DC Drive _ Output Current

[lukin1977]

Dear experts

I am trying to calculate the output current of a DC Drive by measuring the input voltage, input current and output voltage

Input Power x %ef = Output Power
%ef = efficiency %

What is a typical %ef of a DC Drive?

 

[jraef]

Depends on the type, but modern DC drives are about 98-99% efficient, when referring to the losses in the drive alone. The losses in the motor are another issue, but for the purposes of determining the motor current, use 98%.

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[wcarter]

lukin1977,

I am unsure if you are looking at a specific drive, but the manual(s) for it should have the rating in it.

 

[Skogsgurra]

There is a rather exact relation between AC current and DC current. No need to do an indirect calculation. For a six-pulse fully controlled thyristor bridge (the type commonly in use) the current flowing in the motor armature comes directly from the grid (no intermediate circuit to store charge) so every DC electron corresponds to an AC electron.

All you need to do is to find what the rectification does to the AC RMS and see what the resulting DC level is.

That has been done by others, so you don't have to do it. The result is (for practical purposes) that Iac=.82*Idc. The relation holds for continuous DC current and there is some dependency on firing angle and grid impedance, but for practical purposes 1000 A DC will take 820 A AC from the grid.

For total AC current, you need to add excitation and auxiliary power.

Gunnar Englund

http://www.gke.org

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