Iacdrive|Automation Solution

How generator designers determine the power factor?

The generator designers will have to determine the winding cross section area and specific current/mm² to satisfy the required current, and they will have to determine the required total flux and flux variation per unit of time per winding to satisfy the voltage requirement. Then they will have to determine how the primary flux source will be generated (excitation), and how any required mechanical power can be transmitted into the electro-mechanical system, with the appropriate speed for the required frequency.
In all the above, we can have parallel paths of current, as well as of flux, in all sorts of combinations.

1) All ordinary AC power depends on electrical induction, which basically is flux variations through coils of wire. (In the stator windings).
2) Generator rotor current (also called excitation) is not directly related to Power Factor, but to the no-load voltage generated.
3) The reason for operating near unity Power Factor is rather that it gives the most power per ton of materials used in the generating system, and at the same time minimises the transmission losses.
4) Most Generating companies do charge larger users for MVAr, and for the private user, it is included in the tariff, based on some assumed average PF less than unity.
5) In some situations, synchronous generators has been used simply as VAr compensators, with zero power factor. They are much simpler to control than static VAr compensators, can be varied continuously, and do not generate harmonics. Unfortunately they have higher maintenance cost.
6) When the torque from the prime mover exceeds a certain limit, it can cause pole slip. The limit when that happens depends on the available flux (from excitation current), and stator current (from/to the connected load).

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

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.

Techniques contribute in control system

1. Any successful methodology is not a simple thing to come by and typically requires a huge commitment in time and money and resources to develop. It will take several generations to hone the methods and supporting tools.

2. Once you get the methods and tools in place, you then face a whole separate challenge of indoctrinating the engineers in the methods.

3. Unique HMI text involves a lot of design effort, implementation, and testing.

Many of the techniques contributed by others in the discussion address faults, but how do you address the “normal” things that can hold up an action such as waiting for a process condition to occur, such as waiting for a level/pressure/temperature to rise above/fall below a threshold or waiting for a part to reach a limit switch?

Some methods allow for a text message that describes each step. When developing these text messages, I focus on what the step’s transition is waiting for, not the actions that take place during the specific step. This helps both the operator to learn the process as well as diagnose what is preventing the machine from advancing to its next step.

I have seen sequencing engines that incorporate a “normal” step time that can be configured for each step and if the timer expires before the normal transition occurs, then you have “hold” condition. While effective, this involves a lot of up-front development time to understand the process and this does not come cheaply (with another nod to John’s big check!).

(Side note on sequential operations: I have used Sequential Function Charts (SFCs/GRAFCET) for over 20 years and find them to be exceptionally well-suited for step-wise operations, both from a development perspective as well as a troubleshooting perspective.)

I have seen these techniques pushed by end users (typically larger companies who have a vested interest in standardization across many sites) as well as OEMs and System Integrators who see these as business advantages in shortening development, startup, and support cycles. Again, these are long-term business investments that require a major commitment to achieve.

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.