# dc machines

Discussion in 'Homework Help' started by manasa993, Jul 5, 2012.

1. ### manasa993 Thread Starter New Member

Jun 28, 2012
5
0
why is the drop across reactance neglected in dc machine while resistive drop is considered?

2. ### wmodavis Distinguished Member

Oct 23, 2010
739
151
The only voltage drop across a reactive circuit element in a purely DC energized circuit is due to the direct current flow through the resistance of the reactive component. The voltage drop may be neglected if it very small compared to other voltage drops in the circuit.

What do you mean by a "dc machine"?

3. ### #12 Expert

Nov 30, 2010
18,076
9,678
There is also the time factor. Voltage across an inductance is temporary in a DC circuit but the voltage drop across a resistance is constant.

4. ### manasa993 Thread Starter New Member

Jun 28, 2012
5
0
@ wmodavis: i mean a motor or generator.... if you see the voltage equation only the drop across armature resistance is considered but not the drop across the reactance even though the actual voltage generated is ac and only after commutation you get dc...

5. ### Wendy Moderator

Mar 24, 2008
21,528
2,973
Not sure, but I think the OP is referring to amount of wattage dissipated. Resistors limit current by converting energy to heat, Reactance limits current by other means, making voltage lead or lag current. Both drop voltage, only one creates heat and uses energy, while the other prevents the flow of current through phase shifts. This only applies to AC though, not DC.

6. ### wmodavis Distinguished Member

Oct 23, 2010
739
151
@Bill_ The beginning phrase of your post tells a lot.
"Not sure, but I think the OP is referring to...."

I difficulty arises when a questioner does not provide a clear enough question or provide enough basic information for responders to do other than crystal ball it and some of us(myself for sure) are limited in that area. If I was good at it I'd be a climate 'scientist'.

7. ### WBahn Moderator

Mar 31, 2012
23,857
7,382
If you want to look at the mechanism that resistors limit current as being because they are converting energy to heat, then it is probably reasonable to think of reactive elements in a similar vein: they all limit current by converting energy from electrical to some other form, be it heat, an electric field, or a magnetic field. The difference is that the process is reversible in a reactive element.

Makes me wonder if you could make a thermal element that is reactive by using a Peltier junction. During part of the cycle energy is dumped into the thermal mass and during part of the cycle thermal energy is extracted from it. It would be very lossy, to be sure. I don't know if it would be linear or, even if it is, if you could come up with a reasonable differential equation that would allow you to have a simple complex impedance for it. But it would be an interesting project for someone that had way too much time on their hands to diddle with.