ABBdriveX – Page 28 – AC DRIVES SOLUTION.

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: 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: 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: 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: Synchronous generator operating frequency

When synchronous generators (alternators) are connected in parallel with each other on an AC grid, they are all operating at a speed that is directly proportional to the frequency of the AC grid. No generator can go faster or slower than the speed which is proportional to the frequency.

That is, when a synchronous generator and its prime mover is operated in parallel with other synchronous generators and their prime movers, the speed of all of the generator rotors (and hence their prime movers if directly coupled to the generator rotors) is fixed by the frequency of the grid. If the grid frequency goes up, the speed of all the generator rotors goes up at the same time. Conversely, if the grid frequency goes down, the speed of all the generator rotors goes down at the same time. It is the job of the grid/system operators to control the amount of generation so that it exactly matches the load on the system so that the frequency remains relatively constant.

Isolated or is landed generators that are not in parallel with other generators have an added limitation in that keeping exactly 50Hz is somewhat difficult, or puts too much demand on controlling/governing systems. In such environments it is normal to accept some small deviation from the nominal frequency.

The vast majority of power for industry is supplied by large rotating AC generators turning in synch with the frequency of the grid. The frequency of all these generators will be identical and is tied directly to the RPM of the generators themselves. If there is sufficient power in the generators then the frequency can be maintained at the desired rate (i.e. 50Hz or 60Hz depending on the locale).

An increase in the power load is accompanied by a concurrent increase in the power supplied to the generators, generally by the governors automatically opening a steam or gas inlet valve to supply more power to the turbine. However, if there is not sufficient power, even for a brief period of time, then generator RPM and the frequency drops.

By operating transformers at higher frequencies, they can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight. Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with ‘volts per hertz’ over-excitation relays to protect the transformer from overvoltage at higher than rated frequency.

AQ: Special Protection System Advantages and Disadvantages

Quite a few yrs. ago around 1988, I was a Protection and Control Engineer at a large utility in the SE. We were doing our planning to bring the final unit of a large 4 unit plant on line, when it was discovered that we could encounter some unusual instability scenarios. The funny thing was that with all units on-line and above a certain MW output, all that would need to occur would be opening of a remote 500kV breaker on one of the particular lines and the event could trigger, eventually bringing ALL 4 units at the plant out-of-step and tripping off all of the generation in just a few minutes (3600 MW).

The studies were performed numerous times by internal and external experts but the results were always the same. The key problem seemed to be the existing network configuration of 4 units and only 3 transmission lines. Adding a 4th 500kV line from the plant seemed to cure the problem under all conditions, including close in 3 phase faults with breaker failure. Unfortunately, the cost and timeline to build a new t-line was a real challenge!

In order to proceed with commissioning the 4th unit and remediating any scenarios for tripping all generation, a Special Protection System (SPS) was developed. A transfer trip channel was installed at the remote substation, keying on the breaker contact opening. At the plant, a Unit Trip scheme was installed that had a MW meter supervising tripping of any one unit selected by the plant operator (U1-U2-U3-U4). If all units were on line and generation was above 2500 MW (margin of safety added), then a receipt of remote breaker opening would trip the selected unit to avoid having all units cascade into out-of-step condition.

Advantages: Clearly, this Special Protection System saved the day, and bought time until an additional line was added 4 years later.

Disadvantages: The downside was the challenge of installing and testing such a complicated scheme with the potential for mis-operation. I don’t recall any mis-operations occurring, but it was still a bit “dicey”. I have been at that same plant during a full load unit trip (Generator differential) and it was an “exciting” experience to say the least! While I did recommend that we conduct a “live test” to see what would really happen and perhaps test our system BLACK START procedures, this suggestion was not well received by management (LOL).

This was my only encounter with such a special protection system scheme in my 35 years of utility work, but it was very interesting to be involved with this project.

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.

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…

AC DRIVES SOLUTION.

Author: ABBdriveX