Iacdrive|Automation Solution

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?

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,

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.

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).

UPS systems commissioning test and inspection procedures

The UPS systems commissioning test and inspection procedures are to conform to;

• BS EN 50091-1:1993 – Specification for Uninterruptible Power Supplies (UPS). General and Safety Requirements, AND

• IEC 62040-3 (Draft Edition – 2) in particular the Efficiency test procedures outlined in its “Annexure-J”.

These procedures to include:

1. Visual Inspection:
a. Visually inspect all equipment for signs of damage or foreign materials.
b. Observe the type of ventilation, the cleanliness of the room, the use of proper signs, and any other safety related factors.

2. Mechanical Inspection:
a. Check all the power connections for tightness.
b. Check all the control wiring terminations and plugs for tightness or proper seating.

3. Electrical Pre-check:
a. Check the DC bus for a possible short circuit.
b. Check input and Bypass power for proper voltages and phase rotation.
c. Check all lamp test functions.

4. Initial UPS Startup:
a. Verify that all the alarms are in a “go” condition.
b. Energize the UPS module and verify the proper DC, walkup, and AC phase on.
c. Check the DC link holding voltage, AC output voltages, and output waveforms.
d. Check the final DC link voltage and Inverter AC output. Adjust if required.
e. Check for the proper synchronization.
f. Check for the voltage difference between the Inverter output and the Bypass source.
g. Perform full-load, step-load, and battery discharge tests using supplier furnished load bank.

Why your project failed?

I have contracted with lots of different groups and moving within the same company to save failed projects or project in trouble or impossible to implement and helped these groups to achieve company goal. What I have noticed is that less the managers or groups know less they realize more knowledge or experience can help them. Less they know, less they understand they need help because they don’t know what they need. They think they are just fine until it is too late and a group or company goes under because of it.

I give you an example of one of the project I worked on at Nortel. I was assigned to write project specification for a product working with a director group with 100 designers and testers. During my research to get information to write the product specification I discover deficiency in the hardware they wanted to use that would cause the system reliability and availability unacceptable to the customer and did not meet customer requirement. I proposed design change in one of the interface card and firmware used in the system. The management did not agree with me on this item so I refused to write the specification the way they want it to not expose this deficiency. We had a large meeting with the president of the company with 20 people in that meeting looking at two different presentation to see if they need to change direction or stay on course and move me out of the way to another project activity. I am not the greatest in politics and making things look good when they are not.

The result was that I was moved to different project for 1 year implementing and releasing one more product that made the company lots of money. After a year development, they complete the project and released it to the customer. The customer starts validating the product and had lots of the test cases failing in the area that I proposed to change.
This was a large project and lots of money involve. The customer rejected the product and they went back on the drawing board after getting lots of this equipment on order for this project. The management came back to me and one of my team members to come back to the team and help.

Me and my team member both having experience over 15 years at that point came back and have a solution designing a new complex interface card with microcode firmware and some software to save the project. I and he had to work for 4 months for day and night having design review between two of us at 3 am in the cafeteria to get it done (defining specification to validated working product).

This project was completed and customer accepted this solution. The director group was dismantled and all people in the group were laid off and absolved in other groups in the company and some in the same group. The management groups were smart people with good intention, they were with software background and good intention for the project. They just did not have the knowledge and background to manage the system and hardware level because of lake of knowledge and experience in that area.

You see this in lots of companies when a software designer or manager is successful in their area, they get promoted and manage groups that are out of their area and lots of time they destroy groups or project because of not having the background to identify good or bad direction to go. You will always have engineers to not agree with each other and managements have to make decision to go one way or another. The wrong decision in these cases can destroy a project or company. Not all engineers can present a case in 1 hour to sell you their point of view, Remember they are not lawyer or sales man, they are engineers. So what do you do, Follow the sales man or lawyer to save your project or the reason to drive your decision and if you don’t have the knowledge or experience to lesson to the reason then you will make the wrong decision.

What if

Many years ago we used to call this the “what ifs?”. Part of the design phase is when we model what we think the system is meant to do. Just as important is how the system is meant to react when things are not going well, the abnormal situations or what ifs?

Your client will tell you how their machine or process works, well he will describe how he thinks it works. This is OK as a starting point but we need to consider the scenarios of “what If” something goes wrong? Scenarios is also a good word as scenarios paint a situation that can be described to the customer for his comment.

For example on a compressor control project what if the lube oil pump fails on the compressor, how do we alarm this to the Operator, should we trip the compressor or do we start the back-up oil pump (if there is one). As you look at the system you can pick out various components and generate likely scenarios that you can discuss with the client. Using this approach gives more of a real world feel to your client meetings that are likely to generate a deeper insight into how the system is meant to work.

All scenarios do not have to be centered around abnormal situations but can also relate to things that need to be considered as part of normal operation. For example we might look at how a duty/standby pump system works? One scenario might relate to duty pump failure but another scenario might consider rotating the duty and standby pumps to even out wear and tear? You might also have manual mode and auto mode scenarios to consider?

What you have to remember is that most clients are not control systems experts. They might and probably will struggle with flow charts or any other pseudo code expression type formats that describe how you think the client’s system is meant to work? You have to tailor your approach to match your audience and that is also very important when you produce your documentation, you do not want to lose valuable information just because the client doesn’t fully understand what you are trying to tell him? Also make sure that you spend time with your client. Walk the client through your design, do this face to face as much as possible and do it more than once! Getting feedback on a regular basis helps to eliminate the dreaded word “REWORK”! Also taking this partnership approach builds a good relationship with your client.

For the machine builders your client might be in your own company? Remember same company or not they are your client and your success in no small way depends on your relationship with them.

Add the modes of operation and abnormal situations to your system model and develop the methods of how you will flag these situations to the Operator and Maintenance Engineer. Alarm Management is dealt with by EEMUA 191. If you do nothing else then read this document it will help you to set up alarm workshops, alarm reviews, alarm prioritization and rationalization and how to develop an effective alarm management structure for your system.

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.

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.

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