Blog – Page 23 – AC DRIVES SOLUTION.

AQ: PPE (Personal Protective Equipment)

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

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

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

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

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

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

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

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

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

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

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: 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: 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