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AQ: Self Excited Induction Generator (SEIG)

The output voltage and frequency of a self excited induction generator (SEIG) are totally dependent on the system to which it is attached.

The fact that it is self-excited means that there is no field control and therefore no voltage control, instead the residual magnetism in the rotor is used in conjunction with carefully chosen capacitors at its terminal to form a resonant condition that mutually assists the buildup of voltage limited by the saturation characteristics of the stator. Once this balance point is reached any normal load will cause the terminal voltage to drop.

The frequency is totally reliant upon the speed of the rotor, so unless there is a fixed speed or governor controlled prime mover the load will see a frequency that changes with the prime mover and drops off as the load increases.

The above characteristics are what make SEIGs less than desirable for isolated/standalone operation IF steady well regulated AC power is required. On the other hand if the output is going to be rectified into DC then it can be used. Many of these undesirable “features” go away if the generator is attached to the grid which supplies steady voltage and frequency signals.

The way around all the disadvantages is to use a doubly fed induction generator (DFIG). In addition to the stator connection to the load, the wound rotor is provided with a varying AC field whose frequency is tightly controlled through smart electronics so that a relatively fixed controllable output voltage and frequency can be achieved despite the varying speed of the prime mover and the load, however the costs for the wound rotor induction motor plus the sophisticated control/power electronics are much higher than other forms of variable speed/voltage generation.

AQ: 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.

AQ: DC Drives Basic Operation Principles

DC drives vary the speed of DC motors with greater efficiency & speed regulation than resistor control circuits. Since the speed of a DC motor is directly proportional to armature voltage & inversely proportional to field current, either armature voltage or field current can be used to control speed. To change the direction of rotation of a DC motor, either the armature polarity can be reversed, or the field polarity can be reversed.

DC drive diagram

The block diagram of a DC drive system made up of a DC motor & an electronic drive controller. The shunt motor is constructed with armature & field windings. A common classification of DC motors is by the type of field excitation winding. Shunt wound DC motors are the most commonly used type for adjustable-speed control. In most instances the shunt field winding is excited, as shown, with a constant-level voltage from the controller. The SCR (silicon controller rectifier), also known as thyristor, of the power conversion section converts the fixed-voltage alternating current (AC) of the power source to an adjustable-voltage, controlled direct current (DC) output which is applied to the armature of a DC motor. Speed control is achieved by regulating the armature voltage to the motor. Motor speed is directly proportional to the voltage applied to the armature.

The main function of a DC drive is to convert the fixed applied AC voltage into a variable rectified DC voltage.

SCR switching semiconductors provide a convenient method of accomplishing this. They provide a controllable power output by phase angle control. The firing angle, or point in time where the SCR is triggered into conduction, is synchronized with the phase rotation of the AC power source. The amount of rectified DC voltage is controlled by timing the input pulse current to the gate. Applying gate current near the beginning of the sine-wave cycle results in a higher aver age voltage applied to the motor armature. Gate current applied later in the cycle results in a lower average DC output voltage. The effect is similar to a very high speed switch, capable of being turned on & off at an infinite number of points within each half-cycle. This occurs at a rate of 60 times a second on a 60-Hz line, to deliver a precise amount of power to the motor.

AQ: System operation

Our PSA unit (meaning Pressure Swing Adsorption) uses 5 adsorption vessels. The process itself is a batch process, but in order to run in a continuous process plant, each of the 5 vessels can complete all the adsorption process but at the same time, each of them is in a different status of the sequence (i.e. gas in, gas out, adsorption, pressurizing, depressurizing, cleaning, etc). The sequence is mainly controlled by time and pressure condition in each step of the sequence, by managing several valves (I think 5 by vessel, but I’m not sure right now).

Panel operator experienced some problems with valve 1 (gas entry) in vessel 2 because it should open but immediately it received the close command. Instruments technician check that orders coming from the DCS were OK, and also check the valves by injecting the open order, so, they and operation staff concluded that “the program has some kind of problem”.

Some time ago, I spent a lot of time studying the operation manual of this unit and the code written to control it and I wrote a document merging both knowledge. In page 9, I described a condition (an exclusive pressure difference between vessel and gas coming in the vessel) avoiding valve 1 opening during adsorption stage. I explained this condition to operation staff and they confirm that the values were right and that the excessive delta P really exist so, the decided to check back the valve 1 (already checked), discovering a problem (the stem moved, but the disk not).

Conclusion:
– If operation staff know properly the process, they know about this condition, but this could be solved with a properly designed HMI (i.e. including and alarm indicating “valve 1 closed by excessive deltaP”).
– The initial inspection of the valve didn’t show anything wrong, but stem and disk were disconnected.
– If we didn’t dig into the code, this problem, solved in less of an hour could take several hours.

AQ: How is frequency inverter saving energy?

This studies show that up to 80 percent of the energy from the power source to the industrial consumer can be lost. Energy conversion—converting energy into useful work via motors, heat exchangers, process heaters, pumps, motors, fans, compressors, and so forth—represents a large opportunity for energy savings in manufacturing. Industrial electric motor-driven systems represent the largest single category of electricity use in China.

The industrial sector consumes approximately one-third of the energy used in China (see Fig. 1).

Studies show that up to 80 percent of the energy from the power source to the industrial consumer is lost through the transition of raw material to the point of useful output—much of that at the point of conversion from electrical to mechanical output (see Fig. 2).

Rising energy costs, a sense of environmental responsibility, government regulation, and a need for energy reliability are driving efforts for energy efficiency in manufacturing.

Energy is lost primarily in three areas:

  • Generation
  • Distribution
  • Conversion

The third area, energy conversion—converting energy into useful work via motors, heat exchangers, process heaters, pumps, motors, fans, compressors, and so forth—represents a large opportunity for energy savings in manufacturing.

Industrial electric motor-driven systems represent the largest single category of electricity use in the China—more than 65 percent of power demand in industry. Consequently, motor-driven systems offer the highest energy savings potential in the industrial segment.

Supporting this statistic, studies show that 97 to 99 percent of motor life cycle costs are expended on the energy that the motor uses. This fact alone should be a driving motivation for companies to perform a periodic energy consumption analysis on the motor systems they use in their facilities.

Inefficient and ineffective control methods in two areas waste motor systems’ energy:

  • Mechanical flow control (pumps, fans, compressors)
  • Energy recovery (regeneration of braking energy or inertial energy)

An inverter is an effective tool in conserving and recovering energy in motor systems.

What Is a frequency inverter? Why Use It?

A frequency inverter controls AC motor speed (see Fig. 3). The frequency inverter converts the fixed supply frequency (60 Hz) to a variable-frequency, variable-voltage output to enable precise motor speed control. Many frequency inverters even have the potential to return energy to the power grid through their regenerative capability.

A frequency inverter’s precise process and power factor control and energy optimization result in several advantages:

  • Lower energy consumption saves money
  • Decreased mechanical stress reduces maintenance costs and downtime
  • Reduced mechanical wear and precise control produces more accurate products.
  • Lower consumption lowers carbon emissions and helps reduce negative impact on the environment.
  • Lower consumption qualifies for tax incentives, utility rebates, and, with some companies, energy savings finance programs, which short

AQ: How to improve troubleshooting techniques?

The guy asked for suggestions on how to improve troubleshooting techniques. I mentioned this earlier as a “suggestion” for starters but the idea got lost in all the complaining and totally irrelevant responses like the one above.

Proper lay out of inputs and outputs and a “Troubleshooting guide” or flow chart. I have an Aris cablem modem and Netgear wireless router for internet If loose Internet service I can do three things.

A. Pick up the phone, call tech support and wait two days for someone to show up

B. Take them apart and ‘DIG INTO THE PROGRAMMING”

C. Read the instructions someone took the time to write. Before I can get an output identified by the LEDs, I have to have the correct inputs identified by the LEDs. It’s a waste of time tearing in the “programming” over a loose cable connection somewhere. Same for the wireless router and a bad LAN cable connection or network service issue on the computer. I’m already familiar with the proper LEDs for normal operation. When one goes out it gives me an idea where to start looking before even opening up the instructions which I’ve downloaded in PDFs for quick access to their “troubleshooting” guides. Maybe the service is off line – there is an LED for that. No TVs either, no service or common upstream cable connection problem, no-brainier. The first thing a Xfinity service tech does is go outside and look for a signal at the house customer jack. It’s either in his cable or my house. Once their cable had to be replaced. It mysteriously got damaged right after AT&T dug a big hole in my backyard to upgrade their Uverse service – go figure.

In order to get something to operate output wise, you need a certain amount of inputs to get it. If you don’t have a particular output, then look at the trouble shooing guide and see what inputs are required for it. If there are four direct sensor inputs required for a particular output, group them together.

Grouping internal interlocks together helps also when digging into a program like ladder logic instead of hopping through pages of diagrams or text to find everything it takes to get one output. It’s a common program development issues to throw in ideas as you program depending on where you are sequentially.

AQ: Non-regenerative & Regenerative DC Drives

Non-regenerative DC drives, also known as single-quadrant drives, rotate in one direction only & they have no inherent braking capabilities. Stopping the motor is done by removing voltage & allowing the motor to coast to a stop. Typically nonregenerative drives operate high friction loads such as mixers, where the load exerts a strong natural brake. In applications where supplemental quick braking and/or motor reversing is required, dynamic braking & forward & reverse circuitry, may be provided by external means.

Dynamic braking (DB) requires the addition of a DB contactor & DB resistors that dissipate the braking energy as heat. The addition of an electromechanical (magnetic) reversing contactor or manual switch permits the reversing of the controller polarity & therefore the direction of rotation of the motor armature. Field contactor reverse kits can also be installed to provide bidirectional rotation by reversing the polarity of the shunt field.

All DC motors are DC generators as well. The term regenerative describes the ability of the drive under braking conditions to convert the generated energy of the motor into electrical energy, which is returned (or regenerated) to the AC power source. Regenerative DC drives operate in all four quadrants purely electronically, without the use of electromechanical switching contactors:

  • Quadrant I -Drive delivers forward torque, motor rotating forward (motoring mode of operation). This is the normal condition, providing power to a load similar to that of a motor starter.
  • Quadrant II -Drive delivers reverse torque, motor rotating forward (generating mode of operation). This is a regenerative condition, where the drive itself is absorbing power from a load, such as an overhauling load or deceleration.
  • Quadrant III -Drive delivers reverse torque, motor rotating reverse (motoring mode of opera tion). Basically the same as in quadrant I & similar to a reversing starter.
  • Quadrant IV -Drive delivers forward torque with motor rotating in reverse (generating mode of operation). This is the other regenerative condition, where again, the drive is absorbing power from the load in order to bring the motor towards zero speed.

A single-quadrant nonregenerative DC drive has one power bridge with six SCRs used to control the applied voltage level to the motor armature. The nonregenerative drive can run in only motoring mode, & would require physically switching armature or field leads to reverse the torque direction. A four-quadrant regenerative DC drive will have two complete sets of power bridges, with 12 con trolled SCRs connected in inverse parallel. One bridge controls forward torque, & the other controls reverse torque. During operation, only one set of bridges is active at a time. For straight motoring in the forward direction, the forward bridge would be in control of the power to the motor. For straight motoring in the reverse direction, the reverse bridge is in control.

Cranes & hoists use DC regenerative drives to hold back “overhauling loads” such as a raised weight, or a machine’s flywheel. Whenever the inertia of the motor load is greater than the motor rotor inertia, the load will be driving the motor & is called an over hauling load. Overhauling load results in generator action within the motor, which will cause the motor to send cur rent into the drive. Regenerative braking is summarized as follows:

  • During normal forward operation, the forward bridge acts as a rectifier, supplying power to the motor. During this period gate pulses are withheld from reverse bridge so that it’s inactive.
  • When motor speed is reduced, the control circuit withholds the pulses to the forward bridge & simultaneously applies pulses to reverse b

AQ: Machine tool

Ahh I see the words machine tool and shop floor; now I can see where you guys are coming from. The type of machines that you talk about were controlled by relay logic and then when technology arrived the electrical drawings were probably “converted” into ladder logic. The techs had lots to do because you cannot translate relay based systems into ladder logic 100% successfully as they behave differently.

The guys doing this work are just that programmers. They are probably NOT software engineers and are closer to the shop floor techs who are fiddling about with your machines.

I can and have designed many control systems for automotive type machines such as hobbing machines, milling and borers. Very easy code to write if you do not translate the relay logic directly but use the existing documentation as a reference. All of the systems that I did work really well. I did some similar type of machines in a pharmaceutical plant but that was after another company was kicked out after failing to make the machines work. I had to redesign the whole control philosophy as the machine tool world methods used were really a bad fit for the intended application.

But that is only one facet of the work that we Industrial Automation Engineers do. I work in many different industries where the demands for quality deigned, controlled and maintained systems is paramount. We go through proper project life cycles and we deal with the project from inception through design, build, test and commissioning. We even do the maintenance of the systems. We do not sit in Ivory Towers but do the work at the customer site no matter where that is on the planet.

Electrical engineers are tasked with doing all things electrical and we are tasked with all things control. Programming, that is writing the actual code is only one part of what we do and not necessarily the most time consuming part.

I am here in Kazakhstan at the sharp end of a multi-billion dollar project a long way from any ivory tower. I fix other engineers software too, why? Because the vendor may use offshore resources to code much of the systems that are installed at site. Kazakhstan has extreme Summers (up to 60degC) and Winters (down to -50degC), most of the people are friendly but English is not so prevalent. A long way from your shop floor environment. Far more dangerous too as the plant processes H2S or will when first Oil & Gas comes onshore.

Here I have supported technicians performing loop checks and other engineers doing logic tests. I can diagnose many loop problems without even looking in the code but just by looking at what is happening. I have found that if a loop doesn’t work then the techs approach us first as a one stop shop to give them an answer rather than actually trouble shooting the loop themselves.

I said to you guys before you need to get out and look at other industries and see what is going on in the rest of the world. Much of what I have seen would go a long way to improving your world too! Engineers like myself are far away from the “programmers” you have.

AQ: DC drive typical applications

DC drive technology is the oldest form of electrical speed control. The speed of a DC motor is the simplest to control, & it can be varied over a very wide range. These drives are designed to handle applications such as:

Winders/coilers – In motor winder operations, maintaining tension is very important. DC motors are able to operate at rated current over a wide speed range, including low speeds.

Crane/hoist – DC drives offer several advantages in applications that operate at low speeds, such as cranes & hoists. Advantages include low-speed accuracy, short-time overload capacity, size, & torque providing control. A typical DC hoist motor & drive used on hoisting applications where an overhauling load is present.

Generated power from the DC motor is used for braking & excess power is fed back into the AC line. This power helps reduce energy requirements & eliminates the need for heat-producing dynamic braking resistors. Peak current of at least 250 percent is available for short-term loads.

Mining/drilling -The DC motor drive is often preferred in the high-horsepower applications required in the mining & drilling industry. For this type of application, DC drives offer advantages in size & cost. They are rugged, dependable, & industry proven.

AQ: Floor programmer and office programmer

The biggest differences between the floor programmer and the office programmer is often a piece of paper (knowledge and experience do not replace a piece of paper in the mind of HR person that has no understanding of the position they are seeking to fill) and that the floor programmer must produce a working machine. Also many an excellent programmer will never put up with the office politics seen in many companies. To appear right for me is worthless when being right is the goal. In a physical world it can be shown that a program is right or wrong because the machine works or does not. In the theory driven world of the office that can not happen, so appearing correct as well as being correct is necessary.

If you are the best programmer in your company or the worse. If you are the worse one then maybe you are correct. But if you are the best then please take a close look at the worse programmer’s work and tell us if there is not a need for some improvement.

I have cursed out more than one officer programmer for missing logic which on the floor is easy to see is necessary. The office programmer was more than once, myself. Making logic to control machine in theory is far more difficult a task than modifying that logic on a real running machine. Maybe your imagination and intelligence can create a theoretical image that matches the physical one.

Many office programmers are not up to that level. They lack the intelligence, imagination, experience or time to take an offline program that can be loaded and run a machine without help. But no fear, most start-up techs cannot debug a machine after the build is complete and remove all issues that will surface when the machine enters a customer’s plant and full production.

A good program will grow as time passes. To fill in the gaps in the software, to change the design from what design intended to what production requires and to cover the design changes as product models evolve. Static is not the floor condition of a good company, products and machines evolve and grow. More reliable, durable, quicker tool changes or device swaps, lower cycle times or more part types. There are examples of logic once written it never changes but that is not the whole of the world just one part of it.