Understanding DC Brushless Motor Efficiency & How to Test For It

Brushless DC motors are some of the most efficient and long-lasting motors available on the market today. But what exactly makes them so efficient compared to brushed motors, and how can you test for it? Let’s take a closer look.

What is Brushless DC Motor Efficiency?

Efficiency refers to the percentage of the total electrical energy that is converted into useful mechanical energy. In other words, it’s a measure of how much power is being wasted as heat. The higher the efficiency, the less power is wasted, and the more efficient the motor is.

Brushless motors have higher efficiency rates of up to 85 to 90. This means that the mechanical power of a BLDC motor can be up to 90% of the total electrical power input. This efficiency rate is much better than those of brushed DC motors, which have an efficiency output power rating of around 75 to 80.

How is BLDC Motor Efficiency Tested?

To test the efficiency of a BLDC motor, you’ll need to measure the mechanical power and electrical power of the motor. In addition, you’ll need to measure current in amps, voltage, and RPM. Here are the steps to measure;

1. To measure current, you can use a multimeter. First, ensure the multimeter is set to the correct current (Amps) mode for the expected range of current. Then, disconnect the circuit at one point and connect the multimeter in series with the circuit so that the current flows through the multimeter. This means one lead of the multimeter should be connected to the open point of the power source, and the other lead should be connected to the open point leading to the load (e.g., a motor).
2. To measure the voltage across a motor using a multimeter, set the multimeter to ‘Volts DC’ mode if you’re working with a DC motor. Then, carefully touch the multimeter’s leads to the motor’s positive and negative terminals, with the red lead on the positive terminal and the black lead on the negative terminal. This will allow you to read the voltage across the motor directly from the multimeter’s display.
3. To measure RPM, you can use a tachometer, a device designed to measure the rotational speed of a shaft or disk in a motor. There are two main types of tachometers: contact and non-contact. For a non-contact tachometer, simply aim the sensor at the spinning part of the motor, often requiring a reflective marker for accurate detection. For a contact tachometer, the sensor must make physical contact with the rotating element. Many tachometers feature an LED or LCD digital screen, facilitating easy reading of the RPM. Accurately measuring the rotational speed is a crucial step in evaluating the performance of a motor. To calculate the motor’s efficiency, however, you will also need to consider the power input and output, as efficiency is determined by the ratio of mechanical output power to electrical input power, expressed as a percentage.

How do you test the efficiency of a brushless motor?

To test the motor efficiency of a brushless DC motor more scientifically, it’s important to first consider the types of loss that might affect system efficiency. Measuring the loss means you will need to take into account:

• Copper loss
• Eddy current loss
• Hysteresis loss
• Friction loss
• Viscous loss
• Inverter loss

Doing so will give you a more accurate number and let you know which areas need improvement.

Copper Loss

Copper loss is caused by the windings in the motor being resistant to the current flow. The higher the current, the higher the copper loss. This type of loss is measured by finding the input power required to run the motor at a specific speed. The faster the speed, the higher the input power required and the higher the copper loss.

Eddy Current Loss

Eddy current losses are caused by the rotating magnets in the motor. These permanent magnets create a magnetic field that induces a current in the stator windings. The current flowing through the stator windings creates a magnetic field of its own, which opposes the original magnetic field. This opposing field causes energy to be lost as heat.

Hysteresis Loss

The magnets cause hysteresis losses in the rotor being magnetized and demagnetized as they rotate past the stator windings. This process causes energy to be lost as heat.

Friction Loss

Friction losses occur when the friction between the motor’s moving parts slows it down. These losses are typically minimal but can add up over time and cause the motor to run less efficiently. Brushless motors are even smaller since brushless motors don’t have brushes that create friction.

Viscous Loss

Viscous losses occur when the fluid in the motor (usually oil) creates resistance to the moving parts of the motor. These losses are typically minimal but can add up over time and cause the motor to run less efficiently.

Inverter Loss

Inverter losses occur in the inverter that’s used to control the BLDC motors. These losses are typically very small but can add up over time and cause the motor to run less efficiently.

Testing for all these different types of loss can be difficult, so the most common way to test for efficiency is by measuring the input power and output power using a multimeter and a drill or some other device that allows you to spin the motor while taking a measurement.

1. To start, you’ll need to connect the multimeter to the positive and negative leads of the motor. Then, set the multimeter to measure DC volts and amps.
2. Next, use the drill (or another device) to spin the motor at a constant speed. As the motor is spinning, take note of the voltage and current readings on the multimeter.
3. Then, measure the RPM of the motor using a tachometer. Since all tachometers measure differently, you’ll need to calibrate the tachometer before taking your measurement.

Now that you have the voltage and current readings, you can use them to calculate the input power and output power of the motor.

Brushless Motor Efficiency Equations & Calculation

Plug in your calculations using the equation:

Motor efficiency = Mechanical Power / Electrical Power

Where Mechanical Power is Torque x RPM

Electrical Power is Current x Voltage

Brushless Motor Efficiency Curve

The brushless motor efficiency curve describes the relationship between efficiency and torque. As you can see in the chart, the motor is most efficient when it’s operating at high speeds with minimal torque.

There are a few things that you can do to improve the efficiency of BLDC motors and land into the yellow, high-efficiency section of this graph. One is to make sure that the windings in the motor are made of high-quality copper. Another is to use good-quality bearings and lubricants. You can try to reduce the friction between the moving parts of the motor. In addition, ensure you continue to inspect your brushless motor for any dirt or debris that can cause the motor to run less efficiently.

If you keep these tips in mind, you can improve the efficiency of your brushless DC motor and get the most out of it.

Conclusion

By understanding the different types of losses that occur in brushless DC motors, you can take steps to improve the efficiency of your motor. At Assumotors, our goal is to educate our customers about the benefits of BLDC motors, help them choose the best motors, and improve their projects using maximum efficiency! Share this article with others to help them learn more about BLDC motor efficiency.

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You might be interested in reading:

1. 10 Common DC Motor Failures That You Should Know!
2. How Long do DC Brushless Motors Last?
3. How to tell if a DC Motor is Brushed or Brushless?