Demagnetisation in a sensorless brushless DC drive

This article explains the ‘demagnetisation’ issue that challenges sensorless brushless DC drives that depend on Zero Crossing (ZC) detection to synchronise the next commutation phase.

Fig 1.

Above is a scope capture the ‘A’ terminals of a Multistar 4220-650Kv with 1045SF propeller running at about 50% throttle on 3S (about 4000rpm). The motor is quite lightly loaded for the purpose of illustration. The motor drive is low side complementary PWM modulated, and drive is advanced by 15° and the FETs are all N-FETs.

We will focus on the detail starting at about 5 divisions on the time axis (2500µs). The explanation will detail behaviour of the ‘A’ section of the drive, but the same thing happens on the B and C sections which follow each 120° electrical later respectively.

Fig 2.

Above is a capture of the A terminal focussing on the end of one of six commutation phases. The A terminal is the ‘low side’ terminal on the left hand side (and C is the high) and the ‘sense terminal’ on the right hand side (C is the high and B is the low). Continue reading Demagnetisation in a sensorless brushless DC drive

Last update: 21st February, 2017, 7:28 PM

Surecom SW-102 VSWR meter review

I recently purchased a Surecom SW-102 VSWR meter. It looked a little like a supercharged RedDot copy.

sw102-02

Above the Surecom SW-102 VSWR meter with backlight and photographed under normal interior lighting. The display lacks contrast, and overall is difficult to read due to size of text, fonts used, and lack of contrast. (The pic is taken with a screen protector installed, but the evaluation is based on the bare meter with original protective film removed as it degraded readability.) Continue reading Surecom SW-102 VSWR meter review

Last update: 18th May, 2018, 12:25 PM

Hobbyking Multistar 4220-650Kv

This is a report on a series of tests performed on a Hobbyking Multistar 4220-650Kv sensorless brushless DC motor.

Above, a top view of the bare motor. It is a 12S16P disc form or pancake form motor, a style that is very popular though inclined to sync problems.

Above, the underneath of the bare motor.

The prop adapter is not shown, it had almost 1mm runout and would need to be replaced to actually fly the motor as propeller induced vibration would be unacceptable.

The motor would appear to be similar to the Hengli 42 20 650Kv, in fact so similar that it would seem possible, even likely that Hengli is the OEM.

Induced voltage waveform

The motor was driven at 970rpm and the voltage between two motor wires observed on a scope.

The line-line voltage is the sum of two phase voltages, one of which is a time delayed copy of the other and whilst the resultant is sinusoidal when the components are purely sinusoidal, the transformation is less obvious for complex waves such as shown above.

The phase voltage is of interest as it drives the sense process that provides motor timing crucial to commutation.

Direct measurement of the phase voltage using a star of 12kΩ resistors to establish a neutral reference yields the waveform above.

The induced voltage waveform somewhat the result of the 12N16P configuration. Period of the waveform is 7.7ms, equivalent to 60/0.0077=7790erpm indicating 7790/970=8 pole pairs or 16 poles.
Continue reading Hobbyking Multistar 4220-650Kv

Last update: 13th February, 2017, 12:47 PM

Does common mode current flow inside coax?

The term “common mode current” applied to coaxial transmission lines is bandied about with abandon these days in online fora, awareness of its existence has increased if not understanding.

A simplistic analysis is that in TEM mode, ONLY differential current is supported inside a coaxial line, ie that at any point the current on the outer surface of the inner conductor is exactly equal to a current in the opposite direction on the inner surface of the outer conductor.

But, lets look at the wider context of the meaning of common mode current when a uniform coaxial line is connected to an antenna system. Whilst an antenna might have an obvious two terminal connection to the feed line, in the presence of ground, the current in those two terminals are not necessarily equal and opposite. Continue reading Does common mode current flow inside coax?

Last update: 12th June, 2017, 1:24 AM

Exploiting your antenna analyser #26

Find coax cable velocity factor using a very basic analyser

A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that lacks facility to backout cable sections or measure SOL calibration (as discussed in other articles in this series). The older models and newer budget models often fall into this category.

The manuals for such instruments often explain how to measure coaxial cable velocity factor, and the method assumes there is zero offset at the measurement terminals (whether they be the built-in terminals or some fixture / adapters). In fact even the connectors are a source of error, especially UHF series connectors.

It is the failure to read exactly Z=0+j0Ω with a S/C applied to the measurement terminals that adversely impacts efforts to measure resonant frequency of a test line section.

The method described here approximately nulls out offsets in the instrument, measurement fixture, and even in the connectors used and for that reason may sometimes be of use with more sophisticated analysers.
Continue reading Exploiting your antenna analyser #26

Last update: 30th January, 2017, 7:41 AM

PD7MAA’s BN43-202 matching transformer for an EFHW – full measurement set

I have written some recent articles about or relevant to PD7MAA’s BN43-202 EFHW matching transformer. At about the same time a discussion started on and through that discussion, one ‘online extra expert’ stated that my analysis was bogus (dictionary meaning: not genuine, faked, a misrepresentation).

This article presents detail that was not included in the earlier articles as it distracts from the issue for most readers. Continue reading PD7MAA’s BN43-202 matching transformer for an EFHW – full measurement set

Last update: 28th January, 2017, 8:30 AM

Thoughts on binocular ferrite core inductors at radio frequencies

Binocular ferrite cores are widely used, but not so widely understood.

Understanding inductors is an important first step to understanding transformers are they are coupled inductors.

The usual use of them is to make a winding of several turns around the central limb. One turn is a pass through both sides of the core around the central limb. Figures given in datasheets for Al or impedance rely upon that meaning of one turn.

A common assumption is that L=Al*n^2.

Note that published Al values are obtained by measurement typically at 10kHz and are not directly applicable at radio frequencies for core materials where the permeability µ is significantly different to µi (most ferrites). Notwithstanding this fact, most inductance calculators assume µ is not frequency dependent.

Let us measure a one turn winding on a practical binocular core for reference

Above is a measurement of R,X of a BN43-202 core with a one turn winding at 10MHz. X is  59.57Ω implying inductance of 0.95µH (assuming a simple two component model which does not capture self resonance effects). Datasheets for this core specify Al as 2200nH for one turn, yet we measure 950nH at 10MHz… proof of problems in simple application of Al.

Of course it is possible to make an inductor by passing a conductor once though one side of the binocular, a half turn if you like, but don’t let that label imply the impedance relative to a one turn winding.

Above is a measurement of a BN43-202 core with a ‘half turn’ winding.

If inductance followed the formula L=Al*n^2 and this was truly a half turn winding, we would expect the inductive reactance X ( 37.16Ω) to be one quarter or 25% of that of the single turn inductor (59.57Ω). Clearly it is not, it is 62%, the notion of a half turn or the formula or both have failed badly in this case.

Well on the back of that failure, lets try 1.5t.

Would we be brave or foolish to predict inductance will be 1.5^2 times that for one turn?

Above is a measurement of a BN43-202 core with a ‘one and a half turn’ winding.

If inductance followed the formula L=Al*n^2 and this was truly a half turn winding, we would expect the inductive reactance X ( 155.2Ω) to be 1.5^2 or 2.25 times that of the single turn inductor (59.57Ω). Clearly it is not, it is 2.60 times, the notion of a half turn or the formula or both have failed badly in this case.

Now let us look at Q, the ratio of X/R. The Q of the half turn inductor is 1.051, the one turn inductor is 1.022, and the one and a half turn inductor is 1.016. The quite small decrease in Q may be entirely due to the lower self resonant frequency as more turns are added and may not indicate a significant increase in core loss because of ‘half turn effects’ as sometimes claimed.

The error in conventional n^2 estimates of odd half turns becomes less significant with higher turns.

Conclusions

The traditional formula L=Al*n^2 does not apply to ferrite binocular cores at radio frequencies for odd half turns, and does not account for variation of permeability with frequency or influence of self resonance.

Understanding inductors is the first step to understanding transformers are they are coupled inductors.

Last update: 28th January, 2017, 8:54 AM

Common failings of EFHW matching transformers

I have written many reviews of published EFHW matching transformers, and in most cases the reviews have reported estimated or measured losses that are appalling and not disclosed by the ‘designers’.

Why is it so?

I am asked, why is it so?

Up front, I do not know the answer definitively, but let me offer some thoughts based on the designer’s own articles and discussions by ‘online experts’.

Apparent reasons include:

  1. lack of understanding of ferrite and powdered iron core material behaviour;
  2. lack of understanding of coupled coils, and mutual inductance;
  3. use of inductor design tools that are inadequate at radio frequencies;
  4. lack of competency in basic linear circuit theory analysis for AC circuits;
  5. failure to make meaningful measurements of the built article;
  6. focus on input VSWR as a single metric indicating goodness;
  7. reliance on QSOs for evidence of performance;
  8. an attitude that antenna system radiation efficiency doesn’t matter, particularly for QRP (if the term antenna system radiation efficiency is even understood as a quantitative metric).

Continue reading Common failings of EFHW matching transformers

Last update: 26th January, 2017, 6:04 PM

PD7MAA’s BN43-202 matching transformer for an EFHW – measurement of a prototype

At PD7MAA’s BN43-202 matching transformer for an EFHW I gave an estimate of the core loss in PD7MAA’s transformer.

This article reports measurement of a prototype built to his design.

 

Above is PD7MAA’s graphic for his transformer. It is a little confusing as an 11t wind will start and finish with ends as the blue wind, so the red winding must have and odd number of half turns which suggests the windings are actually 1t and 5.5t (pity he did not show a picture of the real transformer).

PD7MAA gives some measurements for his transformer with a 3300Ω load, but he does not give loss measurements. This experiment is to replicate his configuration, measure the loss and compare it to the estimate given at PD7MAA’s BN43-202 matching transformer for an EFHW.

The prototype uses 1t primary and 5.5t secondary. The secondary load is a 3300Ω resistor in series with the VNA 50Ω input port.

Above is a screen shot of a sweep from 6 to 8MHz. The key data is that shown for the marker at 7.1MHz. Continue reading PD7MAA’s BN43-202 matching transformer for an EFHW – measurement of a prototype

Last update: 9th May, 2017, 11:17 AM