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AQ: Switching frequency selection

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

  1. Lower frequency (Eg 30kHz) means bulkier magnetics and capacitors; Higher frequency (Eg 1Mhz)) means smaller parts, hence more compact PSU.
  2. 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.
  3. 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.
  4. Higher frequency results in higher switching losses; To control that, you will need faster switching FETs, Diodes, capacitors, magnetics and control ICs.
  5. 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.

AQ: How to get confidence while powering ON an SMPS prototype?

I never just put power to a first prototype and see what happens. Smoke and loud sounds are the most likely result and then you just know that something was not perfect. So how would you test the next prototype sample?

A good idea is to put supply voltage to your control circuit from an external supply first – often something like 12V. Check oscillator waveform, frequency, gate pulses etc. If possible, use another external power supply to put a voltage to your output. Increasing this voltage slowly, you should see the gate pulses go from max. to min. duty cycle when passing the desired output voltage. If this does not happen, check your feedback path, still without turning main power on.

If everything looks as expected, remove the external supply from the output but keep the control circuit powered from an external source. Then SLOWLY turn up the main input voltage while using your oscilloscope to monitor the voltage waveforms in the power circuit and a DC voltmeter to monitor output voltage etc. Keep an eye on the ampere-meter on the main power source. If something suspicious occurs, stop increasing input further and investigate what’s happening while the circuit is still alive.

With a low load you should normally expect the output voltage to hit the desired value soon, at least in a flyback converter. Check that this happens. Then check what happens with a variable load – preferably electronic.

If you did not calculate your feedback loop, very likely you will see self oscillation (normally not destructive). If you don’t, use the step load function in your electronic load to check stability. If you see a clear ringing after a load step, you still have some work to do in your loop. But feedback and stability is another huge area which Mr. Ridley has taught us a lot about.

And yes – the world needs powerful POWER ENGINEERS desperately!

AQ: Hysteretic controller

We can see that the hysteretic controller is a special case of other control techniques. For example, “sliding mode control” usually uses two state variables to determine one switching variable (switch ON or OFF). So the hysteretic controller is a special case of “1-dimensional” sliding mode. In general, there are many techniques under the name of “geometric control” that can be used to prove the stability of a general N-state system under a given switching rule. So I believe that you can apply some of these techniques to prove the stability of the hysteretic controller, although I have not tried to do this myself. The book “elements of power electronics” by Krein discusses that in chapter 17.

But I can talk more about one technique that I have used and in my opinion is the most general and elegant technique for non-linear systems. It is based on Lyapunov stability theory. You can use this technique to determine a switching rule to a general circuit with an arbitrary number of switches and state variables. It can be applied to the simple case of the hysteretic controller (i.e. 1 state variable, 1 switching variable) to verify if the system is stable and what are the conditions for stability. I have done this and verified that it is possible to prove the stability of hysteretic controllers, imposing very weak constraints (and, of course, no linearization needed). In a nutshell, to prove the system stable, you have to find a Lyapunov function for it.

What can expand is to go beyond a simple window comparator for hysteretic control.

#1) control bands, or switching limits can be variable and also part of a loop, especially if one wants to guarantee a nearly fixed frequency.

#2) using a latch or double latch after the comparator(s), one can define (remember) the state and define operations such as incorporating fixed Ton or Toff periods for additional time control… this permits the “voltage boost” scenario you previously said could not be done. This also prevents common “chaos” operation and noise susceptibility that others experience with simpler circuits.

#3) additional logic can assure multiphase topologies locked to a system clock and compete very well with typical POL buck regulators for high-end processors that require high di/dt response.

Time or state domain control systems such as this, can have great advantages over typical topologies. There really is no faster control method that provides a quicker load response without complete predictive processing, yet that can also be applied to hysteretic control.

AQ: Experience: Power Supply

My first big one: I had just joined a large corporation’s central R and D in Mumbai (my first job) and I was dying to prove to them that they were really very wise (for hiring me). I set up my first AC-DC power supply for the first few weeks. Then one afternoon I powered it up. After a few minutes as I stared intently at it, there was a thunderous explosion…I was almost knocked over backwards in my chair. When I came to my senses I discovered that the can of the large high-voltage bulk cap had just exploded (those days 1000uF/400V caps were real big)…the bare metal can had taken off like a projectile and hit me thump on the chest through my shirt (yet it was very red at that spot even till hours later). A shower of cellulose and some drippy stuff was all over my hair and face. Plus a small crowd of gawking engineers when I came to. Plus a terribly bruised ego in case you didn’t notice. Now this is not just a picturesque story. There is a reason why they now have safety vents in Aluminum Caps (on the underside too), and why they ask you never never to even accidentally apply reverse polarity, especially to a high-voltage Al cap. Keep in mind that an Al Elko is certainly damaged by reverse voltage or overvoltage, but the failure mechanism is simply excessive heat generation in both cases. Philips components, in older datasheets, used to actually specify that their Al Elkos could tolerate an overvoltage of 40% for maybe a second I think, with no long-term damage. And people often wonder why I only use 63V Al Elkos as the bulk cap in PoE applications (for the PD). They suggest 100V, and warn me about surges and so on. But I still think 63V is OK here, besides being cheap, and I tend to shun overdesign. In fact I think even ceramic caps can typically handle at least 40% overvoltage by design and test — and almost forever with no long term effects. Maybe wrong here though. Double check that please.

Another historic explosion I heard about after I had left an old power supply company. I deny any credit for this though. My old tech, I heard, in my absence, was trying to document the stresses in the 800W power supply which I had built and left behind. The front-end was a PFC with four or five paralleled PFC FETs. I had carefully put in ballasting resistors in the source and gates of each Fet separately, also diligently symmetrical PCB traces from lower node of each sense resistor to ground (two sided PCB, no ground plane). This was done to ensure no parasitic resonances and good dynamic current sharing too. There was a method to my madness it turns out. All that the tech did was, when asked to document the current in the PFC Fets, placed a small loop of wire in series with the source of one of these paralleled Fets. That started a spectacular fireworks display which I heard lasted over 30 seconds (what no fuse???), with each part of the power supply going up in flames almost sequentially in domino effect, with a small crowd staring in silence along with the completely startled but unscathed tech (lucky guy). After that he certainly never forgot this key lesson: never attempt to measure FET current by putting a current probe in its source— put it on the drain side. It was that simple. The same unit never exploded after that, just to complete the story.

AQ: Constant on-time control

There are three different more or less widely used types of constant on-time control. The first one is where the off-time is varied with an error signal. A loop with this type of control has a control-to-output voltage frequency response (or Bode plot if you prefer) similar to that of the constant-frequency voltage-mode control. The second one is where the off-time is terminated with a comparator that monitors the inductor current, and when that current goes below a level set by the error signal, the switch is turned on. This control (also called constant on-time valley-current control) has a control-to-output voltage frequency response similar to the constant-frequency valley-current control. The main difference is that its inner current-control loop does not suffer from the subharmonic instability of the constant-frequency version, so it does not require a stabilizing ramp and the control-to-output voltage response does not show the half-frequency peaking. The third version is where the off-time is terminated when the output voltage (or a fraction of it) goes below the reference voltage. This control belongs to the family of ripple-based controls and it cannot be characterized with the usual averaging-based control-to-output frequency response, for the reason that the gain is affected by the output ripple voltage itself.

As for the hysteretic control, the current-mode version is a close relative of the constant on-time valley-current-control. The version that uses the output ripple voltage instead of the inductor current ripple for turning on and off the switch (also called “hysteretic regulator”) is a close relative of the constant on-time ripple-based control.

Although the ripple-based control loops cannot be characterized with the usual Bode plots, the converters can still be unstable, but not in the meaning of the traditional control-loop instability that power-supply engineers are used to. Furthermore the hysteretic regulator is essentially unconditionally stable. The instabilities with ripple-based control are called “fast-scale” because the frequency of the instability is closely related to the switching frequency (either subharmonic, similar to the inner-loop instability of some of the current-mode controller, or chaotic in nature).

The paper I wrote a couple of years ago (“Ripple-Based Control of Switching Regulators—An Overview”) is a good introduction to ripple-based control and discusses some of the stability issues. There are also quite a few papers with detailed analyses on the stability of converters with feedback loops where the ripple content of the feedback signal is significant.

AQ: 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

AQ: Industrial automation process

My statement “the time it takes to start or stop a process is immaterial’ is somewhat out of context. The complete thought is” the time it takes to start or stop a process is immaterial to the categorization of that process into either the continuous type or the discrete type” which is how this whole discussion got started.

I have the entirely opposite view of automation. “A fundamental practice when designing a process is to identify bottlenecks in order to avoid unplanned shutdowns”.

Don’t forget that the analysis should include the automatic control system. This word of advice is pertinent to whichever “camp” you chose to join.

Just as you have recognized the strong analogies and similarities between “controlling health care systems” and “controlling industrial systems”, there are strong analogies between so-called dissimilar industries as well between the camp which calls itself “discrete” and the camp which waves the “continuous” flag.

You may concern about the time it takes to evaluate changes in parameter settings for your cement kiln is a topic involving economic risks which could include discussions of how mitigate these risks, such as methods of modeling the virtual process for testing and evaluation rather than playing with a real world process. This is applicable to both “camps”.

The same challenge of starting up/shutting down your cement kiln is the same challenge of starting up/shutting down a silicon crystal reactor or wafer processing line in the semiconductor industry. The time scales may be different, but the economic risks may be the same — if not more — for the electronics industry.

I am continuously amazed at how I can borrow methods from one industry and apply them to another. For example, I had a project controlling a conveyor belt at a coal mine which was 2.5 miles long – several millions of pounds of belting, not to mention the coal itself! The techniques I developed for tracking the inventory of coal on this belt laid the basis for the techniques I used to track the leading and trailing edge of bread dough on a conveyor belt 4 feet long. We used four huge 5KV motors and VFDs at the coal mine compared to a single 0.75 HP 480 VAC VFD at the bakery, and startups/shutdowns were order of magnitudes different, but the time frame was immaterial to what the controls had to do and the techniques I applied to do the job.

I once believed that I needed to be in a particular industry in order to feel satisfied in my career. What I found out is that I have a passion for automation which transcends the particular industry I am in at the moment and this has led to a greater appreciation of the various industrial cultures which exist and greater enjoyment practicing my craft.

So these debates about discrete vs. continuous don’t affect me in the least. My concern is that the debates may impair other more impressionable engineers from realizing a more fulfilling career by causing them to embrace one artificial camp over the other. Therefore, my only goal of engaging in this debate is to challenge any effort at erecting artificial walls which unnecessarily drive a damaging wedge between us.

AQ: 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.

AQ: 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.

AQ: PPE (Personal Protective Equipment)

When I think of using PPE as a controls engineer, I think
about electrical shock and arc-flash safety in working with electrical devices.

The PPE (Personal Protective Equipment) requirements to work on live electrical
equipment is making doing commissioning, startup, and tuning of electrical
control systems awkward and cumbersome. We are at a stage where the use of PPE
is now required but practice has not caught up with the requirements. While
many are resisting this change, it seems inevitable that we will need to wear
proper PPE equipment when working on any control panel with exposed voltages of
50 volts or more.

With many electrical panels not labeled for shock and arc-flash hazard levels,
the default PPE requires a full (Category 2+) suit in most cases, which is very
awkward indeed. What can we do to allow us to work on live equipment in a safe
manner that meets the now not so new requirements for shock and arc-flash
safety?

Increasingly the thinking is to design our systems for shock and arc-flash
safety. Typically low voltage (less than 50 volts), 120VAC, and 480 VAC power
were often placed in the same control enclosure. While this is cost effective,
it is now problematic when wanting to do work on even the low voltage area of
the panel. The rules do not appear to allow distinguishing areas of a panel as
safe, while another is unsafe. The entire panel is either one or the other. One
could attempt to argue this point, but wouldn’t it be better to just design our
systems so that we are clearly on the side of compliance?

Here are my thoughts to improve electrical shock and arc flash safety by
designing this safety into electrical control panels.

1. Keep the power components separate from the signal level components so that
maintenance and other engineers can work on the equipment without such hazards
being present. That’s the principle. What are some ideas for putting this into
practice?

2. Run as much as possible on 24VDC as possible. This would include the PLC’s
and most other panel devices. A separate panel would then house only these shock
and arc-flash safe electrical components.

3. Power Supplies could be placed in a separate enclosure or included in the
main (low voltage) panel but grouped together and protected separately so that
there are no exposed conductors or terminals that can be reached with even a
tool when the control panel door is opened.

4. Motor Controls running at anything over 50 volts should be contained in a
separate enclosure. Try remoting the motor controls away from the power devices
where possible. This includes putting the HIM (keypad) modules for a VFD
(Variable Frequency Drive) for example on the outside of the control panel, so
the panel does not have to be opened. Also, using the traditional MCC (Motor
Control Centers) enclosures is looking increasing attractive to minimize the
need for PPE equipment.

For example “finger safe” design does not meet the requirements for arc-flash
safety. Also making voltage measurements to check for power is considered one
of, if not the most hazardous activity as far as arc-flash goes.