Iacdrive|Automation Solution

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.

Switching frequency selection

Switching frequency selection is actually a tradeoff, and follows the below guidelines:

Lower frequency (Eg 30kHz) means bulkier magnetics and capacitors; Higher frequency (Eg 1Mhz)) means smaller parts, hence more compact PSU.
Stay away from exact 150kHz as this is the low end of any EMI compliance; So, if your frequency happens to be exactly 150kHz, then your PSU will be a strong emitter; For many commercial low cost PSUs, 100 KHz has been used for many years, which is why many inductors and capacitors are specified at 100kHz.
Higher frequency >/= 1MHz converters provide for better transient response. Obviously, the control IC should be capable of supporting. There are plenty of resonant converters available.
Higher frequency results in higher switching losses; To control that, you will need faster switching FETs, Diodes, capacitors, magnetics and control ICs.
Higher frequency MAY result in more broadband noise; its not always true, since noise can be controlled by good PCB layout and good magnetics designs.

Board power DC/DC converters are commonly built using 1MHz switchers.
Chassis power Telecom/Server PSUs seem to stay with 100-300KHz range.

Manufacturers are able to achieve exceptional density by virtue of High frequency resonant topologies, but they have to achieve high efficiencies too; Else, they will generate so much heat that they cannot meet UL/IEC safety requirements.
In some cases, they will leave the thermal problem to the user. Usually, the first few paragraphs of any reference design discusses the tradeoffs.

Determine coefficient of grounding

Determination of required grounding impedance is based on determination of coefficient of grounding which represents ratio of maximum phase voltage at phases which aren’t exposed by fault and line voltage of power network:

kuz=(1/(sqrt(3)))*max{|e(-j*2*π/3)+(1-z)/(2+z)|; |e(+j*2*π/3)+(1-z)/(2+z)|}
z=Z0e/Zde

where are:

kuz-coefficient of grounding,
z-ratio of equivalent zero sequence impedance viewed from angle of place of fault and equivalent direct sequence impedance viewed from angle of place of fault,
Z0e-equivalent zero sequence impedance viewed from angle of place of fault,
Zde-equivalent direct sequence impedance viewed from angle of place of fault.

So, after this explanation, you can get next conclusions:
if kuz=1 then power network is ungrounded because Z0e→∞, which is a consequence of existing more (auto) transformers with ungrounded neutral point than (auto) transformers with grounded neutral point (when kuz=1 then there aren’t (auto) transformers with grounded neutral point),
if kuz≤0,8 then power network is grounded because Z0e=Zde, which is a consequence of of existing more (auto) transformers with grounded neutral point than (auto) transformers with ungrounded neutral point.

Fault current in grounded power networks is higher than fault current in ungrounded power networks. By other side, in case of ungrounded power networks we have overvoltages at phases which aren’t exposed by fault, so insulation of this conductors could be seriously damaged or in best case it could become older in shorter time than it is provided by design what is the main reason for grounding of power networks.
Coefficient of grounding is very important in aspect of selecting of insulation of lighting arresters and breaking power of breakers, because of two next reasons:
1. in grounded power networks insulation level is lower than insulation level in ungrounded power networks,
2. in grounded power networks value of short circuit current is higher than value of short circuit current in ungrounded power networks.

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.

AM & FM radio

For AM & FM radio & some data communications adding the QP filter make sense.
Now that broadband, wifi, data communications of all sizes & flavours exist – any peak noise is very likely to cause interuptions & loss of integrity of data – all systems are being ‘cost reduced’ ensuring that they will be more susceptible to noise.
I can understand the reasons for the tightening of the regulations.
BUT, it links in to the other big topic of the moment – the non-linearity of managers.
William is obviously his own manager – I bet if his customer was to ask him to spend an indefinite amount of time fixing all the root causes to meet the spec perfectly without any additional cost it would be a different matter.

Unfortunately for most of us the realities of supervisors wanting projects closed & engineering costs minimized we have to be careful in the choice of phrasing.
Any suggestion that one prototype is ‘passing’ suddenly can be translated to job finished, & even in our case where the lab manager mostly understands, his boss rarely does & the accountant above him – not at all.

It gets worse than that – at the beginning of a project (RFQ) – the question is “how long will EMC take to fix?” with the expectation if a deterministic answer; the usual response of a snort of derision & how long is a piece of string generally translates to 2 weeks & once set in stone becomes a millstone (sorry mile-stone).

We already have a number of designs that while not intentionally using dithering, do use boundary mode PFC circuits which automatically force the switch frequency to vary over the mains cycle. These may become problematic at some future variation of the wording of the EMC specs.

While I have a great deal of sympathy for the design it right first time approach, the bottom line for any company is – it meets the requirement (today) – sell it!!

Simulation interpretation in automation industry

Related to “automation industry”, there are generally 3 different interpretations of what simulations is:
1) Mechanical Simulations – Via various solid modeling tools and cad programs; tooling, moving mechanisms, end-effectors… are designed with 3D visualizations, connecting the modules to prevent interference, check mass before actual machining…
2) Electronics Simulations – This type of simulations are either related to the manufacturers of “specific instrumentations” used in automation industry (ultrasonic welders, laser marking systems,…) or the designers of circuit boards.
3) Electrical & Controls Simulations.
A) Electrical Schematics, from main AC disconnect switch, down to 24VDC low amps for I/O interface.
Simulation tools allow easy determinations of system’s required amperage, fuse sizes, wire gauges, accordance with standards (CE, UL, cUL, TUV…)…
B) Logic Simulations, HMI interface, I/O exchange, motion controls…
a) If you want to have any kind of meaningful simulations, get in the habit of “modular ladder logic” circuit design. This means, don’t design your ladder like one continuous huge program that runs the whole thing; simulating this type of programs is almost impossible in every case. Break down the logic to sub-systems or maybe even down to stand alone mechanisms (pick & place, motor starter…), simulating and troubleshooting this scenario is fairly easy.
b) When possible, beside automated run mode of the machine or system, build “manual mode logic” for it as well. Then via physical push-buttons or HMI, you should have “step forward” & “step back” for every “physical movement or action”.

Simulating the integrity of the “ladder logic program” and all the components and interfaces will be a breeze if things are done meticulously upfront.