# Magnetic coupling between inductive loads

1. ## Magnetic coupling between inductive loads

I have been working a little bit on a hobby project of mine involving inductive power distribution. I have never done anything like this before and before I hook things together I would like to avoid blowing stuff up.

I plan on powering between 1 and 3 inductive loads (i.e. hand-wound coils about 4-6cm in diameter shaped like a ring with probably around 50-100 turns) in parallel from an H-Bridge. The coils will probably be wound with 28-30awg wire and my target is to transmit up to 2A between coils placed about 2-3mm apart. The 2A figure is the amount of current consumed by the "transmitting" coil and not the current induced in the "receiving" coil. Any of the coils being driven by the H-Bridge may or may not have a corrisponding coil lined up (in other words, some of them may have no "load" and others may have a load). This whole circuit may or may not be powered by an inductive method itself as well which further complicates matters of power.

I plan on running everything somewhere in the neighborhood of 100Khz, but that's really up for debate. I just know that it has to be a higher frequency since the coils are coupled inductively without any sort of core.

I understand that at the point of resonance for the inductor (which has to do with the capacitance between turns as I understand it), the inductor will stop being there in terms of impedance and the only limiting factor for current would be the resistance of the wire. I would imagine this would blow up my H-Bridge rather quickly unless I seriously oversized things, so I know I need to stay away from the resonant frequency of my inductor.

Here are my questions:
- How does the inductor look from the viewpoint of the H-Bridge when nothing is inside its magnetic field other than itself? I imagine it looks just like an inductor
- How does the inductor look from the viewpoint of the H-Bridge when another coil is placed in close proximity to it with a load attached?
- What would happen if of 3 inductors in parallel, only one (or two) was coupled magnetically to a "receiving" coil?

Something I plan on trying out is the possibility of using the same coil for either sending or receiving power depending on sensor inputs (current and voltage on a coil, basically). What would happen if I were to drive an inductor which was inside the magnetic field of another inductor that was also being driven? What would I see from the perspective of either side in terms of impedance?

Thanks very much in advance for any insights.

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2. ## Re: Magnetic coupling between inductive loads

Your basic problem is that you intend to use an inductive load connected to a switch. The switch can be the H-bridge or a simple MOSFET or transistor. H-bridges can drive the load (coil ) with both current directions, that is all.

Coils do resonate with intrinsic as well as circuit capacitances, but often at quite high frequency. It is better to add a capacitance to make sure you know the resonant frequency (or you can tune the coil with a ferrite or iron core).

Current pattern in such inductive load can be determined if you know all circuit parameters. The equation must include resistance, inductance and capacitance as well as the switching frequency.

Inductive loads to switched current sources generate first an overshoot voltage before the current settles to a current value determined by source voltage and circuit resistance (including source and load).

Any switch including the H-bridge must be designed so that it can supply enough steady current and can withstand the overshoot voltage. With a simple switch the current is limited by an added resistor to coil resistance, and the overvoltage is limited by a parallel diode to the source voltage. In a H-bridge switch the overvoltage will have both positive and negative spike so two diodes would have to be used, with additional switches for each polarity.

This complication is usually too much to add, so I would advise not to use the H-bridge for an inductive load. Instead, use a simple MOSFET switch with a protection diode, and then your current-switched coil will work well.

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Adding a "secondary" coil makes your system a transformer. Coupling between the primary and secondary coils and their turn count will determine the secondary voltage. If loaded, you will adjust the P/S coupling closer to get some power output.

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