how to calculate precharge resistor Topic is solved

[chentron]

after reading lots of post i found that i must consider my system voltage and a given value for I of 2A
so V=I x R
in my system 320v = 2 x R, so I must get a resistor about 160ohm
what i can not find is what power choose.
i see cases of 20 to 100 W
so what is the criteria for W ?

I also see info that I can be 4A (Tesla maker choice) , so resistor could be less ... some problem with this ? advantage ?

[mjc506]

The precharge resistor needs only to withstand load for a few seconds - it's limiting charging current for the DC capacitor. The smaller the resistance/higher the current, the greater the thermal load on the resistor and the greater the load on your capacitor, contactors, etc. You could calculate the 'perfect' resistor given max temp, thermal mass etc, but copying an OEM specification is likely to give you some safety margin and reliability!

For resistance, you're charging a capacitor through a resistor, and need to reach udc_switch within 5 seconds (this is the openinverter precharge time). V_cap = V_bat(1 - e^t/RC) where V_cap is the voltage across the capacitor, V_bat is battery voltage, t is time since starting precharge in seconds, RC is the time constant. So you choose a value for R such that V_cap is above udc_switch before 5 seconds for your capacitor and (worst) battery voltage.

A contrived example:

V_bat = 12V
C = 400uF
udc_switch = 10V

10 = 12(1 - e^5/R*400x10-6)
5/6 = 1 - e^12500/R
R = 12500/log6
R ~7kOhms (but slightly less will charge slightly quicker, which will be more reliable for lower battery voltages (when near discharged). Alternatively, you could adjust udc_switch and/or precharge time)

[johu]

I have the bad habit of just whacking in a parts bin resistors for precharging. I've used anything from 39 Ohms (Nissans choice) to a couple of kilo ohms. It turns out lower values also require a higher power rating to handle the high pulse current for prolonged time. I had quite a few precharge resistors fail after 1 year because they must have gradually worn down. Also wire-wound resistors handle those pulse currents better than carbon ones.

[chentron]

and what wattage ?

[johu]

I'd say 5W for 5k up to 100W for 39 Ohms