Understanding RC Motors

Understanding R/C Brushless Motor Ratings

 

kV Ratings

Ok, lets start with kV ratings. The letters kV stand for the the RPM of your motor per volt with no load. For example if you own a brushless motor with a kV rating of 4600 and 12V. Take the 4600, multiply by 12 to get 55,200 RPMs. This is the max RPMs that this motor can reach under no load. Once you get it inside your vehicle, this will come down due to friction.

Almost all brushless motors will have the kV ratings stamped somewhere on them. Some motors will have kV ratings on the motor can, others on the motor leads, but some you will only see on the motor spec sheet.

Ok. now you are this far, so what does this mean to you really?

  • A motor with a higher kV will have more top end speed, but not as much acceleration/torque.
  • A motor with a lower kV will not be as fast, but will accelerate faster.

So, now you can decide which one works best for your kind of racing. You have the room to really crank it up and reach top speeds? A higher kV will get you there. But maybe you are on a shorter track, and what you want is acceleration out of the corners, then look for a lower kV number. Still not sure which way to go? Try something in the middle!

 

Note: If motor heat is an issue then a lower kV rating with a higher voltage battery will give you the same effect.

The big thing to remember when using kV for your Brushless Motor Ratings is that your Brushless Motor and ESC will each have a maximum input voltage (battery cell count) that is allowed. So if either your motor or ESC has a lower maximum voltage then you must use this to calculate your top RPMs. If you go over the recommended voltage then you have a high chance that something will fry in your setup.

 

Motor Turns

Motor Turns is the same for brushed motors and brushless motors. The word turns stands for the amount of wire windings around each of the motor's rotor poles.

  • The higher the number of wirings/turns means less top speed, but higher acceleration/torque.
  • The lower the number of turns equals higher top end speed and lower torque/acceleration.
  • A motor with a turn rating of 5.5 will have less acceleration/torque but higher top speed than a motor with a 12 turn rating.

Current Rating - Amps

 

It is a great idea to find an ESC that has a current rating that is higher than your motor�s by at least 20%. It will be a good safety cushion to make sure that you do not burn up your brushless power plant.

Here's why: the max current rating is the maximum amount of current that a motor is able to handle safely. This current is measured in Amps. The continuous current rating of a motor is the Amps that a motor can handle safely over a long period of time.

The estimated current rating of a motor is usually on the factory specs sheet. However other factors affect the actual current that a Brushless motor will draw. Things like the kV rating, battery voltage, how heavy the RC vehicle is, and gear ratio or prop size. The harder a motor needs to work to reach it's top speed, the higher the Amp draw is.


Watts

 

Watts are the power rating or the horsepower equivalent of your brushless RC Motor. The math here is Amps x Volts = Watts. You will see a watt rating in the brushless motor specs. Your brushless motor should have a watt rating on its spec sheet, something like "180W". This is the amount of "horse power" that it should produce safely. Running anything over this rating could damage your motor, especially over a long period of time.

 

 

Motor Efficiency

The efficiency of a motor determines its quality. Higher efficiency means better design and high quality components. The higher the efficiency of the motor the more power it can produce before it overheats. A 70% efficient motor produces 70% power and 30% heat. A 85% efficient motor produces 85% power and 15% heat. If your battery is sending the ESC 180 watts, your motor will produce 153 watts (85%), the rest is gold ole heat. 27 Watts of heat will melt solder with some soldering irons, so, that is a lot of wasted watts!

 

A cooler running motor will give you much less trouble. To reduce heat you can change your gearing or prop size, use a more efficient motor, reduce your voltage or amps, or try a motor heat sink and motor fan. Keeping the heat down on your motor allows it to run longer, and give you the power it needs.. Efficiency matters folks.

 

 

 
 

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