# Motor Controller doesn't precharge

I want to include a precharge resistor in parallel with my contactor to charge up the capacitors on my motor controller. This way, there won't be a large in-rush current through the contactor when I close it.

The problem I'm having is that the motor controller is powering on from the precharge circuit, and because of that, the capacitors are not charging up.

I'm using 1 kilohm for my precharge resistor. The steady-state voltage I see on the motor controller during precharge is 3V when I want it to reach about 48V.

Is there something I'm missing with my precharge circuit?

Thanks

• Unless the controller has UVP or DC.ok to enable, it will run when it can, so it must be preset to 0 if poss.or add the DC ok enable cct. Dec 3, 2021 at 19:16
• Not sure about UVP/DC.ok/poss.or, but it doesn't have a way to disable itself while I want it to precharge the capacitors. Dec 3, 2021 at 19:26
• If you have an enable input on the motor controller, then use a aux contact from contactor to pass through the enable. Dec 3, 2021 at 19:33
• My motor controller doesn't have an enable input that will switch itself off Dec 3, 2021 at 20:01
• So what's the real problem? Dec 3, 2021 at 22:42

• Establish the on-board capacitance by reading the capacitor values.
• Decide on the charge time you want. To make it easy, decide how long you want it to take to reach 63% of supply voltage.
• From $$\ \tau = RC \$$ you can calculate the required resistance as $$\ R = \frac {\tau}C \$$.

Setting τ = 1 s would seem reasonable.

Test it out and adjust as required. You've already found that the controller is drawing some of the charge but we've no way of guessing how much.

• So it is normal for the motor controller to draw some of the current from the capacitors? I am totally fine with waiting about 30 seconds to 1 minute for my precharge time (tau value). Dec 3, 2021 at 19:15
• "So it is normal for the motor controller to draw some of the current ..." Sure. There is unlikely to be a low-voltage cutout for the logic circuit's power. Dec 3, 2021 at 19:19
• Thanks, I'll test it out Dec 3, 2021 at 19:24
• I managed to get up to 63% supply voltage with a 100 ohm resistor, but I will need a 40W precharge resistor to handle the power dissipation. This seems excessive but does it make sense? Dec 3, 2021 at 22:22
• I have no idea. There is no datasheet link and there are few measurement numbers in your question. Peak power would be at switch on with $P = \frac {V^2} R = 23\ \text W$ with power decreasing as the capacitors charge up. How long did it take to reach 63%? Dec 3, 2021 at 22:29

The arrangement I'd expect for what you describe is something like this.

The ability to precharge the busbars relies on the motor driver having an ENable control that can switch it to a practically-nothing low power state. Without that, your precharge circuit has to supply a relatively high current to pull up the rail while powering your motor driver.

A series pass MOSFET could be added between the capacitor and the motor driver to enable it. That's not difficult, it just depends on your actual motor driver and the currents and voltages involved, which you don't currently specify.

simulate this circuit – Schematic created using CircuitLab

• My motor controller comes in one piece with both the capacitors and motor drivers/logic built as one unit inside a black box. So yes, if I had access to just the capacitors, then I can add a switch on the motor drivers and separately charge the capacitors. Dec 3, 2021 at 23:38
• Is the ground supposed to be switched? Dec 4, 2021 at 6:32
• I just want the power to be switched. I have a contactor on that line - I don't have a contactor on GND. Dec 6, 2021 at 15:39

UPDATE:

Thank you all for your help.

I did some digging around and found out that NTC thermistors work pretty well for me.

I specifically used ones designed for in-rush current limiting (ICLs). I purchased and tested out this one specifically and it charged up my motor controller without heating up significantly.

https://www.digikey.com/en/products/detail/amphenol-thermometrics/CL-90A/698981?s=N4IgTCBcDa4MwFYC0BGOAWAnEgcgERAF0BfIA