Induction Frequency

Purely out of idle curiosity and because I’m an engineer and these sorts of things interest us -

What frequency does the Bullet induction heater operate at; what is the approximate current in the coil; and the operating impedance of the coil?



good question, i’d also add how is the energy applied, is it some form of constant, or is is pulsing to maintain different power settings, but big enough dips and peaks in heat to register in the flavor, or even the roast curve.

I’d reach out to tech support, though i haven’t heard from them in several days. Must be holiday?

You can see the IGBT switching frequency in the information panel that you can call up during the roast. IIRC I’ve seen values in the neighborhood of 22KHz.

Not sure about the voltage across and current through the coil though.

Thanks, John-I!

Not sure why, but I haven’t explored the info panel yet. Sounds like something I’d love to geek over!

Still curious about current and impedance.



I’d still love to know how much the power might pulse as well. it has to throttle in a sense, because the coils might just perpetually heat up, so how does it work for holding temp. Can it do so in a stable way, or does it have slight dips and pulses? These slight variations in heat have some effect of flavor.

Hard to tell from the IBTS, since it filters anomalies, because it has to.

I don’t imagine the power is pulsed or modulated, apart from the changes the user makes. Why would they want to do that?

I presume the coil is made large enough to handle the power needed without some sort of duty cycle coooling. At any rate, most of the power is dissipated in the drum and not the coil.

I don’t see any advantage to making the induction power supply any larger than it needs to be to supply the required power and then shutting it off in order to keep things from overheating. All you need to do is regulate the voltage of the supply so that it produces the power you need.

Even if the supplied power were modulated in some way, it is unlikely that it would affect the roast because of the very large thermal mass of the drum - that is a pretty thick piece of steel. The length of time it takes to get the drum up to temp (IBTS sensor) during preheat tells us that.

As far as I can tell, the Bullet doesn’t control for temp at all, it controls for power dissipated in the drum. The user controls for temp by riding the power level (and other factors). Absent any imposed safety limits, if you just let the the machine go on at a particular power level, the temp would continue to increase until the power dissipated from the drum by radiation and convection equalled the power being put in by the induction system - i.e., very hot!

(One correction to that, the Bullet controls for temp during preheat. You can see it modulating the power level to maintain the drum temp once it reaches the preheat temp.)


I’m saying i don’t know.

I’ll reconnect with tech support tomorrow and see what they say.

Induction heating is not pulsed on and off. The circuit is powered continuously, it creates an oscillating magnetic field (swapping + and - tens of thousands of times per second). This creates chaotic electric currents in the drum, which heats the metal.

To increase the heat, increase the voltage (or maybe current, I’m not an expert. see the link. :slight_smile: ).

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To increase the power dissipated in the drum, you have to increase the current in the coil since the magnetic field is proportional to the current.

However, to increase the current through the coil you have to increase the voltage across it. This is basic Ohm’s law -

Current = Voltage divided by impedance

The impedance is approximately constant. (It may vary some with the temperature of the drum.)


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ok, we’re starting to geek out here and it’s been a long time since I had to think about this stuff, so please hit me with a learning stick if I’m spouting nonsense…

I think the basic sentiment of modulating the voltage (and thus current) amplitude (as opposed to duty cycle modulating which is what the earlier concerns seemed to be) seems reasonable but I don’t know for sure if that’s the case for the Bullet, or induction hobs in general (edit: just noticed the link in the above post, guess I should read it :slight_smile:).

But for an inductor and an AC input, wouldn’t we be talking about this to model what’s happening across the coil?


You might model a real coil as a perfect inductor with a small series resistor; that resistance needs to be as low as possible to keep joule heating there from damaging the electronics.

You want the drum to have significant resistance so that the eddy currents there produce significant heat, hence carbon steel (or stainless could work I guess?) for the drum material as opposed to say copper.

The switching frequency depends on which model you use. We have a few different 230V models and one 120V model. It also depends on your voltage in your socket. Generally about 20kHz at maximum power. The lower the power the higher the switching frequency.

Some induction stoves does pulsing (duty cycling) especially at lower power like 3 seconds ON 3 seconds OFF, but for the Bullet, the whole power range can be controlled just by adjusting the frequency.
As for the inductance I don’t have the specific values with me right now, but they are different for the 120V and 230V Bullets.

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Hey john_i,

Indeed - lotsa good geeking!

You are right, V= L(di/dt) gives you the instantaneous voltage across the inductance as function of the change in current through the inductor.

And you are correct that the equivalent circuit of the inductor alone is an inductance in series with a small resistance (to be pedantically accurate it would also include a shunt capacitance, but, in this case, that capacitance is so small that the resulting reactance can be ignored at these frequencies).

However, in the Bullet, the coil does not act alone, it is part of a coupled circuit that includes the drum, which has its own resistance and inductance. The drum’s resistance and inductance become part of the equivalent circuit, mediated by the mutual impedance between the coil and drum. These will show up in the equivalent circuit as another series resistance and inductance.

The equivalent resistance of the drum will be much, much higher than that of the coil, which is as it should be, since you want to dissipate all your power in the drum and not the coil.

It’s interesting that power is controlled by varying the frequency and not the voltage, and is a very slick way to do it. As the frequency increases the skin depth(the depth to which the magnetic field will penetrate the steel drum) decreases thus increasing the drum resistance in the equivalent circuit. Increasing the resistance without changing the voltage reduces the power dissipated in the drum.

You want the drum resistance to swamp the reactances in the equivalent circuit. The power dissipated in the drum is proportional to the current induced in the drum squared times the resistive component of the impedance of the drum. However, that current is proportional to the voltage across the entire impedance, not just the resistive portion. That means if the inductive reactance is very high, the drive voltage must be very high in order to drive sufficient current through the resistance.


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that’s what I’m talking about! thanks, that helps my understanding a lot, actually

now we just need to build a spice model :smiley:

In its basic form it is just a transformer, but it becomes very very complicated with the coupling factor, which has a huge influence.
The tank capacitance is 6x390nF (for the 120V boards) and with 20kHz you should be able to calculate the inductance and build a spice model. LT spice works well.

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Thanks for indulging us, Jacob!

For me it’s just a curiosity, I’d rather roast the coffee and leave building the roaster to the experts :smiley:

As a converted EE it’s the software that typically draws my interest more…

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Yes, thanks, Jacob!

A bit of something (other than coffee roasting) to keep a semi-retired EE’s mind engaged.


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:nerd_face: serious nerd topic (not an EE :laughing: just an “old” CS, not even EECS) Seems like the Bullet attracts a certain type :wink:

Happy Roasting