I am a tad bit confused by the specs on ESC's and Brushless Motors.
- When an ESC is said to be a 12amp ESC, what does this mean? Is the maximum output amperage to the motor 12amps?
- What does the motor's "current capacity" mean? For instance, the motor I'm using has a current capacity of 7.5A/60s. What exactly does this refer to? Was the motor tested at 7.5 amps for 60 seconds?
- Why is my brushless motor, with a current capacity of 7.5amps/60s and a Max current of 9.5amp, suggested to work with a 10amp ESC?
My motor specs are found here if you need them.
Thanks for the help,
Chronzerg
Replies
You pose excellent questions about specifications. To my knowledge there is no one standard to define what a manufacturer means when they state a spec. I will try to answer based on my education, training, and experience.
Starting with the last question:
The size of the wire in the windings of a motor are one limiting factor on the amount of current (measured in amps) the device can handle. The Max Current value is easy enough to understand. If you exceed that value, 9.5A in this case, you will overheat the wire in the device and probably begin melting any insulation that is keeping the windings electrically insulated. It may even begin melting the wire itself depending on the concentration of heat. So why would a suggestion be to use the 10A ESC? Because if you went with a lower rated ESC, the motor would overload it, drawing too much current and cause the ESC to overheat and fail.
The above answer somewhat addresses the second question:
I think you have it correct. In this case, I believe the manufacturer claims you can operate the motor at 7.5A for 60 seconds maximum. This is probably due to the ability of the motor to dissipate the heat that builds up as the motor is used. What is not stated is how long you must wait before doing this maximum loading. You want to use an ESC that can deliver slightly more energy to the motor than you need, in the event you want to provide a burst of the previously mentioned 9.5A. Racing applications often run devices at the maximum ratings..and beyond if they can get away with it. Most flying applications usually do not push those boundaries.
And finally:
Yes. A point usually not discussed is the fact that the drive current from the ESC to the motor is 3 phase AC. It may be a square wave, pseudo sine wave, triangle wave, or a general mess but, somehow the manufacturer has set the maximum current, in this case, at 12A. This is often specified based on the power controlling devices used in the ESC -- the FETSs (Field Effect Transistor).
I just opened up a dead ESC this weekend. The FET side of the circuit board had empty spacing for additional FETs. If populated, the 30A rating of the ESC would have been higher...depending on FETs.
There are many more resources on the web that address this topic but ratings are totally dependent on knowing the test conditions and equations used to calculate the values. There are power meters available that help tell you how your motor/ESC is operating...if you really need to know the details. What is used to determine the numbers is often not revealed.
I hope this helped.
-=Doug