How To: Field Weakening

How to setup Field Weakening

What is it and how does it work

Field Weakening is a technique used in PEVs to increase top speed by sacrificing torque (only while at those high speeds). High torque isn’t needed at high speeds so this is okay.
An additional current is injected separately from the normal current used to drive the motor.

If you need to know more, google it.

How to enable it
Any 5.3 firmware supports field weakening, it’s a standard feature!

In FOC - Field Weakening

  • set FW Max Current to 30A (a little more may work too, but be mindful of your BMS cutoffs!)
  • set FW Duty Start to 65% (some people go as low as 50%)
  • set Ramp Time to 500ms (this helps avoid nosedips when it kicks in)

In Motor General Advanced

  • make sure max duty cycle is 95% or 96%

Tiltback

  • set duty tiltback to 85%

FAQ

  • Why would anyone take such a risk? If you’ve ridden an XR or Plus above 16-17mph then you’ve been using this feature already, FM just doesn’t tell you about it
  • How much speed can I reach? About 30 percent more speed than without. I’ve been able to cruise at 19-20mph with a stock XR battery at 50-70% battery (flat road) while duty cycle remained at or below 80%
  • Is this an experimental feature? It’s been in the VESC firmware for a year now, ESK8 and Ebikes have used it for a while, but it’s still new for VESC balance vehicles - Update 8/8/22 many of us have been riding this now for 2 months without any issues
  • Is it safe? Field weakening is safe - going faster than you can handle is not though!
  • Any downsides? The current firmware will apply field weakening solely triggered by duty cycle. This can cause loss of torque at ultra-low speed high-torque situations, e.g. slowly crawling over a door step
  • Do I need special firmware? No, stock 5.3 supports it but ATR-firmware only enables it above a minimum ERPM (5000)