You can place all the switching noise up in the ultrasonic domain with PWM, this method can't do that...
No offence, but that is not entirely true, to be honest. The spectrum of a PWM waveform typically contains components at (fc ± n.fo) where fc and fo are the carrier / modulating frequencies respectively and n = 1, 2, 3 etc. In other words, a PWM waveform carries sidebands well below the carrier frequency at substantial strengths, as shown in the picture below (Courtesy: DGHolmes, TA Lipo, PWM for power converters Principles and Practice).
Besides, lower order harmonic components of the fundamental (causing THD) are also generated by high frequency PWM and no harmonic elimination is guaranteed.
Besides, according to TI (pg 15 of below), the THD of an 8-bit waveform itself is around 0.5% and performing high frequency PWM with this waveform is not going to significantly improve this THD anyway. So what is the takeaway, especially with a filter and an inductive load in place ?
https://www.ti.com/lit/an/snoa665c/snoa665c.pdf
HF PWM maybe convenient (especially to the uninitiated) and Harmonic Elimination PWM may be old, but it still remains a principal method among others in what is today known as 'Programmed PWM'.
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Not sure what 'fast PWM' is but I guess 'phase perfect' refers to an advancement by half a carrier cycle. And in a 3-phase environment, the third harmonic is easily rejected and is therefore of no consequence.
Yes, different kinds of PWM result in different harmonic profiles. However, the one that is recommended is asymmetrically sampled sine-triangle PWM in which the sine reference is sampled at the both the peaks and troughs of the triangular carrier as shown below. The phase advancement is by a quarter carrier cycle. The advantages are that the odd / even harmonic sidebands around odd / even multiples of the carrier frequency (respectively) are automatically eliminated.
Further, there could be some improvement if an odd carrier / fundamental frequency ratio (fc / fo).
(Courtesy: DG Holmes, TA Lipo, PWM for power converters Principles and Practice)
The term comes from microcontroller datasheets. Most support various PWM modes. These are synthesized waveforms, not analog generated.
Motor drives rarely use analogue-generated waveforms and most of what is mentioned above also implies digital PWM, much like what is put out by microcontrollers, utilising a high-modulo counter in place of the triangular waveform.
These work quite well (https://www.ebay.co.uk/itm/296035655659)
after modifying the variable resistor values, with my small BLDC motors
https://www.ebay.co.uk/itm/235536635718?var=535735062836.
Any thoughts?
after modifying the variable resistor values, with my small BLDC motors
https://www.ebay.co.uk/itm/235536635718?var=535735062836.
Any thoughts?
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It is certainly not a 'toy' motor. I have used these motors to replace both the Premotec and Thorens with excellent results. When driven by a 3 phase sine wave drive there is very good torque with little vibration, certainly a lot smoother than either of the original motors.
Yes it can and here is a demo for something along those lines. All harmonics up to the 59th are eliminated using SHE using 20 switching angles giving an effective switching frequency of just (20*2 + 1 )*50 = 2050 Hz. Not quite ultrasonic but the point is that by simply using more switching angles for eliminating more harmonics places the switching higher up in frequency, just like PWM.
However, SHE does not require as many switchings to compete with regular PWM in terms of THD, as the switchings are carried out strategically, so as to eliminate selected harmonics and obtain spectral performance.
Switching pattern (one phase)
Phase Voltage with and without filter
FFT (phase voltage)
FFT (filtered phase voltage)
The harmonic elimination performance is clearly visible in the FFT of the phase voltage. Besides, the FFT of the filtered phase voltage shows a THD value comparable to that of a linear amplifier.
Once again, SHE is not complicated as the switching angles have already been worked out and disclosed by many scholars and those used above were 'copied and pasted' from Appendix B of the following paper, also given below for completeness.
P. N. Enjeti, P. D. Ziogas, and J. F. Lindsay, “Programmed PWM techniques to eliminate harmonics: a critical evaluation,” IEEE Transactions on Industry Applications, vol. 26, pp. 302–316, Mar./Apr. 1990.
Switching angles in degrees:
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Assuming perfect timing and perfect edges and no supply droop...
But even if that's valid assumption, if the 59th harmonic is audible, you've not eliminated the problem, just moved it up the audio spectrum. 59 * 50Hz = 2950Hz, where the ear is most sensitive!
Why not push all the harmonics clean out to ultrasonic by pushing the fundamental up there? Simpler, much more robust.
But even if that's valid assumption, if the 59th harmonic is audible, you've not eliminated the problem, just moved it up the audio spectrum. 59 * 50Hz = 2950Hz, where the ear is most sensitive!
Why not push all the harmonics clean out to ultrasonic by pushing the fundamental up there? Simpler, much more robust.
This is a DIY forum, the idea is to let others know what you're doing so they can try things out themselves.
I'm not against good theoretical information, but most members are not theoreticians.
@ralphcooke: Had you tried sinusoidal drive on the Premotec motor? Many years ago, Connoisseur used a decent motor (very similar to the Philips/Premotec/McLennan) and had a screw under the shaft to ensure the the rotor had the same height in each half of the (double) motor. If you have that, then adjust for equal drive, a phase tweak can make the vibration disappear. Doing the same for the Thorens motor doesn't work because it has rectangular pole pieces, whereas the Premotec has triangular.
Beware that the motor on the BD1 was dreadful.
Beware that the motor on the BD1 was dreadful.
The sample time used in all the simulations in this thread is 1 microsecond, which is more than ten times higher than the switching times exhibited by many modern MOSFETs.
This is why you have bus capacitor reservoirs. Besides, it is also applicable to regular PWM.
It is worth remembering that there's a filter between the motor and the inverter which would simply clean out the energy around such frequencies. Besides, a higher switching frequency may not be all that good when it comes to radiated EMI.
Anyway my point in #22 was that a higher switching rate pushes the energy upward in frequency in all pulse modulation schemes (PAM, PWM, PDM, ∆Σ, PCM and so on) and there's no reason why SHE cannot do the same. However, only SHE can guarantee the absence of harmonics in the region between the fundamental and the 'switching frequency'. Modulations like PWM only result in higher THD when compared to the initial sinusoidal waveform.
If I were you, I wouldn't use a breakout board clocked from a 555 chip in this application. Those are more suitable for RC toys like drones etc.
Those BLDC motors should work very well.
People here seem to forget that maintaining 33 and 45 rpm requires very little power.
I turned down the power supply to 18v on LAB-500 to get 0.01 wrms. I also lowered the vcc on the unobtainable PWM chip.
People here seem to forget that maintaining 33 and 45 rpm requires very little power.
I turned down the power supply to 18v on LAB-500 to get 0.01 wrms. I also lowered the vcc on the unobtainable PWM chip.