Blog – Page 23 – AC DRIVES SOLUTION.

AQ: Induction machines testing

Case: We got by testing 3 different machines under no-load condition.
The 50 HP and 3 HP are the ones which behave abnormally when we apply 10% overvoltage. The third machine (7.5 HP) is a machine that reacts normally under the same condition.
What we mean by abnormal behavior is the input power of the machine that will increase dramatically under only 10% overvoltage which is not the case with most of the induction machines. This can be seen by the numbers given below.

50 HP, 575V
Under 10% overvoltage:
Friction & Windage Losses increase 0.2%
Core loss increases 102%
Stator Copper Loss increases 107%

3 HP, 208V
Under 10% overvoltage:
Friction & Windage Losses increase 8%
Core loss increases 34%
Stator Copper Loss increases 63%

7.5 HP, 460V
Under 10% overvoltage:
Friction & Windage Losses decrease 1%
Core loss increases 22%
Stator Copper Loss increases 31%

Till now, we couldn’t diagnose the exact reason that pushes those two machines to behave in such way.
Answer: A few other things I have not seen (yet) include the following:
1) Are the measurements of voltage and current being made by “true RMS” devices or not?
2) Actual measurements for both current and voltage should be taken simultaneously (with a “true RMS” device) for all phases.
3) Measurements of voltage and current should be taken at the motor terminals, not at the drive output.
4) Measurement of output waveform frequency (for each phase), and actual rotational speed of the motor shaft.

These should all be done at each point on the curve.

The reason for looking at the phase relationships of voltage and current is to ensure the incoming power is balanced. Even a small voltage imbalance (say, 3 percent) may result in a significant current imbalance (often 10 percent or more). This unbalanced supply will lead to increased (or at least unexpected) losses, even at relatively light loads. Also – the unbalance is more obvious at lightly loaded conditions.

As noted above, friction and windage losses are speed dependent: the “approximate” relationship is against square of speed.

Things to note about how the machine should perform under normal circumstances:
1. The flux densities in the magnetic circuit are going to increase proportionally with the voltage. This means +10% volts means +10% flux. However, the magnetizing current requirement varies more like the square of the voltage (+10% volt >> +18-20% mag amps).
2. Stator core loss is proportional to the square of the voltage (+10% V >> +20-25% kW).
3. Stator copper loss is proportional to the square of the current (+10% V >> +40-50% kW).
4. Rotor copper loss is independent of voltage change (+10% V >> +0 kW).
5. Assuming speed remains constant, friction and windage are unaffected (+10% V >> +0 kW). Note that with a change of 10% volts, it is highly likely that the speed WILL actually change!
6. Stator eddy loss is proportional to square of voltage (+10% V >> +20-25% kW). Note that stator eddy loss is often included as part of the “stray” calculation under IEEE 112. The other portions of the “stray” value are relatively independent of voltage.

Looking at your test results it would appear that the 50 HP machine is:
a) very highly saturated
b) has damaged/shorted laminations
c) has a different grade of electrical steel (compared to the other ratings)
d) has damaged stator windings (possibly from operation on the drive, particularly if it has a very high dv/dt and/or high common-mode voltage characteristic)
e) a combination of any/all of the above.

One last question – are all the machines rated for the same operating speed (measured in RPM

AQ: Industrial automation process

My statement “the time it takes to start or stop a process is immaterial’ is somewhat out of context. The complete thought is” the time it takes to start or stop a process is immaterial to the categorization of that process into either the continuous type or the discrete type” which is how this whole discussion got started.

I have the entirely opposite view of automation. “A fundamental practice when designing a process is to identify bottlenecks in order to avoid unplanned shutdowns”.

Don’t forget that the analysis should include the automatic control system. This word of advice is pertinent to whichever “camp” you chose to join.

Just as you have recognized the strong analogies and similarities between “controlling health care systems” and “controlling industrial systems”, there are strong analogies between so-called dissimilar industries as well between the camp which calls itself “discrete” and the camp which waves the “continuous” flag.

You may concern about the time it takes to evaluate changes in parameter settings for your cement kiln is a topic involving economic risks which could include discussions of how mitigate these risks, such as methods of modeling the virtual process for testing and evaluation rather than playing with a real world process. This is applicable to both “camps”.

The same challenge of starting up/shutting down your cement kiln is the same challenge of starting up/shutting down a silicon crystal reactor or wafer processing line in the semiconductor industry. The time scales may be different, but the economic risks may be the same — if not more — for the electronics industry.

I am continuously amazed at how I can borrow methods from one industry and apply them to another. For example, I had a project controlling a conveyor belt at a coal mine which was 2.5 miles long – several millions of pounds of belting, not to mention the coal itself! The techniques I developed for tracking the inventory of coal on this belt laid the basis for the techniques I used to track the leading and trailing edge of bread dough on a conveyor belt 4 feet long. We used four huge 5KV motors and VFDs at the coal mine compared to a single 0.75 HP 480 VAC VFD at the bakery, and startups/shutdowns were order of magnitudes different, but the time frame was immaterial to what the controls had to do and the techniques I applied to do the job.

I once believed that I needed to be in a particular industry in order to feel satisfied in my career. What I found out is that I have a passion for automation which transcends the particular industry I am in at the moment and this has led to a greater appreciation of the various industrial cultures which exist and greater enjoyment practicing my craft.

So these debates about discrete vs. continuous don’t affect me in the least. My concern is that the debates may impair other more impressionable engineers from realizing a more fulfilling career by causing them to embrace one artificial camp over the other. Therefore, my only goal of engaging in this debate is to challenge any effort at erecting artificial walls which unnecessarily drive a damaging wedge between us.

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: 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.