# Is there a purely resistive energy storage load?

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1. Dec 1, 2015

### tim9000

Hi,
So is there a type of battery or ANYTHING that stores electric charge or potential that is purely real and does not look reactive in a circuit?
Cheers

2. Dec 1, 2015

### manzana

Good question. What is your circuit? Normally you would put a bypass capacitor across the battery to keep AC out of it. Chances are, I don't understand the problem.

3. Dec 1, 2015

### Staff: Mentor

Storing energy and being reactive are somewhat synonymous. So at first glance, I say no.

But a better answer depends on how strictly you mean pure. Can you elaborate?

4. Dec 1, 2015

### tim9000

Hi,
Well I don't really have a problem, I was just thinking about a specific type of use for an induction motor where rather than varying the resistance of the rotor to vary the slip, some of the resistance could be saved as energy instead of disipated as resistive heat. Maybe vary the amount of stored energy rather than vary resistance of rotor.

Not really, I accept that what I'm asking there probably doesn't exist any appropriate component or product (with a purely real characteristic). I'm sure you could do it via some sort of conversion, but that's not what I'm after.

Because as far as I'm aware it's the real part of the impeadance of the induction rotor that needs to be varied, not the reactance.

5. Dec 1, 2015

### Staff: Mentor

I'm going to make a general statement here that applies to many PF questions.

Whenever someone uses the word pure with respect to real life electric circuits, he is begging for a contradiction of terms.

Could your question be rephrased as "How can I reduce losses in an induction motor?"

6. Dec 1, 2015

### tim9000

Pure was a poor choice of word, I mean such that the resistive component is comparitively so large that any reactive component can be ignored.

I'm not trying to reduce losses as such, I'm trying to increase the resistance of the rotor and instead of losing the energy, capture it....Unless...: As far as I know you need to increase the resistance of the rotor to modify the slip, if this can be done in a way that doesn't result in loss, THEN your statement would apply. But as far as I know it can't. Not that I'm knowledgeable on the subject, no doubt you may well be about to enlighten me.

7. Dec 1, 2015

8. Dec 1, 2015

### tim9000

Yeah, I am aware of a variable frequency drive, what I'm contemplating WOULD require modification of the rotor circuit, I think. I'm thinking about an arrangement that isn't a motor or generator as such, but a type of apparatus which doesn't really have a stator, relative to earth. Where the rotor and stator rotate, without a set frequency.

9. Dec 1, 2015

### Staff: Mentor

10. Dec 1, 2015

### tim9000

Yeah, similar question though: Does anyone know how capacitive nickelâ€“cadmium or lithimum batteries (NiCd, Li battery) appear in an electric circuit?

11. Dec 1, 2015

### Staff: Mentor

A battery is a DC voltage source wth internal resistance. A capacitor is a capacitor. I don't know what capacitive batteries means.

12. Dec 1, 2015

### Averagesupernova

I think you want to look into double fed induction machines. https://en.wikipedia.org/wiki/Doubly_fed_electric_machine
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There is an older type of brush type motor called repulsion/induction. It was known for being able to start heavy loads. When it came up to speed the brushes were short circuited so it basically became an induction motor. If you can find one of these you may be able to do some experimenting.

13. Dec 3, 2015

### CWatters

If you look at the equations for a capacitor..

Q = CV
I = Cdv/dt
C = I / (dv/dt)

For a voltage source dv/dt is zero (because the voltage is constant) so it's not unreasonable to compare a voltage source with an infinitely large capacitor. In the small signal model of a transistor circuit the power supply is usually treated as a short circuit.

However real world batteries aren't ideal voltage sources. You would need to measure the specific battery over the frequency range of interest.

14. Dec 3, 2015

### tim9000

15. Dec 3, 2015

### tim9000

Ah, interesting points.
So if I was to measure the source impedance of a battery, would that give me only the real part?

16. Dec 3, 2015

### CWatters

Google the difference between resistance and impedance.

17. Dec 3, 2015

### tim9000

Ha, look, with respect I know the difference between impedance and resistance. I can remember off the top of my head:
Zo = RL * ( V-VL ) / VL
however that Zo of the battery may be the magnitude of a complex impedance, and I'm wondering how to tell how much of it is real and how much is reactive.

18. Dec 3, 2015

### CWatters

Depends what equipment you have. You can buy meters that will give you an answer or you can do it with a scope, signal generator and some maths. This from Tektronix explains how to measure the capacitance and resistance of an unknown capacitor.

http://goo.gl/6uN4u7

19. Dec 3, 2015

### tim9000

Right'o. Thanks for the document!