Blog – Page 27 – AC DRIVES SOLUTION.

AQ: 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

AQ: Negative Impact of Accelerated Depreciation on the Indian Economy

For argument sake or as an illustration, if we assume that 1 MW solar will generate 1.6 Mkwh and rs. 1.2/kwh is rebate for AD taken by the investor = 16 x 1.2 = Rs. 19.2 lakhs/year

[Now, Adani and Tata Power have been negotiating the firm Contract PPA to get more, like wise biomass people who based their PPA on LCOE, but, are asking more money from Government, hence, Solar PV developers may also follow the same route after few years, wherein this rebate of AD given will not have any meaning!!]

Total rebate given = 19.2/year x 25 years = Rs. 480 lakhs = Rs. 4.8 Crore (that too year wise depreciated / devaluated rupee value, which has no meaning !)

But, the tax saved is = 80% of investment = 0.8 x 10 cr = 8 Crore, upfront, right in the first year, which is great value, which government would have used as Equity to develop many more MWs.

Is this POLICY of providing 80% Accelerated Depreciation correct by any standards and why Finance Secretaries or policy makers can’t take note and issue corrective measure for INDIA FIRST Culture??

MNRE, in its Draft policy has proposed 20 to 40% Viability Gap Funding, which will further worsen the LOSS to the government !!

If Mahagenco (with 50% subsidy) goes ahead with the proposed business model, then, how and why State and hence Central government has to take the burden due to such errant policies??
We must put an end to the Scrupulous Project Development, which avails the Capital Subsidy (or Viability Gap Funding) and the Accelerated Depreciation and then the Promoters Sell the Project to a prospective buyer, who in turn approaches the Government for the Tariff hike in the 25 years tenure (please note the Politics dynamics or change of administrative set up will hamper the sustainability), thus, the nation is a great loser

Policies and the enabling tax advantages to few promoters (who claimed Capital Subsidy without creating good quality asset or with NON functional biomass power plants) have made a big dent on Indian Economy without any good results esp in Renewable energy sector.

Government or its administration through such policy (without checks or being accountable) transferred the Public Property to the Private Companies in the Form of Renewable Energy Generation through Capital Subsidy (or Viability Gap Funding) coupled with Accelerated Depreciation along with Low cost Debt fund to these Corporate companies (like EXIM etc) / Project Developers – entrepreneurs, which are not paid back as few of these projects are not functioning and still no action taken to recover the Capital Subsidy paid or Tax recovery which was availed through Accelerated Depreciation (AD).

If Government would have established all these projects from the Tax collections (which are doled out as free through AD), it would have needed only a fraction i.e only Rs. 51,504 Crores, which could have been managed from the taxes of Rs.137,344 Crores while retaining the land and property in Government’s name and could have generated lot of employment.

But, by giving an opportunity to Private sector, many have failed to deliver and no Action to recover the Capital Subsidy or the Debt (due to Tribunals etc…. Please be informed that Indian Parliament had to pass an act in Dec 2012 to recover debt (through wrong business cases of Project Promoters, approved by many banks which were certified by National and International Advisors or Consultants) which is around a whopping 40 Billion USD!!)

Total estimated Renewable energy project capacity = 12% of total installed 220GW = 26000 MW
Cost/ MW Investment Equity Debt Cap Sub AD
Source MW installed Total 30% 70% Rs(Cr) 80%adj
Biomass 6 4,500 27,000 8,100 18,900 6,750 21,600

Wind 7 20,160 131,040 39,312 91,728 104,832

Solar PV 10 1,300 13,000 3,900 9,100 VGF? 10,400
(Ground)

AQ: Power factor of a generator connected to national grid

Q: What should be the power factor of a generator connected to national grid in order to have maximum stability? Whether it should be high or low?

Steady State Stability:
1. National grid is like a infinite bus for an average size Generator. We can observe stable operation of generator within its capability limit for all ranges of power factor for infinite time , irrespective of power factor.
2. Observe the load cycle, The generators operate in overexcitation mode (lagging pf) during the day & during night ,when transmission lines generate enough reactive the same generators operate stable in underexcitation mode (leading pf).
3. Therefore as long as there is no instance of large disturbance, we can observe stable operation of generator within its capability limit for all ranges of power factor.

Transient Stability:
1. Depends upon the initial condition of the generator operation (see on Power vs Sin-delta plot)
2 The level of power thrown-off causing the disturbance & Equal area criterion of the energy balance & Inertia.
3 During transient/disturbance, the stability is ensured better if the angle delta (rotor angle or power angle) is small, meaning the amount of store energy in the rotating system is high. Theoretically this means delta angle =0 to have robust stability, but it is practically impossible to have power generation at that value.
4 In order to have maximum stability & power generation simultaneously , the value of rotor angle has to be non zero , on positive side. (negative means motor operation).
To Conclude : It means over-excited mode.(lagging pf ). Many colleges in discussion chain above have written near about 0.9 – 0.94 lagging . They are correct.

AQ: 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.

AQ: India renewable energy

Refer to the REI seminar, wherein Government of India representative stated that the VGF payment is spread over 5 year period.

1) Any profit Making Company, must have had the benefits from the Government (subsidies etc) / Eco system.

The profit must be taxed for the improvement of the Economy of the Country.

2) Present renewable energy policy is allowing these profit making companies to avoid paying taxes, and own the assets due to such FREE EQUITY, which belongs to the Government, thus Accelerated Depreciation (AD) is a killer of Economy.

Thus, we are unable to develop the NICHE technology as unrelated industries are owning the project due to avoidance of paying taxes and just to own the assets due to such loop hole in the policy, later making an early exit to make quick money without serving the Nation.

AD promotion is not a level playing field apart from Tax loss to the Government.

3) The Tax thus saved, is again allowed to earn 19 to 24% Return On Equity (ROE), which is very unfair (actually this should have been disallowed to have rs.3/kwh less tariff), due to a fact that, this is public money, hence, should not be allowed to have such wind fall gains.

4) By loading ROE, showing high CAPEX and taking back more than 30% project equity, getting EXIM Bank or such low cost funding to reduce the interest burden, but, Tariff claimed of rs.18 or 15 or 10/kwh is once again a kind of Tariff subsidy, thus, a common man is paying more money for RE power tariff, which is a great killer of economy and making people poor.

5) Viability Gap funding in addition to AD will be an Economic suicide as a project promoter will be allowed to take back 60 to 70% of project cost without paying tax on profit earned !!

This is likely encourage poor equipment buying / its maintenance due to such immediate undue / windfall gains.

6) Despite taking such huge wind fall gains, again these project promoters will be allowed to sell the project to others, to make further wind fall gain to make few existing companies to get undue benefits due to such wrong policy guidelines, despite many representations made to the Government, which states that they have go clearance from Finance Ministry to further ruin the Economy !!

AQ: Current transformer selection

When you want to select current transformer with appropriate rated power for your power system, you need to consider that value of rated power of selected current transformer should be higher from sum of values of load and Joules’ losses which are a consequence of flow current through conductors which connect current transformer with relay.

So, if you have a long distance between current transformer and relay, then you need to consider one of two following manners for solving this problem:
1. replacing existing current transformer with current transformer with higher power,
2. replacing existing conductors with conductors with lower cross-section.

This solution is a consequence of necessity for reducing of Joules’ losses which are a consequence of flow current through conductors which connect current transformer with relay. If you have conductors whose value of rated current is 5A, you will have Joules’ losses P=R*I^2=R*5^2=25*R. Otherwise, if you have conductors whose value of rated current is 1A, you will have Joules’ losses P=R*I^2=R*1^2=R.
On this way, Joules’ losses in your selected conductors will be reduced 25 times and selected current transformer will be unloaded by reducing additional load.

AQ: 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.

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

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