with the new generation of BLDC’s prompting renewed discussion of real world performance under current limits, I’m wondering about the design space the motor designers are working in. instead of using a high kv mirror with current limiting and reduction, wouldn’t the more natural thing be a lower kv motor? like a larger diameter “pancake”. what would the trade offs be for such a thing?
Low KV motors are expensive, not very powerful for their size, and inefficient. To run many FRC mechanisms without gearing, you’d need about about a 10-50ish kv motor @ 400w which will run less efficiently and take up more space than 500kv motors with a stage or two of gearing.
Generally speaking, reducing kV also reduces availabile power output proportionally. It’s not 1:1, but if someone made a Vortex with 200kV instead of 580kV, the stall torque would decrease to the point where it would become a 300W motor instead of 600W (or whatever the exact values are).
This is because, while kV*kT is a global constant, the phase resistance tends to increase with kT on your average BLDC outrunner motor. So your stall current goes down, and with it your stall torque and peak power.
It’s not impossible, but it’s difficult to make a good pancake motor for FRC.
My understanding is that equal power at lower kV motors aren’t necessarily less efficient. To get a lower kV the number of windings in the motor has to increase (this makes your motor larger), which would increase the motor’s resistance and worsen the efficiency. You can decrease the motor’s resistance by increasing the wire gauge, but this would again make your motor even larger.
Increasing the loop count + diameter could reduce efficiency because more wire is farther away from the center of the winding, and the magnetic field produced by a current-carrying loop is inversely proportional to the loop’s radius.
The potential advantage to a lower kV motor is having less mechanical drag since the parts spin slower, and having less losses due to gear efficiency since fewer reduction stages are needed.
Some other potential issues could include startup torque requirements and limitations in output shaft torque carrying capacity. My dream is to one day have a pancake motor that can direct drive an arm on a live axle, but that would obviously not work so well on 1/2" hex. Maybe one day we’ll get a max spline thru bore pancake motor.