Iacdrive|Automation Solution

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.

Resistance to ground

Resistance to ground is greatly influenced by the ambient conditions and the state of the motor when tested.

Factors Affecting Insulation Measurement:
First, it is important to understand that we are measuring a motor circuit. We are connecting our test instrument at a point where we can measure the majority of the de-energized circuit. As such, we do not necessarily know where an insulation anomaly is located when identified. We also have the motor circuit potentially exposed to differing environments. Ambient temperature and humidity can have a significant effect on any insulation measurements. When a motor circuit’s insulation is tested is also a major variable. Testing a motor circuit immediately after shut down will most likely yield good results. This is because the motor is warm and dry. Testing a motor after it has been shut down for a while may indicate insulation problems, but if the motor is allowed to reach ambient temperature, the insulation integrity may appear normal. This is because while cooling, particularly in somewhat humid conditions, moisture (condensation) will accumulate within the motor and lessen ground resistance. Is this a problem? Yes, particularly if starting from a partially cooled state. Most motor failures occur during starting. This is when the insulation is exposed to the most stresses. If your motors are only down for a few hours at a time, then this is when insulation testing should be conducted.

When conducting insulation testing, the most important consideration is consistency. Always test at the same location, use the same test voltage, perform the test for the same amount of time, and use the same test instrument. It is also important to note the motor temperature, ambient temperature, and relative humidity. It is also helpful to compare like motors and the motors that are operating within the same environment.

Insulation testing is somewhat ambiguous. Although there are reference standards, they cannot be rigidly followed because they do not factor in all of the potential variables that may be encountered. Temperature is the biggest variable. Temperature of the motor and the ambient temperature are of primary concern. One method to help negate the influence of temperature is performance of a “Timed Resistance Test.” This testing is comprised of “Dielectric Absorption,” “Polarization Index,” and “Step Voltage” testing. Dielectric Absorption is a 1 minute test. The resultant values at 30 seconds and 1 minute are logged and the ratio of the 30 second value divided into the 1 minute value, is a relative indicator of insulation integrity. A polarization index is a 10 minute test with the resultant ratio derived from the 1 minute value divided into the 10 minute value.

So, if ground resistance is low after prolonged shutdown and it is at ambient conditions, then you probably have an insulation issue. Conditioning of the insulation may be required. A motor shop can perform a “Clean, Dip and Bake.” process which will prolong the motor longevity. If the motor is several years old you may want to HiPot the insulation but if you are not using one of the newer units that automatically shut down upon a jump in current, you may cause insulation failure and that would necessitate a rewind.

Generator reactive power

After the generator connected to grid, the generator will be more stable than before connected to grid, because in this situation the frequency and voltage are fixed and controlled by the grid, not the independent generators. How much active and reactive power you can contribute to the grid depends on the grid requirement, such as when the grid shorts of active power, the frequency of the grid will drop, and then the grid will ask you or other generators to contribute more active power, and if short of reactive power, voltage will drop, then you could be asked to contribute more reactive power, and vice versa, which depend on the balance of power which is generated from generators and consumed by the users.

From generator side, the less reactive power, the better, as this power increase the VA and then the current to increase the losses on the transmission line which will be carried by the plant. But from grid side, as not too many equipment can generate the reactive power, the more contribution of the reactive power, the better.

At the full load operation of generator, the maximum contribution of reactive power should depend on the PF of the generator at full load (manufacturer provided for each generator). If your PF is too low and it could affect your active power transfers to the grid and will be punished by the grid. At the not full load situation of the generator, the PF could not be decided by the generator, if the grid does not need too much active power from you, but needs more reactive power and asks you to contribute more, PF could be more than 1 at the moment, but never over the Max reactive power calculated from full load.

Solar power

On a purely theoretical level and ignoring interrelated economics and energy usage, it makes sense to charge EVs during the day – though never in non-distributed environments, IMO.

In reality, and the reality for likely the rest of my life, it makes more economic and particulate emissions sense to distribute solar power during the day to decrease, and ultimately decommission, fossil fuel sources used for peak demand supply that occurs during the day.

Thus, using solar output distributed to offset the dirtiest, most expensive and most distribution grid loading power enhances and optimizes the value and worth of that solar generated power – both economically and ecologically. Attempting, therefore, to do all of ones’ EV charging off peak is the optimal solution until the mix of energy sources changes dramatically – likely a 20 plus year process even in the most environmentally friendly “energy generation mix” regions of the world. Even if one charges during “peak”, it is better to simply charge from the grid as the distributed energy is allowed to go to areas of peak demand. Again, for at least my lifetime, I don’t project a more optimal use of that generation even assuming the archaic state of most “grids” persist.

Right now, even for a 1 story commercial building, solar cannot supply the energy needs used in the office, much less a manufacturing facility. In fact, it can normally only supply 1/3 or less for the most energy and resource intensive commercial environment in a UV intense region (and that is quite an optimistic calculation, more likely 1/5th). Once you get to two or more stories on the building, one is not even close. On a modest tower with a tower parking garage, the footprint is likely to small to even generate the needs on a theoretical basis. Distributing the energy to location of greatest needs will allow us to dial down and decommission peak sources, which again are the dirtiest and most wasteful.

At some point, we will hit a new equilibrium where the energy generation mix is much cleaner, solar generation specifically is much more efficient, and peak power generation is handled more efficiently and ecologically cleaner. I still believe, however, that distributed power is better than “off grid” type of scenarios as it allows the energy to go where it is being demanded at the moment, decreasing the need for redundant sourcing. And, even in the cleanest energy generation mix, redundancy means building more of something and is by definition more energy wasteful and ecologically wasteful than a scenario where the redundancy buffer that is required is lesser.

Much of this type of debate reminds me of the consumer sort recycle versus the destination sort recycle debate. Even with the advances in trash collection and recycling processes, 20 years later we are suboptimizing the recycling process. Much of the reason for that is the “style” statement, making people feel like they are contributing by sorting themselves. It may make some people “feel” better by imagining “independent” off grid or semi off grid solutions. In reality, however, we live in an interconnected world where “sharing” or distributing solutions to leverage scale and minimize redundancies is far more advantageous, economic, and a faster route to a solution to both particulate emissions issues and energy independence for groups of people.