Help with BACK EMF measurement

Hey everyone! 👋

I'm new here and still learning, so please be kind. 😊

I'm currently trying to measure the back EMF from a PMSM motor, and I wanted to double-check if my measurements make sense. I’ve attached a picture showing the back EMF signals I'm seeing.

🔧 Setup:

• Control mode: Open-loop (three sinusoidal signals with a 120° phase shift)
• Hardware: DRV8301 driver board + STM32 Nucleo board
• Motor: Surpass Hobby C2826-14 1350KV Outrunner BLDC Motor
• Oscilloscope: Basic 2-channel scope, so only two phases are visible at a time

❓ My question:

In the oscilloscope screenshot, I see a waveform that resembles a sine wave — but I'm not 100% sure if it's truly sinusoidal.

Since I'm using sinusoidal open-loop drive, I would expect the back EMF to also be sinusoidal, right?

However, visually it looks not so much a clean sine wave. That makes me wonder if I'm reading something wrong, or if the setup affects the signal.

🙏 Could someone more experienced take a quick look?

I'd love to know:

• Does the waveform look sinusoidal to you?
• Am I on the right track with how I'm measuring the back EMF?
• Any suggestions or corrections to improve the measurement?

Thanks so much in advance!

Answer 1

[Arushi]

Hi Virag,

It looks like the oscilloscope screenshot you mentioned didn’t come through with your message. If you’re able to share it, that would make it easier to give specific feedback.

Based on your description, though, here are some general thoughts:

- Back EMF Shape: For many hobby PMSM/BLDC motors (like the Surpass C2826-14), it’s normal for the back EMF waveform to not be a perfect sine wave. Small distortions or a more trapezoidal shape are common due to how these motors are constructed.

-Measurement Method: Using a 2-channel oscilloscope to view two phases at a time is a typical way to check back EMF. Just make sure your probe connections and grounding are solid.

- Improving the Measurement: If you want a cleaner view of the back EMF, try spinning the motor by hand (with the driver powered off) and measure the phase voltages. This avoids interference from the drive electronics.

If you’re able to upload the oscilloscope screenshot, I (or others here) can provide more targeted advice. But from what you’ve described, your results and approach sound typical for this kind of setup.

Hope this helps!

[Virag]

Hi!

I'm really glad about your answer—it was really helpful. You're absolutely right; somehow the picture wasn’t attached. But now I managed to upload it. Could you please take a quick look at it?

Thank you so much in advance! :)