Blog – Page 25 – AC DRIVES SOLUTION.

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.

AQ: DC Drives QUIZ

1. List three types of operations where DC drives are commonly found.

2. How can the speed of a DC motor be varied?

3. What are the two main functions of the SCR semi conductors used in a DC drive power converter?

4. Explain how SCR phase angle control operates to vary the DC output from an SCR.

5. Armature-voltage-controlled DC drives are classified as constant-torque drives. What does this mean?

6. Why is three-phase AC power, rather than single phase, used to power most commercial & industrial DC drives?

7. List what input line & output load voltage information must be specified for a DC drive.

8. How can the speed of a DC motor be increased above that of its base speed?

9. Why must field loss protection be provided for all DC drives?

10. Compare the braking capabilities of nonregenerative & regenerative DC drives.

11. A regenerative DC drive requires two sets of power bridges. Why?

12. Explain what is meant by an overhauling load.

13. What are the advantages of regenerative braking versus dynamic braking?

14. How is the desired speed of a drive normally set?

15. List three methods used by DC drives to send feed back information from the motor back to the drive regulator.

16. What functions require monitoring of the motor armature current?

17. Under what operating condition would the mini mum speed adjustment parameter be utilized?

18. Under what operating condition would the maxi mum speed adjustment parameter be utilized?

19. IR compensation is a parameter found in most DC drives. What is its purpose?

20. What, in addition to the time it takes for the motor to go from zero to set speed, does acceleration time regulate?

AQ: “critical” operation with a double-action cylinder, hydraulic or pneumatic

If I had a “critical” operation with a double-action cylinder, hydraulic or pneumatic, I’d put proximity sensors on both ends of travel, typically with small metal “marker” on the shaft. Each input “in series” with the “output” to each coil, time delayed to give the cylinder a chance to reach its destination. The “timer” feeds the “alarm.” If you want to spend the money for a pressure switch (or transducer) on each solenoid output, that’s a plus.

Now you can tell if there was an output to the solenoid from internal programming, if not another interlock prevented it from actuating. If there is an output to the solenoid and no pressure, then the signal did not reach the coil (loose wire somewhere), if it did the coil may be bad, if the coil is good and no pressure, the solenoid may be stuck or no pressure to it from another supervised failure or interlock. If there was sufficient pressure and the cylinder travel not reached, then the cylinder is stuck.

As a technician crawling over all kinds of other people’s equipment since 1975, I could figure out a lot of this from an old relay logic or TTL control system. A VOM confirms whether there is an output to the correct solenoid at the control panel terminals. This lets you now which direction to head next. If there is no power, it’s “upstream” of there, another interlock input that needs to be confirmed, time to dig into the “program.”

If there is power and the cylinder does not move it’s a problem outside of those terminals and the control system. I’d remove the wiring and check for coil resistance, confirming the coil and field wiring integrity while still at the panel. If everything checks out then go to the cylinder and see if a pressure gauge shows pressure on the line with the coil energized – presuming there is pressure to the valve. No pressure would be another “input alarm” from another pressure switch. If there is pressure and power to the valve and no pressure, the valve is bad. If there is pressure on the output side and the cylinder does not move – the cylinder is stuck or mechanically overloaded.

I&E “technicians” may know a lot about programming and code, but if they don’t know how a piece of equipment operates I/O wise then they don’t have a clue where to start looking. Then I guess you need all the sensors and step by step programmed sequences to “spell it out” for them on a screen. A device sequence “flow chart” may help run I/Os out for something like above. I/O status lights on the terminals like PLCs can easily confirm at a glance if you have the proper inputs for a sequence to complete, then you should have the proper outputs. Most output failures are a result of correct missing inputs. The more sensors you’re willing to install, the more the sequence can be monitored and spelled out on an HMI.

From a factory tech support in another location, being able to access the equipment remotely is a huge plus, whether directly through modem, or similar, or indirectly through the local technician’s computer to yours i.e. REMOTE ASSISTANCE. A tablet PC is a huge plus with IOMs, schematics and all kinds of info you can hold in one hand while trouble-shooting.

AQ: Industrial Ethernet vs. Fieldbus technologies

Where we really need digital communication networking, in my personal opinion, is down at the sensor/transmitter and positioner/actuator/valve level to take the place of 4-20 mA and on/off signals. Down at the level 1 of the Purdue reference model you need a fieldbus, not one of the “H2” types of fieldbus, but one of the “H1” types of fieldbus. When first introduced, these technologies were not as fast and not as easy to use has they could have been, but after many years of refinement these technologies are finally becoming sufficiently easy for most plants to use.

An “H1 fieldbus” is the most practical way to digitally network sensors/transmitters and positioners/actuators/valves to the DCS. Options include FOUNDATION fieldbus H1, PROFIBUS-PA, CompoNet, ASI, and IO-link. These protocols can take the place of 4-20 mA and on/off signals.

Note that “H1 fieldbus” should not be confused with the very different “H2 fieldbus” category of protocols used at level 1-1/2 of the Purdue reference model to connect remote-I/O,

AQ: Different brushes at same ring

Recently I had to do a report explain why is impossible join brushes, at same time, from different companies, even with same characteristics.
I used the follow points:
1 – Even with same characteristics the final results is different because tue proportion of material and/or manufacturing process different lead to a different brushes;
2 – Guarantee, because our machine is new, and is a good practice use brushes recommended by Manufacturer;
3 – The film, that is formed on the rings by the brushes could change (but I don’t have any sure if chage for bad);

Unfortunately my report was based on experience for old engineer and recommendation of Manufacturer.

One
of the most important thing about brushes in high current density
environments is uniformity. If there are any variations in material
composition, manufacturing methods, dimensions, porosity, density,
surface hardness, friction coefficient, pig-tail attaching means, size
of pig-tail conductor, etc., there will be a variation in the current
division and/or wear.

Ultimately some brushes will carry more current than others and the increased current density in those brushes will lead to overheating, pitting, scoring, and ultimately costly repairs to the commutator/slip-rings. You might also accidentally mix brush grades when dealing with multiple vendors.

Although manufacturers publish data for brush materials which may prove to be very close to one another, mixing them on a collector surface is not a good practice. Any signs of undesirable performance would be difficult to identify the root cause for and small differences in electrical resistance can produce staggeringly varied performance from each brush.

While the materials used have good material data supplied with them, the manufacturing of the cable connection does not which can account for many times the resistivity differences of the material. Brush manufacturers do use a variety of materials here also and so some brushes, even of the same grade and from the same supplier but with different connection material, cannot be used together.

Mixing of grades is an uncontrolled practice which leads to variable surface conditions especially where the numbers of each grade used is not controlled.

Lower resistance brushes will “grab” the current possibly over filming the collector surface leaving the higher resistance brushes to run at lower than prescribed minimum current densities which results in higher coefficients of friction at the brush/collector interface. You would never know when your film is stable which endangers machine life.

Most machine manufacturers select a grade of carbon to use which is useful at the machines fully rated capacity. However, manufacturing tolerances, specifications etc can produce a machine vastly over rated for your application. Running the manufacturers supplied brushes at reduced load can be very damaging. Most Manufacturers will accept that you need another brush grade for your specific use and will maintain warranty provided they have been consulted regarding any changes.

Many overlook that by moving a machine from one position in their plant to another, that they well need to consider the brush grade at that time also. Sometimes a simple and cost effective reduction of brushes (of the same grade) within the machine can increase plant reliability and longevity dramatically. Other times a consultation with a brush expert can lead to an alternative grade to produce better performance.

AQ: Variable Air Volume System Optimization

Variable Air Volume Systems (VAV) can be optimized to increase energy savings by maximizing the efficiency of the equipment at part-load conditions. The goal with the optimization strategy is to run each subsystem (chiller, cooling tower, Airhandler, etc) in the most efficient way possible while maintaining the current building load requirement.

VAV System Optimization

As each Variable Air Volume terminal controls the space temperature – based on flow the worst case zone can easily be identified by an automation system. The supply fan speed can be reduced by resetting the static pressure (see following page). As the load drops and the fan meets a preset minimum flow, the system resets the air temperature up, so less chilled water is needed. In a variable flow chiller system, this reduces pumping energy.

If the system load continues to drop, the system will reset the chiller supply water temperature upward which will then reduce the energy requirements of the chiller. Changes in the chiller head pressure and loads can then reset the cooling tower fan speed.

The key to optimizing the system operation is communication and information sharing through the entire system equipment. With the reduced cost of variable frequency drives and Building Automation Systems, (BAS) complete system optimization can be implemented as a cost effective option.

In VAV systems where the individual VAV boxes and the AHU are on a building automation system, additional savings can be achieved by implementing static pressure reset. The static pressure sensor in a VAV system is typically located two-thirds of the way downstream in the main supply air duct for many existing systems. Static pressure is maintained by modulating the fan speed.

When the static pressure is lower than the setpoint, the fan speeds up to provide more airflow (static) to meet the VAV box needs, and vice-versa. A constant set point value is usually used regardless of the building load conditions.

Under partial-load conditions the static pressure required at the terminal VAV boxes may be far less than this constant set point. The individual boxes will assume a damper position to satisfy the space temperature requirements. For example, various VAV box dampers will be at different damper positions, (some at 70% open, 60% open, etc) very few will be at design, ie 95% -100% open.

RESET STRATEGY
Essentially, resetting supply air static pressure requires that every VAV box is sampled with the static reset set to the worst case box requirement. For example, each box is polled, every 5 minutes. If no box is more than 95% open, reduce duct static pressure set point by 5%. If one or more boxes exceed 95% open, increase static pressure set point by 5%.

With a lower static set point to maintain, fan speed reduces. The result is increased energy savings in the 3 to 8% range. See figure below. If the BAS system is already installed, implementing this strategy is relatively free.
Variable Air Volume System energy savings

AQ: Renewable Energy in India

Holistic and Combined i.e Hybrid Renewable Energy Generation per Taluka / District of Each state with Energy Potential study with Investment seeking proposal with land (barren) identified with Revenue department clearances and also with a clear MAP of Evacuation with existing Transmission lines and future lines to planned, which shall be appended to RfP and not ask each developer to identify the location and struggle with Government Administration (which will increase time and Costs (read wrong costs)) complying to Land Acquisition bill and also eliminate the real estate babus to relinquish 5000 acres of land per state, which is BENAMI now…..I do not know how this excess land in BENAMI exist when we have Land ceiling Act!!

In order to do an extensive and credible study to explore renewable energy potential in each Taluka, State and Central Government Can hire international Consultancies with Video Documentation with GPRS MAPS to know the real truth and there shall not be much difference between reports and the ground reality, otherwise, hold these agencies responsible with necessary punitive clauses.

These costs can be recovered in the form of Bid document charges, which any serious developer will pay. However, the Equity selling proxy promoters, who have access to the power corridor and bid with Net worth Financial capacity, but, not worthy of any Renewable energy promotion as we saw in JNNSM wherein a large corporate bought equity from the other bidders and later an investigation took place…..

Following is the excerpts of the Mail written to MNRE and KREDL, in Jan 2012 (now we see their web site showing Biomass study is under progress):

For Power evacuation, we need to know the following (as we can’t use the existing data):

a). Distance from the Power generation site, which normally comes under KREDL (single window agency) i.e where one can put up the plant by undergoing NA or KREDL has identified land bank in Yadgir, but, how many km is the Substation from these sites, which we verified, was difficult to ascertain due to patch lands and the distance was over 10 km in certain cases.

b). Whether these substations can accept 20 MW or 10 MW or 5 MW of intermittent Solar PV load (non firm power which at times may create grid related disturbances etc). Biomass power is firm power as long as Firm biomass feed stock is available.

Therefore, we have been writing to many agencies involved to come out with a common approach, wherein the bidding documents identify clearly the SLDCs where the Project Developer can upload (evacuate) the energy generated with an in principle approval (with location MAP with transmission distances etc) from SLDC and ESCOM to accept such Renewable energy as the States are bound to buy the RE under RPO.

If the investor or RE Generator has to run around to know the fundamentals, then, please try to imagine how many man hours will be wasted and how much money gets drained from many participants for the same location? Instead, these data is available with KPTCL / KREDL / KERC / ESCOMs or such multiple organisation, but, Single window agency KREDL does not produce such VITAL information in their bid documents, hence, we as entrepreneurs are trying to tie the loose ends and make things happen for the good of our state.

I hope you understand our concern and append the finer details of evacuation, project site, land bank, the maximum capacity of MWh the substation can take or any upgrade is needed etc be appended in the bidding documents or even in your web sites also.

Further, any new substations are under development, the same with a clearly identified MAP with distances will help the people to understand the grid network to ensure the grid sustainability, reduction in transmission lines and hence the losses can be planned while making the bids, which otherwise will be a