Er Neutral i fart skadelig?

[Erik Ola Ulvestad]

I eit anna forum er det ein del skepsis til bruk av frikobling med Neutral (feks Nissan Leaf) i fart. Dette har blitt brukt ved nedbremsing for å få slipt litt på bremseskiver. Iflg manual bør visstnok ikkje N brukast i fart
Er dette skadelig?

Ref "There is nothing mechanical. It is purely electronic. If you select 0kW on the go pedal and then select N you won't feel anything.

But do not do this above 20mph or so, and only at your own risk. For PM motors there is a fundamental limit where the EMF becomes higher than the battery/inverter supply voltage, which limits speed. To get past this point a DC overlay is put onto the waveform which creates an opposing magnetic field in the motor which actually suppresses the magnetic fields of the permanent magnets. Then, the back EMF is not so high and the motor can be run faster. This is called 'field weakening'.

Now imagine you are at very fast speed, the field weakening is at a maximum but the motor is still running at maximum back EMF, and you go and switch off that field weakening current to the motor. What happens next?

I am sure the components have been sized to tolerate this as an accidental incident, but we'll just wait around for the first case where someone at top speed decides to select neutral at the top of a hill, the reverse diodes dump the excess voltage for a long time, overheat, and, poof ... oh, look, ma, no inverter .....

It may be that well designed systems the neutral merely means 0kW. That is what I would do. The driver has no control over that, really, it is just a way to select 0kW. But if it is a complete shut down of the inverter then the only other way to protect it is to maintain the DC field weakening current.

For reasons related to this, and more complexity I won't mention now, if you try to select neutral in an Ampera in CD2 mode, the power consumption to the motors actually goes up."

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[Mateusz]

Trur det er mange som seter N i farta på en Leaf. Eg gjørt det av og til på en 1 gen leaf i 3 år.

[cra]

Kan ikke tenke meg at det er et problem å gjøre dette innimellom for å få brukt bremseskivene litt. Gjør det både på Leafen ('12) og Teslaen. Om det var et problem så burde det rett og slett ikke tillates i software.

[Jay3s]

Det ble gjort/nevnt i en test Autogeful gjorde av BMW i3 på YouTube. Ble ikke nevnt noe om hverken farer eller gevinster ved å gjøre noe sånt, dog... Bare at det var mulig. (Så vidt jeg kan erindre.) Men argumentet med at man kanskje trenger det for å få "renset" opp bremseskiver/klosser en gang i blant høres jo ikke urimelig ut.