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Three Phase Input DC Drive

Controlled bridge rectifiers are not limited to single-phase designs. In most commercial & industrial control systems, AC power is available in three-phase form for maxi mum horsepower & efficiency. Typically six SCRs are connected together, to make a three-phase fully controlled rectifier. This three-phase bridge rectifier circuit has three legs, each phase connected to one of the three phase voltages. It can be seen that the bridge circuit has two halves, the positive half consisting of the SCRs S1, S3, & S5 & the negative half consisting of the SCRs S2, S4, & S6. At any time when there is current flow, one SCR from each half conducts.

The variable DC output voltage from the rectifier sup plies voltage to the motor armature in order to run it at the desired speed. The gate firing angle of the SCRs in the bridge rectifier, along with the maximum positive & negative values of the AC sine wave, determine the value of the motor armature voltage. The motor draws current from the three-phase AC power source in proportion to the amount of mechanical load applied to the motor shaft. Unlike AC drives, bypassing the drive to run the motor is not possible.

Larger-horsepower three-phase drive panels often consist of a power module mounted on a chassis with line fuses & disconnect. This design simplifies mounting & makes connecting power cables easier as well. A three phase input DC drive with the following drive power specifications:

  • Nominal line voltage for three-phase-230/460 V AC
  • Voltage variation-+15%, -10% of nominal
  • Nominal line frequency-50 or 60 cycles per second
  • DC voltage rating 230 V AC line: Armature voltage 240 V DC; field voltage 150 V DC
  • DC voltage rating 460 V AC line: Armature voltage 500 V DC; field voltage 300 V DC

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.

Single Phase Input DC Drive

Armature voltage-controlled DC drives are constant torque drives, capable of rated motor torque at any speed up to rated motor base speed. Fully controlled rectifier circuits are built with SCRs. The SCRs rectify the supply voltage (changing the voltage from AC to DC) as well as controlling the output DC voltage level. In this circuit, silicon controlled rectifiers S1 & S3 are triggered into conduction on the positive half of the input waveform & S2 & S4 on the negative half. Freewheeling diode D (also called a suppressor diode) is connected across the armature to provide a path for release of energy stored in the armature when the applied voltage drops to zero. A separate diode bridge rectifier is used to convert the alternating current to a constant direct current required for the field circuit.

Single-phase controlled bridge rectifiers are commonly used in the smaller-horsepower DC drives. The terminal diagram shows the input & output power & control terminations available for use with the drive. Features include:

  • Speed or torque control
  • Tachometer input
  • Fused input
  • Speed or current monitoring (0-10 V DC or 4-20 mA)

“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.

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)