Category: Blog

Home  ›  Blog  ›  Page 28

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: Can I operate a 50Hz transformer at 60Hz power supply?

Well first let get one thing straight for transformers: the higher the line frequency, the lower the core (iron) losses! The core power loss are proportional to kf*B^2 approximately for any machine, dynamic or static. But transformers are self-excited static machines, meaning the flux density B is reverse proportional to the line frequency, therefore Pcoreloss = kB^2*f=k*(1/f)^2*f=k/f… so the higher f, the lower the losses. However, increasing the frequency also increases the magnetizing inductance – lowering the magnetizing current. For if you increase the frequency you may want to increase the voltage. But of course this is not usually practical, as line voltage of 60Hz systems is usually lower than those of 50Hz systems. So operating a 50Hz motor at 60Hz should be safe, but may result in higher voltage drop because of lower magnetizing current and because of higher leakage inductance (the series inductance).

It is true that the higher the frequency, the higher the hysteresis (and eddy current) losses will be. But is it a common misconception to assume higher power losses when frequency increases in a transformer. Simply because the hysteresis losses depends not only on frequency, but on the max magnetic flux density as well (Bmax^2). The flux density is reversely proportional to the line frequency, which eventually causes lower core losses as you raise the frequency. This holds true for low and mid frequency ranges. For higher frequencies, skin effect and eddy currents dominates, so the picture may be different. However, iron core transformers do not operate in such high frequencies. We use ferrite core instead. In a practical transformer model, the core losses are represented by a parallel resistor (Rc). The resistor’s value is linearly dependent of the line frequency (Rc=k*f), and the core losses are given by Pc=U^2/Rc… Of course this model is limited to mid-low frequencies…

AQ: Electrical drives for off-highway vehicles

I’ve seen some attempt of electrical driven prototypes in the field, but is still not an enough big sector that let you find specific literature. Excluding the large dumpers for mining, probably the only machine that is built in series is D7E from CAT.

One of largest engineering challenge that you will face on a similar application, is the cooling to the power electronic. You can consider that you will have to dissipate 3-5% of the power that your driver is processing and the max temperature of IGBT’s is not so far from the max temperature in that your vehicle can operate. A small temperature delta, mean a large heat exchanger and/or pretty high speed of air through it. (That with all the problems related to that). A possible solution is liquid cool the IGBT’s mounting them on the aluminum plate. You can’t use the engine cooling fluid because it too warm, but you may can use hydraulic oil (that should never get warmer of 55C).

If you are thinking to expand some gas from the AC, please take in account the possible condensation issues (your voltage on the DC bus can arrive around 800V when the vehicle is breaking, you do not want condensation around). Using SR motors is opening another challenge. For take max advantage of the technology, you want the motor spinning pretty fast (motor get smaller for same size of rotor and with that design, no problems retaining magnets). That means use high ratio gears. In off road vehicle are often used planetary gears because they are compact and cheap. As soon you rise the input speed, the efficiency of those kind of gears drop because you incur in hydrodynamic loss (for a series of problems that are connected to the level of oil that you need to keep in the gear housing). Probably if you are using an SR motor, you want consider to use an angular stage like first reduction after the motor.

I’m not too sure if I would use a battery like energy storage. Batteries take time for convert from electrical to chemical. Most of the braking will happen in a short time so you will end up burning most of the regenerated energy trough a braking resistor (the DC bus can’t go up to infinite about voltage). If you are driving a dozer that has a very low efficiency (most of the vehicle kinetic energy will be burnt in the tracks etc. and very little will arrive to the SR motor to be regenerate), probably the regeneration is not too important, on other vehicle is maybe more important so look to capacitors or flywheels for storage is probably more appropriate.