Category: Blog

Home  ›  Blog  ›  Page 17

AQ: Simulation on EMI

As a mathematical tool eventually, simulation can help to quickly approach the results that we need. If everything is done in right way, simulation can give us reliable conductive EMI results at the low frequency range.

Differential mode conductive EMI can be simulated with good accuracy at the low frequency range. The accuracy of common mode conductive EMI depends on the accuracy of a few parasitic parameters that need to be measured.

Personally for research, I would like to use simulation as a validation tool for calculation, and test results of prototypes can be used as proof for simulation.

E.g. for EMI filter:

1. Do the calculation for the differential mode conductive EMI filter;
2. Do the calculation for common mode conductive EMI filter base upon the parasitic parameters in the hand or estimation;
3. Use the simulation to check and validate if the calculation is right or if something is wrong and needs to be corrected;
4. Use prototype test results to check and validate if the simulation results are right.

Some other issues that caused by EMI filter can be found during system level simulation before prototyping. E.g. audio susceptibility and EMI filter damping problems.

AQ: Excitation system in generator

The excitation system requires a very small fraction of the total power being generated. If we could simply increase the excitation (a very small amount of power) and increase the generator’s real power output, the world’s energy problems would be solved, because we would have a perpetual motion machine.

In the case of a generator connected to a large grid, the generator will inject any desired amount of power into the grid if its prime-mover is fed the desired power (plus a small additional amount of power to take care of losses). This is true, regardless of the total load on the grid, because the generator’s output is an extremely small fraction of the total grid power, and it alone cannot make drastic changes to the grid’s frequency.

Normally, the load varies by a very small fraction of the total grid power. If the load increases, the frequency of the entire grid (including the generator in question) lowers a very small amount, generally less than one-hundredth of one Hz. The frequency slew (that is, the rate-of-change of frequency) is very low, because there is a massive amount of energy that is stored as the kinetic energy of the rotors of all of the generators. At this point, nothing needs to be done; the system simply runs a little faster or slower.

Over time, as the load changes a greater amount, the frequency moves further from the nominal frequency (50 Hz or 60 Hz). When the difference between the actual frequency and the nominal frequency becomes greater than about 0.01 Hz, action is taken to make changes to the output of the grid’s generators.

The specific action may be determined by the regulating authority (for instance a power pool in the US) and it is usually based on economics, subject to other constraints. If the load has increased (and the frequency is less than the nominal frequency), the generators that have the lowest incremental cost of power will be asked to increase their output, or if all generators are near their limits, new generators (with the lowest incremental cost) are asked come on line. It’s important to note that a generator’s limit is usually 80% or 90% of its rating. The 10% or 20% of unused capacity is the system’s “spinning reserve”, which is used to maintain grid stability for sudden, large power variations.

The same thing happens with a generator connected only to its load or a weak grid with just a few other generators. However, because there is relatively little kinetic energy stored in the rotors of the one or few generators, the change in frequency associated with a load change is much greater, so frequency variations are much greater and corrective actions may not be implemented before the frequency varies by more than a few Hz.

AQ: High starting torque, synchronous motor, induction motor or DC motor?

It depends on so much more than the simple requirements listed of high starting torque and variable speed. What kind of application are you using it for? Is it on an automobile (where you have DC already), a factory, and do you have the budget and/or space for a variable frequency drive. A synchronous servo motor gives great dynamic control and great starting torque per volume, but its speed range is limited (unless you’re field weakening by the back EMF). Servo-motors are also the most expensive due to their position sensors and more intelligent drives.

With a proper soft drive you can go with an induction motor, but it depends. if power is small you can go to step motor also. But dc series motor’s starting torque is high as expressed others.
DC Series motors have high starting torque but induction motors have wide range of speed control. So, If DC motor is used, then DC drives you can use, although it will be expensive and DC motors are tough to maintain than ac motors due to commutation Problem.

DC series motor would provide both the high starting torque and adjustable speed BUT beware that DC motors have high maintenance cost and also require AC-DC conversion. You could use other available options e.g. double wound induction motors etc, depending upon your requirements.

But today, there is no application where you cannot apply AC motors, asynchronous or synchronous. If the motor and the associated power electronics are correctly rated, you can have any starting torque you want.

The typical application of DC series motors was in locomotives. This technology has been replaced by AC motors since 20 years. The latest generation of high speed trains use synchronous, permanent magnet motors.

AQ: EMI & EMC

EMI/EMC is rather a subjective topic than theoretic, but we shall look at it with start from noise prevention then noise suppression.

Prevention or design in the solution is needed to concentrate on noise making part/component or its mechanism play in the circuit. These are referring to those part and circuit that directly involve in switching, like PFC mosfet and its driver, PFC diode, DC/DC switching mosfet and its driver, and its output diode, do not left out the magnetic part and layout design, bad design will cause ugly switching then give you headache in EMC problem.

Part/component and topology selection is somehow important in which had some level predetermine your EMI/EMS need to take care, like what Stephen had explained; phase-shift is better than none phase shift.

Mosfet would have higher noise at high frequency but it can be somehow compensated or tolerated or trick by driving speed, by using snubber and may be shielding. The output Diode should be carefully selected so that its high frequency noise is within your output noise spec else is an issue, please make sure this noise is not able to be transmitted out as Radiated noise, or it is not couple into your Primary circuit, else it will all the way out to the input AC then transmitted as Radiated noise. Trr is the parameter to look at, sure the lower the best. Anyhow, some snubber (RC or feerite bead,..) shall be determined and add-in.

Noise suppression is what refer to Filter, energy dumping circuit,.. but somehow is basic need, one of it Input filter that give noise isolation between what generated internal in psu not pass to input supply system that could interfere other system/supply environment (EMI), or what noise environment that could enter into your power supply and interfere your power supply (EMS). Input filter is definitely a must for your switching frequency and its sub-harmonics, which is fall into the EMI standard range.

There are many technique to suppress the noise and is depend on what location, nature of the circuit, switch and diode, like what you means by RCD, is not mistaken is refer to RCD that add across the main transformer, DC/DC switcher and typically at the output Diode, you are on right direction with using this snubber around these component.

Shielding may be needed for your main transformer if you have some gap in it (but is not needed if your controller and your switcher is so call good part), but may be needed after you have made some study it the samples.

Good layout always give peaceful mind, whereby noise part have to be some distant away from noise sensitive controller or decision making circuit, decision making connection point have to wise at right termination point that prevent sense the high noise content signal, but if no choice some RC filter is unpreventable, anyhow and mostly RC is commonly located even is known clean in noise to those decision making circuit.

AQ: Phase rotation errors

Phase rotation errors are not as rare as they ought to be. I’ve seen more than one building with a systematic phase rotation error. This can be prevented by carefully following the color coding system (Yellow Orange Brown and Red Blue Black for 480 volt and 208 volt systems in the US for example) and tagging feeders at both ends to assure proper connections.

To check for proper phase rotation sequencing (ABC and not ACB) you can use a phase rotation meter. Without that you can bump a three phase motor that should be correctly connected to see if it turns in the right direction. If it’s wrong, reverse any two phase wires from the source to the distribution equipment. However, if you have a tie breaker and intend to operate the secondaries of two transformers in parallel by closing it that is not good enough. Both transformer distribution networks have to be connected correctly on all three phases. You have to check the voltage across each corresponding pair of terminals on the tie breaker and be certain they are all about zero volts. If you don’t and there is an error, closing the tie breaker if that is possible at all (some electronic breakers may lock you out) will result in a phase to phase bolted fault that can result in severe damage to your distribution equipment. Phase rotation errors are invariably the result of incompetent installation, inadequate specifications for feeder identification, and inadequate inspection.

There are times when the phase rotation error is made on the primary side of the transformer. If this happens it can be compensated for by reversing the phase rotation error from the secondary side. This is less desirable but it will work. If you have multiple phase rotation errors in the same distribution network you have quite a mess to clean up. It will be time consuming and expensive tracking all of them down to be certain you have eliminated them. False economies by cutting corners on the initial installation of substations and distribution equipment will result in necessitating very expensive and inconvenient repairs. If it is not corrected you risk severe damage to three phase load equipment.

AQ: High AC current inductors

There are several issues at work here. For high AC current inductors, you want to have low core losses, low proximity loss in the windings, and low fringing effects.

At normal frequencies, ferrites are by far the lowest core loss, much better than MPP and other so called “low-loss” materials. So you would like to use them from this aspect.

A toroid gives the greatest winding surface for the magnetic material, letting you use the least number of layers and hence minimizing proximity loss. The toroid also has the advantage of putting all the windings on the outside of the structure, facilitating cooling. This is very important.

However, you can’t easily gap a toroid of ferrite, it’s very expensive.

Some aerospace applications actually cut the ferrite toroid into segments and reassemble them with several gaps to solve the problem. The multiple gaps keep fringing effects low. It might be nice if you could buy a set of toroidal segments so you don’t have to do the cutting because that is a big part of the cost. I don’t know if that is a reasonable thing to do, maybe someone can comment.

Once you go to MPP, the core loss goes up, but the distributed gap minimizes the fringing losses.

The MPP lets you run somewhat higher on current before saturation, but if you have high ac you can’t take full advantage of that due to the core losses.

All these tradeoffs (and quite a few more not mentioned for brevity) are the reason that so many different solutions exist.

AQ: Moving data around within memory of an individual PLC

The first question would have to be – why do want to do it? If the data already exists in one location that is accessible by all parts of the program, why are you going to use up more PLC memory with exactly the same data?

Well, there are a couple of candidate reasons. One might be recipe data. You have an area of memory with a set of stored recipes for different products, and at an appropriate moment you want to copy a specific recipe from the storage area to the working area. The first thing to be said about that is that if your recipes are at all complex and you have a requirement to have a significant number of different recipes, then PLC memory is probably not the right place to be storing them. The ultimate, these days, of course, is that recipes are created by techies on PCs away from the production area, in nice quite, comfortable labs or whatever, and are stored on a SQL server. Only the recipe for today’s actual production run gets transferred to the PLC. But there are some applications where there is a limited number of different recipes required and the recipes themselves are quite simple, when it can be reasonable to store the recipes in PLC memory.

A second reason for copying memory areas within the same PLC is for procedures, sub-routines or whatever. But again, these days, all PLC languages have some sort of in-built facility for procedures – what Rockwell uniquely call Add On Instructions, what everyone else calls UDFBs – user defined function blocks. In any case, the point is that these facilities usually make all that memory management stuff transparent to the programmer. You just configure the UDFB and call it as required. The compiler takes care of all the memory data moves for you.

Another reason for copying memory, actually related to the previous, is a technique much used by PLC programmers where they use an area of memory as a ‘scratch-pad’. So they will copy some unprocessed data to the scratchpad area, all of the operations performed on the data take place using the scratchpad, and at the end, they copy the processed data back again. Again, it is questionable how much this technique is actually required these days, I would suggest that it most cases, there probably is a better way using a UDFB. But I have seen some programmers who routinely include a scratchpad area within any UDFBs they define.

AQ: Creepage in thermal substations

The term creepage distance is specifically associated with porcelain insulators used in the Air Insulated substations. Insulator surface attracts dust, pollution (in industrial areas) and salt (along the sea coast) and these form a conducting layer on the surface of the insulator body when the surface is wet. As long as this surface is dry, there is not much problem. But when it becomes wet during early morning or during winter season the outer surface forms a conducting path along the surface from high voltage terminal to earthed metal fitting at the end of metallic structure and may lead to surface conduction and finally external flash-over. The insulators are provided with Sheds to limit the direct exposer to mist or dew. The protected area of the sheds will not allow formation of continuous conducting layer along the surface of insulator as the part of surface which is under the sheds may not become wet due to mist or dew and this part (length along the bottom surface) of the insulator surface is called protected creepage.

Measurement of corona inception and extinction voltages give a fair idea about the possible flashover even with protected creepage. But these will change under different levels of pollution.
This problem is not present with Composite insulators as the Silicone rubber sheds surface does not allow formation of continuous wet conducting layer as the surface of these insulators is Hydrophobic. Hence higher creepage is not considered for composite insulators.

However air density is also a limiting factor for deciding the creepage of insulators, necessitating higher creepage in case of higher altitudes.
You may have to assess the level of pollution and altitude of substation and select the creepage accordingly.
Medium pollution levels may be 25mm/kV
Very high pollution areas like on the sea coast and chemical and pharmaceutical industrial areas 31mm/kV where the insulators may become expensive alternatively periodic hot line washing is also another solution for cleaning of pollution on insulators.
In case of very high pollution levels GIS may be safe solution (if cost is not an issue)

Thermal substations where there are no electrostatic precipitators may also experience equipment failures due to pollution. Pressurized equipments like SF6 gas circuit breaker experienced external flash-overs during winter months in Northern India The utility was not accepting the theory of insulation failure due to pollution initially but they had to accept the cause of failure as pollution when they had similar failure in the consecutive year during the same winter months and they have resorted to hot line washing since then and there are no more such failures. Sometimes these deposits may not be seen glaringly but failure may happen.

AQ: FETs in ZVS bridge

Had run into a very serious field failure issue a decade ago due to IXYS FETs used in a phase-shifted ZVS bridge topology. Eventually, the problem was tracked to failure of the FETs’ body diode when the unit operated at higher ambient temperature.

When FETs were first introduced for use in hard switching applications, it was quickly discovered that under high di/dt commutating conditions, the parasitic bipolar transistor that forms the body diode can turn on resulting in catastrophic failure (shorting) of the FET. I had run into this issue in the mid ’80s and if memory serves me correctly, IR was a leader in making their FET body diodes much more robust and capable of hard commutation. Having had this experience with FET commutation failures and after exhausting other lines of investigation which showed no problem with the operation of the ZVS bridge, I built a tester which could establish an adjustable current through the body diode of the FET under test followed by hard commutation of the body diode.

Room temperature testing of the suspect FET showed the body diode recovery characteristic similar to that of what turned out to be a more robust IR FET. Some difference was seen in the diode recovery as the IXYS FET was a bit slower and did show higher recovered charge. However, was unable to induce a failure in either the IXYS or IR FET even when commutating high values of forward diode current up to 20A when testing at room temperature.

The testing was then repeated in a heated condition. This proved to be very informative. The IXYS FETs were found to fail repeatedly with a case temperature around 80C and forward diode current prior to commutation as low as 5A. In contrast, the IR devices were operated to 125C case temp with forward diode currents of 10A without failure.

This confirmed a high temperature operating problem of the IXYS FETs associated with the body diode. Changing to the more robust IR devices solved the field failure issue.
Beware when a FET datasheet does not provide body diode di/dt limits at elevated ambient.

A more complete explanation of the FET body diode failure mechanism in ZVS applications can be found in application note APT9804 published by Advanced Power Technology.

I believe FETs can be reliably used in ZVS applications if the devices are carefully selected and shown to have robust body diode commutation characteristics.

AQ: 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.