Motors

High-Level Practical Knowledge

In order to understand a motor, you must understand that a motor is a load, which has a non-ohmic property as well as an effective resistance.  You'll learn about this in ECE 207 or 220; don't worry if that doesn't make sense quite yet.  In our context, the fundamental understanding is that motors draw current.  There are properties of motors that you should be familiar with:

• Power

• Rated voltage

• Rated current

• Stall current

• KV (voltage contant)

Power is a dodgy thing to read from a spec sheet.  Avoid using it for your calculations because it can be unclear whether this is the power output or simply the operating voltage multiplied by the operating current; which of course is dependent on other things that are likely not listed.  Rated voltage and rated current, which are the maximum "safe" values, are great to find on spec sheets as they make your work easy.  Stall current is the current drawn when the shaft is restrained or stopped, which causes damage to the motor, but that's not hugely applicable in our context.

How to pick motors

Your drive motors for beetleweights will be pretty standardized to the brushless mega spark motors found on Fingertech.  We just like those, there's nothing really wrong with them unless you have some strange needs.  You have much more freedom in your weapon motor choice, which really comes down to whether you use a vertical spinner, a horizontal spinner, or neither.

Rated Current

You'll notice somewhere in the spec sheet of your motor, wherever you buy it from, there should be a max current listed.  If not, look up the exact part number of the motor in Google and find the manufacturer's spec sheet; this can take some digging.

You may also estimate this by recording the power in watts, and the operating voltage of your robot (recall the way that LiPo voltage changes... 11.1V battery produces 12.6V when fully charged).  You'll want to use the lower voltage value, so the one listed, because that will produce the most current for your watt (we're just estimating here).  This means that your motor consumes:

"<power / voltage> amps at max power."  

This is a little bit different conceptually to stall current, which you might also see.  This refers to the current drawn when the motor shaft cannot spin, whereas max current is when the motor is spinning as "hard as it can".  Don't get too hung up on this if you don't understand.  Think of the max current as the "greatest amount of current the motor will draw when you put them at full throttle."

Sidenote: in most other applications, you'll see a rated current range, which is far simpler, it's just the operating range of current the motor will nominally draw.

Troubleshooting / Quick Tips

• You can plug the three wires of your motor into your ESC anyway that you like.  If the motor is not turning the correct direction, you can switch any two wires between the ESC and the motor.

• If your motor is twitching, but not spinning, it may:

  • Have something getting in its way / rubbing against it

  • Be getting too little voltage (typically low battery)

KV (voltage constant)

Sometimes, people call it the "kilovolts."  This is picked up largely from the radio-controlled car-racing community, and that's why I started saying it back when I didn't know better.  It's not that.  I still say it.  It's my cross to bear.

Really, it is the RPM produced by a motor for each given volt.  Your operating voltage, multiplied by your KV, is your max RPM.