Iacdrive|Automation Solution

Power supply prototype failures

I remember my very first power supply. They threw me in the deep end in 1981 building a multi-output 1 kW power supply. I was fresh from college, thought i knew everything, and consumed publications voraciously to learn more. Exciting times.

But nothing prepared me for the hardware trials and tribulations. We built things and they blew up. Literally. We would consume FETs and controllers at an alarming rate. The rep from Unitrode would come and visit and roll his eyes when we told him we needed another dozen controllers since yesterday.

The reasons for failure were all over the map . EMI, heat, layout issues, design issues, bad components (we had some notorious early GE parts – they exited the market shortly afterwards.)
Some of the issues took a few days to fix, some of them took weeks. We had two years to get the product ready, which was faster than the computer guys were doing their part, so it was OK.

90% of the failure issues weren’t talked about in any paper, and to this day, most of them still aren’t.

So, fast forward to today, 32 years later. I still like to build hardware – you can’t teach what you don’t practise regularly, so I keep at it.

With all the benefit of 3 decades of knowledge I STILL blow things up. Everything progresses along fine, then i touch a sensitive circuit node, or miss some critical design point and off it goes. I’m faster now at finding the mistakes but I still find there are new ones to be made. And when it blows up with 400 V applied, it’s a mess and a few hours to rebuild. Or you have to start over sometimes, if the PCB traces are vaporized.

So my first prototype, while on a PC board, always includes the controller in a socket because I know I will need that. Magnetics too, when possible, I know I’ll revise them time and again to tweak performance. PC boards will be a minimum of two passes, probably three.

Is it worth to built-in batteries in electric cars

Energy storage is the issue. Can we make batteries or super caps or some other energy storage technique that will allow an electric car to have a range of 300-500 miles. Motors and drives are already very efficient, so there is not much to be gained by improving their efficiency. As far as converting the entire fleet of cars to electric, I don’t expect to see this happen any time soon. The USA has more oil than all the rest of the world put together. We probably have enough to last 1000 years. Gasoline and diesel engines work very well for automobiles and trucks and locomotives. The USA also has a huge supply of coal, which is a lot cheaper than oil. Electricity is cheaper than gasoline for two reasons: Coal is much cheaper than oil, and the coal fired power plants have an efficiency of about 50%. Gasoline engines in cars have a thermal efficiency of about 17%. Diesel locomotives have an efficiency of 50%+.

I don’t believe the interchangeable battery pack idea is workable. Who is going to own the battery packs and build the charging stations? And what happens if you get to a charging station with a nearly dead battery and there is no charged battery available?

Who is going to build the charging stations; the most logical answer is the refueling station owners as an added service. The more important question is about ownership of the batteries. If as an standard, all batteries are of same size, shape, connectors as well as Amp-Hour (or kWh) rating and a finite life time, lets say 1000 recharging. The standard batteries may have an embedded recharge counter. The electric car owners should pay the service charges plus cost of the kWh energy plus 1/1000 of the battery cost. By that, you pay for the cost of new batteries once you buy or convert to an electric car and then you pay the depreciation cost. This means you always own a new battery. The best probable owner of the batteries should be the battery suppliers or a group or union of them (like health insurance union). The charging stations collecting the depreciation cost should pass it on to the battery suppliers union. Every time a charging station get a dead battery or having its recharge counter full, they will return it to the union and get it replaced with a new one. So, as an owner of electric car you don’t need to worry about how old or new replacement battery you are getting from the charging station. You will always get a fully charged battery in exchange. The charging stations get their energy cost plus their service charges and the battery suppliers get the price of their new battery supplies.

Buddies, these are just some wild ideas and I am sure someone will come up with a better and more workable idea. And we will see most of the cars on our roads without any carbon emission.

Heavily discontinuous mode flyback design

With a heavily discontinuous mode flyback design, the transformer’s ac portion of current can be larger than the dc portion. When a high perm material is used for the transformer core, the required gap can be quite large in order to reach the low composite permeability required while the core size will likely be driven by winding and core loss considerations rather than just simply avoiding saturation. Normally the gap is put in the center leg only (with E type topology cores) in order to minimize the generation of stray fields. However, in designs such as yours (high ac with a high perm core) the needed core gap can lead to a relatively large fringing zone through which foil or solid wire may not pass without incurring excessive, unacceptable loss. Possible solutions are to use Litz wire windings or inert spacers (e.g., tape) around the center leg in order to keep the windings far enough away from the gap (the rule of thumb is 3 to 5 gap lengths, which can eat up a lot of the window area).

It is mainly for these reasons that placing half the gap in an E type core’s outer legs might be worth the trouble of dealing with the magnetic potential between the core halves (and you have seen first hand what trouble an ill designed shield band can be).

To avoid eddy current losses, the shield band should be spaced well away from the outer leg gap, probably 5 gap lengths or more. Also to be a really effective magnetic shield, it should be 3 to 5 gap lengths thick.

Bear in mind that with a high frequency, high ac current inductor design proximity effects in the winding may become very significant. This is why many of these type of inductors have single layer windings or winding wound with Litz wire (foil is the worst winding type here). One advantage of an equally gapped E type core design is that the proximity effect on the windings is significantly less because there are two gaps in series (a quasi distributed gapped core design). Not only layer-to-layer, but turn-to-turn proximity effects can sometimes be problematic in an ac inductor (or flyback) design. Just as with the gap, these are reduced by adding appropriate spacing, for example making the winding coil loose or winding it bifilar with a non-conductive filament.

Remote diagnostic

Remote diagnostic is a must now a days. All CNC machines must be able to undergo remote access to undergo diagnostic and it must be two way. The problem mostly with remote diagnostic is it has to be two way and you have to have a qualified technician or an operator who is well verse with machine operations and its features, always on your machine he must be trained on how to be able to recover from lost of communication and the most important is to be able to engage E-stop when needed. The remote operator is a trained technician as well and knows a procedures and protocols that will help prevent accidents that can harm both man and machine. Mostly remote access is good for updates and upgrades, training and assistance needed. We offer the first year as free to make sure we can get the customer up an about during the learning curve on how to familiarize with control functions. We also need a land line or cell phone to be able to have a voice interchange. We use Webex for remote and another pc laptop or desktop as a dedicated bridge with controls that run with older versions of Windows such as windows XP. The dedicated PC is primarily secured as level four security compliance and must be turned off when remote diagnostic is needed. You can add assign a dedicated that is level four compliant as part of the control you will have two computers one on standby for remote diagnostic primiraly use for remote diagnostic, another for CNC function.

In regards to data collection new CNC’s are monitoring activities such as error messages that are categorized in different areas. This can be with the communication between PLC’s, CNC and station cards, lost of communication or timing problems errors common with the system, CNC errors due to plc warnings and prompts, operator prompts to name a few. Mostly this is error messages have a day and time stamp so it can easily be cyphered if the condition of errors are intermittent or consistent. We can all set up the option of recording what nc programs are run and how long it took to complete a job. It can also be set to count the number of hours the tools is used. Since this is a text format you design a spread sheet that can put them in named cells. The extent of data is a chosen through the logging option and in our case is stored in the Logging directory. It helps with monitoring intermitent problems and monitor if this is a NC program error, System error, human error, machine problem etc. It is a must now a days for ease of data gathering for management and troubleshooting.

flyback & boost applications

For flyback & boost applications, powder cores such as Kool-mu, Xmu, etc… are usually best performing and lowest cost. Even these may need to be gapped and if CCM operation is required, a “stepped-gap” is preferred to allow a large load compliance. Center stepped gaps reduce the fringe flux greatly as there is never a complete gap, only localized saturation. This permits the inductor’s value to “swing” more and accommodate the required operation.
With only the center leg with a gap, the outer copper band can be applied without significant loss.

To explore further, dissimilar core materials can be used in parallel, ferrite & powdered types, such that different materials provide function at different operating points within the same construction. Some decades ago, we had some high power projects that utilized fixed magnets within a ferrite’s gap to provide a flux bias offset for a forward topology.

Abe Pressman wasn’t big on exploring magnetic losses, however he operated at lower frequencies than are typical today. MPPs are great with large DC bias, but suffer high loss if AC swing is large and fast. Toroids also have the least efficient winding window, however, they are best to mitigate emi.

How/where do we as engineers need to change?

System Design – A well designed system should provide clear and concise system status indications. Back in the 70’s (yes, I am that old), Alarm and indicator panels provided this information in the control room. Device level indicators further guided the technician to solving the problem. Today, these functions are implemented in a control room and machine HMI interface. Through the use of input sensor and output actuator feedback, correct system operation can be verified on every scan.

Program (software) Design – It has been estimated that a well written program is 40% algorithm and 60% error checking and parameter verification. “Ladder” in not an issue. Process and machine control systems today are programmed in ladder, structured text, function block, etc. The control program is typically considered intellectual property (IP) and in many cases “hidden” from view. This makes digging through the code impractical.

How/where do we as engineers need to change? – The industry as a whole needs to enforce better system design and performance. This initiative will come from the clients, and implemented by the developers. The cost/benefit trade-off will always be present. Developers trying to improve their margins (reduce cost – raise price) and customers raising functionality and willing to pay less. “We as engineers” are caught in the middle, trying to find better ways to achieve the seemingly impossible.

Sensorless control

I am curious about the definition of “sensorless control”. When you talk about sensorless control, are you in fact meaning a lack of physical position sensor such as e.g. a magnet plus vane plus hall effect? i.e. not having a unit whose sole objective is position detection.
Is the sensorless control based around alternative methods of measurement or detection to predict position using components that have to exist for the machine to function (such as measuring or detecting voltages or currents in the windings)?

I had long ago wondered about designing a motor, fully measuring its voltage and current profiles and phase firing timings for normal operation (from stationary to full speed full load) using a position sensor for getting the motor to work and to determine the best required phase firing sequences and associated voltage/current profiles then program a microprocessor to replicate the entire required profile such that I would attempt to eliminate the need for any sensing or measurement at all (but I concluded it would come very unstuck for any fault conditions or restarting while it was still turning). So in my mind don’t all such machines require a form of measurement (i.e. some form of “sensing”) to work properly so could never be truly sensorless?

A completely sensor-less control would be completely open-loop, which isn’t reliable with some motors like PMSMs. Even if you knew the switching instants for one ideal case, too many “random” variables could influence the system (just think of the initial position), so that those firing instants could be inappropriate for other situations.

Actually, induction machines, thanks to their inherent stability properties, can be run really sensor-less (i.e. just connected to the grid or in V/f). To be honest, even in the simple grid-connection case there is an overcurrent detection somewhere in the grid, which requires some sensing.

But there can also be said the term sensorless relates to el. motor itself. In another words, it means there are not any sensors “attached” to the el. motor (which does not mean sensors cannot be in the inverter, in such a case). In our company we are using the second meaning, since it indicates no sensor connections are needed between the el. motor and the ECU (inverter).

What is true power and apparent power?

KW is true power and KVA is apparent power. In per unit calculations the more predominantly used base, which I consider standard is the KVA, the apparent power because the magnitude of the real power (KW) is variable / dependent on a changing parameter of the cos of the angle of displacement (power factor) between the voltage and current. Also significant consideration is that the rating of transformers are based in KVA, the short circuit magnitudes are expressed in KVA or MVA, and the short circuit duty of equipment are also expressed in MVA (and thousands of amperes, KA ).

In per unit analysis, the base values are always base voltage in kV and base power in kVA or
MVA. Base impedance is derived by the formula (base kV)^2/(base MVA).

The base values for the per unit system are inter-related. The major objective of the per unit system is to try to create a one-line diagram of the system that has no transformers (transformer ratios) or, at least, minimize their number. To achieve that objective, the base values are selected in a very specific way:
a) we pick a common base for power (I’ll come back to this point, if it should be MVA or MW);
b) then we pick base values for the voltages following the transformer ratios. Say you have a generator with nominal voltage 13.8 kV and a step-up transformer rated 13.8/138 kV. The “easiest” choice is to pick 13.8 kV as the base voltage for the LV side of the transformer and 138 kV as the base voltage for the HV side of the transformer.
c) once you have selected a base value for power and a base value for voltage, the base values for current and impedance are defined (calculated). You do not have a degree of freedom in picking base values for current and impedance.

Typically, we calculate the base value for current as Sbase / ( sqrt(3) Vbase ), right? If you are using that expression for the base value for currents, you are implicitly saying that Sbase is a three-phase apparent power (MVA) and Vbase is a line-to-line voltage. Same thing for the expression for base impedance given above. So, perhaps you could choose a kW or MW base value. But then you have a problem: how to calculate base currents and base impedances? If you use the expressions above for base current and base impedance, you are implicitly saying that the number you picked for base power (even if you picked a number you think is a MW) is actually the base value for apparent power, it is kVA or MVA. If you insist on being different and really using kW or MW as the base for power, you have to come up with new (adjusted) expressions for calculating base current and base impedance.

And, surprise!, you will find out that you need to define a “base power factor” to do so. In other words, you will be forced back into defining a base apparent power. So, no, you cannot (easily) use a kW/MW base. For example, a 100 MVA generator, rated 0.80 power factor (80 MW). You could pick 80 as the base power (instead of 100). But if you are using the expressions above for base current and base impedance, you are actually saying that the base apparent power is 80 MVA (not a base active power of 80 MW).

PMBLDC motor in MagNet

You can build it all in MagNet using the circuit position controlled switch. You will have to use motion analysis in order to use the position controlled switches. You can also use the back EMF information to find what the optimal position for the rotor should be with respect to the stator field. The nice thing about motion is that even if you do not have the rotor in the proper position you can set the reference at start up.

Another way of determining that position is to find the maximum torque with constant current (with the right phase relationship between phases of course) and plot torque as a function of rotor position. The peak will correspond to the back EMF waveform information.

If you want to examine the behavior of the motor with an inverter then another approach works very well. There are 2 approaches you can use with MagNet: 1) co-simulation, and, 2) reduced order models. The former can be used with matlab with Simulink or Simpower Systems and runs both Matlab and MagNet simultaneously. The module linking the two systems allows 2 way communication between the modules hence sharing information. The latter requires that you get the System Model Generator (SMG) from Infolytica. The SMG will create a reduced order model of you motor which can then be used in Matlab/Simulink or any VHDL-AMS capable system simulator. A block to interpret the data file is required and is available when you get the SMG. Reduced order models are very interesting since they can very accurately simulate the motor and hook up to complex control circuits.

Transformer uprating

I once uprated a set of 3x 500KVA 11/.433kv ONAN transformers to 800KVA simply by fitting bigger radiators. This was with the manufacturers blessing. (not hermetically sealed – there were significant logistical difficulties in changing the transformers, so this was an easy option). Limiting factor was not the cooling but the magnetic saturation of the core at the higher rating. All the comments about uprating the associated equipment are relevant, particularly on the LV side. Increase in HV amps is minimal. Pragmatically, if you can keep the top oil temperature down you will survive for at least a few years. Best practice of course is to change the transformer!

It is true that you can overload your transformer say 125 %, 150 % or even greater on a certain length of time but every instance of that overloading condition reflects a degradation on the life of your transformer winding insulation. Overload your transformer and you also shorten the life of your winding insulation. The oil temperature indicated on the temperature gauge of the transformer is much lower than the hotspot temperature of the transformer winding which is a critical issue when considering the life of the winding insulation. Transformers having rating of 300 KVA most probably do not even have temperature indicating gauge. The main concern is how effectively can you lower the hotspot temperature in order that it does not significantly take away some of the useful life of your transformer winding insulation.