Is my BLDC trapezoidal or sinusoidal? - Page 1
Forum section: electronics / beginners

author: Power-Electronics
author_rank: Contributor
timestamp: November 13, 2021, 01:25:46 am
content: I have a 250W Bafang e-bike hub motor that I'm trying to figure out. Looking at the windings and based on what I've read about BLDCs, I expected to see trapezoidal back EMF. But when I look at the oscilloscope, I'm unsure whether the waveform is a sinusoid or a trapezoid. My measurement technique was to connect an oscilloscope probe and ground clip across two of the phases and spin the motor at a fairly constant speed with a drill.Can this winding pattern produce sinusoidal back EMF? Is the waveform actually more of a trapezoid than a sinusoid? Or did I mess up the measurement?

author: WattsThat
author_rank: Frequent Contributor
timestamp: November 13, 2021, 05:40:02 am
content: Uh, that’s 100% sinusoidal.

author: Power-Electronics
author_rank: Contributor
timestamp: November 13, 2021, 01:36:11 pm
content: Thank you for confirming. It subverted my expectation, and I didn't know how obvious or subtle the distinction should be.

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2021, 07:33:36 pm
content: The back EMF doesn'y really say anything about the drive electronics. They may be trapezoidal.Does the motor have Hall sensors?

author: langwadt
author_rank: Super Contributor
timestamp: November 13, 2021, 07:38:37 pm
content: Quote from: Power-Electronics on November 13, 2021, 01:36:11 pmhttps://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/909911/faq-trapezoidal-motors-vs-sinusoidal-motors

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2021, 09:18:06 pm
content: @Langwadt, nice, but very short summary page link.My question still stands: does the motor have Hall feedback or not?BLDC and PMSM are two radically different things (although the motors are almost the same).

author: Power-Electronics
author_rank: Contributor
timestamp: November 13, 2021, 09:43:26 pm
content: Quote from: Benta on November 13, 2021, 07:33:36 pmYes, it has three Hall sensors.Not sure if you need more info beyond just the existence of them, so here's a bit extra. I tied the three phases together with three resistors to create a virtual ground. Then I measured the back EMF from line to neutral of one phase (yellow) and a Hall sensor output (blue). Screenshot attached.Quote from: Benta on November 13, 2021, 07:33:36 pmThis project is to make the drive electronics. Do I not need to know the back EMF shape to choose the right drive scheme?

author: xavier60
author_rank: Super Contributor
timestamp: November 13, 2021, 10:28:22 pm
content: That doesn't look right. Possibly crosstalk from the hall sensors. Try lower value resistors or disable the hall sensors.

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2021, 10:46:01 pm
content: Quote from: Power-Electronics on November 13, 2021, 09:43:26 pmNot really. The Hall sensors will normally take care of commutation. Your task is to provide a variable voltage for speed control.Imagine it like this:The Hall sensors control the MOSFETs in your three-phase bridge directly, thus replacing the mechanical brushes.On top of that, you can vary the supply voltage to control the motor speed.That won't work in this case, so the solution is to modulate the Hall-sensor feedback signals to the MOSFET gates with a PWM waveform. The important thing to understand is, that the Hall-sensor feedbacks have priority and are directly responsible for motor operation.

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2021, 10:47:17 pm
content: Quote from: xavier60 on November 13, 2021, 10:28:22 pmThat looksexactlyright to me. Example:https://toshiba.semicon-storage.com/ap-en/semiconductor/product/motor-driver-ics/brushless-dc-motor-driver-ics/detail.TB6585FG.htmlp.16 and onwards.

author: xavier60
author_rank: Super Contributor
timestamp: November 13, 2021, 11:27:09 pm
content: The Modulated waveform in the TB6585FG data sheet is mirrored pairs of 120° sine wave portions generated inside of the IC.How did Power-Electronics get the same waveform?

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2021, 11:33:36 pm
content: Quote from: xavier60 on November 13, 2021, 11:27:09 pmWell, as we haven't seen her/his schematic: no idea. But the waveform is normal.

author: xavier60
author_rank: Super Contributor
timestamp: November 13, 2021, 11:45:24 pm
content: This might be interesting. I just scoped an output of my recent motor drive project with respect to a virtual Neutral.The motor is a 3 phase induction type which I assume would prefer a sine wave.I'm using 180° conduction mode instead of the usual 120°. Another option was 150° conduction mode, but the switching pattern gets a bit complicated, needing 12 steps.I'm assuming that the stator windings are Star connected.

author: langwadt
author_rank: Super Contributor
timestamp: November 14, 2021, 01:09:00 am
content: Quote from: xavier60 on November 13, 2021, 11:27:09 pmlooks like space vector drive?

author: xavier60
author_rank: Super Contributor
timestamp: November 14, 2021, 01:33:32 am
content: Quote from: langwadt on November 14, 2021, 01:09:00 amI wasn't aware of the term. Interesting though.

author: Power-Electronics
author_rank: Contributor
timestamp: November 14, 2021, 12:46:42 pm
content: Didn't mean to cause confusion with the m-shaped waveform. There's no public documentation for the motor, and it was kind of difficult to match the Hall sensors to the phase windings by measuring line-to-line. I thought it might be easier measuring line-to-neutral, but the motor is Delta-wound and doesn't have a neutral pin. So I created a virtual ground by connecting the Delta motor to a Wye resistor network. I read somewhere that the m-shape is expected, at least for certain motors. But I couldn't find it again, so I added that info in case it's distinctive enough to identify.

author: Benta
author_rank: Super Contributor
timestamp: November 14, 2021, 03:14:18 pm
content: Quote from: langwadt on November 14, 2021, 01:09:00 amIt's just a normal BLDC sine wave drive.

author: langwadt
author_rank: Super Contributor
timestamp: November 14, 2021, 09:51:38 pm
content: Quote from: Benta on November 14, 2021, 03:14:18 pmbut the double peak rather than sines makes it look like space vector

author: xavier60
author_rank: Super Contributor
timestamp: November 14, 2021, 11:06:22 pm
content: I wonder if this statement always holds true for square-wave drive."A star-connected BLDC machine with rectangular line currents of 180 electrical degrees width has the same current distribution as a delta-connected BLDC machine with rectangular line currents of 120 electrical degrees width."https://www.emetor.com/glossary/brushless-dc-machine/

author: Benta
author_rank: Super Contributor
timestamp: November 14, 2021, 11:14:13 pm
content: Quote from: langwadt on November 14, 2021, 09:51:38 pmI think you are confusing three-phase drive (PMSM) with BLDC. On a BLDC, only two phases are driven at any time. This results in the waveform when measured in respect to the (imaginary) neutral, which doesn't really exist.It's probably easier to visualize with square-wave drive. Look here:https://www.onsemi.com/pdf/datasheet/mc33035-d.pdfp. 19.

author: Benta
author_rank: Super Contributor
timestamp: November 14, 2021, 11:24:17 pm
content: Quote from: xavier60 on November 14, 2021, 11:06:22 pmThe text you refer to is ambiguous, in that it suggests that drive for wye vs. delta connected motors are different.That is not true.Also, the reference to "rectangular line currents" is suspicious to me. Rectangular line voltages I'd accept.Drive is the same, the result is the same, but the analysis is a bit different. A delta-wye or vice-versa transformation can always be done.The difference is, that with wye you have a true neutral reference point for measurements. With delta you don't.

author: langwadt
author_rank: Super Contributor
timestamp: November 14, 2021, 11:27:28 pm
content: Quote from: Benta on November 14, 2021, 11:14:13 pmno, sine vs. space vectorhttps://jp.mathworks.com/help/physmod/sps/ref/pwm_three_phase_two_level_continuous.png

author: Benta
author_rank: Super Contributor
timestamp: November 14, 2021, 11:29:51 pm
content: Yes, wonderful for you that you've found a similar waveform. And?Space vector is three-phase drive.

author: langwadt
author_rank: Super Contributor
timestamp: November 14, 2021, 11:59:31 pm
content: Quote from: Benta on November 14, 2021, 11:29:51 pmso is sinewave ...

author: Benta
author_rank: Super Contributor
timestamp: November 15, 2021, 12:17:15 am
content: Quote from: langwadt on November 14, 2021, 11:59:31 pmNo.Not as BLDC drive. PSMS, yes.

author: Power-Electronics
author_rank: Contributor
timestamp: November 15, 2021, 12:31:03 am
content: Space vector is a driving technique, isn't it? I'm not driving the motor yet. I'm just spinning it with a drill.

author: ace1903
author_rank: Regular Contributor
timestamp: November 16, 2021, 01:20:39 pm
content: I have checked several motors from different manufacturers for one project. In all cases backEMF was nice sinewave.I think that BLDC motors all have sinusoidal back EMF. This is because of motor shape.Trapezoidal maybe can be found in axial motors like in old floppy drives or other low torque price sensitive applications.But I never saw one myself.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 16, 2021, 06:38:47 pm
content: Indeed, I have ran some cheap hoverboard-type BLDC (supposedly trapezoidal) motors, which come with hall sensors and are supposed to be driven with the typical 6-step "two phases driven at a time" commutation, using FOC, sine PWM, rotor position interpolated to higher resolution, and they perform better in all regards as far as I can tell.From practical standpoint, it seems to me you can driveallBLDC/PMAC/PMSM motors with sine wave and that is the best way to drive any of them, but you canalsodriveanyof those types with 6-step commutation only driving two phases at once based on rotor angle sensing (typically hall-based), so that the position directly decides the output stage state. The benefit is simplicity in drive logic (nowadays irrelevant because we have processing power available in cheap MCUs), also lower switching losses in MOSFETs. But switching loss isn't that big of a deal anyway because motor controllers run at low f_sw. Downside is torque ripple, because the magnetic flux angle is always off from the optimum. Supposedly the whole idea of trapezoidal motors is to compensate for this in physical motor design, but I have never seen such motor in wild. BLDC motors I have used produce sinusoidal BEMF, produce ripply torque when driven like they are "supposed" to (6-step), and provide smooth torque when driven with sine wave. So are they BLDC? Or should I call them PMSM?"more torque" is one of the stupidest points IMHO seen in drive comparison tables, actually that only means "more torque for a given DC link voltage assuming you have designed no headroom in voltage and need to max the motor out with what you have", which usually isn't the case.So is my BLDC trapezoidal or sinusoidal? I have no idea; I drive them all with FOC and sine wave.

author: xavier60
author_rank: Super Contributor
timestamp: November 18, 2021, 08:36:55 pm
content: Fromhttps://www.digikey.com.au/en/articles/why-and-how-to-sinusoidally-control-three-phase-brushless-dc-motors"Practical sinusoidal control implementations don’t drive each phase with a pure sine-wave voltage. Instead a saddle-shaped voltage is applied which results in a sine-wave differential voltage between two terminals, phase-shifted by 120˚ for commutation. The result is higher torque and speed for a given voltage and improved efficiency. (Image source: NXP)The saddle profile technique offers two advantages: First, the maximum differential voltage generated is higher than can be produced with a pure sinusoidal signal, offering higher torque and speed for a given input. Second, each terminal output is zero for 1/3 of the time, further reducing switching losses in the power stage."I'm finding it difficult to visualize how this works. And also how one would get the saddle waveform out of a motor being spun externally.

author: xavier60
author_rank: Super Contributor
timestamp: November 18, 2021, 10:02:20 pm
content: I got this waveform by connecting the 3 phases to lightly biased clamping diodes to ground and spinning the motor with a power drill.Looks familiar.It's a PM 3 phase washing machine pump motor.

author: Power-Electronics
author_rank: Contributor
timestamp: November 19, 2021, 01:39:59 am
content: Quote from: ace1903 on November 16, 2021, 01:20:39 pmSo much educational material focuses on trapezoidal back EMF that I thought it would be more common. They also say you can tell the difference by looking at the windings, but I guess that's not true either.Quote from: Siwastaja on November 16, 2021, 06:38:47 pmThat's what I was hoping for. Makes it both relatively easy to start and interesting to continue after I get it working.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 19, 2021, 09:06:21 am
content: AFAIK, this "saddle" thing is known as "fifth harmonic injection" in academic literature if you want to study further. It can be used in induction motor VFDs as well. It has nothing to do with BLDCs per se, or the BEMF shape; its only purpose is to get away with lower DC link voltage. Think about it as a similar mechanism to get more power out of an audio amplifier by letting the signal clip. It sounds more distorted but you get more power out, at the expense of the ear pain of a high-end listener.But the motor doesn't care, it's fine with such distortion, specifically the 5th harmonic is "harmless". This doesn't mean adding 5th harmonic is very beneficial or required. If you have the voltage available, you can just use sine wave. Or you can just amplify the sine to let it clip, the result is very closely the same, but adding the 5th harmonic in a more controlled way ("saddle" instead of "clipped sine") does not add other higher harmonics which would only contribute to some noise and possibly some minor efficiency loss.When I built an ACIM VFD (it was FPGA based) based on slip control algorithm (ACIM FOC with flux estimation was too much black magic voodoo to me at the time; slip control worked pretty well), I added the 5th harmonic because in that particular project, it really lacked voltage (it was an EV conversion vehicle with motor rewound for 300V li-ion yet we only had some 96V Pb on the proto). Maybe this extended the top speed from 40km/h to 45km/h. I don't remember how it went exactly.

author: xavier60
author_rank: Super Contributor
timestamp: November 19, 2021, 10:50:16 am
content: Actually there is no distortion to the phase to phase waveform with the Saddle Wave drive.The voltage of all 3 phases are simultaneously shifted up and down to accommodate the peaks within the available DC link voltage.I saw a good illustration yesterday. Ill post the link if I find it.

author: xavier60
author_rank: Super Contributor
timestamp: November 27, 2021, 01:00:14 pm
content: I have modified my motor drive to do 12 step 150° conduction instead of 6 step 180°. It is supposed to greatly reduce the DC supply current ripple.The waveform is phase to virtual Neutral using resistors. Looks really nice.I have yet to see how the induction motor likes it.The motor didn't like 120° conduction mode at all, caused the speed to rapidly bounce up and down until it caused an over-current trip.Update: That was a big waste of time. There is so much reactive current flow that nothing changes. Turning a MOSFET off in an attempt to float a phase just causes the voltage to swing across to the other rail causing the opposing MOSFET's body diode to clamp for the whole 30° step.Ill have to live with the high DC ripple current.