How to Measure Back EMF of BLDC Motor

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Last Updated on 8 months by Francis

In this article, we will discuss the concept of back electromotive force (emf) and how it can be measured in brushless DC (BLDC) motors. Back emf is a crucial parameter for controlling the speed and torque of these motors, and understanding how to measure it is essential for proper motor control and performance. We will explore the different methods of measuring back emf, the equipment required, and the steps involved in the process. Whether you are an experienced engineer or a hobbyist, this guide will provide valuable insights into the measurement of back emf in BLDC motors.


Understanding Back EMF

Before we dive into the topic of measuring back EMF of BLDC motor, let’s first understand what back EMF is. Back EMF, also known as counter-electromotive force, is a voltage that is generated in a motor’s winding when the rotor rotates. The back EMF opposes the supply voltage that is applied to the motor, and this opposition determines the speed of the motor.

How Back EMF Works

The back EMF is created due to the motor’s magnetic field. When the motor rotates, the magnetic field generated by the rotor cuts through the motor’s windings, generating a voltage. This voltage is proportional to the speed of the motor and opposes the supply voltage, leading to a reduction in the current flowing through the motor.

Importance of Back EMF

Back EMF is essential for the proper functioning of the motor. It helps in controlling the speed of the motor and also protects the motor from damage due to excessive current flow. The higher the back EMF, the lower the current flow, leading to efficient motor operation. Thus, measuring back EMF is crucial for motor control and optimization.

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Measuring Back EMF of BLDC Motor

Measuring back EMF of BLDC motor is essential to determine its speed and position accurately. There are various methods to measure back EMF, and we will discuss some of them below.

Oscilloscope Method

One of the most common methods to measure back EMF is the oscilloscope method. In this method, an oscilloscope is connected to the motor’s winding, and the voltage waveform is observed. The back EMF waveform appears as a negative voltage spike when the motor commutates. The amplitude of this spike is proportional to the motor’s speed and can be used to determine its position accurately.

Phase Voltage Method

Another method to measure back EMF is the phase voltage method. In this method, a voltage is applied to one phase of the motor, and the voltage generated in the other two phases is measured. The back EMF is proportional to the difference between the applied voltage and the measured voltage. This method is simple and effective and can be used for low-cost motor control systems.

Indirect Method

The indirect method of measuring back EMF involves measuring the motor’s current and voltage and then calculating the back EMF using the motor’s equations. This method is more complex but can be used for high-precision motor control systems.

Hall Sensor Method

Another method to measure back EMF is the Hall sensor method. In this method, Hall sensors are placed in the motor’s stator, and their output signals are used to measure the back EMF. This method is commonly used in high-performance applications where high-precision control is required.

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Encoder Method

The encoder method involves using an encoder to measure the motor’s speed and position accurately. The encoder generates pulses as the motor rotates, and the number of pulses generated is proportional to the motor’s speed and position. The back EMF can be calculated from the encoder output signals, leading to accurate motor control.

FAQs – How to Measure Back EMF of BLDC Motor

What is back EMF?

Back EMF, or electromotive force, is the voltage that is produced by the rotation of the rotor in a BLDC (brushless DC) motor. This voltage is opposite in polarity to the applied voltage that powers the motor, and its amplitude varies with the speed of the motor.

Why is it important to measure back EMF in BLDC motors?

Measuring back EMF in BLDC motors is a critical step in determining the speed of the motor. This information is used to adjust the pulse width modulation (PWM) signal that controls the current flowing through the motor windings, allowing the motor to run smoothly and efficiently.

What equipment do I need to measure back EMF in BLDC motors?

To measure back EMF in BLDC motors, you will need an oscilloscope, a voltage probe, and a BLDC motor that is disconnected from any load. You may also need a means of rotating the motor shaft at different speeds, such as a variable frequency drive or a manual speed controller.

How do I connect the voltage probe to the motor?

The voltage probe should be connected to the motor’s phase lead, which is the wire that carries the current to one of the motor’s three windings. The probe should be connected in parallel with the winding, and care should be taken to ensure that the probe’s ground lead is connected to a common ground point with the motor.

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How do I use the oscilloscope to measure back EMF in BLDC motors?

To measure back EMF with an oscilloscope, you will need to connect the voltage probe to one of the motor’s phase leads, as described above. Then, you will need to set up the oscilloscope to display a waveform of the signal, and adjust the sweep rate and trigger level until the waveform is clearly visible. You can then measure the amplitude and frequency of the waveform to determine the back EMF voltage and speed of the motor.

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