soft start resister value

I'm not a motor guy so just spitballing...

I don't know the motor you are using or its power rating, but if you figure out the impedance of the starter winding at the supply frequency, and knowing the capacitance and supply voltage, you figure out the power versus resistance equation for your resistor.

Alternatively you could figure out how much starting torque you need and try to work back to how much power would be needed  to achieve this.

Or perhaps you can just guess based on the physical size of the mystery component and the voltage applied. For example if the resistor looks like a 5W size and there is 110Vrms applied and the start winding is taking equal power to the resistor, you might come up with 600 ohms. The numbers and ratios I used are completely fictitious since I don't know any of them, but it just provides some sort of practical method to improve your guesswork.

( Im a VFD guy)   the pre charge resistor is only in the circuit while the DC bus charges on first power up. Once the DC bus reaches a certain % of target, usually a 230VAC VFD has about a 350V DC bus, 460 Vac drive is about 735 Vdc, the precharge resistor gets bypassed. Its a simple RC circuit with the resistor watts sized for that operation. Sometimes those resistors are mounted on a heatsink. The resistor size depends on the short term watts, and the instantaneous amps. The cap is a short when totally discharged so the instantaneous amps it sees is dependent on upstream AC impedance. Some resistors are good with high instantaneous current, others are not. Wirewound resistors act like chokes. usually lower inductance resistors are used. Resistor ohms are sized to be the current limiter for the caps.  The watts are based on several seconds , repeated only xx times an hour.  

So a burnt resistor could indicate too many motor starts in a short time period?

as i said at the start  this had a dead IGBT module 

as i said at the start  this had a dead IGBT module 

this unit does have 400v  caps . so maybe it is 350v 

The caps will be rated for v-peak, not v-rms.  That's what I've seen the DC buss charge to (+/-).

acdc90 said:

dead IGBT module 

Dead doesn't mean shorted, it could also be blown open.

Possibly. Normal failure mode is short, at least for a short time. If stays short too long, either it melts down inside and explodes or the IGBT blows itself open. Upstream fusing is intended to limit the energy on a ground/short/ other failure ......A blown pre charge resistor would tell me the bridge was probably shorted somewhere un;ess it was a cap that originally failed..

robogary said:

Upstream fusing

yes the IGBT module was shorted