burn out

13 years 10 months ago #15191325 by dbmoore
burn out was created by dbmoore
<P style="MARGIN: 0px">an unusual event, I was doing some testing of some electric motors, all basically uni-direction type motors. As I tested first without loads all the motors behaved as expected. Then I introduce a load and my controller smoked? I have never seen this before? usually if here s an over current the controller would just stop sending power.
<P style="MARGIN: 0px">Has anyone had this happen?

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13 years 10 months ago #15364599 by cosma
Replied by cosma on topic Re:burn out

Beware that current limiting works only up to a point.
<P style="MARGIN: 0px">The issue is that what is measured and limited is the current flowing out of the battery.
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<P style="MARGIN: 0px">Current flowing into the motor can actually be much higher. This is quite counter intuitive and works as follows:
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<P style="MARGIN: 0px">If for example, the PWM cycle is 10% and the battery current is measured to be 10A, with an oscilloscope you would see that the battery current is 100A for 1/10 of the time and 0 the rest of the time (thus averaging to 10A).
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<P style="MARGIN: 0px">However, because of the flywheel effect in the motor;s inductance, during the off time, the 100A continues to flow across the opposite mosfet the remaining 9/10th of the time. The MOSFET that is on the opposite leg of the bridge and that is always on will see 100A all the time.
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<P style="MARGIN: 0px">Such a situation of high amps at low PWM will occure when you have a short or when driving a stalled motor of very low impedance.
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<P style="MARGIN: 0px">This is not necessarily what happened in your case, but it is good to be aware of this phenomenon.
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<P style="MARGIN: 0px">Note that we are considering changing the current limiting algorithm to limit the current flowing in the motor rather the current flowing from the battery.

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13 years 10 months ago #15372723 by dbmoore
Replied by dbmoore on topic Re:burn out
<P style="MARGIN: 0px">I have not open the device yet but my guess is I will have to replace the mosfets at least. I hope that the circuit board is okay. Any suggestions on the best way to get back up and going?
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13 years 10 months ago #15403079 by cosma
Replied by cosma on topic Re:burn out
<P style="MARGIN: 0px">It is generally quite difficult to repair a controller when MOSFETs are dmaged. It is quite common for the driver and other circuits to have failed too.
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<P style="MARGIN: 0px">Best is to contact us to arrange for a repair/replacement. Please contact us directly via our web site.

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