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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.
Variable torque load

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.
Constant torque load

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.
Constant power load

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.

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

Low impedance fault

A low impedance fault is usually a bolted fault, which is a short circuit. It allows a high amount of fault current to flow, and an upstream breaker or fuse usually senses the high current and operates, ending the event. A high impedance fault, usually an arc fault, is a fault of too high of an impedance for overcurrent protection to detect and operate, so the fault exists for long period of time without tripping upstream protection. Examples of arc faults are: A high or medium voltage distribution utility wire falling to earth in a Y grounded system and arcing to earth where no breaker or fuse will clear; another example is any fault tracking through a substance such as cable insulation or even air….this could be wiring within a building wall with a fault that lasts long enough to ignite the building wall it is installed in, which happens all the time somewhere (sometimes called “arc through char”). Another high impedance fault is one within a transformer secondary coil, arcing through the coil insulation and transformer oil (oil cooled units)…the arc will boil the oil into component gases such as acetylene and hydrogen and if the arc fault lasts long enough and gets to the gases, the gases may explode…and the primary fuse protection will likely not detect this for some time. There are many other examples of high impedance faults. One way to tell a high impedance fault or arc fault is if there is a protecting breaker or fuse that did not operate for a fault…if the breaker or fuse are correctly sized and working properly and did not operate that usually indicates a high impedance fault….a short circuit usually generates high enough current to trigger breaker/fuse operations (assuming normal circuit impedance is low). Another way to look at it is any fault in a power circuit with an impedance such that less than “available” fault current flows.

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

Negative sequence

Negative sequence will not cause a physical rotation. This component creates a field which, though not strong enough, tries to counter the primary field, An increase in this component will cause the motor to overheat due to the opposition. a physical rotation is not likely to occur.

Negative sequence currents are produced because of the unbalanced currents in the power system. Flow of negative sequence currents in electrical machines (generators and motors) are undesirable as these currents generates high temperatures in very short time. The negative sequence component has a phase sequence opposite to that of the motor and represents the amount of unbalance in the feeder. Unbalanced currents will generate negative sequence components which in turn produces a reverse rotating filed (opposite to the synchronous rotating filed normally induces emf in to the rotor windings) in the air gap between the stator and rotor of the machines. This reverse rotating magnetic field rotates at synchronous speeds but in opposite direction to the rotor of the machine. This component does not produce useful power, however by being present it contributes to the losses and causes temperature rise. This heating effect in turn results in the loss of mechanical integrity or insulation failures in electrical machines within seconds. Therefore it is undeniable to operate the machine during unbalanced condition when negative sequence currents flows in the rotor and motor to be protected. Phase reversal will make the motor run in the opposite direction and can be very dangerous, resulting in severe damage to gear boxes and hazard to operating personnel.

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.