# I have a general question about LC circuits

1. Feb 14, 2010

### zakbrown0308

Setting up an LC circuit is difficult because I find it hard to make a reliable inductor and do not have the resources to get one. However, if I used an electric motor, which has an induction coil, would that suffice? Or is there something obvious I am missing? Also, if this is feasible, how could I accurately measure the inductance of the motor?

2. Feb 14, 2010

### Okefenokee

The inductance of the motor is going to vary depending on the speed and load. You could take the rotor out and the stator coils would have a consistent inductance by themselves.

To measure the inductance you'll need an alternating source. DON'T USE A WALL SOCKET AND GET ELECTROCUTED! You could actually make a safe AC source that is powered by a wall socket and has ground fault detection + current limiting but it would be better to go onto Ebay and find yourself a cheap function generator. You'll also want a cheap oscilloscope or a true RMS multimeter. It's important that you don't use a cheap \$20 multimeter. The cheapos use a math trick to measure AC power which assumes that you'll only meausre 120 Hz sinusoidal power. It doesn't work for anything else (the cheapos are not that accurate for what they're designed for anyway).

So with the gear ready, http://technologyinterface.nmsu.edu/fall96/electronics/induct/induct.html" [Broken].

Last edited by a moderator: May 4, 2017
3. Feb 14, 2010

### Bob S

The best (lowest loss) inductors are air core coils. You can wind one using enamelled copper wire around a small bottle. Inductance formulas are on the web. What inductance do you need?

Bob S

4. Feb 14, 2010

Hello zakbrown,

The coils of an electric motor might be useful for some purposes but generally would be a long way from being ideal inductors. It would be easier to help if you could explain more about what you are trying to do. For instance, what resonant frequencies and signal levels are required?

When you say that you have found it hard to make a reliable inductor, do you mean that you have not been able to make coils with the right performance, or that you have been making coils which have worked initially but later failed in some way?

5. Feb 14, 2010

### vk6kro

Motor coils do have inductance, but there are problems with using this in the way you mention.

Firstly, the inductance is going to be very high. A fan motor I have measures 2 Henrys which would not be suitable for typical L/C circuits.

Also, the inductance will change with frequency due to the properties of the iron core.

And the iron core will become quite lossy as the frequency rises.

However, if the motor is faulty or no longer required, it would be possible to use some of the wire from it to make inductors. Field coil wire may be easy to remove.

Many other discarded appliances will either have usable wire or usable inductors in them. Computer power supplies and old radios are especially good for this.

The following article in Wikipedia gives some formulae for calculating inductance from the dimensions of a coil.:
http://en.wikipedia.org/wiki/Inductor
That is a good resource. I've never seen some of those formulae before.

Measuring inductance is not easy. You need at least a suitable signal generator and preferably an oscilloscope. The AC voltage reading on a multimeter would probably not be much use at high frequencies.

To do the measurement, you could put the inductor in series with known capacitor and apply a signal generator to the series combination. Then measure the voltage across either the capacitor or the inductor and tune the signal generator for a maximum. Use this resonant frequency to calculate the inductance.

6. Feb 15, 2010

### zakbrown0308

basically I'm trying to build a tesla coil. But I was hoping to be able to tune the primary circuit. This is easier said then done. The second part of my question is, to do this, would it be easier to adjust the inductance or capacitance?