How would you make a very simple and rotating magnetic field starting from a three phase power supply?

Recently I had a brilliant idea :D why not make a rotating magnetic field that can rotate a needle of a compass? (or any other magnetic needle for that matter).

Ok but the design must be very basic. One possible option would be the following:

This is, probably, one of the easiest analog choices for a very low power and easy three phase power supply.

How does it work?

At the x1-1 terminal a sinusoidal voltage must be applied. Ideally a 1 V sinusoidal voltage. Let’s call this voltage V1

1. The first branch (C1 + R6 plus all the other series components) applies a 120° phase shift (60° + 60°) through the use of two carefully tuned RC filters. In order to compensate for the loss in amplitude, two non-inverting op amps are added after each RC filter. Then a buffer ensures is used to apply the voltage to the load and two transistors in a push and pull configuration supply all the current needed. The voltage applied to the load, let’s call it V2 is shifted 120° with respect to V1.
2. The second branch (C3 + R11 plus all the other series components) applies a –120° phase shift (60° -180°). The voltage applied to the load, let’s call it V3, is shifted -120° with respect to V1.
3. The third branch simply applies V1 to the output.

And there we have it. It’s worth noting that this circuit is very low power. The inductors L1, L2 and L3 are home made using copper wire. The resistors R7, R9 and R16 are used to limit the current absorbed by the inductors. You can choose the current through the inductors by dimensioning these resistors. Just be sure to not overload the transistor pair. After having built the circuit, all that is left is to postion the three inductors in a star configuration at 120° one from the other and there you have a two pole rotating magnetic field.

Below is the simulation with LTspice that I’ve run. Please don’t mind the name of the components which are different from the ones in the drawing above. Also, in the example below, the limiting resistors are set to a value of 2.5 Ohms letting through the inductors a current of 400 mA peak to peak. The resistance of the inductors is assumed to be negligible. The inductance values assigned are just for reference, they will depend on how you build the inductors, for example, if you build a simple coil of wire the inductance will be approximately equal to $L =\frac{\mu N^2 S}{l}$ where $S$ is the section of the coil, $l$ its length, $N$ the number of turns and $\mu$ the magnetic permeability of the air.

Pros:

• Easy to set up.
• Easy to understand.
• Uses common components. No special requirements for the op amps.

Cons:

• Dual supply rails and low power.
• Requires sinusoidal voltage input. However, this can be either built using an oscillator, or a mobile phone app can be used to generate a sinusoidal wave.

All in all I like this circuit. It’s analog and very simple. I hope you like it and had fun reading this post. If you have any suggestion or you’d like to show me your circuits, then leave a comment below.