The good ole’ 2nd gen Prius boost inductor. Extra large, extra cheap. I was unable to find a measurement of the saturation current of this inductor on the internet, so I set out to measure it myself.

This inductor is one of the beefiest I’ve ever set my eyes upon, at about 400uH, and I estimated it had a saturation current somewhere between 150 and 200 amps. I measured the line-to-line resistance at 19 milliohms. At 150 amps, this equates to over 400W of resistive losses alone. I plan on using this inductor for the big dyno, where it will need to handle peak DC bus currents of around 150 amps.

The usual strategy for measuring saturation current is to charge up a cap, then instantly connect the two and observe the LC ringdown. The electrical engineer’s way of doing this measurement would be to use a large IGBT plus a diode for the reverse current. I did not own a large enough IGBT to build this device. Therefore, it was time to build the device using mechanical engineering, and mechanically switch in the capacitor. I ended up using a 420uF film cap, as an electrolytic won’t work here as the inductor will force a reverse voltage across the capacitor. In order to really push the inductor into saturation, the cap needed a good 250 volts.

The craparattus in question. A paint stick kept a thin piece of aluminum separated from a bolt sticking out of the inductor. The paint stick was pulled out quickly, and a rubber band forced the aluminum stick into the bolt. The pop generated here was actually pretty quiet, because it’s not cap-on-cap but rather cap-on-inductor so the instantaneous current is zero. Enough to create some sparks though. I used a 300A LEM current sensor. The current ran through 10 gauge wire, with the alligator clips just being used for charging the cap through a resistor.

Of course this was a great excuse to take some long exposures.

Here is the ringdown. The current sensor gain equates to a gain of 75 amps per division. Noticeable saturation is present in the first few peaks. It is also clear that the saturation is rather soft, on the second peak (about 240A), it is clear that although saturation has occurred there is still some amount of inductance left. All saturation has disappeared below 150A. Cool!

A shifted view. It is clear that below 150A no saturation is present.

Bottom line: Saturation begins to occur around 150A, but there is still enough inductance to keep things in line up til about 200A. Additionally, the resonant frequency of the system is 400 Hz. Therefore, the inductor has an inductance of about 380 uH if the capacitor was indeed 420 uF, which the label guarantees it is to within 10%. Cool! Next up: time for some big power converters.