Starter Kit Motor Upgrade to 37mm Questions? Higher voltage Lower RPM on the 37mm?

#1

Hello,

I recently built my starter robot kit and I was considering upgrading to the 37mm motors but I am confused about their specs. I see the 37mm is a 12v 50rpm motor and the original 25mm motor that came with the kit is a 6v 185rpm motor. So am I correct in assuming it will be a much slower motor overall? Will it even be a torque upgrade considering the limited current available?

Also how does the motor driver board determine how much voltage the motors get? What kind of voltage range should I see on the driver board pins?

#2

Hi,

Based on the specs:

  • Indeed, the speed will be lower. 50rpm at 12v will be about 25rpm at 6v.
  • rated torque of the 37mm motor is about 3 times the torque of the 25mm motor at their nominal voltage. A rough estimate is torque of a given motor is be proportional to the voltage, so the 37mm motor will have slightly more torque at the same voltage.
  • Current requirement of the 12V motor is about half of the 6V motor at 12V voltage and even lower at 6V.
  • The driver board uses the supply voltage, regardless of the motor connected. The max voltage of the driver is 15V.
  • The voltage on the driver pin is a fast pulse. You need an oscilloscope to properly measure this. Or set the output at 100%. It will be around the supply voltage.

Bottom line: the 37mm motor is slower, uses less current and supplies significant more torque. When run at 6V, it will be even slower, use even less current and still provide slightly more torque than the 25mm motor.
I think the 37mm motor is intended for slow moves, like lift an arm via a belt.

Joep

#3

Thanks for the reply! I just wanted to ask a follow up question or 2.

To be clear about the driver board when programmed to 100% speed should output the same as the input voltage? Based on this wouldn’t the 6aa batteries in the kit be overpowering the 6v motor? Could this be a problem?

I know you said the driver max voltage is 15v, but the boards max is 12v, correct? I was hoping to use a 14v source for my board, but based on what I have read I am likely to have problems and even blow the voltage regulator. Do you agree?

Thanks!

#4

Hi,

  • The max output voltage is probably .2 or .3V below the input voltage, due to D8 (see Orion schematics).
  • Actually, a motor is quite less sensitive to overvoltage than it is to overcurrent. A few volt extra will just make it spin a bit faster. And my guess is that it is quite hard to get one of these motors ruined by overcurrent with normal (alkaline or NiMh) batteries. Won’t put it to the test with my own motors though :wink:
  • Not sure why you would think you get problems over 12V and would be happy to look into this if you have more info. For now, I just have the schematics and I don’t see any real limit on particular components below 30V. It could however that max heat is the limit, but it seems MakeBlock has done a good job here too, like on their mechanics. (Software, and more particular, version control of this, seems their main chalenge…) Anyway, I think you will be fine with 14V when using the 12V motors and I would check the regultator and motor controller temperature to be sure (up to 60 degrees Celcius is fine). I would recommend against running 6V motors with a 14V supply, even at low speed percentage, due to high peak currents.

Regards,
Joep

#5