Iacdrive|Automation Solution

Variable Frequency Drive Basics (Working Principle)

Variable Frequency Drive (VFD) Basic Configuration
The basic configuration of a variable frequency drive is as follows.
VFD Basic Configuration
Fig. 1 Basic configuration of variable frequency drive

Each part of a variable frequency drive has the following function.

Converter: Circuit to change the commercial AC power supply to the DC
Smoothing circuit: Circuit to smooth the pulsation included in the DC
Inverter: Circuit to change the DC to the AC with variable frequency
Control circuit: Circuit to mainly control the inverter part

Principle of Converter Operation
The converter part consists of the following parts as following figure shows:

Converter
Inrush current control circuit
Smoothing circuit

Fig. 2 Converter part

Method to create DC from AC (commercial) power supply
A converter is a device to create the DC from the AC power supply. See the basic principle with the single-phase AC as the simplest example. Fig. 3 shows the example of the method to convert the AC to the DC by utilizing a resistor for the load in place of a smoothing capacitor.

Fig. 3 Rectifying circuit

Diodes are used for the elements. These diodes let the current flow or not flow depending on the direction to which the voltage is applied as Fig. 4 shows.
Diode
Fig. 4 Diode

This diode nature allows the following: When the AC voltage is applied between A and B of the circuit shown in Fig. 3, the voltage is always applied to the load in the same direction shown in Table 1.

Table 1 Voltage applied to the load

That is to say, the AC is converted to the DC. (To convert the AC to the DC is generally called rectification.)

Fig. 5 (Continuous waveforms of the ones in Table 1)

For the three-phase AC input, combining six diodes to rectify all the waves of the AC power supply allows the output voltage as shown in Fig. 6.

Fig. 6 Converter part waveform

Input current waveform when capacitor is used as load
The principle of rectification is explained with a resistor. However, a smoothing capacity or is actually used for the load. If a smoothing capacitor is used, the input current waveforms become not sine waveforms but distorted waveforms shown in Fig. 7 since the AC voltage flows only when it surpasses the DC voltage.

Fig. 7 Principle of converter

Inrush current control circuit
The basic principle of rectification is explained with a resistor. However, a smoothing capacitor is actually used for the load. A capacitor has a nature to store electricity. At the moment when the voltage is applie

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.

The cause of harmonics in variable frequency drive

Before you attempt to dissipate causative factors of harmonics verbally, you take a look at several studies done by NEMA regarding such, and look into variable frequency drive (VFD) a bit better. You can view articles and studies by subscribing to the NEMA newsletter, and find other sources quite readily through NEMA. It’s an easily accessible place for many current dissertations on this and other electrical topics, with excellent subject matter.

Categorizing all VFDs into the same bucket doesn’t get it. You can also look at EPRI reports done better than 15 years ago on this and other VFD oriented subjects. Of course, all VFDs use Pulse Width Modulation to create the AC type wave form output (AKA ‘Sinusoidal Flows) and of course all have rectifiers at the top end, as do all computers, PLCs, and many solid state control components. The differences of transient creation on the outputs of variable frequency drives depend upon the quality of the wave form output. The more transients or ‘spikes’ in the wave form, the more disruption potential. The quality of outputs of variable frequency drives can clearly be seen in testing with oscilloscopes. Several VFDs on the market significantly reduce this effect with chokes up front, and on the output. It really is a garbage in/garbage out situation that lesser drives don’t bother to address.

Anytime AC is rectified to DC a field is created, and this is at best an elementary statement. The solution is good grounding to bleed it off. It isn’t a problem to do so as long as the grounding pathway is adequate, a simple and proven fix. All drives employ capacitors. Motor field generation, field collapse of any wound coil has the potential of creating conductive/inductive reactance, and capacitors create capacitive reactance. To claim otherwise flies in the face of electrical fact. Phase balancing capacitor banks serve to bring about the same effect. As far as ‘putting drives on a pedestal’, you seem far more inclined to pursue a defensive posture than to take a better look at the correlation between capacitive and inductive/conductive reactance. Again, when these two factors meet the same frequency is when the distortion issue is brought to a peak, with these harmonics becoming the face of disruption.

I successfully remedied these situations by working with engineers in DOD and DOE facilities, as well as with a host of different independent companies, Iacdrive, General Electric, Shaw Nuclear, being a few among them.

Renewable Energy in India

Holistic and Combined i.e Hybrid Renewable Energy Generation per Taluka / District of Each state with Energy Potential study with Investment seeking proposal with land (barren) identified with Revenue department clearances and also with a clear MAP of Evacuation with existing Transmission lines and future lines to planned, which shall be appended to RfP and not ask each developer to identify the location and struggle with Government Administration (which will increase time and Costs (read wrong costs)) complying to Land Acquisition bill and also eliminate the real estate babus to relinquish 5000 acres of land per state, which is BENAMI now…..I do not know how this excess land in BENAMI exist when we have Land ceiling Act!!

In order to do an extensive and credible study to explore renewable energy potential in each Taluka, State and Central Government Can hire international Consultancies with Video Documentation with GPRS MAPS to know the real truth and there shall not be much difference between reports and the ground reality, otherwise, hold these agencies responsible with necessary punitive clauses.

These costs can be recovered in the form of Bid document charges, which any serious developer will pay. However, the Equity selling proxy promoters, who have access to the power corridor and bid with Net worth Financial capacity, but, not worthy of any Renewable energy promotion as we saw in JNNSM wherein a large corporate bought equity from the other bidders and later an investigation took place…..

Following is the excerpts of the Mail written to MNRE and KREDL, in Jan 2012 (now we see their web site showing Biomass study is under progress):

For Power evacuation, we need to know the following (as we can’t use the existing data):

a). Distance from the Power generation site, which normally comes under KREDL (single window agency) i.e where one can put up the plant by undergoing NA or KREDL has identified land bank in Yadgir, but, how many km is the Substation from these sites, which we verified, was difficult to ascertain due to patch lands and the distance was over 10 km in certain cases.

b). Whether these substations can accept 20 MW or 10 MW or 5 MW of intermittent Solar PV load (non firm power which at times may create grid related disturbances etc). Biomass power is firm power as long as Firm biomass feed stock is available.

Therefore, we have been writing to many agencies involved to come out with a common approach, wherein the bidding documents identify clearly the SLDCs where the Project Developer can upload (evacuate) the energy generated with an in principle approval (with location MAP with transmission distances etc) from SLDC and ESCOM to accept such Renewable energy as the States are bound to buy the RE under RPO.

If the investor or RE Generator has to run around to know the fundamentals, then, please try to imagine how many man hours will be wasted and how much money gets drained from many participants for the same location? Instead, these data is available with KPTCL / KREDL / KERC / ESCOMs or such multiple organisation, but, Single window agency KREDL does not produce such VITAL information in their bid documents, hence, we as entrepreneurs are trying to tie the loose ends and make things happen for the good of our state.

I hope you understand our concern and append the finer details of evacuation, project site, land bank, the maximum capacity of MWh the substation can take or any upgrade is needed etc be appended in the bidding documents or even in your web sites also.

Further, any new substations are under development, the same with a clearly identified MAP with distances will help the people to understand the grid network to ensure the grid sustainability, reduction in transmission lines and hence the losses can be planned while making the bids, which otherwise will be a

Variable Frequency Drive Load Types

The potential for variable frequency drive (VFD) energy saving from slowing down the load depend on the characteristics of the load being driven. There are three main types of load: variable torque, constant torque and constant power.

Variable torque load
Variable torque loads are typical of centrifugal fans and pumps and have the largest energy saving potential controlled by variable frequency drives. They are governed by the Affinity Laws which describe the relationship between the speed and other variables.

The change in flow varies in proportion to the change in speed:

Q1/Q2 = (N1/N2)

The change in head (pressure) varies in proportion to the change in speed squared:

H1/H2 = (N1/N2)²

The change in power varies in proportion to the change in speed cubed:

P1/P2 = (N1/N2)³

Where Q = volumetric flow, H = head (pressure), P = power, N = speed (rpm)

The power – speed relationship is also referred to as the ‘Cube Law’. When controlling the flow by reducing the speed of the fan or pump a relatively small speed change will result in a large reduction in power absorbed.

Constant torque load
Typical constant torque applications controlled by variable frequency drives include conveyors, agitators, crushers, surface winders and positive displacement pumps and air compressors.

On constant torque loads the torque does not vary with speed and the power absorbed is directly proportional to the speed, this means that the power consumed will be in direct proportion to the useful work done, for example, a 50% speed reduction will result in 50% less power being consumed.

Although the variable frequency drive energy savings from speed reduction are not as large as that with variable torque loads, they are still worth investigating as halving the speed can halve the energy consumed.

Constant power load
On constant power loads the power absorbed is constant whilst the torque is inversely proportional to the speed. There are rarely any energy savings opportunities from a reduction in speed. Examples of constant power applications include center winders and machine tools.

Why designing an ethernet network IP scheme?

Depends on the size of the network (# of devices planned on connecting), for medium to large corporate networks go 10.x, for home and small business 192.168.x, or to 172.16.x. I would think the IP plan would be looking at least 10 – 20 years out. Changing IP schemes is hard, especially on a controls LAN, you wouldn’t want to undertake this task to frequently. Also consider any routing / firewalling / DMZing that you may want to do between the controls LAN and the business network (ideally these are separated networks).

Here’s some things to consider:

Number of devices or potential devices on the network
You may want to use a Class A subnet when you have or will have a large number of devices or a Class C when you have or will have a small number of devices.

Amount of traffic
A large subnet will more likely expose devices to more traffic. A smaller network may be employed to segment and/or control the amount of data that must be handled by a device.

Security
A large network (e.g., Class A) network may be more difficult to restrict access to or exposure of devices.

Simplicity
A Class A network is a flatter architecture and may be simpler to manage because you don’t have to worry about routing, gateways, and/or firewalls as much. This has to be balanced with security and traffic issues though.

Others
There are other considerations too…

In my experience, connecting with the “business” side of things is not technically difficult with an appropriate firewall/router. However, I have often found that the political challenges are more difficult. I have often butted heads with IT folks who have a fortress mentality and don’t understand the constraints, limitations, restrictions, and special considerations needed for industrial control systems. Many times, the best solution is to have a well defined line of demarcation where the IT folks take care of their side and the control guys take care of the control side. Most IT folks are OK with that as long as they can quarantine the control side to their satisfaction.

When it comes to selecting the firewall/router, you will need to take into consideration the protocols passing through it. If it’s the nominal business protocols like http, ftp, rdp, ssh, etc., then any business class device will typically work. However, if industrial protocols like CIP, Ethernet/IP, or OPC will be passing through, you will need to confirm that the firewall/router supports them specifically. When making the link, the important thing is the type of packet filtering and address translation rules that are configured in the firewall router. The IT folks might be more happy if they can setup a VLAN just for the controls.

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…

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.