Brushless motors are reliable, but they can still fail. Many RC hobbyists have experienced a motor suddenly losing power, overheating or releasing the familiar cloud of smoke.
When we look at an electrical motor failure, the two main parameters people normally consider are voltage and current. Both affect how the motor operates, but they do not damage the motor in the same way. In this article, we will look at what each one does and what usually causes the motor to fail.
Can Voltage Electrically Damage a Brushless Motor?
Under normal RC operating conditions, voltage by itself is usually not what burns up a brushless motor. The more immediate electrical cause of failure is normally excessive current and the heat that current produces.
However, this does not mean that voltage can be increased without limits. Excessive voltage can increase motor speed, increase the load placed on the motor and cause the motor to draw more current. At extremely high voltage, it may also exceed the electrical insulation limits of the motor. Within the normal voltage range used for RC systems, though, overheating from excessive current is usually the more important concern.
Every brushless motor has a Kv value. Motor Kv describes the approximate unloaded motor speed produced for each volt applied to the motor.
The basic relationship is:
Unloaded RPM = Motor Kv × Applied Voltage
For example, a 1,000 Kv motor supplied with 10 volts would have a theoretical unloaded speed of approximately 10,000 RPM. The actual loaded speed will be lower because of motor losses and the load being driven.
If you do not know the Kv of your brushless motor, it can be measured using a relatively simple test.
What Happens When Motor Voltage Is Increased?
Increasing the voltage supplied to a brushless motor increases its potential operating speed. This is why changing from a lower-cell battery to a higher-cell battery can produce a large increase in vehicle performance.
Most RC power systems operate within a relatively low voltage range. Depending on the ESC and application, this could range from a small single-cell LiPo system to a large 12S system or beyond. At these voltages, insulation breakdown inside a properly manufactured motor is not normally the first limitation.
The more likely problem is that the increased motor speed also increases the load. The motor may then draw more current than it can safely handle.
There are also mechanical limits. Excessive RPM can damage bearings, loosen rotor components or exceed the safe rotational speed of the rotor. These are mechanical failures rather than winding failures, but they still need to be considered before increasing voltage.
Can Current Electrically Damage a Brushless Motor?
Current is much more directly connected to electrical motor failure. The current drawn by a brushless motor depends heavily on the load placed on it.
As the motor load increases, the motor must produce more torque. Producing more torque requires more current. That additional current creates more heat inside the motor windings.
This is why changing a propeller, gear ratio, vehicle weight or operating speed can dramatically change the amount of current a motor draws, even when the battery voltage remains the same.
Brushless Motor Copper Loss
The copper windings that make up the stator have electrical resistance. This resistance is often identified as Rm in motor specifications. The motor manufacturer may provide this value, or the motor winding resistance can be measured.
Current flowing through this winding resistance produces copper loss. Copper loss is calculated using:
Copper Loss = Current² × Winding Resistance
This is also written as:
P = I²R
The important detail is that current is squared. Doubling the current does not merely double the copper loss. Assuming winding resistance remains the same, doubling the current produces four times the copper loss.
This is why a small increase in current can create a much larger increase in motor temperature.
Motor current can be measured using an external wattmeter or an ESC with onboard data logging. Current should always be checked under realistic operating conditions because unloaded testing does not represent the load seen during actual use.
What Happens When a Brushless Motor Overheats?
Not all electrical power entering a motor becomes useful mechanical power. Some of it becomes waste heat through copper loss, iron loss, bearing friction and other losses.
When the motor produces heat faster than it can remove it, the internal temperature begins to rise. The windings may eventually become hot enough to damage the enamel insulation surrounding the copper wire.
Once the winding insulation is damaged, turns within the winding can short together. This reduces the effective winding resistance and can cause the motor to draw even more current. The resulting increase in heat can quickly turn a minor problem into a complete motor failure.
Excessive temperature can also weaken or partially demagnetize the rotor magnets. If the magnets lose strength, the motor may attempt to draw more current to produce the same torque. Motor performance will drop while motor temperature continues to increase.
Wait—Increasing Voltage Killed My Motor
It is common for a motor to fail shortly after a higher-voltage battery is installed. It may appear that voltage directly destroyed the motor, but the actual sequence is usually more complicated.
Consider an RC car that is changed from a lower-cell battery to a higher-cell battery without changing the gearing. The higher voltage increases motor RPM. The motor must now turn the same drivetrain and tires at a higher speed.
That higher operating speed can significantly increase the load and current draw. The increased current produces more copper loss and heat. If the motor cannot dissipate that heat, the winding temperature continues to rise until the motor is damaged.
The voltage increase started the chain of events, but excessive current and temperature were the more direct causes of the electrical failure.
The same principle applies to airplanes and boats. Increasing battery voltage while keeping the same propeller can increase RPM, power demand and current. The full system must be checked whenever battery voltage is increased.
Common Causes of Excessive Motor Current
- Using a propeller that is too large or has too much pitch
- Using overly aggressive gearing in an RC car
- Increasing battery voltage without reducing the motor load
- Operating a boat with an oversized or overly aggressive propeller
- Restricted cooling or blocked airflow
- Mechanical binding in bearings, gears or the drivetrain
- Running the motor at high load for too long
- Using a motor that is too small for the application
How to Prevent Brushless Motor Failure
The best way to protect a brushless motor is to measure the system rather than relying only on the motor’s advertised wattage or current rating.
- Measure current under realistic operating conditions.
- Monitor motor temperature after short test runs.
- Use the correct propeller or gear ratio for the battery voltage.
- Make sure the motor receives adequate cooling.
- Inspect the drivetrain for resistance or binding.
- Do not assume that a short burst current rating can be sustained continuously.
Motor temperature should be checked gradually while testing a new setup. Begin with a short run, allow the motor to stop and then check its temperature. Increase the run time only after confirming that the motor, ESC and battery remain within a safe operating range.
Conclusion: What Kills a Brushless Motor?
Excessive current and the heat it produces are the most common causes of electrical brushless motor failure. The copper loss inside the motor increases with the square of current, which means even a moderate increase in current can produce a large increase in heat.
Voltage is still important because increasing voltage increases potential motor speed and can indirectly increase the motor load and current draw. It can also create mechanical RPM concerns and, at extreme levels, electrical insulation concerns.
The safest approach is to treat the battery, motor, ESC, propeller or gearing as one complete system. Whenever one part of that system changes, current and temperature should be checked again.
Keep the motor properly loaded and adequately cooled, and you greatly reduce the chance of letting out the magic smoke.
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