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generating and detecting back E.M.F.

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Thunderchild

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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 ?
 
It's likely a mechanical switch will generate both higher positive and negative spikes, depending upon the stray capacitance of you setup. Be aware that a mechanical switch has contact bounce and that will generate a series of spikes, rather than just one.

Not sure what level of spikes you will see but I wouldn't be surprised if they were 100V or more.
 
I'm not concerned about how many spike I get, I need to capture at least one and measure it, my scope will measure up to 300 V and is protected for up to 3000 V (CAT I) - when using the probe set to x10 attentuation.

I have been told that the spike is 120 V but then I seem to be dealing with a bunch of people that hardly know what they are talking about and not surprisingly have turned to the amateur (me) for answers (and equipment)
 
The fan might use a brushless DC motor which will have an internal driver and won't produce and high voltage spikes because the it will have internal suppression, otherwise the driver would self destruct.

Spinning a fan at high speeds can destroy the driver, for example using a vacuum cleaner to spin a CPU fan is a good way to kill it which is why it's important to avoid the fan when you clean your computer.
 
hm thats a possibility I'll have to look up the fan specs but it is quite a compact motor so I would think not,

As you probably know, semiconductors are tiny and I'd think that if they can pack a driver into the hub of a CPU fan, they'll have no problem in a larger automotive fan.
 
thats true, I'll have to have a look, there are some centrifuge type fans we use that have a resistive speed control so presumably they are brush motors so i may try one of those next
 
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In that case it sounds more like they're brushed.

I wouldn't expect to see any high voltage spikes during normal operation, the only spikes you'll get is when it's turned off.

The commutator on a brushed motor produces wideband noise from the rotational frequency, all the way up to the microwave band.
 
well the project uses 2 kinds of motors, I'll try the resistive speed controlled ones next as they seem like better candidates
 
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