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Choose PLC base on top PLC Brand?

Always the top brands will be the most popular PLC and over many years it is my opinion that this is because of their marketing strategy, history, reputation and worldwide acceptance more than any other reasons. This does not mean they are better or worse in any way, just means they are more accepted world wide and more people are experienced with their software. Thus there is some security for the owner in respect to programmer support or future resources etc (people come, people go) and a basis on which management may dictate what hardware is used. There is also the consideration on the capital outlay for programming software which can be very expensive.

Choice most often depends on your application and infrastructure. Example: if an entire factory or whatever was “x-brand” and communicating with each other through “y-protocol”, it may be wise to keep to the same-same. Other brands PLC may talk same protocol but then you need to think about software and the experience of your programmer resources,  spares etc.

The alternative may be a more task or machine specific PLC that can communicate the same protocol but at the cost of the programmer not knowing the device or software, or the costs of additional software and also there may be less skilled programmers in this hardware choice constricting the owners future options in using this alternative.

Experienced programmers fall into two basic categories. Just like Joe-Builder who has had 25yrs experience – now Joe, was that 25years experience doing different things or was that 1years experience 25 times? I have encountered this so often, fantastic CV but doesn’t know anything because has been in same job, day in day out, year after year. Very good at THAT job mind you but no real (other) world experience. PLC programmers are often the same, know x-plc (or software language) inside out but nothing else.

Just my opinion but a good programmer is someone skilled in ladder logic, functions / function blocks, structured text, CRC etc and knows when to use it. Someone also familiar with the hardware and its associated costs. Someone who knows how the hardware device scans and can makes efficient use of its resources through the above mentioned skills. Someone also who is mind-full of who will maintain / modify and what can be modified and what should not… etc. Bit of a mouth full I know, but such a person can then make choices of hardware based on the end result required and not be constrained in his/her thinking based on what already exists or what they themselves know or what they or their management consider to be the current reality.

So, a long story to ask another question. Are you really asking which is the most popular brand PLC because a quick google search using the a brand name would tell you that in seconds based on the number of millions of pages available for THAT brand or are you asking which PLC should you choose?

As further comment…
Today I would go task specific by choice. If you want ultra speed, complex math or fast analogue and. or heavy processing etc… then you are looking at a soft logic PLC that will talk the same protocol as the other PLC’s in the factory. If the task is simple logic and minimal analogue and does not require ultra fast scan times (i.e. 10ms+ is acceptable) then many top brands offer a range that will do this.

There are many things you can do in ladder logic that will satisfy a situation admirably. There are lots of things you can do in structured text that is impossible / impractical to do in ladder logic. All soft-logic PLC’s I have experienced are totally useless at complex ladder logic. This is WHY I choose by what the task requires as opposed to choosing because of what constrains my current reality thinking or comfort zone.

The end result is a functional task, machine or project that is maintainable – not what a particular

Frequency inverter with AC line reactor or DC choke?

Is it AC line reactor more important than DC choke in a frequency inverter? If AC line reactor is missing in the inverter, what are possible impacts to the inverter? And how about DC choke?
Quality frequency inverters incorporate either an AC Reactor or DC Reactor (choke). Their inclusion in the basic design of the frequency inverter allows the design engineer to maximize the advantages of the choke. Their function is to reduce the current distortion caused by the input stage rectifiers by slowing the rate of change of current, and thus charging the internal capacitor at a slower rate over a longer time.

The Harmonic Distortion caused by a frequency inverter is related to its size & load, choke size, and the supply network parameters.
With no AC Reactor or DC Choke, the harmonic distortion will be greater.

Another consideration should be a properly sized source transformer that provides enough impedance. The sized source transformer used as an isolation transformer (although a bit more of an investment) should provide 3 to 5% impedance yet also provides Voltage Transient mitigation with ten to one reduction in impulse peaks, as well as noise reduction through the use of a Delta primary to Wye secondary with center tap ground. It provides additional protection for the frequency inverter front (Converter) end while proper ground of the Source to inverter, frequency inverter to Motor and Motor to Voltage Source assists in mitigating high frequency noise, especially when flat braid is used as the grounding straps. This protects your investment and assists in keeping the variable frequency inverter from generating noise into the supply that can compromise your nearby instrumentation, and PLC power supplies, etc. As well you can tap up the transformer giving you a higher input voltage mitigating the voltage drop issues resulting from the higher impedance.

The DC link assists in mitigating DC Bus Ripple and increasing the input impedance enabling a slower inrush for power on and sudden demand current requirements furthering the life of your capacitors, while a sized supply transformer protects the front end of the frequency inverter drive by providing voltage noise protection and adding input impedance for smoother current and adding a capability to change taps to prevent a voltage drop, while input reactors slow inrush current furthering the life of your input components and capacitors but add no protection from Voltage impulses or noise to the drive converter components, and add voltage drop increasing stress on those components. The important thing to remember is that “Proper” systemic design protects your frequency inverters and system components investment.

variable frequency drive

Synchronous motors VS induction motors

1- Synchronous motors generally offer more efficiency than induction ones, and hence in higher ratings (about 5000 hp and higher) they may be more cost effective considering Life Cycle Costs. The exact size of preference to switch to Synchronous shall be determined based on LCC analysis of specific application.
2- A Large reciprocating compressor is a highly variable load and a synchronous motor will keep its speed in this situation while the induction motor would respond with fluctuating speed.
3- Based on API 618 (with reference to IEC and NEMA), a synchronous motor used for reciprocating compressor may tolerate 66% variation in current, while an induction motor is allowed to have only 40% variation in current which in larger compressors may be exceeded (because of variable load).Also Higher efficiency induction motors with less slip, cause more current variations and are prohibited.

Synchronous motors are characterized by limited starting torque, the ability to actively control power factor and less current in-rush than the induction motor. The synchronous motor also requires active matching of torque demand with motor output. Synchronous motors started “across-the–line” also produce oscillatory torques at the twice slip frequency during acceleration (i.e., starting at 120 Hz and decreasing to 0 Hz at full speed). These torques generally require additional transient torsional analysis because of the potential for damage.
Synchronous motors are usually advantageous on slow speed applications (e.g., low speed reciprocating compressors operating from 200-400 RPM) and also on machines larger than about 10,000 to 15,000 HP.  With both motor types, it is important to match the compressor torque versus speed requirements with motor torque versus speed capabilities as discussed in Sections 6.0 and 7.0. Both induction and synchronous motor types can be coupled with a VFD for variable speed operation.

If the motor is being driven by a variable frequency drive with sophisticated drive algorithms, i.e. controllers that can track the load torque variations, then both the efficiency and transient stability problems can be solved together.

The other significant thing is the starting problem. The transient load torque is also present at starting so the motor has to be able to accelerate through the load transients and be capable of starting when the compressor is sitting at the highest load.

Cable length between VFD and Motor | Iacdrive

The dU/dt at the output of the variable frequency inverter drive combined with the motor cable length will result in very high voltage peaks at the motor terminals. This is a concern for the isolation in motors not designed to be driven by VFDs.
On the other hand the maximum motor cable length depends also on the switching frequency used due to the charging effect of the motor cable capacitance (this is a limitation on the variable frequency drive side, not on the motor isolation).
The dU/dt at motor terminals normally is very different from the dU/dt that you can calculate from IGBT and its driving characteristics (turn on time, gate resistor, etc) at variable frequency drive terminals. As the cable acts like a distributed LC impedance, the dU/dt calculation on VFD terminals will give you very high values that can be apparently dangerous, but in practice, will not happen at motor terminals.

For long cables, the combination of cable impedance, high frequency input impedance of motor and VFD switching frequency can lead to reflection of voltage pulses that gives origin to large voltage overshoots on motor terminals. The problem increases as increasing switching frequency because the time between voltage pulses will be smaller, so, a voltage pulse reaching the motor will add to the pulse being reflected. This “double pulsing” can results in extreme voltage overshoot and dU/dt that will result in motor insulation failures. For the variable frequency drives side the increasing switching frequency will be a problem (besides power losses) if you have a big capacitor filter at converter output, that can lead to high current pulses at inverter side.

The determination of the resulting dU/dt at motor terminals from the dU/dt at VFD drive terminals is very difficult if you try to use simulations. For this task you’ll need the high frequency parameters of cables (that also depends on installation details) and motor, that will not be available from standard datasheets and are very difficult to obtain from measurements. In practice almost all VFD manufacturers make extensive measurements and establish some criteria in order to orient applications. The approach is to determine if it is necessary or not to have an output filter for a known application (cable length).

For instance, a common specification is:

For cable lengths up to 100 meters (and motor suitable for variable frequency drive applications) it is not necessary a filter; for lengths from 100 to 200 meters, a series reactance can be used; for greater lengths it is necessary an LC filter at VFD terminals. The limit lengths can be different from different manufacturers and voltage levels (LV/MV). Iacdrive, for instance, can give complete orientation for application of its drives considering the needed cable length for the application.

frequency inverter

Can soft starters create shaft voltages similar to VFD’s?

We recently evaluated a 500 HP 4 pole motor on a pump application. The motor is started with a soft starter. Upon examination of the bearings we discovered fluting inside both the variable frequency drive and opposite drive end bearings.
If it were shaft currents, especially on a pump, the fluting would be typically on the non-drive end only, excess shaft current would be drained through the apparatus attached to the drive end shaft. We would more likely suspect a vibration issue with the assembly while inactive. What is base condition for the pump? Is it on a stable foundation or is it mobile? If mobile, and transported you need to “lock” the shaft to avoid axial or radial motion.
PAM winding is still a feasible alternative to VFD where simply two or three discrete speeds are necessary without the need for servo-like control, mostly for high power applications as was mentioned above. Only several extra leads and contactors but no nasty harmonics, reduction of insulation life and no additional variable frequency drive that takes space & is not cheap to buy or maintain, might become obsolete and most likely will not last as long as the motor.
Note that some shaft couplers are insulating; and therefore, won’t drain shaft motor voltage controls.

However, all of the soft starters that I have used are line (mains) frequency phase angle modulating. Hence they act as three phase variacs (variable autotransformers). I have not run across any stray voltage problems with these units. However, some soft starters modulate only two of the three phases. I don’t know what this will cause.

Regarding VFD’s, three steps are needed to protect the motor: 1) High enough winding voltage withstand voltage (dielectric strength), 2) Adequate thermal capability to counter the extra (5% or so) winding heading due to the harmonics, and 3) protecting the bearings from developed stray voltage (grounding, bypassing or insulating).

A soft starter is in the circuit for so short a time, it is not likely that the fluting is coming from the drive. My logic is that fluting is a low current long time event. Bearing damage that could occur from the very short and very infrequent duration of starting would have to be a very high energy (for that short time), and would more likely be pitting.

In evaluating all possible sources:
There have been instances where the external current is coming from the plant piping. This would be eliminated by insulating the piping from the pump (if a flanged connection, use an insulative gasket [no metal fibers or rims], and plastic sleeves & washers for the bolt set).

Other motor related sources: the API motor specs say to insulate one end where the shaft voltage exceeds 500 mV. This can be done many ways, and usually done on the non drive end. (Have you measured the shaft voltage?)

I am not a big fan of shaft grounding brushes, and grounding the plant piping may not be enough. Brush contact is not reliable, and may not drain all the current (same for grounding the pipe).

Anecdotally: an electric utilitie had system grounding problems that elevated the potential of “ground” in a dairy. The path to lowest potential was through the cow to the milking machine to “ground”. Milk production went down, it took a while for the farmer to get the utility to check their system. Finally they did, fixed the transmission system grounding, and the problem disappeared.

motor voltage control

Electrical machine software

You can categorize the electrical machine software into 2 basic types:

Learn more about pump.

1) FEA packages that may or may not have a front end for analyzing motors. These are available from companies like Vector Field (now Cobham), Infolytica and a few others.
2) Motor design specific software such as the SPEED software, RMxprt and MotorSolve from Infolytica.

In the first category, the FEA packages are expensive because they are general purpose modeling packages. The motor add-on is usually limited mostly to the building the model and perhaps some specialized post-processing for motors. Their main advantages are:

1) 2D and 3D versions.
2) The user is free to define what analysis he wants to perform since they have very advanced general post-processors.

Their main disadvantages are:
1) Cost, they can get very expensive depending on the options you require.In some cases, the motor design module is a cost option.
2) Although they have general post-processors, many users require a lot of training in order to be able to get useful information.
3) Geometry input can be a lot more complicated since the front-ends typically have a limited number of geometries available.

The second category, the motor design software, is specifically designed for motor analysis. It can be magnetic circuit based such as SPEED and RMXprt or full finite element based such as MotorSolve. The magnetic circuit type of software has been available for a long time but it has only been recently that full FEA based motor design packages have become available.

The general advantages of software of this type are:

1) Template based input so the user simply chooses the motor geometry, stator and rotor and sets the parameters for the geometry. The input is therefore very simple but limited to the templates that are implemented in the package.
2) Post-processing is specialized and presented in a form that a motor designer can use it.

The general disadvantages of this type of software is:

1) No specialized post-processing is available directly from these packages unless added by the software provider in a new release.
2) Geometries are limited to the templates and adding templates may be very difficult and has to be done by the software provider.

What factors cause Current unbalance

1. Voltage unbalance in supply side (1% volts could easily be 10% current).
2. Physical differences between individual stator coil shapes and connections causing small (but noticeable) resistance changes.
3. Unsymmetrical magnetic circuit – not as big a deal in the smaller “ring” lamination designs, unless highly saturated.
4. Lightly loaded machines will exhibit far higher unbalance than those loaded closer to the full nameplate rating (mostly due to the magnetizing current requirements and associated core/stray loss).

For quick solution measure the current in the three phases, then change the three supply terminals by shift the three terminal to rotate the motor in the same direction, and measure again the current, if the high current move with a certain phase (example: phase L1 of supply read high current in the two case above) the problem is from supply, you can then measure the voltage at motor terminal to be sure that the control circuit and cable are good.

Variable frequency drive power anomalies

variable frequency drive power anomalies can be divided into following three types: phase loss, low voltage and power off, sometimes they maybe appear mixed. The main reasons for these anomalies are transmission line impact by wind, snow and lightning, sometimes it’s the power supply system appear ground wire and phase short circuit. The lightning is very different due to geographical and seasonal factors. In addition to voltage fluctuations, some power grid or self-generation units will have frequency fluctuations, and these phenomena maybe appear repeated in short times, in order to ensure normal operation, the variable frequency drive power supply also need to make corresponding requirements.

If there is a direct-start motor or cooker or other equipment near the variable frequency drive, to avoid voltage decrease when these devices power on, those devices power supply should be separated with the VFD power supply to reduce influence each other.

For the applications require continues operation in instantaneous power off, in addition to select appropriate VFD drives, we also need to consider the motor load deceleration ratio. When the variable frequency drive and external control loop are adopted instantaneous power off compensation, we need to prevent over current during acceleration by detect motor speed when power on.

For the application requires continuous operation, it’s better to install additional automatic switching uninterrupted power supply devices. Like adopt diode input and single-phase control power variable frequency drives, it can continue work even if in phase loss status, but individual rectifier device current is too high, and the capacitor pulse current also high, it’s not good for the variable frequency drives reliability and service life in long time running, so we should handle it the early the better.

3 phase induction motor designs

For 3 phase motor designs, there is hardly any slot combination that will yield a perfectly smooth torque-speed curve. Keeping the following rules in mind will (mostly) avoid the combinations that tend to amplify magnetic noise, harmonics, and parasitic torques.

Let the number of stator slots be S, and the number of rotor slots be R, and the number of poles be P. Undesirable combinations occur when any of the following are true:

1. S – R = 0
2. S – R = +1 OR -1
3. S – R = +2 OR -2
4. S – R = +P or -P
5. S – R = +(P + 1) or -(P +1)
6. S – R = +(P + 2) or -(P + 2)
7. S – R = -(P * 2)
8. S – R = -(P * 5)
9. S – R = +(P * 3) or -(P * 3) .. or multiples of +/-(3 * P).

We know the stator should have an even number of slots to make winding easier – although for certain pole counts, it too can be an odd integer value. And except for a few cases, the number of rotor slots can be either even OR odd.

Then it comes down to the accuracy of the compound die or indexing die for the slot stamping.

Variable frequency drive installation requirements

variable frequency drives are electronic devices, they have stringent requirements in installation environment which is specified in its user manual normally. In exceptional circumstances, if it does not meet these requirements, we must adopt appropriate suppression measures: vibration is the main reason to cause electronic devices mechanical damaged, for big shock and vibration occasions, we should use rubber anti-vibration measures; moisture, corrosion gas and dust will cause electronic devices such as corrosion, poor connection, insulation reduced and then cause short circuit, as a precautionary measure, we should do dust treatment and corrosion control for the control panel, and adopt closed structure; temperature is the key factor to affect electronic devices life and reliability, especially semiconductor devices, we should install the variable frequency drive according to its required installation environment or install additional air conditioning and avoid direct sunlight.

In addition to the above points, inspect the variable frequency drives air filter and cooling fan periodic is also very necessary. For special alpine occasions, to avoid the microprocessor can’t work properly due to temperature too low, we should take necessary measures such as setting the air heater.