Iacdrive|Automation Solution

Experience: Flyback

My first SMPS design was a multiple output flyback. This was in 1976, when there were no PWM controllers. So I used a 556 (1/2 osc -30 kHz, and 1/2 PWM generator) plus used a 3904 NPN where the VBE was the reference and also provided gain for the error amp function. I hap-hazardly wound the windings on a 25 mm torroid. It ranglike a tank circuit. I quickly abandoned the transformer and after a year, and many hours on the bench, I had a production-grad SMPS.
Since it went into a private aircraft weather reader system, I needed an exterier SMPS which was a buck converter. I used an LM105 linear regulator with positive feedback to make it oscillate (one of nationals ap notes). It worked, but I soon learned that the electrolytic capacitors lost all of their capacitance at -25 deg C. It later worked with military-grade capacitors.

I had small hills of dead MOSFETs and the directly attached controllers. When the first power MOSFETs emerged in 1979, I blew-up so many that I almost wrote them off. They had some real issues with D-S voltage overstress. They have come a long way since.

As far as very wide range flyback converter, please dig-up AN1327 on the ONSEMI website. This describes a control strategy (fixed off-time, variable on-time) and the transformer design.
The processor to that was a 3W flyback that drove 3 floating gate drive circuits and had an input range of 85 VAC to 576 VAC. It was for a 3 phase industrial motor drive. The toughest area was the transformer. To meet the isolation requirements of the UL, and IEC, it would have required a very large core, and bobbin plus a lot of tape. The PCB had the dimensions of 50 mm x 50 mm and 9 mm thick A magnetics designer named Jeff Brown from Cramerco.com is now my magnetics God. He designed me a custom core and bobbin that was 10 mm high on basically an EF15 sized core. The 3 piece bobbin met all of the spacing requirements without tape. The customer was expecting a 2 – 3 tier product offering for the different voltage ranges, but instead could offer only one. They were thrilled.

Can be done, watch your breakdown voltages, spacings and RMS currents. I found that around 17 -20 watts is about the practical limit for an EF40 core before the transformer RMS currents get too high.

Experience: Design

I tell customers that at least 50% of the design effort is the layout and routing by someone who knows what they are doing. Layer stackup is very critical for multiple layer designs. Yes, a solid design is required. But the perfect design goes down in flames with a bad layout. Rudy Severns said it best in one of his early books that you have to “think RF” when doing a layout. I have followed this philosophy for years with great success. Problems with a layout person who wants to run the auto route or doesn’t understand analog layout? No problem, you, as the design engineer, do not have to release the design until it is to your satisfaction.

I have had Schottky diodes fail because the PIV was exceeded due to circuit inductance causing just enough of a very high frequency ring (very hard to see on a scope) to exceed the PIV. Know your circuit Z’s, keep your traces short and fat.

Fixed a number of problems associated with capacitor RMS ratings on AC to DC front ends. Along with this is the peak inrush current for a bridge rectifier at turn on and, in some cases, during steady state. Unit can be turned on at the 90 deg phase angle into a capacitive load. This must be analyzed with assumptions for input resistance and/or a current inrush circuit must be added.

A satellite power supply had 70 deg phase margin on the bench, resistive load, but oscillated on the real load. Measured the loop using the AP200 on the load and the phase margin was zero. Test the power supply on the real load before going to production and then a random sampling during the life of the product.

I used MathCAD for designs until the average models came out for SMPS. Yes, the equations are nice to see and work with but they are just models none the less. I would rather have PsPice to the math while I pay attention to the models used and the overall design effort. Creating large closed form equations is wrought with pitfalls, trapdoors, and landmines. Plus, hundreds of pages of MathCAD, which I have done, is hard to sell to the customer during a design review (most attendees drift off after page 1). The PsPice schematics are more easily sold and then modified as needed with better understanding all around.

Power supply prototypes is the best way to learn it

I have been designing power supplies for over 15 years now. We do mostly off line custom designs ranging from 50 to 500W. Often used in demanding environments such as offshore and shipping.
I think we are the lucky ones who got the chance to learn designing power supplies using the simple topologies like a flyback or a forward converter. If we wanted to make something fancy we used a push-pull or a half bridge.

Nowadays, straight out of school you get to work on a resonant converter, working with variable frequency control. Frequencies are driven up above 250kHz to make it fit in a matchbox, still delivering 100W or more. PCB layouts get almost impossible to make if you also have to think about costs and manufacturability.
Now the digital controllers are coming into fashion. These software designers know very little about power electronics and think they can solve every problem with a few lines of code.

But I still think the best way to learn is to start at the basics and do some through testing on the prototypes you make. In my department we have a standard test program to check if the prototype functions according to the specifications (Design Verification Tests), but also if all parts are used within their specifications (Engineering Verification Tests). These tests are done at the limits of input voltage range and output power. And be aware that the limit of the output power is not just maximum load, but also overload, short circuit and zero load! Start-up and stability are tested at low temperature and high temperature.

With today’s controllers the datasheets seem to get ever more limited in information, and the support you get from the FAE’s is often very disappointing. Sometime ago I even had one in the lab who sat next to me for half a day to solve a mysterious blow up of a high side driver. At the end of the day he thanked me, saying he had learned a lot!
Not the result I was hoping for.

Grain Storage system

A Grain Storage system usually consists of the following elements

1. A means of measuring Grain coming in and out- Usually a truck scale or a bulk weighing system. In addition some applications require measuring grain between transfers to different bins and a bulk weighing system is usually used for that.

2. A means of transferring between different operations or storage location- Conveyors, screws, buckets, pneumatic, wheel house.

3. Dust Collection

4. And Equipment for the operations that will be performed, drying, cleaning, screening, grading, sampling, roasting, steaming, packaging etc.

A system I have just completed was 24 containers + 3 buildings for storage with 76 conveyors, 3 drop-off and 3 loading points. Connection to ERP system and weigh scales to weigh trucks and send them to the correct bay. Local HMI on each bay ensured correct lorry goes to correct bay. Main conveyor runs are automatically selected. Manual option to run all conveyors to move grain around.

System used Ethernet infrastructure with hubs mounted strategically around tank farm. Also implemented soft starter with Ethernet connectivity, thus allowing easy monitoring of current consumption + for maintenance.

The future-proof design will allow customer to install level and humidity measurement in the future using the Analogue IO connected on Ethernet.

Caution is the key to success in power converters

I work across the scale of power electronics in voltages and currents. From switchers of 1W for powering ICs to 3kW telco power supplies up to multi-megawatt power converters for reactive power control in AC transmission networks and into power converters for high voltage transmission.

There is a difference in how you can work on these different scale converters. This difference is down to how much the prototype you are destroying costs, how long it takes to rebuild it and how easily it will kill you. When you spend more than 2 million on the prototype parts then you do not ever blow it up. If the high voltage on your converter is 15kV or more then there is no way to probe it with an oscilloscope directly and no possibility to be anywhere near that voltage without being hurt. So the level of care at these bigger power levels is higher and the consequence of a mistake is so high that the process needs to be much more detailed and controlled mostly for safety’s sake. We find that our big power converter processes really help when working on smaller converters. The processes include sign offs for safety, designed and prescribed safety and earthing systems for each converter, no scope probes put on and off live parts and working in pairs at all times with agreed planned actions. Pair working is one thing that may save you in the event of an electric shock. These processes seem very slow and cumbersome to engineers who work on low voltage (<1000V) but they are very useful even at low voltage.

Having said all that, experienced cautious engineers prevent converter blow ups. Add just a little bit of process and success can go up significantly. I think that an analysis of Dr Ridley’s failure list will point to actions that will improve success.

As my boss at one of those really large converter companies used to say “Stamp out converter fires”.

High voltage power delivery

You already know from your engineering that higher voltages results to less operational losses for the same amount of power delivered. The bulk capacity of 3000MW has a great influence on the investment costs obviously, that determines the voltage level and the required number of parallel circuit. The need for higher voltage DC levels has become more feasible for bulk power projects (such as this one) especially when the transmission line is more than 1000 km long. So on the economics, investment for 800kV DC systems have been much lower since the 90’s. Aside from reduction of overall project costs, HVDC transmission lines at higher voltage levels require lesser right-of-way. Since you will be also requiring less towers as will see below, then you will also reduce the duration of the project (at least on the line).

Why DC not AC? From a technical point of view, there are no special obstacles against higher DC voltages. Maintaining stable transmission could be difficult over long AC transmission lines. The thermal loading capability is usually not decisive for long AC transmission lines due to limitations in the reactive power consumption. The power transmission capacity of HVDC lines is mainly limited by the maximum allowable conductor temperature in normal operation. However, the converter station cost is expensive and will offset the gain in reduced cost of the transmission line. Thus a short line is cheaper with ac transmission, while a longer line is cheaper with dc.
One criterion to be considered is the insulation performance which is determined by the overvoltage levels, the air clearances, the environmental conditions and the selection of insulators. The requirements on the insulation performance affect mainly the investment costs for the towers.

For the line insulation, air clearance requirements are more critical with EHVAC due to the nonlinear behavior of the switching overvoltage withstand. The air clearance requirement is a very important factor for the mechanical design of the tower. The mechanical load on the tower is considerably lower with HVDC due to less number of sub-conductors required to fulfill the corona noise limits. Corona rings will be always significantly smaller for DC than for AC due to the lack of capacitive voltage grading of DC insulators.

With EHVAC, the switching overvoltage level is the decisive parameter. Typical required air clearances at different system voltages for a range of switching overvoltage levels between 1.8 and 2.6 p.u. of the phase-to-ground peak voltage. With HVDC, the switching overvoltages are lower, in the range 1.6 to 1.8 p.u., and the air clearance is often determined by the required lightning performance of the line.

How generator designers determine the power factor?

The generator designers will have to determine the winding cross section area and specific current/mm² to satisfy the required current, and they will have to determine the required total flux and flux variation per unit of time per winding to satisfy the voltage requirement. Then they will have to determine how the primary flux source will be generated (excitation), and how any required mechanical power can be transmitted into the electro-mechanical system, with the appropriate speed for the required frequency.
In all the above, we can have parallel paths of current, as well as of flux, in all sorts of combinations.

1) All ordinary AC power depends on electrical induction, which basically is flux variations through coils of wire. (In the stator windings).
2) Generator rotor current (also called excitation) is not directly related to Power Factor, but to the no-load voltage generated.
3) The reason for operating near unity Power Factor is rather that it gives the most power per ton of materials used in the generating system, and at the same time minimises the transmission losses.
4) Most Generating companies do charge larger users for MVAr, and for the private user, it is included in the tariff, based on some assumed average PF less than unity.
5) In some situations, synchronous generators has been used simply as VAr compensators, with zero power factor. They are much simpler to control than static VAr compensators, can be varied continuously, and do not generate harmonics. Unfortunately they have higher maintenance cost.
6) When the torque from the prime mover exceeds a certain limit, it can cause pole slip. The limit when that happens depends on the available flux (from excitation current), and stator current (from/to the connected load).

Induction machines testing

Case: We got by testing 3 different machines under no-load condition.
The 50 HP and 3 HP are the ones which behave abnormally when we apply 10% overvoltage. The third machine (7.5 HP) is a machine that reacts normally under the same condition.
What we mean by abnormal behavior is the input power of the machine that will increase dramatically under only 10% overvoltage which is not the case with most of the induction machines. This can be seen by the numbers given below.

50 HP, 575V
Under 10% overvoltage:
Friction & Windage Losses increase 0.2%
Core loss increases 102%
Stator Copper Loss increases 107%

3 HP, 208V
Under 10% overvoltage:
Friction & Windage Losses increase 8%
Core loss increases 34%
Stator Copper Loss increases 63%

7.5 HP, 460V
Under 10% overvoltage:
Friction & Windage Losses decrease 1%
Core loss increases 22%
Stator Copper Loss increases 31%

Till now, we couldn’t diagnose the exact reason that pushes those two machines to behave in such way.
Answer: A few other things I have not seen (yet) include the following:
1) Are the measurements of voltage and current being made by “true RMS” devices or not?
2) Actual measurements for both current and voltage should be taken simultaneously (with a “true RMS” device) for all phases.
3) Measurements of voltage and current should be taken at the motor terminals, not at the drive output.
4) Measurement of output waveform frequency (for each phase), and actual rotational speed of the motor shaft.

These should all be done at each point on the curve.

The reason for looking at the phase relationships of voltage and current is to ensure the incoming power is balanced. Even a small voltage imbalance (say, 3 percent) may result in a significant current imbalance (often 10 percent or more). This unbalanced supply will lead to increased (or at least unexpected) losses, even at relatively light loads. Also – the unbalance is more obvious at lightly loaded conditions.

As noted above, friction and windage losses are speed dependent: the “approximate” relationship is against square of speed.

Things to note about how the machine should perform under normal circumstances:
1. The flux densities in the magnetic circuit are going to increase proportionally with the voltage. This means +10% volts means +10% flux. However, the magnetizing current requirement varies more like the square of the voltage (+10% volt >> +18-20% mag amps).
2. Stator core loss is proportional to the square of the voltage (+10% V >> +20-25% kW).
3. Stator copper loss is proportional to the square of the current (+10% V >> +40-50% kW).
4. Rotor copper loss is independent of voltage change (+10% V >> +0 kW).
5. Assuming speed remains constant, friction and windage are unaffected (+10% V >> +0 kW). Note that with a change of 10% volts, it is highly likely that the speed WILL actually change!
6. Stator eddy loss is proportional to square of voltage (+10% V >> +20-25% kW). Note that stator eddy loss is often included as part of the “stray” calculation under IEEE 112. The other portions of the “stray” value are relatively independent of voltage.

Looking at your test results it would appear that the 50 HP machine is:
a) very highly saturated
b) has damaged/shorted laminations
c) has a different grade of electrical steel (compared to the other ratings)
d) has damaged stator windings (possibly from operation on the drive, particularly if it has a very high dv/dt and/or high common-mode voltage characteristic)
e) a combination of any/all of the above.

One last question – are all the machines rated for the same operating speed (measured in RPM

Active power losses in electrical motor

Equivalent active power losses during electrical motor’s testing in no-load conditions contain next losses:
1. active power losses in the copper of stator’s winding which are in direct relation with square of no-load current value: Pcus=3*Rs*I0s*I0s,

2. active power losses in ferromagnetic core which are in direct relation with frequency and degree of magnetic induction (which depends of voltage):
a) active power losses caused by eddy currents: Pec=kec*f*(B)x
b) active power losses caused by hysteresis: Ph=(kh*d*d*f*f*B*B)/ρ

3. mechanical power losses which are in direct relation with square of angular speed value: Pmech=Kmech*ωmech*ωmech,

Comment:
First, as you can see, active power losses in ferromagnetic core of electrical motor depend of voltage value and frequency, so by increasing voltage value you will get higher active power losses in ferromagnetic core of electrical motor.

Second, you can’t compare two electrical motors with different rated voltage and different rated power because active power losses in the ferromagnetic core, as I have already said above, depend of voltage value and frequency while active power losses in the copper of stator’s windings depend of square of no-load current value which is different for electrical motors with different rated power.

Third, when you want to compare active power losses in no-load conditions of two electrical motors with same rated voltage and rated power, you need to check design of both electrical motors because it is possible that one of them has different kind of winding, because, maybe in the past, one of them was damaged, so its windings had to be changed, what could be the reason for different electrical design and that has a consequence different no-load current value.

Friendly system without technicians diagnose

How to make our systems so friendly that they do not need technicians to help diagnose problems? Most of the more obvious answers have been well documented here but to emphasize these points it is normally the case that diagnostics and alarms can dominate the amount of code constructed for an application. That is the amount of code required to fully diagnose an application may be as much if not more than the code required to make the application work in the first place!

I have seen HMIs with diagnostic screens showing an animated version of the Cause & Effects code that allows users to see where the trip condition is. I have also seen screens depicting prestart checks, Operator guides, etc all animated to help the user. Allen-Bradley even have a program viewer that can be embedded on an HMI screen but you would probably need a technician to understand the code.

From a control system problem perspective it is inevitable that you would need to use the vendor based diagnostics to troubleshoot your system. Alarms may indicate that there is a system related problem but it is unlikely that you could build a true diagnostics application in your code to indicate the full spectrum of problems you may encounter. For example if the CPU dies or the memory fails there is nothing left to tell you what the problem is 🙂

From an application/process problem perspective if you have to resort to a technician to go into the code to determine your problem then your alarm and diagnostics code is not comprehensive enough! Remember your alarming guidelines an Operator must be able to deal with an abnormal situation within a given time frame, if the Operator has to rely on a Technician to wade through code then you really do need to perform an alarm review and build a better diagnostics system.