by Eric Cohen
It’s likely you’ve heard of an “MRI” – in fact, many of you have probably had an experience having an MRI. If so, you remember being in a long tube where you hear a lot of noise, don’t feel anything, and end up with some cool pictures of your body. That’s Magnetic Resonance Imaging (MRI). And, because so many of you are familiar with an MRI, we often get questions about the safety of WiTricity’s magnetic resonance power transfer. Despite the similar names, the two technologies are not related in any way. Let’s take a look at the differences.
Magnetic resonance imaging (MRI) is a medical imaging technique that uses a very high and homogenous (uniform) magnetic field as well as computer-generated radio waves to create detailed images of the organs and tissues in your body. Our bodies are made of many hydrogen atoms (from water and fat). Hydrogen atoms act like tiny magnets with a north and south pole. MRI machines are large, tube-shaped magnets. When you lie inside an MRI machine, the magnetic field temporarily realigns the hydrogen atoms so that the protons’ axes are all aligned in your body. In this condition, radio waves are added to the magnetic field that cause the nuclei of the hydrogen atoms to resonate and produce faint signals, which are used to create cross-sectional MRI images — like slices in a loaf of bread. It’s a safe and efficient way for your doctor to examine your organs, tissues, and skeletal system, but due to the strength of the (non-oscillating) magnetic field used, no metals can be nearby otherwise they will be pulled with force by the magnet. That’s why pacemakers and MRIs don’t mix
Magnetic resonance as a technology for wireless charging, despite its name, is nothing like an MRI. Magnetic resonance wireless power transfer uses very low oscillating magnetic fields that do not attract metals. These oscillating (time-varying) magnetic fields are used to transfer power safely and efficiently across a gap using the principle of resonance (nothing to do with hydrogen atoms!).
In its simplest form, magnetic resonance wireless power works by utilizing two coils to transfer power via a magnetic field. Both coils are tuned to resonate as similar frequencies with one producing an oscillating magnetic field and the other changing the field into electrical energy (i.e., Ampere’s Law and Faraday’s Law). For your electric vehicle, one coil is placed in the underside of your vehicle while the other coil is placed on or in the ground – either in your garage, in a parking lot, or curbside. Either way, magnetic resonance wireless power for your EV is just as efficient as a Level 2 plug-in charger.