Blog – Page 25 – AC DRIVES SOLUTION.

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: Design and Implementation

The owner of the system should provide clear requirements of what the system should do and should define what constitutes “maintainability” of the system. This places a burden on the owner of the system to consider the full life-cycle of the system.

1. You need good design documentation.

2. All source code should be well-documented.

3. Coders should be trained on the techniques used and mentored,

4. The use of “templates” helps ensure that coders and maintenance alike are familiar with routine functions.

5. The HMI should provide clear indication of faults and interlocks.

6. The HMI should provide clear indication of equipment statuses.

7. Any code that is hidden must “work as advertised”. This means that it must be completely and unambiguously documented for all inputs, outputs, statuses, and configurations. It must be thoroughly tested and warranted by the vendor,

8. All code should be well-tested. (I have found that the first line of defense is to simply read the code!)

Post-Startup
1. The owner should have a change-control procedure to manage modifications.

2. All users and maintenance support personnel should have adequate training. Training needs to be periodically refreshed as it can become stale through lack of use.

AQ: Benefits of Having products and services in the same company

Having products and services in the same company can either be treated as an opportunity or as a constraint. I strongly believe that having services and products in the same company should be treated as an opportunity, and that any potential constraints should be eliminated.

Here are the things that I have learned.

First: Never limit the product sales to the capacity of your service organization:
I see some companies that develop products that are so great that they want to be the only organization delivering, implementing and maintaining them. They believe that the products are a competitive advantage that will allow them to dominate the services market. This almost always fails; your example from Xerox is one of many. One of two things tend to happen: Either the product does not reach its full market potential due to limited services capacity, or the product organization limits their innovation and product development so that it can continue a lucrative services business. Both may be good short term, but fails on a longer term basis.
My recommendation is that companies that have both products and services should allow their products to be delivered, implemented and maintained by other companies that compete with themselves in the services market.

Second: Never limit the services that you offer to the products that you have in your own portfolio:
Service organizations are typically focused on delivering, implementing and maintaining solutions for their customers. They deliver more than just the product. If you limit the services to only focus on the products in the in-house portfolio, then you are either going to miss opportunities to sell services or you are going to get a portfolio that is too broad. Neither of them is good.
My recommendation is that companies that have both products and services should allow their services organization to deliver products from everywhere, even products that directly compete with the products in their own portfolio. This will ensure that the services organization stays competitive.

Third: Leverage the synergies between products and services:
You may ask “why have both products and services in the same organization if they need to be kept separate?”. The answer lies in the synergies. Companies need to create a culture where the product and services organizations can collaborate even though they are independent. Good organizations can make good decisions about when to expand their own portfolio and when to solve the same customer problems through services and/or third party products. I have seen great innovations come from organizations that master this.

Having products and services in the same organization creates a great foundation for innovation. The key to success is to have the right company culture.

AQ: Operate low speed generator and high speed generator in the same terminal

Can we operate low speed generator and high speed generator in the same terminal? Is there a mechanical effect?

First, specify that this is an isolated system with two generators feeding the same bus. Operation of an isolated system is different than a grid connected system, and the mode setting of the governors have to be set to accommodate this. Depending upon the prime mover type and governor model, improper tuning will manifest itself in speed variations. The size of the two machines relative to each other, as well as their size relative to the load, can have measurable impact as well. The best way to tell whether it is mechanical or electrical in nature is to look at the time-frame of the phenomena relative to the time constants of the various control and response loops.

Second, “…In large power system, generators are not connected in the same terminal…” is not generally true, there are many power plants where multiple generators feed the same bus before the power is utilized.

Third, “…frequency oscillation is about 1.5-2 Hz…”, if you mean that the frequency swings between 48 and 52 Hz routinely, that usually indicates a governor setup/tuning problem or a non-uniform load.

Fourth, reactive current compensation takes place in quadrature from real power and should have minimal effect on real power and only affect the terminal voltage if not set properly. Droop compensation is the means for ensuring that the AVRs do not fight with each other since you cannot have two independent controllers attempting to control the same control variable.

Fifth, regarding different types of prime movers, some are inherently more likely to induce mechanical vibrations, especially reciprocating engines, especially if they are not all of the same size and/or number of cylinders. The same is true of the loads, non-uniform, cyclic loads can cause very severe problems especially on isolated systems where the load is a significant percentage of the prime movers’ output power. The analysis of, and solution to, such problems is an interesting area of study.

AQ: Why we need Engineers?

Even the humble motor car runs diagnostics that the garage read to see the problems with your car. This doesn’t involve technicians looking at the code that controls the car but is 100% driven by the faults flagged by the car’s management system programs. These could even be displayed to the users, the drivers like me and you but the manufacturers don’t want amateurs hacking around their management systems and you know that is exactly what we would do.

Do we ask for this functionality from our car manufacturer? Do we complain about it and ask for them not to fit it? Would we like to go back to the “golden age” of motoring where we spent as much time under the hood as we did on the road?

You do?…. Yeah right and neither do I nor do I want a plant where I need a guy with a laptop to diagnose a blown fuse, sticking valve, overload trip, etc .

We need to change as Engineers by selling systems to customers that fulfill their needs, that are safe and reliable, that follow industry and international best practices and are user friendly. The notion of having to wait for a blank cheque from the customer to fulfill these goals is really a cop out, you either do what is right or just walk away because at the end of all this it is you who are under scrutiny when things go wrong not the customer who will plead ignorance.

AQ: DC Drives Parameter Setting / Programming

Programming parameters associated with DC drives are extensive & similar to those used in conjunction with AC drives. An operator’s panel is used for programming of control setup & operating parameters for a DC drive.

SPEED SETPOINT
This signal is derived from a closely regulated fixed voltage source applied to a potentiometer. The potentiometer has the capability of accepting the fixed voltage & dividing it down to any value, For example, 10 to 0 V, depending on where it’s set. A 10-V input to the drive from the speed potentiometer corresponds to maximum motor speed & 0 V corresponds to zero speed. Similarly any speed between zero & maximum can be obtained by adjusting the speed control to the appropriate setting.

SPEED FEEDBACK INFORMATION
In order to “close the loop” & control motor speed accurately, it’s necessary to provide the control with a feed back signal related to motor speed. The standard method of doing this in a simple control is by monitoring the armature voltage & feeding it back into the drive for comparison with the input setpoint signal. The armature voltage feedback system is generally known as a voltage regulated drive.

A second & more accurate method of obtaining the motor speed feedback information is from a motor mounted tachometer. The output of this tachometer is directly related to the speed of the motor. When tachometer feedback is used, the drive is referred to as a speed regulated drive.

In some newer high-performance digital drives, the feedback can come from a motor-mounted encoder that feeds back voltage pulses at a rate related to motor speed.

These pulses are counted & processed digitally & compared to the setpoint, an error signal is produced to regulate the armature voltage & speed.

CURRENT FEEDBACK INFORMATION
The second source of feedback information is obtained by monitoring the motor armature current. This is an accurate indication of the torque required by the load.

The current feedback signal is used to eliminate the speed droop that normally would occur with increased torque load on the motor & to limit the current to a value that will protect the power semiconductors from damage. The current-limiting action of most controls is adjustable & is usually called current limit or torque limit.

MINIMUM SPEED
In most cases, when the controller is initially installed the speed potentiometer can be turned down to its lowest point & the output voltage from the controller will go to zero, causing the motor to stop. There are, how ever, situations where this is not desirable. E.g.,, there are some applications that may need to be kept running at a minimum speed & accelerated up to operating speed as necessary. The typical minimum speed adjustment is from 0 to 30 percent of motor base speed.

MAXIMUM SPEED
The maximum speed adjustment sets the maximum speed attainable. In some cases it’s desirable to limit the motor speed (and machine speed) to something less than would be available at this maximum setting. The maximum adjustment allows this to be done.

IR COMPENSATION
Although a typical DC motor presents a mostly inductive load, there is always a small amount of fixed resistance in the armature circuit. IR compensation is a method used to adjust for the drop in a motor’s speed due to armature resistance. This helps stabilize the motor’s speed from a no-load to full-load condition. IR compensation should be applied only to voltage-regulated drives.

ACCELERATION TIME
As its name implies, the acceleration time adjustment will extend o

AQ: Power industry engineers

The power industry has many tentacles. Energy production is one key subset, the design, manufacture, installation and operation of hydro, nuclear, fossil, renewables, etc is continuing to grow especially in the renewable area. Then there is the transmission of energy which includes the design/manufacture/construction/maintenance of substations, protection and control systems, overhead and underground lines, series and shunt compensation, etc. Last there is the distribution of the energy to the customers at the lower voltages which includes many of the transmission opportunities but introduces other niche areas like power quality, smart metering, distributed generation, etc.

It’s not as simple as stating you want a PHD in the power industry with hands on experience without first knowing the ins and outs of the business. As has been previously mentioned, get your BS in EE with a slant toward power. Get a job in a utility and learn the business top to bottom so you can actually make an intelligent decision on what area of the business floats your boat. Once you know that then pursue an advanced degree in that specific area (the real bonus is most companies will pay for it).

AQ: Why designing an ethernet network IP scheme?

Depends on the size of the network (# of devices planned on connecting), for medium to large corporate networks go 10.x, for home and small business 192.168.x, or to 172.16.x. I would think the IP plan would be looking at least 10 – 20 years out. Changing IP schemes is hard, especially on a controls LAN, you wouldn’t want to undertake this task to frequently. Also consider any routing / firewalling / DMZing that you may want to do between the controls LAN and the business network (ideally these are separated networks).

Here’s some things to consider:

Number of devices or potential devices on the network
You may want to use a Class A subnet when you have or will have a large number of devices or a Class C when you have or will have a small number of devices.

Amount of traffic
A large subnet will more likely expose devices to more traffic. A smaller network may be employed to segment and/or control the amount of data that must be handled by a device.

Security
A large network (e.g., Class A) network may be more difficult to restrict access to or exposure of devices.

Simplicity
A Class A network is a flatter architecture and may be simpler to manage because you don’t have to worry about routing, gateways, and/or firewalls as much. This has to be balanced with security and traffic issues though.

Others
There are other considerations too…

In my experience, connecting with the “business” side of things is not technically difficult with an appropriate firewall/router. However, I have often found that the political challenges are more difficult. I have often butted heads with IT folks who have a fortress mentality and don’t understand the constraints, limitations, restrictions, and special considerations needed for industrial control systems. Many times, the best solution is to have a well defined line of demarcation where the IT folks take care of their side and the control guys take care of the control side. Most IT folks are OK with that as long as they can quarantine the control side to their satisfaction.

When it comes to selecting the firewall/router, you will need to take into consideration the protocols passing through it. If it’s the nominal business protocols like http, ftp, rdp, ssh, etc., then any business class device will typically work. However, if industrial protocols like CIP, Ethernet/IP, or OPC will be passing through, you will need to confirm that the firewall/router supports them specifically. When making the link, the important thing is the type of packet filtering and address translation rules that are configured in the firewall router. The IT folks might be more happy if they can setup a VLAN just for the controls.

AQ: Rotary Tube Furnace Efficiency

There are many factors that govern the performance of rotary tube furnaces. A direct fired rotary unit has a potential for much higher thermal efficiency due to the direct contact of the hot gases with the material in process. Cement kilns are the most common large scale unit operation with direct fired units. Any articles you find on this will be helpful. Thermal efficiency can be estimated by dividing the inlet temperature minus the outlet temperature by the inlet temperature minus the ambient temperature in absolute scales either Rankine or Kelvin. Then there is the issue of co-current versus counter-current firing and heat recovery from the hot material and the exit gas for which standard designs are available. Indirect fired rotary kilns have heat transfer limitations due to the thickness and alloys needed for high temperature calcination >500 C. There is no simple way to measure the equivalent of the inlet and outlet temperatures on a direct fired unit. There are simply exit gas temperatures from each zone and an approximate shell temperature on the hot side of the shell which is lower than the zone exit gas temp. These are useful for control purposes and consistent operation. The higher the temperature the material requires to achieve conversion the higher the shell side fired temperature has to be to provide the delta T necessary to drive heat through the shell into the material zone.

Some materials further limit heat transfer by adhering to the inside of the shell and acting as an insulator! This requires trial and error application of “knockers” at the ends of the shell or sometimes internally secured chains that bang around and knock the adhering material loose. This is a potential nightmare as the learning curve to install chains so that the securing lugs and the chains themselves will stay attached for acceptably long service before failing and ending up in the take off conveying equipment with usual breakage and downtime is an uncertain one. From Perry’s one can find thermal efficiencies for indirect fired rotary’s given as less than 35%. The bed fill can be 10-30% depending on the heat demand of the material and the heat transfer limitations. You will want to have real time gas usage metering on the burners so that you know the theoretical energy input. From that you can subtract the theoretical heat needed to complete your reaction and compare that to the input to see how efficiently you have used the energy input.

The few large high temperature direct fired rotary kilns I have seen had view ports for measuring the local wall temperatures by optical pyrometer. It can be a challenge to get a protected thermocouple sheath down into the moving bed for an actual bed temperature and even just to hang it in the gas streams at the outlet or inlet area. See if you can contact cement kiln suppliers for some configurations of temperature sensing elements for your application. Bed fill effects on heat transfer are related to several parameters. Above ~500 C gas and refractory liner temperatures, the main heat transfer mode will be radiative as far as the surface of the bed material. Within the bed it will be conduction and some convection at the surface. A thin bed will reach max. temperature in shorter time, but this reduces through put for a given gas temperature. If you increase bed fill to increase production you will have to increase the firing temperature and the outlet temperature will probably increase lowering your thermal efficiency. This becomes a trade off between production rate and energy efficiency. Countercurrent firing usually maintains the highest driving force for heat transfer along the bed and gives the highest temperature of the bed just before exit of the bed material.

Perry’s may have a useful section on direct fired rotary kilns and lime or cement manufacturing references may help you as well. Please make sure lead emissions to air are properly captured

AQ: Low impedance fault

A low impedance fault is usually a bolted fault, which is a short circuit. It allows a high amount of fault current to flow, and an upstream breaker or fuse usually senses the high current and operates, ending the event. A high impedance fault, usually an arc fault, is a fault of too high of an impedance for overcurrent protection to detect and operate, so the fault exists for long period of time without tripping upstream protection. Examples of arc faults are: A high or medium voltage distribution utility wire falling to earth in a Y grounded system and arcing to earth where no breaker or fuse will clear; another example is any fault tracking through a substance such as cable insulation or even air….this could be wiring within a building wall with a fault that lasts long enough to ignite the building wall it is installed in, which happens all the time somewhere (sometimes called “arc through char”). Another high impedance fault is one within a transformer secondary coil, arcing through the coil insulation and transformer oil (oil cooled units)…the arc will boil the oil into component gases such as acetylene and hydrogen and if the arc fault lasts long enough and gets to the gases, the gases may explode…and the primary fuse protection will likely not detect this for some time. There are many other examples of high impedance faults. One way to tell a high impedance fault or arc fault is if there is a protecting breaker or fuse that did not operate for a fault…if the breaker or fuse are correctly sized and working properly and did not operate that usually indicates a high impedance fault….a short circuit usually generates high enough current to trigger breaker/fuse operations (assuming normal circuit impedance is low). Another way to look at it is any fault in a power circuit with an impedance such that less than “available” fault current flows.