A Demagnetisation Risk Index for a sensorless brushless DC drive

The article Demagnetisation in a sensorless brushless DC drive gave a broad overview of demagnetisation in a sensorless brushless DC drives that depend on Zero Crossing (ZC) detection to synchronise the next commutation phase.

There is no widely recognised method of prediction whether a drive is at risk of excessive demagnetisation time, not even in the steady-state wide open throttle (WOT) scenario.

This article proposes a statistic that might be used as a risk indicator, it is dimensionless and approximately proportional to the ratio of energy stored in the self inductance of a coil to the energy consumed by the motor in the time available for demagnetisation.

The concept is that most of the energy stored in the self inductance of a coil is returned to the motor producing output power and internal losses in much the same proportions as the primary input power.

The calculation of each of these quantities is quite complex, but broadly, making some approximations and ratioing the quantities gives a dimensionless statistic that might well be a good indicator of potential problems.

The method relates to energy, a statistic is proportional to the ratio of energy stored in the winding field due to current to the energy delivered in the available interval for demagnetisation.

Energy stored in the magnetic field due to coil current is equal to 0.5LI^2, and though L is the phase inductance, we can use the line to line inductance and say that energy stored in the magnetic field due to coil current is proportional to LI^2.

This energy is returned to the motor (shaft torque and losses), and this must be achieved in less than MotorPeriod/12. The average energy consumed by the motor in time t =Pin*MotorPeriod/12 =Pin*60/rpm*2/P/12 =Vbat*I*60/rpm*2/Poles/12, we can say that the energy consumed by the motor in the available time is approximately proportional to V*I/rpm/P where P is the number of poles, V is the loaded battery voltage, I is the current under load and rpm is the loaded rpm.

Ratioing these quantities gives the statistic LIrpmP/V, and large values indicate high risk of demagnetisation overrunning the available time, failure of ZC detection and sync loss.

A number of drives were tested on different voltages and with different propellers, and the LIrpmP/V statistic calculated. Experience was that small drives that appeared to have reliable sync even under rapid acceleration had LIrpmP/V well less than 5, and drives that would not make WOT before sync loss had LIrpmP/V well over 5.

A series of SPICE models were run to model the behaviour of some drives and results were consistent with tests on those drives.

In drives that could not make WOT, examination of the statistic gives a hint about possible measured to configure a workable drive. For a given motor, L and P are fixed. Reducing I  by decreasing the propeller load helps but typically reduced output power results in higher rpm for the same battery voltage somewhat offseting the benefit of reduced I. However if I is reduced sufficiently, even at the higher rpm, reliable sync may be obtained.

For example, a Hobbyking 4220-650Kv 16 pole motor with 1245SF propeller on 14V drew 15A at less than WOT and lost sync. The calculated index was 9.0, and would have been higher had sync not been lost.

Changing to 1145SF and 1045SF at 14V did not solve the problem, but a 0945SF allowed 8190rpm at WOT on 24V, I=6.1A and the calculated index was 3.2. Motor power at 146W is way less than you might expect from the specifications.

All of these tests were steady state, and drives that just make WOT with reliable sync may come undone under rapid acceleration, though the rpm term might be lower, the I term might be much greater.

Issues with usage

It seems that as time passes, fewer and fewer motors are advertised with inductance specified, so it must be measured by the user / community.

Whilst inexpensive inductance meters abound online, I have not verified they are up to the task. All measurements of L for this study were done with a quality LCR meter at 10kHz.

ic9350 updated to support 3040

Announcement: ic9350 is updated to v1.03 to add support for the Codan 3040 automatic antenna.

The ic9350 is protocol converter to permit use a Codan 9350 or 3040 automatic antenna with an Icom radio. Most Icom radios support the Icom AH-4 ATU, so the approach is to design a protocol converter that converts the protocol used by the 9350 and 3040 to the AH4 protocol to allow full integration with the Icom radios that support the AH-4.

The ic9350 Protocol Converter uses the interface uses the radio’s ATU interface in the way that Icom intended, and will not disrupt radio operation . Continue reading ic9350 updated to support 3040

Modifications for Jasic 200A TIG welder and pedal

In about 2009 I purchased a Uni-mig Jasic 200A TIG welder (though these are sold under many brands).

The welder came with unusual 2 and 5 pin connectors for the torch trigger switch and an optional pedal. The optional pedal was quoted at around $500, probably partly as the seller had locked the market up with the unusual connectors. (I note that the XS12K2P etc connectors are now available on Aliexpress.)

There is no standardisation of these connectors, but the pedal internals are pretty common. A quite common configuration is a 2 pin Foster (microphone) connector for the trigger switch and 3 pin Foster (microphone) connector for the current pot.

A further usability issue is that the pedal varies current from 5 to 200A, it is not possible to set the maximum current when the pedal is fully depressed. Continue reading Modifications for Jasic 200A TIG welder and pedal

NH7RO 7-foot diameter QRO STL for 40M

NH7RO describes his loop project at Building a 7-foot diameter QRO STL for 40M in my HOA backyard.

The loop appears to be made from 7/8″ copper tube, and is 7′ in diameter. He estimates its efficiency to be 66% and initially reports I’ve got it less than 4 feet above ground yet it tunes flat to 1.1>1 with roughly 10kHz bandwidth.. Curiously, 10kHz is the result calculated by AA5TB’s spreadsheet, though I have written elsewhere it is deeply flawed (Small transmitting loop calculators – a comparison).

Let us assume that these figures are correctly reported, and that the unqualified bandwidth means the half power bandwidth of a matched loop.

We can estimate the efficiency of a Small Transmitting Loop (STL) in free space.

Before getting excited about the results, let us question the validity of the model. There are three important factors that question the validity of the model:

  • bandwidth;
  • size of the loop; and
  • proximity to ground.

Continue reading NH7RO 7-foot diameter QRO STL for 40M

LNR Precision small transmitting loop

LNR Precision have announced a small transmitting loop for amateur radio.

On their site, they state:

No images or text are to be used without permission of LNR Precision.

(LNR Precision  2016)

This is a clear attempt to suppress analysis and discussion of the technical aspects of the antenna which is based on the work of others. They are not alone in attempting this, and as a buyer you might ask what have they to hide.

Though entitled to quote under fair use terms of copyright law, I will refrain from creating the basis for argument with a company that sets out to constrain discussion of their product… you can find the referenced graphs and tables in the references below. Continue reading LNR Precision small transmitting loop

Check / calibrate frequency accuracy of IC-7300

The IC-7300 is a transceiver where all heterodyning oscillators are derived from a single master oscillator.

This type of radio makes for very easy checking and calibration of frequency accuracy.

The video below demonstrates the technique.

The video used a local GPS disciplined source at 50.1MHz. The frequency was chosen to provide the greater resolution in setting the oscillator, though setting it to within 1 part in 50,000,000 or 0.02ppm is better than the stability of the oscillator (specification is 0.5ppm or 5Hz at 10MHz).

Any accurate known reference can be used, it could be WWV or the like, or even a MW broadcast station, though an accurate signal at 10MHz or higher is better.

The technique can be applied to the much older IC-7000, and many transceivers released since then, of various brands. The important thing is that ALL oscillators are derived from a single master oscillator.

 

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

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

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