Iacdrive|Automation Solution

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.

OPC drivers advantage

A few years back, I had a devil of time getting some OPC Modbus TCP drivers to work with Modbus RTU to TCP converts. The OPC drivers could not handle the 5 digit RTU addressing. You need to make sure your OPC driver that you try actually works with your equipment. Try before you buy is definite here. Along with some of the complications, like dropping connections due minor network cliches, a real headache and worth a topic all its own, is the ability us tag pickers and the like. The best thing to happen to I/O addressing is the use of Data Objects in the PLC and HMI/SCADA. The other advantage OPC can give you the ability to get more Quality Information on your I/O. Again, check before you buy. In my experience, the only protocol worse than Modbus in the Quality Info department is DDE and that pretty well gone. This still does not help when the Modbus slave still reports stale data like its fresh. No I/O driver can sort that out, you need a heartbeat.

A shout out to all you Equipment manufactures that putting Modbus RTU into equipment because its easy, PLEASE BUILD IN A HEATBEAT us integrators can monitor so we can be sure the data is alive and well.

Also, while you try before you buy, you want your HMI/SCADA to be able to tell the difference between, Good Read, No Read and Bad Read, particularly with a RTU network.

DC Drives Basic Operation Principles

DC drives vary the speed of DC motors with greater efficiency & speed regulation than resistor control circuits. Since the speed of a DC motor is directly proportional to armature voltage & inversely proportional to field current, either armature voltage or field current can be used to control speed. To change the direction of rotation of a DC motor, either the armature polarity can be reversed, or the field polarity can be reversed.

DC drive diagram

The block diagram of a DC drive system made up of a DC motor & an electronic drive controller. The shunt motor is constructed with armature & field windings. A common classification of DC motors is by the type of field excitation winding. Shunt wound DC motors are the most commonly used type for adjustable-speed control. In most instances the shunt field winding is excited, as shown, with a constant-level voltage from the controller. The SCR (silicon controller rectifier), also known as thyristor, of the power conversion section converts the fixed-voltage alternating current (AC) of the power source to an adjustable-voltage, controlled direct current (DC) output which is applied to the armature of a DC motor. Speed control is achieved by regulating the armature voltage to the motor. Motor speed is directly proportional to the voltage applied to the armature.

The main function of a DC drive is to convert the fixed applied AC voltage into a variable rectified DC voltage.

SCR switching semiconductors provide a convenient method of accomplishing this. They provide a controllable power output by phase angle control. The firing angle, or point in time where the SCR is triggered into conduction, is synchronized with the phase rotation of the AC power source. The amount of rectified DC voltage is controlled by timing the input pulse current to the gate. Applying gate current near the beginning of the sine-wave cycle results in a higher aver age voltage applied to the motor armature. Gate current applied later in the cycle results in a lower average DC output voltage. The effect is similar to a very high speed switch, capable of being turned on & off at an infinite number of points within each half-cycle. This occurs at a rate of 60 times a second on a 60-Hz line, to deliver a precise amount of power to the motor.

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.

How is frequency inverter saving energy?

This studies show that up to 80 percent of the energy from the power source to the industrial consumer can be lost. Energy conversion—converting energy into useful work via motors, heat exchangers, process heaters, pumps, motors, fans, compressors, and so forth—represents a large opportunity for energy savings in manufacturing. Industrial electric motor-driven systems represent the largest single category of electricity use in China.

The industrial sector consumes approximately one-third of the energy used in China (see Fig. 1).

Studies show that up to 80 percent of the energy from the power source to the industrial consumer is lost through the transition of raw material to the point of useful output—much of that at the point of conversion from electrical to mechanical output (see Fig. 2).

Rising energy costs, a sense of environmental responsibility, government regulation, and a need for energy reliability are driving efforts for energy efficiency in manufacturing.

Energy is lost primarily in three areas:

Generation
Distribution
Conversion

The third area, energy conversion—converting energy into useful work via motors, heat exchangers, process heaters, pumps, motors, fans, compressors, and so forth—represents a large opportunity for energy savings in manufacturing.

Industrial electric motor-driven systems represent the largest single category of electricity use in the China—more than 65 percent of power demand in industry. Consequently, motor-driven systems offer the highest energy savings potential in the industrial segment.

Supporting this statistic, studies show that 97 to 99 percent of motor life cycle costs are expended on the energy that the motor uses. This fact alone should be a driving motivation for companies to perform a periodic energy consumption analysis on the motor systems they use in their facilities.

Inefficient and ineffective control methods in two areas waste motor systems’ energy:

Mechanical flow control (pumps, fans, compressors)
Energy recovery (regeneration of braking energy or inertial energy)

An inverter is an effective tool in conserving and recovering energy in motor systems.

What Is a frequency inverter? Why Use It?

A frequency inverter controls AC motor speed (see Fig. 3). The frequency inverter converts the fixed supply frequency (60 Hz) to a variable-frequency, variable-voltage output to enable precise motor speed control. Many frequency inverters even have the potential to return energy to the power grid through their regenerative capability.

A frequency inverter’s precise process and power factor control and energy optimization result in several advantages:

Lower energy consumption saves money
Decreased mechanical stress reduces maintenance costs and downtime
Reduced mechanical wear and precise control produces more accurate products.
Lower consumption lowers carbon emissions and helps reduce negative impact on the environment.
Lower consumption qualifies for tax incentives, utility rebates, and, with some companies, energy savings finance programs, which short

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.

Non-regenerative & Regenerative DC Drives

Non-regenerative DC drives, also known as single-quadrant drives, rotate in one direction only & they have no inherent braking capabilities. Stopping the motor is done by removing voltage & allowing the motor to coast to a stop. Typically nonregenerative drives operate high friction loads such as mixers, where the load exerts a strong natural brake. In applications where supplemental quick braking and/or motor reversing is required, dynamic braking & forward & reverse circuitry, may be provided by external means.

Dynamic braking (DB) requires the addition of a DB contactor & DB resistors that dissipate the braking energy as heat. The addition of an electromechanical (magnetic) reversing contactor or manual switch permits the reversing of the controller polarity & therefore the direction of rotation of the motor armature. Field contactor reverse kits can also be installed to provide bidirectional rotation by reversing the polarity of the shunt field.

All DC motors are DC generators as well. The term regenerative describes the ability of the drive under braking conditions to convert the generated energy of the motor into electrical energy, which is returned (or regenerated) to the AC power source. Regenerative DC drives operate in all four quadrants purely electronically, without the use of electromechanical switching contactors:

Quadrant I -Drive delivers forward torque, motor rotating forward (motoring mode of operation). This is the normal condition, providing power to a load similar to that of a motor starter.
Quadrant II -Drive delivers reverse torque, motor rotating forward (generating mode of operation). This is a regenerative condition, where the drive itself is absorbing power from a load, such as an overhauling load or deceleration.
Quadrant III -Drive delivers reverse torque, motor rotating reverse (motoring mode of opera tion). Basically the same as in quadrant I & similar to a reversing starter.
Quadrant IV -Drive delivers forward torque with motor rotating in reverse (generating mode of operation). This is the other regenerative condition, where again, the drive is absorbing power from the load in order to bring the motor towards zero speed.

A single-quadrant nonregenerative DC drive has one power bridge with six SCRs used to control the applied voltage level to the motor armature. The nonregenerative drive can run in only motoring mode, & would require physically switching armature or field leads to reverse the torque direction. A four-quadrant regenerative DC drive will have two complete sets of power bridges, with 12 con trolled SCRs connected in inverse parallel. One bridge controls forward torque, & the other controls reverse torque. During operation, only one set of bridges is active at a time. For straight motoring in the forward direction, the forward bridge would be in control of the power to the motor. For straight motoring in the reverse direction, the reverse bridge is in control.

Cranes & hoists use DC regenerative drives to hold back “overhauling loads” such as a raised weight, or a machine’s flywheel. Whenever the inertia of the motor load is greater than the motor rotor inertia, the load will be driving the motor & is called an over hauling load. Overhauling load results in generator action within the motor, which will cause the motor to send cur rent into the drive. Regenerative braking is summarized as follows:

During normal forward operation, the forward bridge acts as a rectifier, supplying power to the motor. During this period gate pulses are withheld from reverse bridge so that it’s inactive.
When motor speed is reduced, the control circuit withholds the pulses to the forward bridge & simultaneously applies pulses to reverse b

Systems Development Life-Cycle

Step 1. Initiation
Step 2. System Concept Development
Step 3. Planning
Step 4. Requirements Analysis
Step 5. Design
Step 6. Development
Step 7. Integration and Test
Step 8. Implementation
Step 9. Operation and Maintenance
Step 10.Disposition

There are three major players present in this model; Customer (client), System Integrator, and Machine or device manufacturer.

In many instances, the result of step 4 (Requirements Analysis), is an RFQ for the system implementation has been issued to one or more systems integrators. Upon selecting the system integrator, step 5 (Design) begins. Upon completing step 5 (Design), the system or process flow is defined. One of the major outputs from step 5 are the RFQs for the major functional components of the finished system. Based on the RFQ responses (bids), the Machine or device manufacturers are chosen.

Steps 6, 7, and 8 are where all the individual functional components are integrated. This is where the system integrator makes sure the outputs and feedback between to machines or devices is defined and implemented. Step 8 ends with a full systems functional test in a real manufacturing situation is demonstrated to the customer. This test includes demonstrating all error conditions defined by the requirements document and the systems requirements document. If a specific device or machine fails its respective function it is corrected (programming, wiring, or design) by the manufacturer and the test begins anew.

Each of the scenarios presented is correct. The technician role being presented (customer, integrator, or manufacturer) is not clear. System diagnostics are mandatory and need to be well defined, even in small simple machines. There should be very few and extreme conditions under which the customer’s technician should ever have to dig into a machine’s code to troubleshoot a problem. This condition usually indicates a design or integration oversight.

(You can find a complete description here, http://en.wikipedia.org/wiki/Systems_development_life-cycle)

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.