Induction cook-tops are fairly recent in their discovery. But induction heating has been around for ages.
Industrial induction heaters made their debut in the 18th century. They were used to forge and make tough iron and steel. They were also quite easy to work with, as larger metal blocks meant Induction Heating (IH) systems would have to run at a lower frequency . This enabled some pretty neat use of Induction heating by using power right off the 50/60Hz Mains.
It was only in the early 1950’s that semiconductors were developed. These were suitable to the area and switching speeds of household IH cook-tops. With the invention of these semiconductors, making miniaturized electronics suited to households electronics were possible at a fraction of the space and cost.
Since its early days, induction cook-tops have had their fair share of skepticism, however.
At Nymble, we’ve been working extensively on breaking down induction stoves for quite some time now and in the process have learnt a lot and debunked some myths. Here’s what we learned/debunked!
Over the past decade, there have been claims that induction heaters can heat your food with never-before-seen efficiency . However, according to the Electrical Power research institute , the typical efficiency of an Induction cooktop is around 74%. This is the maximum efficiency that has been seen on commercial cooktops in a controlled environment in this decade. But this is still inefficient with about 1/4 of the entire power consumed lost to the environment and those number get larger as we bump up the power output.
The truth is, not all Induction heaters are made equal. Several factors determine efficiency, including the coil to pan coupling, the magnetic permeability of the pan, coil DC/AC resistance, and the frequency of operation, among others. The efficiency of the system is a combination of all of these factors. In fact, most off-the-shelf induction cooktops have the same lossy power stage most of the time. It is often the nature of the pan that matters more than the brand of the cooktop, which brings us to the next myth.
2. Induction heaters can only heat ferromagnetic metals
Pans not only come in all shapes and sizes, but also in different metal and combinations. Historically, they ranged from cast iron to the extinct/posh gold cookwares (which are the least inert and don't leave a metallic taste to your dish). The revolution of SS Cookware brings a democratic shift in giving ordinary food, an elite culinary experience.
Today there are many pans, one of the high performance cookwares are the all-clads. These pans have multiple layers of Copper/Al and SS. So, why are they a great addition to your kitchen? Here’s the gist:
Aluminum Cookware= More even conduction of heat, but not a good absorber of IH energy.
SS Cookware= Less even conduction but great absorber of IH energy.
All-Clads= Best of both worlds!
Here’s the deal, induction heating for industrial purposes does not require a metal that is ferromagnetic. Industrial induction heaters are accustomed to heating all kinds of metals, from copper and aluminum to more uncommon precious metals like gold. So, does this imply that a household induction heater can also support any metal cookware?
The answer is - yes, regular metal cookware can be used on induction heaters, but this will result in reduced efficiency.
This is usually okay if the metals being heated are those that require non contact heating to maintain purity and the concern is not about efficiency . Experiments have shown that metals like copper and Al on a cook-top setup could get you a decent efficiency of around 60%. Keeping this in mind, we can definitely see a future where induction heaters are compatible with any metallic cookware. Maybe gold pans, if that’s your thing?
3. Induction heater..an electrical hazard ?
Currents as low as 30mA can kill an adult human when on its return path to the earth from the AC main. If that is the case, why doesn’t an induction heater actually pose an actual health hazard to its users?
This is explained by what is known as Galvanic isolation (transformer-based). Galvanic isolation differs from insulation. In an isolation, there is always a transfer of power from one side to the other. Galvanic Isolations are used to prevent ground loops (Prevent one ground from interacting to another). This means that a current loop in the primary stays in the primary side and the currents in the secondary, stay in the secondary side of the transformer. This is why technicians repairing old TVs used isolation transformers when probing with any instrument that has is earth reference.
This galvanic isolation that is created in an induction cooktop by the coil (Primary winding with ‘n’ turns) and the pan (secondary winding with a single turn), ensures that the hot side of the system is isolated from the pan, exactly like how in a transformer. This prevents a ground loop back to the earth ground, even if you touch the pan. So, you won’t be electrocuted.
4. Magnetic fields are absorbed by the pan
EM Induction was observed and documented by Michael Faraday a long long time ago. His equations still hold true for how systems work today. In a nutshell, induction power transfer is the transfer of power from one metal to another through the use of varying magnetic fields (by an effect called induction). These varying magnetic fields generate potential differences on a work piece.
Magnetic fields are usually dipole and always make a loop to return back to its opposite half. To understand this better we need to dive into what magnetic permeability means and how it affects how field lines behave. Permittivity of free space in layman’s terms is the ability of a magnetic field to move in free space. It is like conductance for Magnetic fields. A Ferromagnetic pan has more permeability than its surroundings. As a law, magnetic fields move through the path of least resistance , and voila it sees the pan (with a higher permeability), and the fields lines sort of short itself through the Pan. This concentrates large amounts of magnetic fields lines through the pan.
What is actually happening within an induction heater is nothing short of field management for lack of a better word. With the pan close to the coil. The fields effectively short though the pan and back to the coil (closing the magnetic loop in the shortest way possible). Thus, there is no absorption of magnetic fields in the pan. Instead, fields are diverted through the pan and back to the coil.
5. Over-the-counter Induction Cooktops differ in performance
Let’s assume the major points of comparison among induction heaters here are efficiency and power output.
A fact we have now established is that a chunk of efficiency in an induction heater depends on the pan. This is also because the load determines how much power is drawn from the system. A perfect load for an induction heater is a metal that has a high magnetic permeability and resistance. A pan with higher resistance will consequently be able to dissipate higher power (due to I2R Losses) than one with lower resistance. Also higher pan resistance means that the amount of current that needs to be coupled into the pan is much lower.
Yes, design matters. All induction heaters are not made equal. There are several design approaches that are used in induction heaters across different fields. But among induction cook-tops in the market, the most common topology is the Quasi resonant topology .
Asian markets are filled with these kind of outdated induction heaters. These heaters give a reasonable performance for the price you pay. However, they fall behind in efficiency and control. So don’t better alternatives exist? Yes they do, but that's for another post.