Eddy current heating for the bored pickup maker

[Joe Gwinn]

A number of people now have powered coil winders and neo magnets in an old drill press vise. This is enough equipment to do a simple experiment.

Attach an aluminum (or copper, but not steel) disk to the winder, as if to hold a bobbin. Arrange things such that the disk passes between the two neo magnets in the vise, with the full magnetic field passing through the disk more or less perpendicular to the plane of the disk. Feel the temperature of the disk material that will be swept by the magnetic field. Turn winder on and run at 1200 rpm or so for five minutes or so. Stop winder and again feel the disk temperature. It should be palpably warm.

Bonus question: Where did the thermal energy come from?

[Bruce Johnson]

And if you use a copper pick, and play really fast right over a pickup, you'll eventually burn your fingers.....That would be known as an eddy current riff.

[David King]

Bonus answer1)
Overly excited electrons made dizzy from spinning around 6000 times?

[David Schwab]

Bonus question: Where did the thermal energy come from?

Friction? I'm under the impression that's why things with current flowing get hot.

[David Schwab]

That would be known as an eddy current riff.

Eddy Van Current riff.

[Bruce Johnson]

I think I've seen Eddy Van Current play....He always has a pained expression on his face.

Answer to bonus question: the thermal energy actually comes through the motor of the winder or drill press. The eddy current drag isn't mechanical friction, but it has the same effect. It puts a load on the motor, causing it to draw more current. That extra electrical energy gets converted into thermal energy, slightly warming the motor and the copper disk.

[Joe Gwinn]

Answer to bonus question: the thermal energy actually comes through the motor of the winder or drill press. The eddy current drag isn't mechanical friction, but it has the same effect. It puts a load on the motor, causing it to draw more current. That extra electrical energy gets converted into thermal energy, slightly warming the motor and the copper disk.

We have a winner.

[Steve Conner]

I was fascinated by induction heating a few years ago, and I ended up building my own induction heater.

It works similarly to the way Eddy Van Current uses a cordless drill to play Eruption live.

[Joe Gwinn]

I was fascinated by induction heating a few years ago, and I ended up building my own induction heater.

Cool. What's the circuit diagram? I couldn't tell from the photo, but it looks like there is a fullwave rectifier fed directly from 220 vac and feeding a class C oscillator or amplifier (I see what looks like a signal generator in the corner).

[Artur pickupmaker]

Aluminium and copper are diamagnetic, suffer magnetic repulsion but never attraction, maybe it have some relation to heat.

[David King]

Essentially a dynamic brake like a shorted DC motor.

Question II: Will the disc get perceptibly hotter if it wobbles off axis? (we'll put both examples in a vacuum to eliminate convective dissipation as a factor. Oh and the gap will be the same between the magnets)

[Bruce Johnson]

In the world of machinery, you can buy what's called an Eddy Current Brake. It's a brake unit that's installed on a shaft to trim and control its speed. The big advantage to them is that they're all electrical with no friction pads or surfaces that have mechanical wear. They usually have aluminum disks, for cost reasons. Some are electrically actuated; they have electromagnetic coils on either side of the disk. Apply electricity to get braking effect. Other types have fixed permanent magnets.

One unique feature of the eddy current brake is that its braking power (that it, how much torque it can apply to the disk) is enormously related to the disk's speed. The exponential rate depends on the design of the brake, but it's usually huge. For example, double the shaft speed and you get ten times the brake torque; half the shaft speed gives you one tenth the torque. It's very sensitive to the speed between the disk and the magnets. The faster you try to turn the shaft, the harder it resists. But, it won't bring the shaft to a stop. It doesn't do anything at zero speed. For that reason eddy current brakes are commonly used as fail-safe overspeed brakes. On a machine where's there's a real danger of it getting going too fast, a permanent magnet eddy current brake will be installed to make sure that, no matter what happens, that shaft cannot spin too fast. Think of the winch motor in a construction crane or an elevator.

How does this relate to pickups? The thing to understand is that the eddy current phenomenon is critically related to speed; how fast a conductive metal part is moving through a magnetic field. That is, the frequency of the movement of the guitar string (which is acting like a magnet) in relation to a slab of conductive metal. It's not just a linear relationship, but very exponential one. If that string tries to go too fast, the eddy current brake comes on hard and slows it down. If you develop an eddy current condition in your pickup, it will clip off high frequencies above a certain point without doing much to the rest of the curve. You can use this as part of your design. On the pickups I make for my basses, I deliberately surround each coil with a shell of perforated brass that is connected into a full loop and to ground. I specifically want to limit high frequency overtones.

It's also a safety feature to keep bass players from going overspeed and exploding.

David's Question II: If everything else is kept the same, I doubt that there's any significant difference in the the eddy current effect between a disk that's wobbling and one that's running true. I'm sure some poor engineering student has studied it and written a paper.

[Bruce Johnson]

We have a winner.

I admit that I cheated.....I suffered through an engineering degree many years ago because I knew that someday in the future, someone would ask this question, and all that work would be validated. We engineers live for moments like this.

[Steve Conner]

Cool. What's the circuit diagram?

It actually ended up quite complicated. The 240V mains is rectified and filtered to feed a half bridge of ultrafast IGBTs. The RF output from this, at about 250kHz, goes through a DC blocking capacitor into a ferrite-cored transformer with a 20 turn primary and a single turn secondary.

The secondary is inserted in series with the tank circuit, which consists of the work coil and a mica capacitor rated for 200 amps of RF ($40 on Ebay, sourcing this was the hardest part of the project!)

The capacitor, work coil and transformer secondary are all water cooled, otherwise it would melt itself before melting the workpiece.

The IGBTs are driven by a phase locked loop that tracks the resonant frequency of the work circuit. I originally made that circuit for another application.

The power output is about 1kW. I tried self-oscillating circuits with MOSFETs, but I couldn't get above a few hundred watts before they died from parasitic oscillations.

I blew it up trying to do levitation melting, and never got round to fixing it.


Re the original topic, I think the disc would get hotter if it wobbled. The asymmetry means another degree of freedom for induction of eddy currents, so more should be induced.

[Joe Gwinn]

I admit that I cheated.....I suffered through an engineering degree many years ago because I knew that someday in the future, someone would ask this question, and all that work would be validated. We engineers live for moments like this.

Doesn't this bring you back to Electrical Lab I or the like?