Generator Armature Time Constant vs DC Time Constant

[voltagevortex]

I am modeling a generator using SKM and don't really understand the difference between the armature time constant, Ta and the DC time constant Tdc.

I tried using Glover's analysis and design textbook but still cannot determine what the difference is. He provides the equation idc = -sqrt(2)V/Z*sin(alpha - theta)*e^-t/T and says the time constant T = X/R = X/2*pi*f*R

He later uses the armature time constant to calculate idc = sqrt(2)*I''*e^(-t/Ta)

SKM uses the equation X"d /(2pi * f * Ra*1000)... Is the DC time constant the time when the DC contribution has already decayed to 0? So what would the armature time constant be?

 

[Gr8blu]

Ta = armature short-circuit time constant (in seconds). Function of negative sequence reactance, frequency, and DC armature winding resistance.

Not sure what you're referring to with Tdc, since SKM uses Td' (direct axis transient) and Td'' (direct axis subtransient) time constants in its equations. There is also Tdo which is the direct axis open-circuit transient time constant, and is a function of the field circuit impedance, among other things.

Converting energy to motion for more than half a century

 

[voltagevortex]

On SKM's generator ANSI Contribution tab, Tdc is an input in the bottom right, and in the Decrement Curve tab SKM now uses Ta.

 

[Gr8blu]

@voltagevortex From your screen shot, Tdc used to be Tdo in earlier versions of the SKM software. See my previous post for defining the term Tdo.

The only reason I can think of them changing it to Tdc is to reinforce the concept that it is a function of the ROTOR FIELD impedance (which is typically powered by DC).

Converting energy to motion for more than half a century