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Most HVAC systems are designed to perform during peak load conditions that rarely occur during the year. Various flow control methods such as inlet guide vanes, discharge dampers, and throttling valves have been used to regulate flow during the majority of the year when demand is less than full capacity. While these methods can effectively control flow, they are very inefficient and therefore waste a lot of energy because they do not take advantage of the Affinity Laws. The Affinity Laws govern the flow of fluids such as air and water.
The Affinity Laws state the following:
• Flow is directly proportional to speed
• Torque required is proportional to speed squared
• Horsepower required is proportional to speed cubed
The Affinity Laws are shown graphically to the right. This clearly shows that reducing the fan or pump speed, even slightly, results in a large reduction in required horsepower. This adds up to significant energy savings. For example, at 50% flow, the horsepower required is only 12.5% (0.53 = 0.125 = 12.5%). For a 50 Hp motor, this means only 6.25 Hp
is required. It would actually cost less to run two 50 Hp motors at 50% speed than it would to run one 50 Hp motor at 100% speed!
THE MCH SERIES VFD vs. OTHER FLOW CONTROL METHODS
The MCH Series VFD (variable frequency drive) saves energy as shown above by reducing the actual speed of the pump or fan when full flow is not required. In other flow control methods, the motor always runs at full speed and the flow is mechanically restricted. While these methods can save some energy at reduced flows, they cannot match the savings that can be achieved using an MCH Series VFD.
ADDITIONAL BENEFITS OF THE MCH SERIES VFD
PID SETPOINT CONTROL: The built-in PID feature allows the MCH Series VFD to maintain a desired process setpoint (such as PSI or GPM) by constantly adjusting the motor speed based on a process feedback signal. This provides very precise process control, which saves additional energy by exactly matching demand. This feature also reduces installation costs because it eliminates the need for a separate PID controller.
Only a feedback transducer is required to make the MCH Series VFD a complete process control system.
SOFT-STARTING: Ramping the motor up to speed eliminates the peak current and shock-load conditions that stress the driven equipment. This further reduces energy costs, and also reduces maintenance costs and downtime.
Centrifugal fans are commonly used to distribute air in HVAC systems. Two of the most common methods of flow control for fans are discharge dampers and inlet guide vanes. The diagram below compares the power requirements of discharge dampers and inlet guide vanes to direct variable flow control using the MCH Series VFD. Also shown is the theoretical power requirement defined by the Affinity Laws.
• Supply and Return Fans
• Exhaust Fans
• Boiler Fans
• Fume Hood Fans
• Kiln Fans
SIGNIFICANT ENERGY SAVINGS AND FAST PAYBACK
Due to system inefficiencies, the theoretical Affinity Law power requirement cannot be achieved, but controlling flow using the MCH Series VFD is very close. It’s easy to see from the diagram above that the energy savings compared to other flow control methods is dramatic. For example, operating at 50% flow requires only about 18% power using the MCH Series, while the discharge damper requires about 90% and the guide vanes require about 60%. This tremendous energy savings potential results in typical paybacks of less than one year!
BETTER PROCESS CONTROL
Using the built-in PID Setpoint Control feature, the MCH Series VFD can maintain a desired pressure by constantly adjusting the fan speed to match demand.
In the example to the right, a transducer is used to provide a feedback signal that represents the actual system pressure. The VFD compares this feedback signal to the
desired setpoint, and adjusts the fan speed based on the error between the two. By
always trying to eliminate the error, the VFD maintains the desired pressure.
Centrifugal pumps are used in a wide variety of commerical, industrial, and municipal applications, and throttling valves are a common flow control method. The diagram below compares the power requirements of a throttling valve to that of the MCH Series VFD. Also shown is the theoretical power requirement defined by the Affinity Laws.
• Chilled and Hot Water Pumps
• Condenser Water Pumps
• Booster Pumps
• Potable Water Pumps
• Chemical Pumps
SIGNIFICANT ENERGY SAVINGS AND FAST PAYBACK
Due to system inefficiencies, the theoretical Affinity Law power requirement cannot be achieved, but controlling flow using the MCH Series VFD is very close. It’s easy to see from the diagram above that the energy savings is dramatic compared to the throttling valve.
For example, operating at 75% flow requires only about 50% power using the MCH Series, while the throttling valve requires about 90%. This tremendous energy savings potential results in typical paybacks of less than one year!
BETTER PROCESS CONTROL
The built-in PID Setpoint Control feature in the MCH Series VFD allows it to maintain a
desired pressure or temperature by constantly adjusting pump speed.
In the example to the right, a transducer is used to provide a feedback signal that
represents the actual system pressure. The VFD compares this feedback signal to the
desired setpoint, and adjusts the pump speed based on the error between the two. By
always trying to eliminate the error, the VFD maintains the desired pressure.
Cooling towers are common in HVAC systems and are used to cool water. They do this by using fans to blow air through falling water. The water temperature is reduced as the heat is transferred to the air and expelled in the form of evaporation.
To achieve the desired water temperature, some cooling towers simply cycle the fan on and off, or use a 2 speed fan motor. Other cooling towers have multiple fans that are staged according to demand.
The absence of an air flow control method (such as vanes or dampers) means that the cooling tower fans operate at 100% flow and therefore require 100% energy. Also, cycling of the fan (or fans) on and off increases mechanical stress on the driven equipment
and results in high peak currents.
Applying an MCH Series VFD to a cooling tower will result in dramatic energy savings (see the Centrifugal Fans section of this guide for more information). The PID feature in the MCH Series allows the water temperature to be controlled precisely by varying the
speed of the fan (or fans) that control the air flow.
A temperature transducer measures the actual water temperature and is the feedback to the MCH Series VFD. The VFD compares the actual water temperature to the desired temperature setpoint, and adjusts the fan speed accordingly. If the temperature is too high, the VFD will increase the air flow to reduce the water temperature back to the desired setpoint. If the temperature is too low, the VFD will decrease the air flow to reduce the cooling effect.
COOLING TOWER DE-ICING
In cold climates, the movement of air through the cooling tower can result in ice forming, ultimately resulting in either damage to the cooling tower or preventing water
flow in the system. To eliminate ice build up, the fan motor can be operated in reverse, which moves warm air over the ice, causing it to thaw. This procedure needs to be done manually and regularly during icing conditions to keep the ice from reaching a critical build-up.
This method of de-icing is losing favor for a more automated system that senses ice forming conditions and stops the airflow to prevent the condition. The MCH Series VFD can be configured to work in either case. Ice sensors can be easily interfaced to the VFD
to shut down the fan, and the MCH Series has a “deicing” function that allows the motor to be manually operated in reverse rotation.
Often times those using a Variable Frequency Drive (VFD) may find they need to connect a higher horsepower VFD to a single phase input power source. Since most high horsepower VFDs only accept three phase input as a power source, they are left with few options or alternatives. Don’t fret, there is a solution.
If you are using a Variable Frequency Drive (VFD) rated for three phase input and the only power source you have available to you is single phase input, then you can derate the Variable Frequency Drive (VFD) to accept the single phase input power source. You can almost always use a VFD rated for three phase input with a single phase input power source. When only a three phase input VFD is available, it is acceptable and common practice to derate the VFD to work with a single phase input
Before you derate your VFD, it is most important to ensure the VFD you are using is properly suited for your application. The following are some basic guidelines to help you in determining whether or not your Variable Frequency Drive (VFD) is suitable for your application: Gather motor nameplate data including horsepower (HP), current (Amps), motor voltage, input line voltage and power source phase.
Determine which type of VFD your application will require. The type will fall under the category of Volts per Hertz (V/Hz), closed-loop vector, or open-loop vector (Sensor-less Vector). The internal components of the three phase input VFD are rated for the appropriate current expected when three phase input power is applied. When using single phase input the line side current from the single phase is always higher. To “derate” is the process of ensuring that these components are rated for the higher current that will flow from the single phase input instead of the three phase input.
You can derate a VFD by:
Determining the Horsepower of the Motor the VFD will be connected too, then choosing VFD with a Horsepower higher than the Horsepower of the motor to compensate for the additional input current from the single phase power source. The simplest formula used for these types of applications is:
VFD Input Current > Motor Current Rating * 1.73
The VFD input current must be equal to or greater than the Motor Current Rating * 1.73
When installing most three phase input Variable Frequency Drives (VFDs) on an application where single phase input power is used, you will almost always connect the input line leads to L1 and L2 of the VFD. L3 will be left open with nothing connected. Consult with the VFD manufacturer or knowledgeable integrator to be sure.
An application has a 230 VAC single phase input power source and needs to connect it to a conveyor that has a Variable Frequency Drive (VFD) connected to a 10 Horsepower 230 VAC 3 phase induction motor. Let us assume it has been determined that this application will operate well with a simple Volts per Hertz (V/Hz) VFD. The issue is, since there are no VFD manufacturers that offer a 10 Horsepower (HP) single phase input Variable Frequency Drive (VFD), we will need to de-rate a VFD with a three phase input for single phase input. Most manufacturers of VFDs only offer products up to 3 Horsepower (HP) for single phase input. The 10 Horsepower (HP) AC motor nameplate reveals that the motor is rated for approximately 27 amps at 230 VAC. We must use the equation above:
VFD Input Current > Motor Current Rating * 1.73
VFD Input Current > 27 Amps * 1.73
VFD Input Current > 46.71
This application will need a 230 VAC 3 phase Volts per Hertz (V/Hz) Variable Frequency Drive (VFD) with an input current rated at or above 47.0 amps.
Motors connected to VFDs receive power that includes a changeable fundamental frequency, a carrier frequency, and very rapid voltage buildup. These factors can have negative impacts, especially when existing motors are used.
There are a number of potential problems that can become real when a variable frequency drive (VFD) is used to power an existing induction motor. As such, you should carry out a careful study to determine if these problems could be sufficiently bad to cause reconsideration of such an installation. With a VFD, an existing motor normally having a number of useful years left in it could abruptly fail.
Existing motors are designed for 60 Hz only, 50 Hz only, or 60/50 Hz service. As such, you have to question whether or not a new VFD can be matched to your existing motor and still have the motor perform reasonably well. In other words, will the motor be able to handle additional factors that may cause greater vibration, heat rise, etc., and a possible increase in audible noise?
High Frequencies Can Cause Problems
You should be aware of possible side effects caused by high pulsing frequency when applying a VFD to an existing motor. These negative effects include additional heat, audible noise, and vibration. Also, pulse width modulation (PWM) circuitry, which causes a high rate of voltage rise of the carrier frequency, can cause insulation breakdown of the end turns of motor windings as well as feeder cable insulation.
The carrier frequency, a by-product of obtaining current at a variable fundamental frequency, is the cause for having additional watts in the motor; this power is essentially wasted energy that adds heat to the motor. The amount of such loss varies, depending upon the motor’s stator and rotor designs and frequency of the carrier wave.
With frequencies other than the fundamental, a motor runs at very high slippage and, therefore, is running somewhat inefficiently. (Slip is the difference between the rotational speed of the stator’s magnetic field [the synchronous speed of the induction motor] and the speed of the rotor.) Also, numerous lines of magnetic flux are being cut by the rotor; this phenomenon produces additional watts and additional heat. (Note that the high-frequency ripples in the current are at low magnitude, and the additional heat is in the order of 5% to 10% above that produced by a pure sine wave).
The synchronous speed of a four-pole motor served by 60-Hz power is 1800 rpm. This same motor, when considering “overtones” or ripples in the current’s fundamental frequency caused by a voltage carrier frequency of 4 kHz, will have current flowing through it based on that high frequency. Thus, the rotor of a four-pole, 60-Hz designed motor (with a rated full-load speed of 1750 rpm) being supplied power by a VFD adjusted to 10-Hz output will be turning at 1/6 rated speed. If the load’s torque requirements are constant at low- through full-rated speeds, slip rpm remains constant. For the motor above, which is operating at 10 Hz, the shaft will be turning at 250 rpm.
The rotor, while turning at 250 rpm and crossing lines of flux (the magnetic field) based on the 10-Hz fundamental frequency and the synchronous speed of 300 rpm (1/6 of 1800 rpm), is also crossing lines of flux due to the carrier frequency voltage of 4 kHz. The synchronous speed at 4 kHz is 120,000 rpm ([120 x 4000] [divided by] 4).
Based on a synchronous speed of 120,000 rpm and a shaft speed of 250 rpm, you can see that the magnetic lines of flux being cut due to the carrier frequency (4 kHz) are substantial compared with a synchronous speed of 300 rpm caused by the 10-Hz frequency. This additional current, which is transmitted to the rotor bars by the cutting of additional magnetic flux caused by carrier frequency, produces very little useful power. Most of this current is dissipated as heat, adding to the temperature rise of the motor. This additional heat represents about another 5% to 10% thermal buildup in the motor and can place an additional thermal strain on the motor’s rotor bars and stator windings, if it is running at full load. This high frequency power is an inefficient producer of torque.
Because of these and other conditions mentioned, you may wish to derate an existing motor when it’s connected to a VFD. The amount of energy from the carrier frequency that’s dissipated by the motor depends upon the amplitude and the frequency of the voltage, and the reactance and resistance of the motor at the resultant frequency. The amplitude of the current is determined by the ratio of the voltage over the impedance, while the watts lost is a product of the current squared times the resistance.
Other Undesired Side Effects
You also should be aware of other potential side effects caused by high frequency. These include undesirable audible noise, harmful vibration, and bearing problems.
Vibration and noise problems. To avoid noise and vibration problems, it’s recommended that the motor being used not have components that can resonate at the frequencies the motor (and its load) will generate. This is possible on systems where the frequency of the power is known, such as with 60 Hz. However, today’s VFDs have no standard carrier frequency, and the fundamental frequency can range from less than 10% of 60 Hz to 100% of 60 Hz, and beyond. Depending on which brand and model number of VFD is mated to the existing motor, and other factors such as the characteristics of the electrical system at the site, resonances in certain components may or may not be excited.
You must also consider that when a 60-Hz designed motor is operating at a different electrical frequency, various components of the motor might go into mechanical resonance, such as the fan or shaft. Each component has its own natural mechanical frequency, and an electrical frequency going through the coils and rotor bars can cause mechanical vibrations that are different from the initial design parameters. When an electrical frequency matches the natural frequency of a mechanical component, serious problems may occur. This may include the disintegration of a component.
Bearing problems. Another possible problem, which still isn’t fully understood, is the slow disintegration of the roller/ball (anti friction) bearings that support the shaft. It appears this is caused by bearing current and static discharge. What happens is that pitting occurs on the roller/ball surface and, when accumulated, causes the bearing to make noise. If not addressed, vibration will begin to develop.
Air flow problems. An additional factor you should consider when operating a standard 60-Hz motor at very low speed is that the fan, which is fixed and attached to the rotor, may not create enough air flow to effectively cool the motor. This is true because air flow is proportional to shaft speed. Thus, at half shaft speed, the air flow is half normal flow. To compensate for low-volume air flow at low motor speeds, if installation is possible, the attachment of a constant velocity air blower package to the back of the motor will usually provide adequate cooling.
Conductor Insulation Breakdown
As mentioned, PWM circuitry, which causes the high rate of voltage rise at the carrier frequency, can cause insulation breakdown of the end turns of the motor windings, as well as possible breakdown of the feeder cable insulation. This relates to the very high rate of rise of the voltage (rate of voltage change with respect to time) in combination with the very rapidly repeating voltage pulse caused by the VFD. Conductor insulation failures in motors have occurred because of this phenomenon. This subject is not completely understood and is presently being researched. The known facts about the matter are summarized as follows.
Switches in the inverter section of VFDs used today cause instantaneous turn-to-turn voltage inside a motor’s windings to be significantly higher than what an equivalent normal sine wave supply produces.
Each cycle of the fundamental voltage consists of numerous pulses of voltage.
Long distance between a motor and its VFD causes the turn-to-turn voltage to get even higher.
There are different approaches in explaining why there’s an increase of voltage at the motor terminals. Some explain it in terms of resonant capacitance/inductance (LC) circuits; others explain it in terms of standing wave theory. Both approaches end up with a similar result. When the distance between a motor and its VFD exceeds a critical distance (which may be as low as 30 ft), there is a voltage overshoot that may exceed twice the amplitude of the voltage pulse originally delivered at the VFD output terminals.
This higher voltage comes at the motor at such a high rate of change for each of the PWM pulses, from zero volts to its peak value, that it’s unevenly distributed across the winding, causing high turn-to-turn voltages in the turns connected closest to the power leads. The result places very high stress on the conductor insulation, which can cause early breakdown of the insulation.
Special inverter duty motors are available that are designed to meet or exceed the voltage amplitudes and rise times defined in NEMA Standard MG1, Motors and Generators, Section .184.108.40.206., Voltage Spikes. When connecting existing motors to VFDs with long cable lengths, you should consider using a filter to reduce the effects caused by the long cable.
Skin effect contributes to losses
In addition to the problems described above, there’s yet another loss component you should be aware of: skin effect. Skin effect induces the current in an AC system to crowd to the outside surface of a conductor. This phenomenon causes resistance to be directly related to the square root of the frequency of the current. In other words, the greater the frequency, the greater the resistance due to skin effect. The carrier frequencies are usually between 800 Hz to 15 kHz, and currents at these high frequencies will cause [I.sup.2]R losses. While the high frequency currents are relatively nominal, the loss relates to the current’s square power. And the carrier frequency, even at its square root, can be somewhat effective because of its basic high value. The geometry of the rotor bars also determines the degree to which the skin effect impacts rotor losses.
Motor application is very important
You should remember that a motor is a constant torque machine. In other words, at rated speed and rated torque, it will produce a certain horsepower. When speed is reduced through frequency and voltage reduction, the motor, by consuming more current, will try maintaining constant horsepower, if called for by the load. This can be done to a limited extent. As more current flows, more heat is produced, and it will not take long for the motor to overheat.
For situations where, throughout the speed range being used, there is a constant horsepower requirement, it’s critical that the motor be sized to match the horsepower requirement at the lowest shaft speed anticipated. For example, if the required speed range is from 50% to 100% of rated speed and the load’s horsepower requirement is 100 hp, then the motor must still be able to produce 100 hp at 50% speed. This also means that at 100% speed, the motor’s horsepower output, as called for by its load, also will be 100 hp; however, the load’s torque requirement will be reduced by 50%. At full-rated speed, the motor will be capable of producing 200 hp, meaning the motor will be larger than normal.
Using a VFD, with fundamental frequency being reduced to achieve lower speed, the voltage also is reduced in direct proportion to the speed reduction. As mentioned earlier, a 460V motor at half rotor speed will have 230V across its lines. Thus, if the motor’s rating is 100 hp at full speed, its output would only be 50 hp at half speed.
Certain loads, like lathes and grinders, require constant horsepower throughout their operating speed range. Let’s assume a VFD is serving a 20-hp lathe motor that’s operating at a 25% reduction in speed (3/4 rated speed). The lathe’s rotating chuck, which holds some material being worked by a cutting tool, will need constant horsepower over the entire speed range being used. If speed is reduced by 25%, voltage will be reduced by 25%. For the motor to maintain constant horsepower output, it will draw 33% more current (4/3 of normal amperage). Because current produces heat (primarily [I.sup.2]R losses), the motor will have to have sufficient thermal capacity to handle the extra amperage.
Some motors can withstand a certain amount of excess thermal load based on the motor’s service factor (SF). Usually an SF ranges from 1.0 to 1.15; beyond that point, motor damage will occur. Because voltage is reduced using a VFD, a motor’s horsepower rating must be increased to match the load requirement at the lowest speed used, should constant horsepower be required. Of course, this means the motor is overbuilt when used at higher speeds and will have higher losses and lower power factor (PF) at the higher speeds when operating at less than full load. However, the lower PF is compensated for by the VFD. This is a condition that must be accepted. Otherwise, you’re asking for trouble.
When working with motors, you’ll find it helpful to remember the following relationships:
1 hp = 0.746kW = [3 ft-lb x 1750 rpm] [divided by] 5250
Any of these numbers can be changed. When doing so, however, the equality of both sides of the equation must be maintained. Torque is ft-lb. If horsepower remains constant and speed (rpm) is reduced, obviously torque must be increased. Thus, in the above motor application (where there is a 25% reduction in speed), the motor’s torque output must be increased by 33%. And if kW remains constant and the voltage is reduced (which will happen using a VFD to reduce speed), current must be increased. This could lead to overheating. Incorrect application of motors is one of the main reasons why they fail.
If someone recommends getting a VFD for your existing motor, with the idea of making adjustments that would cause the output voltage to be set to any value (with a limit up to the VFD’s incoming voltage) for any particular fundamental frequency, use caution.
Such an adjustment can be made; for example, you can adjust a VFD to produce 460V at 30 Hz. If 460V is the line voltage (thus the maximum voltage), then as the fundamental frequency increases beyond the set point, the voltage going to the motor remains constant.
Let’s look at one of the above examples again. Say 100 hp is required at half speed and the VFD is adjusted for delivering 460V at 30 Hz. If you use an existing motor rated for 100 hp, what will happen? Well, the motor will try to deliver 100 hp at half speed and will continue to try should the fundamental frequency be increased while the voltage remains constant at 460V. (Note that when the fundamental frequency gets below the set value [say 15 Hz], the voltage will be reduced proportionally, in this case to 230V.) At 30 Hz and 460V, the iron in the stator of this existing motor is magnetically saturated, which causes more current to flow and the motor to become excessively hot. This condition may destroy conductor insulation as well as negatively affect other motor components. Motors usually have enough iron in their stators to handle a certain ratio of volts to frequency (V/Hz). But when the ratio increases extensively, more iron is needed; otherwise, there will be overheating.
Still, using 30 Hz at 460V is an effective way of getting adjustable speed at constant horsepower, providing the iron in the motor’s stator is designed to take a higher V/Hz ratio. This means that more iron has to be placed in the motor’s stator. There are certain motors built today that have extra iron in their stator for operation at high V/Hz ratios. You’ll have to pay a premium for them. But for certain type applications, such as above, such motors can be cost effective when compared with using an existing motor of twice the capacity. This is because the premium motor can operate at 30 Hz, 460V, and normal current, whereas the high-capacity existing motor, operating at 30 Hz, 230V, will have to use double the current, and will experience the losses associated with high-current operation.
Are you experiencing mechanical or electrical problems?
The financial consequences are considerable; every technical problem and every breakdown costs money in terms of repairs as well as lost production.
Electrical problems due to voltage and current transient arising from Direct-On-Line or Star-Delta starts. Such transients may overload the local supply network and cause unacceptable voltage variations that interfere with other electrical equipment connected to the network.
Mechanical problems that address the entire drive chain,from motor to driven equipment, causing a big need for service and repair as well as unwanted down time.
Operational problems, such as damage to products on conveyor belts.
Water hammering and pressure surges in pipe systems when starting and stopping pumps.
The easy solution to all of these problems is to install an ABB Softstarter type PSR, PSS, PSE or PST(B). With ABB Softstarters, it is possible to start and stop smoothly while keeping mechanical and electrical stresses to a minimum.
Differences between different starting methods
Graphs showing the basic differences between Direct-On-Line starting (DOL),
star-delta starting and soft starting in terms of the motor voltage (U), motor current (I) and motor torque (T).
Customers expect their equipment to handle their products with care and avoid unnecessary shaking or vibrations. While this requirement is an obvious one for the food and beverages on the production line, it is also true for the electrical supply that keeps the plant working. Electricity networks can easily be affected by harmonics, or higher-order oscillations introduced by various types of equipment. Harmonics can have negative effects, such as overheating and malfunctioning, on other equipment connected to the grid. Although solutions exist to counter or mitigate harmonics, the better solution is to employ equipment that doesn’t cause them in the first place. ABB offers a range of ultra-low harmonic drives.
Many phenomena in nature occur in cycles. Examples include a rotating wheel, waves on the sea or the changing seasons. The term “cycle” suggests a rotation at constant speed, something mathematicians describe using the sine function. However, the above examples would not (with the possible exception of the wheel) be adequately described by this function alone. The aberration takes the form of superimposed higher frequencies, which are themselves also sine functions. One way of picturing this would be that the outdoor temperature follows the slow cycle of the seasons throughout the year but is also affected by the much shorter cycle of day and night.
Harmonics are not in themselves a problem. Without harmonics, musical instruments would all sound the same, musicians wouldn’t be able to play chords and surfers wouldn’t have much pleasure on the waves. But in electrical systems, harmonics could wreak a lot of havoc. Because the generators in power plants rotate at constant and regulated speed, the current in an AC grid is sine shaped in the ideal case. However, in reality it often isn’t because harmonics are introduced into the grid through various effects.
Equipment that introduces harmonics includes motor starters, variable-speed drives, welding equipment, uninterrupted power supplies and computers. The harmonics they cause can negatively affect other devices and systems connected to the grid. In motors, transformers and other equipment they cause heat, which is wasteful of energy, requires additional cooling and can ultimately damage the equipment. Displays and lighting can flicker, circuit breakers can trip and measurement devices can give false readings.
Why does a variable-speed drive cause harmonics? Such a drive converts a fixed voltage and fixed frequency input (from the grid) to a variable voltage and frequency output (typically to control and power a motor). This is usually achieved through the intermediary of a DC link: Two converters are arranged back-to-back with the AC grid input being converted to DC in the first converter and then converted back to AC at the required voltage and frequency in the second.
In conventional drives, the grid-side converter uses a six-pulse diode bridge. The drawback of this solution is that it introduces current harmonics into the grid.
Especially prevalent are the so-called fifth and seventh harmonics (ie, they have five and seven times the grid frequency, respectively). The resulting distortion can account for 30 to 50 percent of total current.
The problem of harmonics is far from new and multiple solutions exist, including active and passive filters, chokes and multi-pulse methods with multi-winding transformers. But prevention is better than cure and thus ABB offers ultra-low harmonic drives created to avoid these harmonics by design. Such a converter, combined with the drive’s built-in active supply unit together with the line filter, can reduce the current distortion to below 5 percent.
The input converters of ultra-low harmonic drives do not use diodes but IGBTs1 that can be used to actively modulate smoother wave-forms.
ABB offers a family of ultra-low harmonic devices, an example of which is the ACS800-31, a wall-mounted drive for up to 110 kW. It includes EMC filters and I/O extension modules and is available with an IP21 protection rating, making it suitable for several applications in a food and beverage environment.
For higher power requirements, the cabinet-mounted ACS800-37 drive for up to 2,800 kW is available with a protection rating up to IP54.
ABB’s drives are easy to set up and configure and are suitable for a broad range of working environments and power classes.
With the cost of energy and water and wastewater services continuing to rise, operators are looking for ways to reduce the costs of these basic, yet crucial to life, utilities. Proposed energy efficiency standards from the US Department of Energy (DOE) and other federal measures could help.
On the water side, DOE has a mandate to develop and enforce national minimum energy efficiency standards for any product if significant savings can be realized in a cost justified and technically feasible manner. During the past year, DOE and stakeholders have been negotiating a regulation for pump efficiency. That rule, now in the Notice of Proposed Rulemaking phase, would apply only to clean water pumps of these types:
- End suction, close-coupled
- End suction, framemounted/own bearings
- Radially split, multi-stage, vertical, in-line, diffuser casing
- Vertical turbine, submersible
With the massive infrastructure of existing water distribution systems, the ability to increase energy efficiency comes down to a few items, including pumps and the motors that operate them. Utilities are incentivizing facilities to install variable frequency drives (VFDs) onto their motors and pumps to reduce the electrical surges caused by starting and stopping motors/pumps which benefits the facility through reduced water hammer and lower maintenance costs.
By increasing the efficiency of the pump itself as well as the motor, accomplished by selecting a premium efficiency motor and operating that pump/motor system at constant pressure or constant flow, a VFD can help reduce the energy consumed. But how do we know we will save energy?
This energy consumption is due to the affinity laws; the three affinity laws describe the relationship between speed of the pump (n), flow (Q), pressure head (H), and electrical power consumption (P):
This shows us that the power to run a pump is relative to the cube of the speed, which means that if we slow down a pump by 20% we enjoy almost 50% in power or energy reduction. So, even small decreases in pump speed can result in energy savings. It is important to note that the maximum speed of the pump may not be the highest efficiency that it can pump when looking at gallons per kWh. It may be experienced at a lower than full speed flow.
But energy consumption may not be the only issue that variable frequency drives may help to resolve. What about non revenue water? Since we can now better regulate pressure, why not reduce our system pressure?
Prior to the now common use of drives, water distribution systems typically used pressure switches to control pumps. When the system hit a low pressure point, the pump turned on, and when it hit a higher pressure point it was turned off. There might have been two pumps: a smaller one for slow night hours and a larger unit for the higher daily usages. Th is type of control causes pressure fluctuations, but is often common practice. With a VFD, pressure is maintained at a constant set point and, as pressure decreases, the motor/pump increases speed to maintain pressure.
By reducing and maintaining a constant pressure of the system, water leakage can be reduced. For example, a single 1/4 inch hole in a distribution pipe can leak as much as 4 million gallons per year at 50 pounds per square inch gauge or over 5 million gallons per year at 80 pounds per square inch gauge. By keeping the pressure more constant, non revenue water losses can be reduced.
While there are opportunities to reduce energy consumption and losses in water distribution systems which could lead to higher revenue and lower operating costs there are additional savings that may not be initially realized:
- Protection of pumps and assets
- Reduction of maintenance cost
- Limiting the risk for bacteria/contamination of tap water
- Limiting the risk for road breaks
- Reducing pipe repair cost
- Extended service life of network
- Postponing investment in system upgrades
- Improved control performance
- Increased redundancy
- Usage of dedicated VFD software features thus reduced load on the SCADA system
Similarly, on the wastewater side of the system, there are more processes, and as a result, a greater number of motors and pumps. Since wastewater pumps operate at different conditions from clean water pumps, they have not been included in the DOE rule, although that does not mean DOE could not develop a rule for wastewater pumps in the future. However, DOE’s schedule for efficiency rulemakings under the present administration appears full and a federal initiative in the near term does not seem likely.
On the other hand, EPA, through its Clean Air Act (section 111d), will require states to develop compliance plans, which could include state energy efficiency plans. Wastewater treatment plants could be included in this. It would not likely state a minimum efficiency for wastewater pumps specifically, but could incentivize plants that reduce their energy use.
So how can wastewater operations work to increase efficiency and reduce costs today? Adding a VFD to the motor and pump can make it more efficient if it is controlled properly, similar to how in clean water applications utility rebates can help with the acquisition and installation costs. Further, the resulting lower energy bill can help to reduce the operating costs, but it’s important to be smart in the control of these systems.
For smaller facilities, using a larger SCADA system may be out of the question to operate the entire system. But today’s VFDs typically include integral controllers that can be used instead to maintain level, pressure, flow, dissolved oxygen, turbidity, or other processes being monitored. Simply by providing an analog signal to the VFD from the monitoring equipment, the VFD has the ability to maintain a constant process output, and through internal communication modules data can be transmitted to other areas to monitor current status.
Additionally, integral cascade controllers can operate multiple pumps allowing for better overall control and when utilized in a variable pumping system, where pumps of different flow capabilities are installed and staggered to meet flow requirements can increase system efficiency. This type of control, master/follower, can off er the most efficient operation of a pumping system and the highest system efficiency.
In most any size system, the use of VFDs on pump motors means greater flexibility and better process control. By reducing the surges that a plant may see during an event or normal loading, the plant can more closely handle the waste influx by regulating the flows into the plant. VFDs can operate as slave units to the existing SCADA or overseeing system.
Drives can also off er features that help maintain pump efficiency such as a deragging function. Th is feature will rotate the impeller backwards (on a pump that can be operated backwards) to dislodge solids, stringy materials, or other debris to keep the impeller better able to pump at optimum performance.
VFD manufacturers off er different features, but drives designed specifically for the water/wastewater market tend to offer additional features to facilitate motor/pump operation such as deragging, initial ramp and/or check valve ramping, flow counting, flow confirmation, no flow or low flow/dry pump protection, and many others.
Additionally, analog and digital output cards, communication cards, and specific application cards are available to assist in control functions. As the water/wastewater industry continues to modernize and expand and issues like reducing energy consumption and water loss become more important and regulated, VFDs will become more commonplace in many applications.