I'm trying to put a system to test to check that it does not create any back EMF, it is a heavy duty vehilce airconditioning system richly populated with fans. To try and get a handle on what I'm working with I got one of the fans used in the system and tried turning it on and off and capturing the "spike" with my scope, I got rather poor results and no back EMF emerged, the best I got was a 2 MHz oscilation that hit +50 volts (24 volt supply) and -2 volts, my suspicion is that the power supply is the cause,
it is a beafy 80 amp bench supply and has a soft on/off button so my guess it that the oscilation was something to do with the output circuitry of the SMPS and the fact that it never really went negative was due to some protection circuitry. Possibly the output of the supply "falls" "slowly" on switch off to help prevent back EMF ?
So I'm now thinking that to generate my back EMF and capture it perhaps i need to use a mechanical switch ? so that the power supply cannot interfere ?
Any ideas of what voltage I'm likely to get from a 24 V 8 amp motor ?