An important motor constant that effects efficiency greatly, is known as the brushless motor no load current. It is also known as the no load brushless motor constant.
What is the Brushless Motor No Load Current
The no load current of a brushless motor is defined as the current the motor draws, with no load applied, at a specific voltage. The no load current is measured in Amps and denoted “Io.” The key point here is that the current is measured at a specific voltage. Manufactures select a voltage for the motor to be tested at. Typically, motor manufactures are testing the motors at 10v. This is important to note as the current measured does tend to vary as voltage is above or below this value. The reason for 10v is the motors no load current does not tend to change significantly when you get outside this value. Motor manufactures can not predict the exact voltage you plan to run. Therefore running all tests at 10v is a great median.
No Load Constant vs Efficiency
When we look at how the no load current effects efficiency it begins to be very obvious. If there is no load applied to the motor, the motor is not doing any work. However, if there is power being consumed by the motor, then we know this directly translates in to waste energy. The wasted energy is the energy that it takes to maintain rotational speed of the motor.
We are able to confirm that the rotational speed of the motor is waste energy by looking at Newtons first law. Newtons first law states that an object in motion, stays in motion. Once our motors are accelerated up to maximum speed at the input voltage, the motors then run at constant speed.
The power being consumed is partially due to losses caused from hysteresis, eddy current in the iron core as well as mechanical losses found in the bearings. This is a summation of all Iron Losses present in a brushless motor.
The power to keep a motor running under its own power at a constant speed is known as the no load current constant.
How to measure the No Load Current
To measure the No Load Current of a brushless motor, there are a few tools required. In this method, we are using the ESC connected to the motor in order to spin up the motor. This method does provide an accurate result, however it does include the current that is being consumed by the ESC. Speed Controls built in today’s age are quite efficient and will have a negligible effect on the results.
The tools required consist of:
- Multimeter capable of minimum 10A.
- Electronic Speed Control (ESC) with Radio System
- LiPo of a voltage that is closest to what you will run using the motor.
The steps to take in order to measure the no load current are:
- In order to measure the no load current, the first thing you must do is connect your radio system to the ESC
- Then plug in the motor you wish to test. Ensure there is no mechanical connection on the output shaft.
- Connect your LiPo battery up to the ESC with the ammeter between one lead of the battery and the ESC. Set your ammeter to measure current. Double check the leads are in the correct spot to measure current up to 10A. Be certain that the LiPo voltage is very close to the battery that you plan to use with the motor being tested.
- Start up the motor and run it to full power carefully observing the current that the motor is drawing.
This motor constant is fairly easy to obtain as it is a direct measurement that can be performed very safely. Use this to better understand the performance capability of your motor. The motor with the least Io will have the least iron losses between two motors with equal internal resistance.