Q. Are the pump motors special for VFD operation or can any motor be used with a VFD?
A. Yes. Today there are special motors that are rated to be used with a VFD. They are commonly called "inverter rated". Older motors can be used with VFDs but there are factors that need to be reviewed such as motor lead length, voltage (460 & 575)(the higher the voltage the bigger the concern, and motor cooling because as a motor slows down it might not be able to cool itself properly especially in a constant torque application like a PD pump.
Q. Are VFD's suited to small (10 HP) motors in a pump station?
A. Yes but for low horsepower motors you should look at the cost benefit ratio especially considering the minimum speed. 10Hp is probably ok.
Q. Can a VFD be used with any motor or do they have to be of a special design?
A. Yes. Today there are special motors that are rated to be used with a VFD. They are commonly called "inverter rated". Older motors can be used with VFDs but there are factors that need to be reviewed such as motor lead length, voltage (460 & 575)(the higher the voltage the bigger the concern, and motor cooling because as a motor slows down it might not be able to cool itself properly especially in a constant torque application like a PD pump.
Q. Can you talk a little bit about any issues using modern VFDs with existing (old) induction motors?
A. Yes. Today there are special motors that are rated to be used with a VFD. They are commonly called "inverter rated". Older motors can be used with VFDs but there are factors that need to be reviewed such as motor lead length, voltage (460 & 575)(the higher the voltage the bigger the concern, and motor cooling because as a motor slows down it might not be able to cool itself properly especially in a constant torque application like a PD pump.
Q. Can a VFD be installed in an outdoor area?
A. Yes. It must be installed properly selected outdoor enclosure (common enclosure rating of UL TYPE 3R). There are numerous variables that need to be considered as well such temperature (hot & cold), direct sunlight, altitude, etc.
Q. Why are VFD outputs 3-phase?
A. 3-phase is needed in order to be able to control the speed or torque of the motor. This cannot be sufficient done with single phase motors.
Q. Any comment on loss of efficiency in the motor and VFD at reduced speeds? Can the vendor provide the wire to water efficiencies?
A. The drive and motor efficiency will go down slightly as a factor of speed reduction. Just like with the motor the energy goes down when the speed is reduced but for the VFD it is not linear. Wire to water efficiencies can certainly be calculated but not all suppliers can offer this. This item will have to be determined from vendor to vendor.
Q. What does PID stands for?
A. Proportional, Integral, Derivative.
Q. In the early days of VFDs there was a rule of thumb not to run the motor below 50% speed. Is this no longer true?
A. No. VFDs can actually float a load at zero speed such as in a hoist or elevator application. For pump applications you typically do not go below 50% speed because of the pump / system characteristics.
Q. Do you recommend passive filters or line reactors to combat the effects of the harmonics or using a 12 or 18 pulse VFDs?
A. Unfortunately there is no simple answer to this. All solutions help to reduce harmonics but what it comes down to is the size of the VFD(s), other linear and non-linear loads and the electrical distribution system. Typically for small HP VFDs reactors are fine and then as they increase in size is when a multi-pulse solution might be considered
Q. Some manufacturers have VFDs to serve 1 phase well pump motors. How does that work for starting?
A. This is a very special VFD and I only know of one manufacturer that has this offer but there could be more. In order for this to work the motor capacitor / start wire must be available to connect to the VFD. Outside of that I am not sure how this technology works.
Q. Clarification - could VFD output single phase? Is there an advantage to using 3-phase outputs or just industry standard?
A. The VFD could have a single phase output situation in the event that the motor wire comes loose or if there is an output contactor on the VFD and one of the contacts fails.
Q. What is the response time between VFD and pump/motor to respond to that change?
A. Considering the speed of the VFD processor it would be less then 100 milliseconds.
Q. Drives can be set for either constant torque or variable torque loads. Which is the proper setting for pumps?
A. For centrifugal pumps the drive should always be set to variable torque. Positive displacement pumps may require constant torque drives, but this depends on the application.
Q. How do you establish the minimum speed setting for a VFD pump system?
A. Minimum speeds are established by analyzing several parameters and computing the min flow based on the most conservative limiting factor. The primary parameters are: fluid velocity in the largest sections of pipe, minimum flow that the pump will run at based on the system curve, and cooling of the motor. For wastewater pumps, the higher propensity to clog at reduced speed must also be factored in and accounted for.
Fluid velocity: compute the flow required to maintain the minimum recommended fluid velocity in the largest pipe sections of the system (don’t forget to consider suction piping on dry pit pumps). The formula for computing velocity is GPM x .408 / Pipe ID squared (pipe ID in inches). For normal wastewater it is recommended to maintain a minimum of 2 ft/sec for horizontal runs and 3 ft/sec for vertical runs.
Minimum flow through pump: Pump manufacturers normally publish the minimum recommended flow for each pump model in their catalog literature. The determination of this minimum recommended flow depends on many factors including impeller type, free passage size, specific speed, bearing structure and overhangs, as well as field experience and other parameters. To check the minimum flow the pump will experience in a particular system at a particular speed, locate the intersection points between the system curve and the reduced speed pump curves, and read the flow at each reduced speed intersection point (see presentation for examples). Determine the minimum speed which meets or exceeds the pump manufacturer’s recommendation for minimum flow. Keep in mind that running at minimum flows is much more damaging to the pump if the rpm at the min flow is high, than if it is low. For systems with a high percentage of static head where the pump rpm will be quite high at low flow, you should be much more conservative with the minimum flow setting (set the min flow higher than you would if the pump rpm was lower).
Motor cooling: The motor must be cooled during operation to prevent overheating. VFD systems typically create a little bit more heat in the motor than direct line power systems. In most cases, motor manufacturers accommodate for this extra heat in the design and service factor of the motor, but in some cases, motors must be de-rated for VFD use. At reduced speed, the power output of the motor is reduced due to the lower power demand from the pump wet end. Therefore you would expect the motor running at reduced speed to run cooler, but unfortunately this is not always the case. At reduced speed the motor cooling system becomes less effective, which can lead to higher temperatures. The cooling system type that is most prone to this problem is the open cooling system, which uses a portion of the process fluid to cool the motor. The pump manufacturer should be consulted to verify the minimum speed or minimum flow that will provide adequate motor cooling.
Clogging: As the speed (and flow through) a wastewater pump is reduced, the propensity to clog with solids increases. This is especially true with pumps that run at reduced speed for long periods of time without stopping, such as treatment plant influent pumps. The rules of thumb vary depending on the impeller types, but generally speaking, fluid velocities through the pump below 5 ft/sec are likely to create more clogging problems. This should be taken into consideration when setting min flow for a VFD based system.
Q. How are VFDs sized or selected for a particular motor?
A. VFDs are classified by horsepower to aid in the selection process, but the best way to select the drive is to compare the maximum drive output current to the maximum motor current. To do this, select a drive where it’s max output current rating (not including its safety margin) exceeds the motor’s full load current rating by about 5 to 10 percent. Be aware that sometimes the max drive output current is de-rated in hot environments or at high elevations, so be sure to correct for environmental conditions.
Q. What effect does slowing the pump down have on the solids passing ability of a wastewater pump?
A. For most impeller types used in wastewater, slowing the pump down reduces the impeller’s ability to pass solids, especially fibrous or rag type material. This means that the pumps are more likely to clog at reduced speed than at full speed. To help prevent clogging at reduced speed, try and maintain at least 5 feet per second fluid velocity through the pump, and ramp the pump up to full speed on each start allowing it to stabilize before dropping the speed to reduced speed control.
Q. For centrifugal pumps is it possible to operate at low frequency, like 20 Hz?
A. Yes, centrifugal pumps can operate at low speeds, provided several critical parameters are accounted for. Please see the answer to the question “How do you establish the minimum speed setting for a VFD pump system?” for details.
Q. How can a VFD allow smaller pumps to be used if you still need to handle peak flow?
A. A VFD system will not necessarily allow small pumps to meet peak flow requirements, but may allow the pumps to operate more efficiently during off peak time (which is most of the time). Selecting the full speed operating point to the right of Best Efficiency Point for VFD systems helps improve the efficiency at reduced speed, and often has the side effect of resulting in smaller pumps. Peak flow requirements however must still be considered and sized for.
Q. Why for fixed speed application is it recommended to place the operating point at the left of pump's BEP?
A. For fixed speed operation the pump should be selected so that the operating point is as close to BEP as possible. It does not matter too much whether you are a little left of BEP or a little right of BEP for most impeller types. As we discussed in the webinar, for variable speed operation it is always preferable to select the pump so that the full speed operating point is to the right of BEP (of course NPSH margin should always be checked).
Q. What is the effect of reverse rotation on VFD operation during flow reversal? How long it can sustain flow reversal phenomenon?
A. Reverse rotation of the pump can occur in systems where no check valve is used and the water flows back through the pump, spinning it backwards at the end of each pump cycle. This type of system is often used in flood control stations with short discharge piping, or in cold climates where the piping may be exposed to freezing temperatures and it makes sense to drain the pipe between cycles.
If a VFD attempts to start a motor that is spinning backwards, in most cases, the VFD will fault out and alarm, either due to its sensing the reverse rotation or due to reaching the preset current limit while trying to start a back spinning motor. The best way to handle this and prevent faults and alarms is by employing a restart timeout period. The restart timeout prevents the pump from restarting for a prescribed period of time. This allows the back flowing water and back spinning impeller to stop before a restart is initiated. This timer would be set during the station startup based on observations of the system in operation. Most modern VFDs have the restart timer function built in, so programming it is quite simple. A word of caution, not all pumps are capable of back spinning without damage. The pump manufacturer should be consulted prior to selecting a pump for this type of application.
Q. Another advantage of VFD systems should be smaller wet wells. What method or parameters are used to size wet wells?
A. VFD systems can be used to help reduce pump cycling with existing wet wells that are too small for the demand. As a result, in theory it is possible to reduce the wet well size for new stations where VFDs will be employed. This must be done with great caution however. In general it’s best to size wet wells assuming start-stop operation, and creating enough storage volume to find that balance between minimizing pump starts, and keeping the wet well active enough to prevent settling, septic wastewater, and odors. Commercial software is available to help with wet well sizing, and some pump manufacturers can supply software for assistance in wet well sizing.
Q. What is the best practice to select the VFD drive size?
A. Please see the answer to the question “How are VFDs sized or selected for a particular motor?” for details.
Q. How can a VFD detect true cavitation?
A. The VFD cannot specifically detect cavitation however it can look at various other parameters and inputs and make “educated assumptions” about what is going on with the pump. A parameter that the VFD may consider would be pump performance at a specific speed, based on a known pump and system curve. It may also receive input from other devices such a vibration sensors to make the determination.
Please address potential limitations on pump speed reduction related to line velocity requirements to prevent solids settling.
Please see the answer to the question “How do you establish the minimum speed setting for a VFD pump system?” for this answer.
Q. How can you develop an accurate system curve?
A. The system curve is developed by carefully inventorying all piping fittings and other loss causing components in a system, then using commercially available software, an Excel spreadsheet, or in some cases, pump manufacturer’s selection software to calculate the curve. Of course, the static head must also be measured and added to the calculation of the system curve.
An alternate way to create a fairly accurate system curve for an existing pump system is to take some field measurements during pump operation. A fairly accurate system curve can be plotted by commercially available software programs (or by hand) using just two points, the static head, and one operating point.
The static head can be estimated by placing a pressure gauge in the discharge pipe on the downstream side of the check valve. Run the pump system until steady state is achieved, then shut off the pump. Once the gauge reading stabilizes with the pump off, the gage will show the static head of the system (some caution is advised here because this gauge reading may only be an estimate of the static head, and may not be accurate for piping systems that vary significantly in elevation over their run, or systems that siphon). To capture the operating point, run the pump in steady state while recording the discharge pressure and flow rate (for dry pit pumps you will need to measure and factor in the suction pressure as well). Be sure and add the necessary manual corrections to the pressure gauge readings to get an accurate total head the pump is operating at. These manual adders include the Velocity Head at the gauge tap, the Friction Loss in all piping components between the gauge tap and the pump flange, and the Elevation between the surface of the water and the centerline of the gauge (not required if a suction gauge is used and the suction readings factored into the total head). There are many publications available that describe this process in more detail than we have space for here.
Q. Please explain the affinity law symbols.
A. The definition of the symbols is as follows:
Q = flow
n = rotational speed of the pump. It is common to substitute VFD output frequency for rpm of the pump, and the results are essentially the same regardless of which parameter is used.
H = Head or discharge pressure
P = Power
In the US water/wastewater industry, the most common units are Gallons per Minute for Q, RPM for n (can also be drive out frequency in Hz), Feet of water for H, and kilo Watts for P.
Q. Is there a speed reduction where recirculation damage to the pump begins?
A. Yes, but it’s not a fixed number for all situations. The damaging effect of recirculation cavitation at reduced speed depends on the impeller type and material, the flow through the pump at reduced speed (usually compared to the flow at best efficiency point), and the rotational speed at the min flow. There are so many factors involved that it is impossible to provide rules of thumb however in general you can say that if the pump will be running at a relatively high RPM at reduced flow, the negative effects of recirculation will be worse than if the rpm was lower. As a result, for systems with a high percentage of static head, you need to be quite conservative about the min flow rate (keep the min flow higher) to avoid pump damage.
Q. Are VFDs ideal for a pump that has a highly varied diurnal pattern?
A.Yes they are, but the effectiveness of a variable speed pumping system in accommodating large swings in flow depends mostly on the percentage of static head in the total head at the duty point. Systems with a low percentage of static head are idea for VFDs, but systems with a high percentage of static head are not. These “high percentage” systems may be better off with a different kind of station design such as a jockey pump system. A jockey system uses one or more small pumps to handle the low to medium flow periods, and one or more larger pumps to handle the higher flow periods. For stations with a high percentage of static head, and which have a highly varied inflow during the day, a jockey pump system with direct on line starting, or reduced voltage starting is normally the most efficient.
Q. Are Design B motors rated to operate above 60 Hz? If so, how much above and for how long?
A. Design B motors are rated to operate at 60Hz only, however many of them are capable of operating above that frequency. The manufacturer of the motor must be consulted whenever you plan to run a motor at above rated speed or frequency. The limitations for over speeding motors are typically as follows:
- Power output. Most loads require much more motor power at higher speeds, so you can quickly exceed the motor’s output capability when over speeding.
- Mechanical stability. Loads and forces on the rotor, shaft and bearings increase rapidly when over speeding, which could lead to mechanical failure.
- Temperature. The motor may run too hot when over speeding, leading to premature winding failure.
- Vibration. The motor may vibrate at unacceptable levels if a natural frequency is reached or approached during over speeding.
The motor manufacturer can provide advice for how much over speeding the motor can take, and for how long. However, as long as the loading is acceptable, the motor is mechanically stable, and the temperatures are within acceptable limits, the motor should be able to operate continuously at the higher speeds.
Q. When considering a VFD retrofit on an existing pump, how does one analyze the VFD energy savings, when the pump and system curves my not be easily obtainable?
A. The short answer is that you need certain minimum information to do the analysis, and without it, the potential energy savings is just a guess. One thing that you can say without having too much information is that if the total head in the system is comprised mostly of static head, you will probably not save much, if any money on energy cost by employing a VFD. If the total head is comprised mostly of dynamic head (a low percentage of static head) and if the pumping demand for the station varies a fair amount over the course of the day, you will likely save on energy costs by employing a VFD. The second requirement of varying pumping demand over the day is important because stations with pumps that run at full speed most of the time just to keep up, will not see much benefit from the VFD even if they have a low percentage of static head. This is because the energy savings comes from reducing the speed, so if the pump must run at full speed most of the time anyway, there will not be much opportunity for savings.
Q. Are 3500 or 1800 rpm motors preferable for VFD operation?
A. Motors of any base speed can be used with VFDs, so there is no particular advantage of one speed motor over another for VFD operation. You should always select pumps with the most appropriate speed motors for the duty point that they must serve, regardless of whether a VFD is being used or not. The rules of thumb are to use higher speed motors for high head (high pressure) applications, and low speed motors for low head (low pressure) applications. There are some small exceptions to this rule, but in general, following this rule will give you the most efficient and longest lasting pump for your duty point.
Q. Any advice about using a VDF on a LOBE Rotary Pumps?
A. Lobe rotary pumps would be considered positive displacement (PD) pumps rather than centrifugal pumps. Most of the considerations are the same for each type, however PD pumps can require much more torque from the motor at reduced speed that centrifugal pumps. This is application dependent, so it may or may not be the case, depending on the application. For loads that can require high torque at startup or high torque at low speeds, constant torque VFDs may be more suitable. The torque demand curve for the pump in the specific application should be shared with the VFD supplier during the selection process to insure that the proper VFD is selected.
Q. Does the motor size determine if a shaft grounding ring is needed to protect bearings when installing a VFD?
A. Yes the motor size is a parameter in this decision. In general, motors below about 100 hp do not need any circulating current mitigation measures such as grounding rings or isolated bearings. Motors greater than 100 hp will probably benefit from grounding rings or isolated bearings. Motors above about 200 hp should always have grounding rings or isolated bearings when used on VFDs.