Hello everyone,
We are currently in the process of evaluating a few new motors.
The motors are low inductance and we need a high PWM drive frequency to avoid getting a big ripple current. The rough specifications are 9uH line to line inductance, 2A max continuous current, and a rated voltage of 72V.
We were looking at the ODrive S1 and Pro as possible candidates for this application. On the surface they have similar specs but the Pro mentions 48KHz ripple current in parenthesis. What does that mean exactly? Can the ODrive Pro drive the motor at 48KHz PWM?
Besides the controller we are planning on adding a choke in series with the motor phases in order to further alleviate the current ripple issue.
Thank you in advance for your time and help.
We were looking at the ODrive S1 and Pro as possible candidates for this application. On the surface they have similar specs but the Pro mentions 48KHz ripple current in parenthesis. What does that mean exactly? Can the ODrive Pro drive the motor at 48KHz PWM?
Both the S1 and Pro switch at a fixed 24kHz. Because of the center-aligned PWM and modulation scheme, the motor sees an effective 48kHz for ripple current purposes – that’s what the note on the Pro page means. The S1 is identical there, it’s just not listed on its page.
So no ripple difference between the two. The Pro’s advantage is voltage and current: 58V/100A max vs ~50V/40A on the S1 (but the S1 can do 80A at voltages under 36V), and 99% vs 78% modulation depth, which is just the effective maximum utilizable percentage of your DC bus voltage. Neither gets to your motor’s rated 72V, but that’ll only affect the top speed.
Besides the controller we are planning on adding a choke in series with the motor phases in order to further alleviate the current ripple issue.
Assuming you mean a series inductor instead of a choke (chokes I usually associate just with ferrite beads/rings/clamps, which won’t do much to increase inductance), that’s a fine strategy – just make sure the inductor’s saturation current is 2-3x your motor’s peak current.
The main risk with low-inductance motors is that the ripple current (which is inversely proportional to motor inductance) adds additional power dissipation. The fact the motor is only rated for 2A continuous means this could be an issue – you can definitely see 5-10A effective of additional heating due to the ripple. So the series inductors are likely required here.
That being said, 72V motor + 2A rated continuous + 9uH line-line is a combo I don’t think I’ve seen before. Are you able to share what motor this is?