What Kills a Brushless Motor – Current or Voltage

This may not be all that common, however, brushless motors can fail. There are many people out there that have seen this happen first hand.  What makes them fail electrically really only comes down to two parameters. Voltage and Current. In this article we will break down each parameter.

Does a Motor Fail Electrically due to Voltage

Short answer here is no, voltage is not responsible for electrically failing a brushless motor.

One parameter that every brushless motor will have is a kv value. A motors kv value is the relationship between voltage and total output RPM. The kv value and the voltage that the motor will run on determines the total amount of unloaded RPM that the motor outputs. RPM = kv x voltage. If you don’t know the kv of your brushless motor, this value can be calculated. Increasing the voltage of the motor will simply increase the total amount of output RPM. We do know that this of course can lead to mechanical failure if the bearings max RPM or the rotors maximum rotation speeds are exceeded. Keep in mind that we are looking at electrical failure here and not mechanical.

Increasing Motor Voltage

Assuming that the rotor or bearings will not explode, what happens if we continuously increase the voltage?In radio control modelling, voltage does fall within a specific domain. We have the ability to use anything from 1 cell LiPo up to 12 cell LiPo based on most ESC that are readily available.  As voltage is increased, the potential for the voltage to jump an insulated gap increases. This is why wire is rated to specific voltages. Luckily for us, the voltage in our RC domain do not come close to becoming an issue. The increase in voltage does increase the total RPM output but this will not destroy a brushless motor.

Does a Motor Fail Electrically due to Current

The current a brushless motor will see, is directly related to the load. As the load of a motor increases the amount of current also increases. Current is what contributes to heat. As current increases the heat built up in the motor also increases.

Brushless Motor Waste Energy

The windings that make up the stator on a brushless motor have a resistance. The motor winding resistance is typically provided by the motor manufacture but can also be determined. The current flowing through a brushless motor can be measured by using an onboard logging device. Many ESC’s have the logging capability built right in.  The equation for waste heat generated by a brushless motor is equal to the current x current x resistance. Yes current is multiplying through twice. You can really see how important current is to the waste heat generated by a motor.

All of this waste heat contributes to brushless motor failure. Heat built up in the windings can get so hot that the rotor magnet is demagnetized. When demagnetization occurs, the kv of the motor increases leading to the motor trying to hit higher RPM’s. And of course these leads to a higher current draw. When enough heat in the windings builds up, the enamel coating on the windings burn away. Once this starts to occur, the motor is freshly cooked.

Wait! Increasing Voltage Killed my Motor

If you increased the voltage of your RC vehicle and experienced a motor failure. I can assure you that it was not due to voltage that your motor electrically failed. What also must be considered is the load applied with a voltage increase. For example, on an RC car, increasing the voltage and keeping everything else equal will in fact increase the load of the motor as well. This occurs as you are demanding the motor turn the same load but at higher RPM’s. This will certainly only place more strain on your motor. This increased motor load is directly related to increased current. Current is what kills the motor, don’t be fooled.

Conclusion – What Kills a Brushless Motor

Current is what ultimately leads to the destruction of a brushless motor. Increasing the amount of current a brushless motor is consuming comes with a cost. The cost is heat. Too much heat in your motor will lead to failure of the motor windings. Don’t let the magic smoke out of your motor, otherwise your motor is toast!

 

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