DC Motor vs BLDC Motor

[aansari97]

Hello,

I'm working on selecting a BLDC motor for a robotics application where I require mean torques of about 3 Nm and peak torques of 4 Nm at an average rate of 2.5 rad/s (24 RPM) continuous.

This is a high torque, low RPM operation.

I have shortlisted a generic large air-gap BLDC motor with a 90KV rating (Motor Link (Aliexpress)). This is a popular motor among roboticists but doesn't have a lot of data.

I intend to use the ODrive S1 BLDC driver in torque control mode.

My questions are:

1. This is rated as a motor requiring a "12S" battery. Can I run the motor/driver with a "6S" battery knowing that the driver can function on that voltage? Will it affect the motor performance?
2. Can I run the motor at my desired torque and speed? I understand brushed DC motor curves enough to do the motor sizing, but selecting a BLDC motor is confusing.
3. Will there be a lot of power loss running this motor at my desired torque given that the winding resistance of the motor is high?

I know some questions may appear amateur, but the literature on BLDC motors is confusing for starters. I have seen this and other motors with higher KV ratings being used in similar applications but I want to have a good justification of why I selected the motor.

Thanks!

 

[BrianPetersen]

Can't open your motor link, and google search of your proposed drive (whose name I've never heard of) reveals a very inexpensive unit. Is this an application that matters? Siemens. SEW Eurodrive. Yaskawa. Mitsubishi. There are other big names. You'll have proper technical manuals and proper application engineers, and you can buy a motor and drive that are matched together so that they work properly together, because they're designed to work properly together. I have no experience with the hobbyist stuff.

Normally, in a robotic application, servo motors are connected to their loads through high-ratio gearboxes that are specially designed to have low backlash (affects motion accuracy) but it also puts the motor speeds and torques into a range where (a) they can be more easily controlled and (b) you don't end up needing magnetic fields of colossal magnitude in order to achieve the required torque - which in your case, is pretty low, but it's still going to be easier to achieve with a small motor spinning fast.

"90 KV" to me, does not make sense. 90 000 as in ninety thousand volts? That's nuclear-power-plant power-transmission-line-magnitude voltage, not "12 battery cells in series" voltage. And battery cells have various voltages depending on what type of cell chemistry they use. It is not sufficient to simply specify how many battery cells are in series. Specify the voltage.

IMO your speed and torque range are low enough that you should probably be looking at a geared stepper motor.

Here is but one source out of many ... https://www.micromotors.eu/en/dc-gear-motor-vs-stepper-gear-motor-all-the-differences/

 

[BrianPetersen]

Doing some more digging at the ODrive site. Seems that "kV" in their world means RPM per volt, which makes more sense.

Have you read this? https://docs.google.com/spreadsheet...1F5VUpc-lJEs3mmkWP1H4Y/edit?pli=1&gid=0#gid=0

I'm pretty sure you're going to need a high-reduction gearbox to get the motor and its drive to be in a happy place with the gearbox output in your desired range.

Strongly recommend buying the motor and the drive from the same vendor to avoid finger-pointing later.