Blog – Page 23 – AC DRIVES SOLUTION.

AQ: Voltage transmission & distribution

If you look back over history you will find how things started out from the early engineers and scientists looking at materials and developing systems that would meet their transmission goals. I recall when drives (essentially ac/dc/ac converters) had an upper limit around 200 to 230 volts). In Edison and Tesla days there was a huge struggle to pick DC or AC and AC prevailed mainly because it was economical to make AC machines. Systems were built based on available materials and put in operation. Some worked great some failed. When they failed they were analyzed and better systems built. Higher and higher voltages lowered copper content and therefore cost as insulators improved. Eventually commitees formed and reviewed what worked and developed standards. Then by logical induction it was determined what advances could be made in a cost effective and reliable manner. A lot of “use this” practice crept in. By this I mean for example, I worked at a company and one customer bought 3,000 transformers over the course of ten years, They had a specific size enclosure they wanted.

Due to high volume purchase the cost of the enclosure was low. Other small jobs came thru and this low cost enclosure was used on them to expedite delivery and keep cost minimum. Guess what, that enclosure is now a standard enclosure there because it was used on hundreds of designs over ten years. Is it the most economical box, probably not in the pure engineering sense but changing something that works is seldom a good idea. Today, they are raising voltage levels to new high values. I read of a project in Germany to run HVDC linesover huge distance. They are working to overcome a problem they foresee. How do you break the circuit with HVDC economically. If you ever put DC thru a small contactor maybe 600VDC you find quickly that the arc opening the contactor melts the contacts. Now, what do you do at 800kVDC or 1.2MVDC. What will the cost of the control circuit be to control this voltage level. (Edison and Tesla all over again)And there you have it, my only push for the subject of history to be taught.

AQ: Motor design

When I was doing my PhD in motor design of reluctance machines with flux assistance (switched reluctance machines and flux switching machines with magnets and/or permanently energised coils) my supervisor was doing research on the field of sensorless control (it wasn’t the area of my research but it got me thinking about it all). At the time I had thought (only in my head as a PhD student daydream) that I would have to initially force a phase (or phases) to deliberately set the rotor into a known position due to the phase firing then start a normal phase firing sequences to start and operate the motor for a normal load without the need for any form position detection (all this was assuming I had the motor running from stationary to full speed at normal expected load with use of a position sensor to start with so I could link phase firing, rotor position and timings all together to create a “map” which I could then try to use to re-program a firing sequence with no position detection at all but only if I could force the rotor to “park” itself in the same position every time before starting the machine properly – the “map” having the information to assume that the motor changes speed correctly as it changes the firing sequences as it accelerates to full speed). But any problem such as unusual load condition or fault condition (e.g. short circuit or open circuit in a phase winding) would render useless such an attempt at control with no form of position detection at all. The induction machine being sensorless and on the grid being measured.

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: Systems Development Life-Cycle

Step 1. Initiation
Step 2. System Concept Development
Step 3. Planning
Step 4. Requirements Analysis
Step 5. Design
Step 6. Development
Step 7. Integration and Test
Step 8. Implementation
Step 9. Operation and Maintenance
Step 10.Disposition

There are three major players present in this model; Customer (client), System Integrator, and Machine or device manufacturer.

In many instances, the result of step 4 (Requirements Analysis), is an RFQ for the system implementation has been issued to one or more systems integrators. Upon selecting the system integrator, step 5 (Design) begins. Upon completing step 5 (Design), the system or process flow is defined. One of the major outputs from step 5 are the RFQs for the major functional components of the finished system. Based on the RFQ responses (bids), the Machine or device manufacturers are chosen.

Steps 6, 7, and 8 are where all the individual functional components are integrated. This is where the system integrator makes sure the outputs and feedback between to machines or devices is defined and implemented. Step 8 ends with a full systems functional test in a real manufacturing situation is demonstrated to the customer. This test includes demonstrating all error conditions defined by the requirements document and the systems requirements document. If a specific device or machine fails its respective function it is corrected (programming, wiring, or design) by the manufacturer and the test begins anew.

Each of the scenarios presented is correct. The technician role being presented (customer, integrator, or manufacturer) is not clear. System diagnostics are mandatory and need to be well defined, even in small simple machines. There should be very few and extreme conditions under which the customer’s technician should ever have to dig into a machine’s code to troubleshoot a problem. This condition usually indicates a design or integration oversight.

(You can find a complete description here, http://en.wikipedia.org/wiki/Systems_development_life-cycle)