I’m running two motors on two separate odrives pro, both powered by a single 4S 6400 mAh 80C LiPo. I’m running into a conflict: the docs say DC max negative current should be greater than motor current limit plus margin, but my battery can only safely handle approx -32 A, so I have to limit dc_max_negative_current below that. With two motors, this effectively limits my max motor current, underpowering them. Is the “negative current > motor current + margin” rule strict, or can it be relaxed?
And if not, can I safely use a Regen Clamp with the battery to allow full motor performance?
the docs say DC max negative current should be greater than motor current limit plus margin
Hmm, this is not fully correct, I’ll mark this for a change.
Motor current is (almost) always greater than the DC bus current, so having that value less than motor current is typically fine. Motor current == DC bus current only when the motor is at maximum speed / current / etc.
Specifically:
power in == power out
power out (mechanical power, to motor) is speed (RPM) * torque (Nm), and speed * torque * pi/30 == power (watts)
power in is bus voltage * bus current
So we can solve for bus current with Ibus == (speed*torque * pi/30)/(n*Vbus) where n is the motor efficiency – 0.5-0.75 is typical values for BLDCs at any significant power.
So as an example, if you have your 4S lipo (14.8V nominal) and an ODrive with a motor where you know the max speed and torque is 1500 RPM and 2Nm, then that’s:
(1500 * 2 * pi/30)/(0.75 * 14.8) = 28.3A Ibus max (or -28.3A Ibus when regenerating).
You may want to consider using the active power limit feature to control/limit motor braking torque / regenerated current in order to not exceed the battery regenerated current limit.
And if not, can I safely use a Regen Clamp with the battery to allow full motor performance?
Yes! Though it would only help if you think your total regenerated power is going to exceed the battery charge current limit.
Oh - small correction. When calculating the regenerated power, the efficiency term should be on the top, since you’ll be losing regenerated power in the motor windings. So it would be: (1500 * -2 * pi/30 * 0.75)/14.8 = -16A Ibus when regenerating, for the example.