I was recently told that if the actual load on a pump motor were less than 50% of rated, there would be great potential for energy savings through the use of VFD’s. Is there a “rule of thumb” or an equation that would help predict savings by using VFD’s on synchronous pumps?
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ASD savings based on pump motor loading?? 13
- Thread starter pumpedup
- Start date
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4
- #2
The VFD actually has nothing to do with the energy savings, its the system the pump (or load) is working in. If you have varied flow or head requirements, then use of a VFD to match the pump to your system needs saves power over using things like recirc lines, throttled valves, and bypasses. I have been told that a lot too by vendors for VFD's, is that where you happened to hear that from?
My question back to the vendor usually is someting like.....you mean if I overdesign the motor to 2 times my need then VFD it down 50 percent to the actual power prequirement I will save more money that running the pump with the correct motor? The answer is no, but that is not usually the response I get....LOL
VFD's are actually another inefficiency in the pump/motor system that must be figured into the efficiency of the pump and motor. Again, if your system varies, they are worth their weight in gold insofar as the electric savings you can realize....
There really is no rule of thumb for the VFD but rather you need to calculate the way the system operates over a period of time and perform a summation of energy requirements at the various locations of the system curve if you throttle, or flows in the bypass lines.
hope this helped....
BobPE
My question back to the vendor usually is someting like.....you mean if I overdesign the motor to 2 times my need then VFD it down 50 percent to the actual power prequirement I will save more money that running the pump with the correct motor? The answer is no, but that is not usually the response I get....LOL
VFD's are actually another inefficiency in the pump/motor system that must be figured into the efficiency of the pump and motor. Again, if your system varies, they are worth their weight in gold insofar as the electric savings you can realize....
There really is no rule of thumb for the VFD but rather you need to calculate the way the system operates over a period of time and perform a summation of energy requirements at the various locations of the system curve if you throttle, or flows in the bypass lines.
hope this helped....
BobPE
- Thread starter
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I agree with Bob. VFD's will only save you money if your running the motor at less than nameplate (to what degree is debatable and determined by application only) then the VFD may save some money as far as power factor penalties and such. If the system was oversized for some reason but the head is determined to be about the same over some period of operation, then it is more sensible to change the pump and motor (depending on cost) than install a VFD. The VFD may create additional headaches with equipment (or utilities depending on size) from the harmonics it places on the utility (and thus other circuits in the building). Line reactors may have to be installed (more money). Anyway, your VFD salesman is not atypical.
I think you will find that most companies are similar, in that controlling flow rates tends to be determined by a flow control valve and not by variable speed. This is 'old' technology. In essence, the pump generates too much head for the required flow and the FCV then has to limit the flow. The energy required by the motor to generate the excess pump head is basically wasted energy.
In saying this, not all applications are suitable for VSD's, so I would suggest you contact your local ABB rep, as they have a free CD that not only calculates the energy saving, but also the CO2 saving that the rest of the world gets from the wasted energy not having to be generated in the first place!
In saying this, not all applications are suitable for VSD's, so I would suggest you contact your local ABB rep, as they have a free CD that not only calculates the energy saving, but also the CO2 saving that the rest of the world gets from the wasted energy not having to be generated in the first place!
electricpete
Electrical
The biggest savings associated with vfd's are reduction in fluid system losses, as discussed.
But, there are devices specifically targeted to improve efficiency of MOTORS operating at or below 50%. They are called NOLA devices. They reduced the voltage to reduce the associated core losses and magnetizing current I^2*R losses. It sounds like perhaps this was the type of device the vendor was referring to.
But, there are devices specifically targeted to improve efficiency of MOTORS operating at or below 50%. They are called NOLA devices. They reduced the voltage to reduce the associated core losses and magnetizing current I^2*R losses. It sounds like perhaps this was the type of device the vendor was referring to.
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pumpedup:
Drives are good for some applications, but not all. If your pump/motor operates at 100% load some of the time and 50% load some time you need to calculate the savings yourself.
You also need to ask your drive supplier what the losses will be for the drive itself. In most cases the drive will have heat losses anywhere from 2% up to 5% depending on the manufacture and type. The drive will constantly have a low(er) power factor that can not be corrected even at 100% motor load.
Drives are the best answer for some applications, but there are a lot of white elephants in operation too simply because the engineer is not smart enough to design a 60 hertz operation.
One thing I consider is that I have seen a lot of D-O-L starters over 20 years old that have not been touched. With drives if you operate 2 to 3 years without repairing it, it is a good drive. You also add heat to the motor with a VSD, so if you add a drive you may need to upgrade the motor to a “VSD” designed or rated motor.
I sell pumping systems, but I don’t like drives. I have a customer show me an absolute demand before I sell one. They cost more up front and cause more headaches than a D-O-L type starter. I will sell them where the need is such, but will let my customer know about all the negatives too. There are times where the system and operation demand a drive, but I would recommend that you review all the added benefits and problems before buying one.
Good luck!
David
Drives are good for some applications, but not all. If your pump/motor operates at 100% load some of the time and 50% load some time you need to calculate the savings yourself.
You also need to ask your drive supplier what the losses will be for the drive itself. In most cases the drive will have heat losses anywhere from 2% up to 5% depending on the manufacture and type. The drive will constantly have a low(er) power factor that can not be corrected even at 100% motor load.
Drives are the best answer for some applications, but there are a lot of white elephants in operation too simply because the engineer is not smart enough to design a 60 hertz operation.
One thing I consider is that I have seen a lot of D-O-L starters over 20 years old that have not been touched. With drives if you operate 2 to 3 years without repairing it, it is a good drive. You also add heat to the motor with a VSD, so if you add a drive you may need to upgrade the motor to a “VSD” designed or rated motor.
I sell pumping systems, but I don’t like drives. I have a customer show me an absolute demand before I sell one. They cost more up front and cause more headaches than a D-O-L type starter. I will sell them where the need is such, but will let my customer know about all the negatives too. There are times where the system and operation demand a drive, but I would recommend that you review all the added benefits and problems before buying one.
Good luck!
David
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The only type type of case where the use of a VFD is usually likely to be an easy choice is where ALL of the following conditions are involved:
(1) The required flow rate varies substantially.
(2) Much of the duty cycle is at significantly reduced flow rates.
(3) The VFD eliminates the use of a throttling valve in the system.
(4) The head required from the pump is simply to overcome frictional losses due to flow through pipes, heat exchangers, etc. with no great head needed such as to charge a pressure vessel.
(5) The pump and motor involved are a reasonable combination for the duty. (The flow rates and corresponding required head are well within the normal design capabilities of the pump, AND the motor is NOT significantly oversized.)
(6) The duty cycle involves at least 2000 hours per year (preferably much more).
The above may seem to be an endorsement of a simplistic selection process, but it should only be interpreted as a general guide for perspective.
When condition (2) involves relatively small flows for relatively large portions of the duty cycle, the energy savings provided by a VFD can appear to be quite dramatic, but prudent system evaluation should take other options into consideration. For example, two (or more) pumps operating in parallel (operating individually or in combination) may actually be able provide better cost and energy saving benefits.
Pumpedup, the simple answer to your question is that there is no simple answer. The implementation of a VFD in a system can provide astounding savings (pay back in under a year may be possible). Conversely, a VFD installation can cost far more than any potential energy savings, and in the case of really poor choices, VFD induced energy burdens may result in a net increase in energy usage.
Another caution to be considered is to pay attention to the range of speeds involved. Care must be taken to avoid operation at frequencies where harmonic effects can be troublesome.
(1) The required flow rate varies substantially.
(2) Much of the duty cycle is at significantly reduced flow rates.
(3) The VFD eliminates the use of a throttling valve in the system.
(4) The head required from the pump is simply to overcome frictional losses due to flow through pipes, heat exchangers, etc. with no great head needed such as to charge a pressure vessel.
(5) The pump and motor involved are a reasonable combination for the duty. (The flow rates and corresponding required head are well within the normal design capabilities of the pump, AND the motor is NOT significantly oversized.)
(6) The duty cycle involves at least 2000 hours per year (preferably much more).
The above may seem to be an endorsement of a simplistic selection process, but it should only be interpreted as a general guide for perspective.
When condition (2) involves relatively small flows for relatively large portions of the duty cycle, the energy savings provided by a VFD can appear to be quite dramatic, but prudent system evaluation should take other options into consideration. For example, two (or more) pumps operating in parallel (operating individually or in combination) may actually be able provide better cost and energy saving benefits.
Pumpedup, the simple answer to your question is that there is no simple answer. The implementation of a VFD in a system can provide astounding savings (pay back in under a year may be possible). Conversely, a VFD installation can cost far more than any potential energy savings, and in the case of really poor choices, VFD induced energy burdens may result in a net increase in energy usage.
Another caution to be considered is to pay attention to the range of speeds involved. Care must be taken to avoid operation at frequencies where harmonic effects can be troublesome.
All good replys. In the lines of ccfowler's 4th point, if the static discharge head is substantial when compared to the frictional losses (for ex. multi storeyed buildings) VSD is not a good option. If the flow rate is continuously 50% then better go for a change in pump.
Regards,
Regards,
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pumpedup
I work for a company in the oilfield that provides both pumps and VSDs, i would challenge the statement made by the vendor. Bob's points are very well made.
d23 and I work for competing companies and I am much more comfortable with drives than he is. The fact that the VSD adds heat to the motor is undeniable, but in the applications he and I specialise in this is already designed into our motors.
Many of the issues that he raises can be addressed by proper set up of the converter firing angle in drives with a controlled front end as opposed to simple diode bridge front ends.
But having said all that, like any engineered product, they have their advantages and limitations. They should not be looked upon as a panacea nor discarded because of a lack if understanding of them. Any good engineer will investigate the advantages they bring and weigh this up against the cost. We may be engineers, but these days we all get to be accountants too!
dadfap
I work for a company in the oilfield that provides both pumps and VSDs, i would challenge the statement made by the vendor. Bob's points are very well made.
d23 and I work for competing companies and I am much more comfortable with drives than he is. The fact that the VSD adds heat to the motor is undeniable, but in the applications he and I specialise in this is already designed into our motors.
Many of the issues that he raises can be addressed by proper set up of the converter firing angle in drives with a controlled front end as opposed to simple diode bridge front ends.
But having said all that, like any engineered product, they have their advantages and limitations. They should not be looked upon as a panacea nor discarded because of a lack if understanding of them. Any good engineer will investigate the advantages they bring and weigh this up against the cost. We may be engineers, but these days we all get to be accountants too!
dadfap
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goutam_freelance
Mechanical
There is one control system benefit of VSD. As the discharge control valve (level control or flow control) will have reduced throttling duty , the life of the valve improves due to lower cavitation and wear. Also the control valve can operate within good controllable range.
goutamiam
goutamiam
goutamiam
I not trying to be smart, but if I paid X amount for a drive I would expect the system to operate without a control valve. If I'm operating a control valve then I wouldn't want a VSD.
With my limited amount of industrial experience I’ve found that with two separate control systems, one electrical and one mechanical, operating on one system it is next to impossible to prevent nuisance oscillations on the system. When you need less pressure or flow both devices attempt to control the system usually causing fluid oscillations.
David
I not trying to be smart, but if I paid X amount for a drive I would expect the system to operate without a control valve. If I'm operating a control valve then I wouldn't want a VSD.
With my limited amount of industrial experience I’ve found that with two separate control systems, one electrical and one mechanical, operating on one system it is next to impossible to prevent nuisance oscillations on the system. When you need less pressure or flow both devices attempt to control the system usually causing fluid oscillations.
David
d23,
You are absolutely right about the matter of duelling control systems, but sometimes the control valve is left in place in retrofit VSD installations. This is done to provide a redundant control system in case of a problem with the VSD. The normal operating mode is with the control valve stroked full-open and the VSD system in full control of the system. This arrangement necessarily involves additional fluid friction losses introduced by the control valve, so savings are necessarily compromised.
I agree with your general suspicion of VSD's. There is way too much of a fad involved in specifying them now. (Just notice how commonly they are simply presumed to be the answer to any and all problems--possibly including bad breath and dandruff.)
The various "electrical noise" and motor heating problems associated with the various adjustable frequency types of drives can eat up much of the expected savings unless they are fully and properly addressed from the very beginning. Because of this, it is usually wise to consider the eddy current types of VSD's. They avoid all the electrical problems of adjustable frequency drives, usually without much net relative energy penalty.
In thoroughly considered and properly designed systems, VSD's can provide excellent control and substantial savings. They should always be given due consideration whenever their use can be of benefit.
You are absolutely right about the matter of duelling control systems, but sometimes the control valve is left in place in retrofit VSD installations. This is done to provide a redundant control system in case of a problem with the VSD. The normal operating mode is with the control valve stroked full-open and the VSD system in full control of the system. This arrangement necessarily involves additional fluid friction losses introduced by the control valve, so savings are necessarily compromised.
I agree with your general suspicion of VSD's. There is way too much of a fad involved in specifying them now. (Just notice how commonly they are simply presumed to be the answer to any and all problems--possibly including bad breath and dandruff.)
The various "electrical noise" and motor heating problems associated with the various adjustable frequency types of drives can eat up much of the expected savings unless they are fully and properly addressed from the very beginning. Because of this, it is usually wise to consider the eddy current types of VSD's. They avoid all the electrical problems of adjustable frequency drives, usually without much net relative energy penalty.
In thoroughly considered and properly designed systems, VSD's can provide excellent control and substantial savings. They should always be given due consideration whenever their use can be of benefit.
electricpete
Electrical
The efficiency of the eddy current type (magnadrive.com) is great at high speed but horrible at low speed.
Efficiency of the magnadrive itself decreases in proportion to speed. At 90% speed it's 90% efficient. At 50% speed it's 50% efficient. Could be a big problem if the machine spends most of its time at low speed.
Efficiency of the magnadrive itself decreases in proportion to speed. At 90% speed it's 90% efficient. At 50% speed it's 50% efficient. Could be a big problem if the machine spends most of its time at low speed.
goutam_freelance
Mechanical
Hi d23,
I am sorry that I did not mention the specific application of the concept in my earlier post. Actually these two controls are seen(VSD and control valves) in power plant boiler feed pumps (bigger capacities). The control of drum level is critical and in case of sudden change in levels the quick acting feed control valves operate to control the level. Subsequently the VSD will change the speed such that the pressure drop across the feed control valve is approximately constant. By making the feed control valve pressure drop approximately constant we are preventing larger pressure drop across the feed control valve at lower flows which results in large energy savinge in many cases.
Thanks,
goutamiam
I am sorry that I did not mention the specific application of the concept in my earlier post. Actually these two controls are seen(VSD and control valves) in power plant boiler feed pumps (bigger capacities). The control of drum level is critical and in case of sudden change in levels the quick acting feed control valves operate to control the level. Subsequently the VSD will change the speed such that the pressure drop across the feed control valve is approximately constant. By making the feed control valve pressure drop approximately constant we are preventing larger pressure drop across the feed control valve at lower flows which results in large energy savinge in many cases.
Thanks,
goutamiam
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PUMPDESIGNER
Mechanical
A few gripes about the typical VSD discussions.
Few people ever mention the natural reduction in current that inductive motors experience when coupled to centrifugal pumps. This reduction in current requires no fancy expensive equipment, and works good when flows are 75% - 100% of BEP on pure centrifugal pumps. Power saving for free with no attention required. Throttling valves in that 75%-100% range do not waste power.
Few people ever get into the high costs associated with drives. Initial purchase costs are high especially when surge protection/ line reactors are included, higher set up costs, exposure to potential replacement costs that are unexpected. Also, regionally there are places where drives are a nightmare because of all the electrical surges from lightning: Central-South Florida, Texas Gulf Coast, and a few other places are big VSD destroyers. I have seen abandoned VSDs hanging on walls replaced by Across the Line stuff in desperation as the money ran out replacing VSDs.
Trusting VSD guys to give the straight scoop on cost savings is just a little like the fox watching the chicken coop if you ask me. Some salesmen have good morals, but many do not and figure it is your problem while they put out the dope.
I have a magazine article that followed a complete change over to VSDs in Venezuela oil field. Immense savings were available so that it had to be done. Savings not fully realized as they continue to replace drives from electrical surges. Project paid off in dollars, but aggravation factor makes it about even. Two experts brought in from the States to install protection. Nothing has worked so far. Money is being saved, but there are problems.
We do drives, but it is hard to analyze stuff because of all the hype.
Want a rule of thumb I read? When comparing VSD to straight centrifugal pump ACL, if the pump can be run at less than 50% for 50% of the time, begin looking at VSD alternative.
PUMPDESIGNER
Few people ever mention the natural reduction in current that inductive motors experience when coupled to centrifugal pumps. This reduction in current requires no fancy expensive equipment, and works good when flows are 75% - 100% of BEP on pure centrifugal pumps. Power saving for free with no attention required. Throttling valves in that 75%-100% range do not waste power.
Few people ever get into the high costs associated with drives. Initial purchase costs are high especially when surge protection/ line reactors are included, higher set up costs, exposure to potential replacement costs that are unexpected. Also, regionally there are places where drives are a nightmare because of all the electrical surges from lightning: Central-South Florida, Texas Gulf Coast, and a few other places are big VSD destroyers. I have seen abandoned VSDs hanging on walls replaced by Across the Line stuff in desperation as the money ran out replacing VSDs.
Trusting VSD guys to give the straight scoop on cost savings is just a little like the fox watching the chicken coop if you ask me. Some salesmen have good morals, but many do not and figure it is your problem while they put out the dope.
I have a magazine article that followed a complete change over to VSDs in Venezuela oil field. Immense savings were available so that it had to be done. Savings not fully realized as they continue to replace drives from electrical surges. Project paid off in dollars, but aggravation factor makes it about even. Two experts brought in from the States to install protection. Nothing has worked so far. Money is being saved, but there are problems.
We do drives, but it is hard to analyze stuff because of all the hype.
Want a rule of thumb I read? When comparing VSD to straight centrifugal pump ACL, if the pump can be run at less than 50% for 50% of the time, begin looking at VSD alternative.
PUMPDESIGNER
- Thread starter
- #19
Since I don't want to trust "the fox watching the chicken coop" let me run these potential disadvantages/ facts by you:
- Harmonic current introduced into the motor windings causing a 5% nominal motor heating.
- Real power is reduced as a result of harmonics created by the VSD which might cause the power factor to decline between 0.75-.8
- Starting torque is lowered to 130% of the rated full-load torque (which is significantly less than what the motor could deliver otherwise).
- Pump should have a service factor of 1.15 for VSD implementation
I didn't receive any of these facts for the sales people, I got them from misc readings.
- Harmonic current introduced into the motor windings causing a 5% nominal motor heating.
- Real power is reduced as a result of harmonics created by the VSD which might cause the power factor to decline between 0.75-.8
- Starting torque is lowered to 130% of the rated full-load torque (which is significantly less than what the motor could deliver otherwise).
- Pump should have a service factor of 1.15 for VSD implementation
I didn't receive any of these facts for the sales people, I got them from misc readings.
guys
at great personal risk I need to admit that I am comfortable with drives and I do sell them. (OUCH....)
Ok, I agaree with many of the comments made by you all regarding the negatives of the VSD and as I said in my previous post, they should NEVER be considered a panacea and in many applications the overall picture needs to be looked at. Additionally, I know that I am still ethical enough not to try and snow my customers, I also get to deal with them face to face on a daily basis if they go wrong!, not cut and run.
Here are some of the reasons I like drives and many of them pertain specifically to the industry in which I work and sell to.
1. Reduction in inventory carrying costs for the end user, they can "make" systems fit wells in their fields
2. Many applications can be on a stand alone generator and the VSD has the added advantage of the soft start capability reducing the expenditure on the engine
3. My applications tend to be very dynamic, well bores with gas slugs, water breakthrough etc and the VSD can be set up by competent technicians to allow the motor to be either sped up or slowed down to maintain FLA and thus protect the equipment.
4. The costs of replacing the "incorrectly" specified pump often outweigh many of the issues raised above. And before anyone takes me to task about inability to size a pump, remember that we get all of the info needed to size the pump from the end user and as d23 can attest, their info is not always the best
5. The enhanced motor protection provided by way of current limits, the ability to alter the speed of the unit to accomodate changing operating conditions etc can factor heavily in the operator's decision to purchase a VSD
6. The VSD with which I deal is rated at 55C, NEMA3r and as such often finds itself in remote, dusty and downright hostile environments, yet with a correctly sized and set up drives, we have a component MTBF of almost 2.6 years
7. Many of my customers like them because they can install smaller equipment and run it faster. Savings are therefor realised in capital expenditure of the expensive submersible pump, the payback is quicker and we still get decent runlives even running equipment at 80Hz.
8. On a personal note, I despise choke valves, they allow lazy engineering and mean that often "wasted" horsepower is installed
As I said in my previous post, we often get to be accountants as well as engineers and it is incumbent upon us to do a damn good job of evaluating the possibilities of all systems available to us.
looking forward to your responses.
dadfap
at great personal risk I need to admit that I am comfortable with drives and I do sell them. (OUCH....)
Ok, I agaree with many of the comments made by you all regarding the negatives of the VSD and as I said in my previous post, they should NEVER be considered a panacea and in many applications the overall picture needs to be looked at. Additionally, I know that I am still ethical enough not to try and snow my customers, I also get to deal with them face to face on a daily basis if they go wrong!, not cut and run.
Here are some of the reasons I like drives and many of them pertain specifically to the industry in which I work and sell to.
1. Reduction in inventory carrying costs for the end user, they can "make" systems fit wells in their fields
2. Many applications can be on a stand alone generator and the VSD has the added advantage of the soft start capability reducing the expenditure on the engine
3. My applications tend to be very dynamic, well bores with gas slugs, water breakthrough etc and the VSD can be set up by competent technicians to allow the motor to be either sped up or slowed down to maintain FLA and thus protect the equipment.
4. The costs of replacing the "incorrectly" specified pump often outweigh many of the issues raised above. And before anyone takes me to task about inability to size a pump, remember that we get all of the info needed to size the pump from the end user and as d23 can attest, their info is not always the best
5. The enhanced motor protection provided by way of current limits, the ability to alter the speed of the unit to accomodate changing operating conditions etc can factor heavily in the operator's decision to purchase a VSD
6. The VSD with which I deal is rated at 55C, NEMA3r and as such often finds itself in remote, dusty and downright hostile environments, yet with a correctly sized and set up drives, we have a component MTBF of almost 2.6 years
7. Many of my customers like them because they can install smaller equipment and run it faster. Savings are therefor realised in capital expenditure of the expensive submersible pump, the payback is quicker and we still get decent runlives even running equipment at 80Hz.
8. On a personal note, I despise choke valves, they allow lazy engineering and mean that often "wasted" horsepower is installed
As I said in my previous post, we often get to be accountants as well as engineers and it is incumbent upon us to do a damn good job of evaluating the possibilities of all systems available to us.
looking forward to your responses.
dadfap
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