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AQ: What is SynRM motor?

Many others thirty years ago, synchronous reluctance motors (SynRM) have finally replacing the traditional AC induction motors in the industry. ABB has claimed achieving IE4 efficiency with SynRM, a great improvement from IE2 efficiency with the traditional induction motors, for the same motor envelope size and input power.

A SynRM is a true AC machine with or without permanent magnets on the rotor. It is totally different from the closed-loop controlled, permanent magnet brushless DC machines (BLDC) in that one would never be able to get rid of torque ripples as that have been achieved in commonly used BLDC machines.

The difference is on the rotor: copper or aluminum bars for inductance motor (squirrel cage after joining end disks) vs. flux barriers (air pockets) in SynRM. The SynRM rotor can be further enhanced by inserting permanent magnets in the air pockets for a machines called PM assisted SynRM. High efficiency is achieved for two reasons: 1) no copper loss due to the lack of rotor bars and end disks; and 2) high inductance difference between d- and q-axes (Ld-Lq) because of flux barriers and motor torque linearly proportional to (Ld-Lq).

In comparison with the traditional AC induction motor, a SynRM motor needs a frequency inverter and when permanent magnets are present in the rotor, a rotor position feedback sensor. The drawbacks of SynRM are the motor torque ripples due to switching operation, inherited small air gap, etc.

AQ: Calculate Capacitors Power

In general, to calculate the necessary Power of Capacitors, we can use the following formula:

Qc = P ( tgφ1 – tgφ2 )

where :
– Qc : the Power of Capacitors.
– P : the total Power of Loads that are running during normal working.
– tgφ1 : the tangent of φ1 ( the angel between current & voltage before compensation )
– tgφ2 : the tangent of φ2 ( the angel between current & voltage after compensation )

In all cases, we should take into consideration the following points :
1- It will be better to oversize the calculated Qc by ” 10 to 15% “.

2- Be careful when compensate the PF of a Motor to avoid the Over-excitation case, but we can verify it by using the following formula : Qc (motor) = 2 x P (1 – Cos φ ), where :
– P : the Motor’s Power.
– Cos φ : the PF of the motor before compensation.

3- After calculation of Qc, the choosing of Capacitors type will be done according to the Harmonic Distortion percentage. Noting that in some case where the Harmonic Distortion percentage is high, we should use ” Detuned Reactors ” with Capacitors, and when this percentage is too high, we can’t install the Capacitors before minimizing or eliminating the harmonics that their percentages are too high.

AQ: The noise of variable frequency drive fed motors

The rotating electrical machines have basically three noise sources:

  • The ventilation system
  • The rolling bearings
  • Electromagnetic excitation

Bearings in perfect conditions produce practically despicable noise, in comparison with other sources of the noise emitted by the motor.

In motors fed by sinusoidal supply, especially those with reduced pole numbers (higher speeds), the main source of noise is the ventilation system. On the other hand, in motors of higher polarities and lower operation speeds often stands out the electromagnetic noise.

However, in variable frequency drive (VFD) systems, especially at low operating speeds when ventilation is reduced, the electromagnetically excited noise can be the main source of noise whatever the motor polarity, owing to the harmonic content of the voltage.
Higher switching frequencies tend to reduce the magnetically excited noise of the motor.

Criteria regarding the noise emitted by motors on variable frequency drive applications
Results of laboratory tests (4 point measurements accomplished in semi-anechoic acoustic chamber with the variable frequency drive out of the room) realized with several motors and variable frequency drives using different switching frequencies have shown that the three phase induction motors, when fed by VFDs and operating at base speed (typically 50 or 60 Hz), present and increment on the sound pressure level of 11 dB(A) at most.

Considerations about the noise of variable frequency drive fed motors

  • NEMA MG1 Part 30 – the sound level is dependent upon the construction of the motor, the number of poles, the pulse pattern and pulse frequency, and the fundamental frequency and resulting speed of the motor. The response frequencies of the driven equipment should also be considered. Sound levels produced thus will be higher than published values when operated above rated speed. At certain frequencies mechanical resonance or magnetic noise may cause a significant increase in sound levels, while a change in frequency and/or voltage may reduce the sound level. Experience has shown that (…) an increase of up to 5 to 15 dB(A) can occur at rated frequency in the case when motors are used with PWM controls. For other frequencies the noise levels may be higher.
  • IEC 60034-17 – due to harmonics the excitation mechanism for magnetic noise becomes more complex than for operation on a sinusoidal supply. (…) In particular, resonance may occur at some points in the speed range. (…) According to experience the increase at constant flux is likely to be in the range 1 to 15 dB(A).
  • IEC 60034-25 – the variable frequency drive and its function creates three variables which directly affect emitted noise: changes in rotational speed, which influence bearings and lubrication, ventilation and any other features that are affected by temperature changes; motor power supply frequency and harmonic content which have a large effect on the magnetic noise excited in the stator core and, to a lesser extent, on the bearing noise; and torsional oscillations due to the interaction of waves of different frequencies of the magnetic field in the motor air gap. (…) The increment of noise of motors supplied from PWM controlled variable frequency drives compared with the same motor supplied from a sinusoidal supply is relatively small (a few dB(A) only) when the switching frequency is above about 3 kHz. For lower switching frequencies, the noise increase may be tremendous (up to 15 dB(A) by experience). In some circumstances, it may be necessary to create “skip bands” in the operating speed range in order

AQ: TIA portal, a nightmare!

I have been using TIA since it was launched and it has come on leaps and bounds since it was first launched. Its a great tool and as already mentioned it has its bad aspects but it also has its good aspects. The biggest improvement (in my opinion) is the drag and drop functionality in the WinCC part and the code editor. Just need a field PG to be launched with screens to fold out so you can have multiple screen!!

They are moving in the right direction and it was always going to be resource hungry WinCC was bad enough for that in previous versions.

New improvements make a long list but one of the most recent is being able to switch a DB to and from optimised. How many times in previous versions did I forget to check the box then have to delete the DB and create it again. PID loop tuning function within TIA is useful and if you look on the Siemens Automation website (UK/Europe) the example files are growing all the time and they have some great examples that can be integrated easily in to application, I have used the ASi maintenance and monitoring example which was very well put together along with a few others. Even if you don’t use them but need some pointers on which way to go they are a good starting point.

I could list the gripes I have, but all in all its coming together nicely, just need a decent well priced Field PG to run it on £5K is a bit steep for an M4 which maybe no great improvement on the M3 which, in my opinion, wasn’t very good.

Few months ago I had a project with TIA Portal v11. Hardware targets: Simatic S7-300 and Simatic Comfort Panels.
Compared to RSLogix 5000 / FactoryTalk View for example, TIA Portal is a nightmare, especially on commissioning and start-up, when the pressure is huge and you have to work FAST.

The main problems:
1. Very slow on every operation (compiling, downloading, on-line editing, project printing/documenting).
2. Requires a very high resolution display (it is almost unusable on a 1366×768 laptop)

3. Weird behavior (HMI display alterations, crashes).

AQ: 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.

AQ: Variable Frequency Drive Harmonics

For the AC power line, the system (VFD + motor) is a non-linear load whose current include harmonics (frequency components multiples of the power line frequency). The characteristic harmonics generally produced by the rectifier are considered to be of order h = np±1 on the AC side, that is, on the power line (p is the number of pulses of the variable frequency drive and n =1,2,3).Harmonics Thus, in the case of a 6 diode (6 pulses) bridge, the most pronounced generated harmonics are the 5th and the 7th ones, whose magnitudes may vary from 10% to 40% of the fundamental component, depending on the power line impedance. In the case of rectifying bridges of 12 pulses (12 diodes), the most harmful harmonics generated are the 11th and the 13th ones. The higher the order of the harmonic, the lower can be considered its magnitude, so higher order harmonics can be filtered more easily. As the majority of VFD manufacturers, Iacdrive produces its low voltage standard variable frequency drives with 6-pulse rectifiers.

The power system harmonic distortion can be quantified by the THD (Total Harmonic Distortion), which is informed by the variable frequency drive manufacturer and is defined as:

THD = √(∑∞h=2 (Ah/A1)2)

Where
Ah are the rms values of the non-fundamental harmonic components
A1 is the rms value of the fundamental component

The waveform above is the input measured current of a 6-pulse PWM variable frequency drive connected to a low impedance power grid.

Normative considerations about the harmonics
The NEMA Application Guide for variable frequency drive systems refers to IEEE Std.519 (1992), which recommends maximum THD levels for power systems ≤ 69 kV as per the tables presented next. This standard defines final installation values, so that each case deserves a particular evaluation. Data like the power line short-circuit impedance, points of common connection (PCC) of variable frequency drive and other loads, among others, influence on the recommended values.

Voltage harmonics
Even components 3%
Odd components 3%
THDvoltage 5%

The maximum harmonic current distortion recommended by IEEE-519 is given in terms of TDD (Total Demand Distortion) and depends on the ratio (ISC / IL), where:
ISC = maximum short-current current at PCC.
IL = maximum demand load current (fundamental frequency component) at PCC.

Individual Odd Harmonics
(Even harmonics are limited to 25% of the odd harmonic limits)
Maximum harmonic current distortion in percent of IL
ISC/IL <11 11<h<17 17<h<23 23<h<35 35<h TDD
<20 4 2 1.5 0.6 0.3

AQ: Stiff voltage sources

Stiff voltage sources are not problematic as long as they don’t get in the way of the solver’s attempts to linearize the behavior of the circuit matrix via step size reduction. It is the highly nonlinear stiff sources that are heavily fed back into the rest of the circuitry that can cause the solver to hang. Linear sources that are ground referenced or nonlinear ones that don’t feed back anywhere are not likely to cause problems.

In the initial versions of SPICE there were a few elements that could not be simulated directly with nodal analysis in the circuit’s admittance matrix, ideal inductors and voltage sources being the most common among them. However, starting with some version of SPICE 2 this deficiency was removed when modified nodal analysis (MNA) was added to the simulation engine (requiring an additional computational enhancement sometimes called the auxiliary matrix, I believe).

Modified nodal analysis is an extension of nodal analysis which not only determines the circuit’s node voltages (as in classical nodal analysis), but also some branch currents. This permits the simulation engine to crunch ideal inductors and voltages sources (true Thevenin circuit elements) but at a cost of incrementally increasing the matrix size and difficultly about twice as much as for when “easy” Norton type elements (e.g., resistors, capacitors and current sources) are added.

In other words, adding one ideal inductor slows down the simulation about as much as adding two ideal capacitors. However, there is a small additional silver lining to this, as it also comes with the possible advantage of “free” (whether you use it or not) automatic sensing of instantaneous inductor current.

LTspice (my simulator of choice) treats inductors in a special way in that they are normally given a default series resistance of 1 m-ohm unless a value of zero is explicitly entered for that parameter. Having a non-zero series resistance allows LTspice to “Nortonize” the inductor such that it can be processed as a normal branch within the circuit matrix, thereby allowing the simulation to run marginally faster. This also makes the inductor “look” like any other of the “easy” elements so that it is not a numerical problem to parallel it with a stiff voltage source. If a series resistance parameter is entered for a voltage source, it also becomes Nortonized by LTspice.

Nortonizing an inductor or voltage source comes at the cost of giving up free sensing of the instantaneous branch current, which is not a cost at all if this current is not being used elsewhere. However, as soon as you call out the inductor current in *any way* in any b-source behavioral expression, LTspice changes the default series resistance for that inductor back to zero ohms and reverts back to the standard MNA way of processing it within the circuit matrix so that it can get access to the inductor’s instantaneous current.

Only true Thevenin type elements have the possibility of being used as the instantaneous current sense for a current controlled switch (or other similar current controlled devices). The SPICE standard is to only allow voltage sources for this purpose, but apparently LTspice accepts zero ohm inductors as well.

One last note, LTspice is indeed able to measure the current in any element, including Norton type devices, but for these devices the current measured will necessarily be a time delayed version that may not be suitable for tight feedback loops (there is a warning about this in the LTspice Help file section on b-sources).

AQ: Cleaning solvent for motor windings

Usually, the dry ice approach is the best bet because it leaves no real residue from the cleaning material. If the insulation is “fluffing”, the likely problem is that the air pressure used to move the dry ice particles is too high.

A second alternative that can be used is “corn cob blasting”. The media is reusable, biodegradable particles of corn husks. Again, a relatively low pressure air stream is required. It WILL damage the insulation if the pressure is too high, just as in the dry ice case.

Most solvents will aggressively attack the insulation systems used for windings: this is specifically true for the larger machines where mica tapes are coated / filled with a resinous material (vacuum pressure impregnation). However, it is equally true for smaller machines where the primary insulation is at the strand level and is essentially a varnish or enamel coating on the wire. If you’re worried about how the solvent will affect the insulation system, get in touch with the motor supplier for their suggested approach.

If a solvent-based cleaner must be used, it should be applied sparingly – BY HAND – on the areas to be cleaned to break up the oily / greasy contaminant and then rewashed with some other (non-solvent) approach to clean away any solvent residue. This also will require a “dry out” of the equipment after the second washing. This three-stage approach tends to minimize damage done by solvent that may be left behind to “eat away” at the varnishes, enamels, and resins comprising the insulation system.

One last thing – pretty much ALL solvents are going to be designated as hazardous materials in most regions, due to health concerns. Therefore it is more a case of “pick your poison”!

AQ: Why companies don’t invest in variable frequency drive control

Investing in energy efficient variable frequency drives (VFD) seems like an obvious path to cutting a company’s operating costs, but it is one that many companies ignore. This article explores some possible reasons for this reluctance to invest in VFD.

There is a goldmine of savings waiting to be unlocked by controlling electric motors, but the reluctance to take advantage of this is a very puzzling phenomenon. Motors consume about two thirds of all electrical energy used by industry and cost 40 times more to run than to buy, so you would think optimizing their efficiency would be a priority. The reality is that this good idea is not always turned into good practice and many businesses are missing out on one of the best opportunities to boost profits and variable frequency drive growth.

It might surprise you to learn that your average 11kW motor may cost about £500 to buy but £120,000 to run at 8,000 hours per year over a 15-year lifetime (and that isn’t even accounting for inevitable increases in energy prices). It’s worth considering the payback on any investment in motor control that will reduce this significant running cost, such as using VFDs to control speed, or implementing automated starting and stopping when the motor is not needed. Payback times can often be less than 1 year and, of course, the savings continue over the lifetime of the system, particularly as energy costs rise.

The question that often arises when I talk about this subject to people is: “If the savings are so great, why don’t more people do this?” It would appear to be something that fits into the nobrainer category, however there are three main barriers to the wider uptake of motor control with variable frequency drive, none of which should stop common sense from prevailing – but all too often they do.

The first barrier is a lack of awareness of how much energy is being consumed, and where, in a business. A surprising number of companies do not have a nominated energy manager, still less have energy management as a dedicated job function or have a board member responsible for this significant cost. Those that do measure their energy consumption often have a financial rather than technical bias, so solutions tend towards renegotiating supply contracts, rather than reducing consumption.

The second barrier stems from the economic climate and the level of uncertainty about future events and policies. Businesses are still reluctant to invest in improvement projects, despite short payback periods and the ongoing benefits. The short-term focus is on cutting costs, not on spending money, even to the detriment of future growth. This make-do-and-mend attitude is often proudly touted as a strength, but it is ultimately a false economy. Saving money by cutting capital budgets, reducing staff and cancelling training is damaging to a business and to morale, making it difficult to grow again when the opportunity arises. Saving money by reducing energy consumption makes a business more competitive, while keeping hold of key skills and resources.

The third barrier is a focus on purchase cost, rather than lifetime cost. Whenever a business invests in a machine, a production line or a ventilation system, you can be sure they will have a rigorous process for getting several quotes, usually comparing price, with the lowest bid winning. Something that is not often evaluated is the lifetime energy cost of the system. Competing suppliers will seek to reduce the capital cost of the equipment but without considering the true cost for the operator, including energy consumption. What if the cost of automation and motor control added £700 to the purchase cost? Many suppliers will consider cutting this from the specification. But what if that control saved £1,400 per year in energy? It co

AQ: Different brushes at same ring

Recently I had to do a report explain why is impossible join brushes, at same time, from different companies, even with same characteristics.
I used the follow points:
1 – Even with same characteristics the final results is different because tue proportion of material and/or manufacturing process different lead to a different brushes;
2 – Guarantee, because our machine is new, and is a good practice use brushes recommended by Manufacturer;
3 – The film, that is formed on the rings by the brushes could change (but I don’t have any sure if chage for bad);

Unfortunately my report was based on experience for old engineer and recommendation of Manufacturer.

One
of the most important thing about brushes in high current density
environments is uniformity. If there are any variations in material
composition, manufacturing methods, dimensions, porosity, density,
surface hardness, friction coefficient, pig-tail attaching means, size
of pig-tail conductor, etc., there will be a variation in the current
division and/or wear.

Ultimately some brushes will carry more current than others and the increased current density in those brushes will lead to overheating, pitting, scoring, and ultimately costly repairs to the commutator/slip-rings. You might also accidentally mix brush grades when dealing with multiple vendors.

Although manufacturers publish data for brush materials which may prove to be very close to one another, mixing them on a collector surface is not a good practice. Any signs of undesirable performance would be difficult to identify the root cause for and small differences in electrical resistance can produce staggeringly varied performance from each brush.

While the materials used have good material data supplied with them, the manufacturing of the cable connection does not which can account for many times the resistivity differences of the material. Brush manufacturers do use a variety of materials here also and so some brushes, even of the same grade and from the same supplier but with different connection material, cannot be used together.

Mixing of grades is an uncontrolled practice which leads to variable surface conditions especially where the numbers of each grade used is not controlled.

Lower resistance brushes will “grab” the current possibly over filming the collector surface leaving the higher resistance brushes to run at lower than prescribed minimum current densities which results in higher coefficients of friction at the brush/collector interface. You would never know when your film is stable which endangers machine life.

Most machine manufacturers select a grade of carbon to use which is useful at the machines fully rated capacity. However, manufacturing tolerances, specifications etc can produce a machine vastly over rated for your application. Running the manufacturers supplied brushes at reduced load can be very damaging. Most Manufacturers will accept that you need another brush grade for your specific use and will maintain warranty provided they have been consulted regarding any changes.

Many overlook that by moving a machine from one position in their plant to another, that they well need to consider the brush grade at that time also. Sometimes a simple and cost effective reduction of brushes (of the same grade) within the machine can increase plant reliability and longevity dramatically. Other times a consultation with a brush expert can lead to an alternative grade to produce better performance.