Brushless Motor No Load Current

An important motor constant that effects efficiency greatly is known as the brushless motor no load current or 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. Generally, manufactures produce results This is very important as the current measured does vary with voltage applied as voltage applied directly translates in to RPM. This we know from the Kv constant article.

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 heat.

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

In order 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 should have a negligible effect on the results

The tools required consist of:

  1. Multimeter capable of minimum 10A.
  2. Electronic Speed Control (ESC) with Radio System
  3. 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:

  1. In order to measure the no load current, the first thing you must do is connect your radio system to the ESC
  2. Then plug in the motor you wish to test.
  3. Connect your LiPo battery up to the ESC with the ammeter between one lead of the battery in order to measure current. Be certain that the LiPo voltage is very close to the battery that you plan to use with the motor being tested. This is important in order to get an accurate reading for the Current.
  4. 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. As with both efficiency constants, it is preferred to have the lowest value of current for the no load current constant.