DC output take a look at the last page of the attached document. There is a table with AC line current and DC output for the Reliance drives.

http://www.reliance.com/pdf/drives/data_sheets/D2882.pdf

Hi GW you can insert images into a CR4 post using the picture button as shown on the left on the editor toolbar. Click on this button and follow the instructions and you will past the selected image into the post. Once the image is inserted you can then drag it to where you want it and resize it by dragging the corners or edges as required.

Missed your filter question.

Normally ac side filters are either a relatively small inductance to limit the DI/DT and DV/DT to the drive and filter out some of the commutation notches.

However, there are strong 5th, 7th, 11th harmonics and some companies (typically government with lots of dollars to waste) will use harmonic filters to suppress these. Total waste of $$$$.

It is much easier to use a transformer with a HV primary and step down to the drive. This becomes a dedicated supply to the drive. Primary side just doesn't see the crap the drive throws out.