Save Money

[jamco23]

Can anyone tell me if there is any evidence to suggest that fitting a Variable Speed Drive to a motor can save money in both electrical energy, life time costs of the motor, if for example it is connected to a pump controlling a wetwell level or similar. I am interested in both the advantages and disadvantages of VSDs against DOL - Star/Delta when using a 3 phase induction motor

Comments Please

Many thanks

 

[CJCPE]

It will be difficult to show a total life cycle cost advantage for using an adjustable speed drive in comparison to full-speed start-stop operation. I would expect that only an adjustable frequency drive is worth considering, other types of adjustable speed drives are less efficient.

POSSIBLE ADVANTAGES OF AFD OR DISADVANTAGES OF DOL STAR/DELTA (Y-D):

The process receiving water from the pump may benefit from constant flow with moderate variations compared to periodic alternation between high flow and no flow. This factor alone may justify using an adjustable speed drive, even an inefficient drive.

Slightly more hydraulic power is required to move water in intervals of higher flow and slightly higher total dynamic head compared to lower average constant flow and slightly lower TDH. This is only a difference of higher pipe friction losses due to higher flow rate.

Pump efficiency difference for different operating points advantage/disadvantage?

Effects of inrush current and transient current and voltage at Y-D transition on motor and power distribution system.

Start/stop mechanical stress on motor, drive train, pump and pipes.

POSSIBLE DISADVANTAGES OF AFD OR ADVANTAGES OF Y-D:

Effects of AFD waveform harmonics and transient voltages on motor.

Effects of AFD input waveform harmonics on power distribution system.

AFD - motor combined efficiency is less than motor alone on sine wave.

 

[Barry1961]

If a VSD will save money enough money to be worth it will depend on your system.

One rough comparison would be to driving a car with the speed limited to 65mph. Does it save money to slowly accelerate the car instead of stomping the gas pedal to floor? What if you are going to be traveling 200 miles non-stop on a highway? What if you are going to be driving downtown, stop light to stop light? You will save money in both cases but probably not enough to notice with highway driving.

There is a fair sized industry just for controlling fans and pumps with VFD’s. There are used a lot with water and sewage where the power usage, changed in demand and life cycle make them very effective. They are also use in HVAC to good effect.

Barry1961

 

[jraef]

jamco23
Your query is a little too general. There are literally thousands of articles and texts on this very subject, Google would be a better place to start. In asking for a "list" of advantages and disadvantages, your question appears to be akin to a classroom assignment, and as you know (or should know), students are not allowed to post their classwork problems here.

If you are not a student and this is a specific project that you are working on, post all of the available details of your project, including motor data, pump type, pump capacity, inflow rates etc. As others have mentioned, this subject is very application specific.

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[jamco23]

Jraef, thanks for your comments. No I am not a student and yes the question is general and was meant to be. I have been told, although I am not an expert on VSDs, that they can save money when fitted to motors in many applications and I was after a general view. I work in the Water Treatment industry

 

[dpc]

If you have pumps that are burning head across control valves, then you might be able to save money.

I agree with jraef - you're asking how long is a piece of rope. Adjustable Frequency Drives can save energy under certain situations, but this advantage is dramatically oversold by some manufacturers by ignoring a lot of other issues and factors.

They can certainly reduce the number of motor starts for a pump on wetwell duty if it has been running in constant speed and cycling on wetwell level.

In general, the process and control advantages in water and particularly wastewater facilities are more significant than the energy savings.

 

[jraef]

Well, giving you the benefit of the doubt that you are not a very clever student, here is my take.

dpc beat me to it a bit. VFDs definately save energy WHEN there s a variable flow rate involved. So using your wetwell application, the prime use of the VFDs there is to match inflow rates by lowering the pump speed, but have higher pumping capacity available for the occasional higher inflow. That way you are not having to stage multiple pumps on and off and using the wetwell as a surge tank all of the time.

How much energy do they save? It is totally dependant on the average running speed. At full speed, VFDs actually WASTE a little more energy, but the energy savings vary by the cube of the speed. So at 70% speed, they are only consuming 34% of the energy compared to full speed, a dramatic diference. I picked 70% for a reason however, because many VFD salesmen will tell you that at 50% speed it uses only 25% of the energy, but fail to mention that at 50% speed you may not be pumping anything! In a wetwell application, it is a little different because when the well is filling up and the first pump has not yet been called for, the pumps consume NO energy, then turn on at full speed and consume a lot for a brief time, then shut off again. So you must consider avarage energy savings over time based on inflow rates, and that is tricky. In spite of that, VFDs generally come out as saving more energy.

But energy cost is only part of the problem. Reliability is another concern, and lets face it, as good as VFDs have become, they contain a lot more critical parts than full speed controls, therefore their MTBF is always lower. So if a remote lift station might take someone 2 hours to get to in an emergncy shutdown situation, it can be a major liability if the VFD dies. If you get fined for spillage, all your energy savings went for naught. A VFD in the headworks of a manned WWTP however is a different story, because someone is there and can deal with any issues immediately.

The comparisons can go on and on, that's why it is better to consider each specific application on it's own merits rather that rely on generalizations.

One generalization I will make however is that it is NEVER beter to consider Y-Delta starters. They save absolutely nothing except some up front costs, and risk the equipment they are connected to as well as the power system. If you are considering full speed operation but want reduced voltage starting to protect the equipment mechanically, consider soft starters. They are less expensive than autotransformers and will not expose the mechanical equipment to risks of damage from switching transients.

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[LionelHutz]

I'm with the others saying that a VFD can be a big advantage in a process control application. ie, where you're trying to maintain a constant flow so your process works better.

If you're trying to move X amount of water into a tower every day then that requires Y amount of work to do it. You can run the pump at full speed and perform this work quickly or you can run it at a reduced speed and perform it over a longer period of time. But, in the end, you still have to perform Y amount of work to move the water. The input power is the output power divided by the efficiency of the pump, motor and VFD combination. This is why the efficiency of the complete system at different speeds is so important since the efficiency is the only thing that will change the amount of input energy.

Now, if you're talking a fan application where you really don't need the amount of airflow that the fan moves at full speed then running it at a lower speed makes perfect sense. A dust collector or the ventilation system in a mine comes to mind as examples of this.

 

[cbarn24050]

Hi Lional, thats a good point you make but you have forgotten to take the very considerable effect of friction into account.

 

[LionelHutz]

When I said you need to look at the efficiency of the complete system does that not include friction as one of the losses???

 

[cbarn24050]

Hi Lional, I must have missunderstood your post. I thought (wrongly) that you were saying it took roughly the same amount of power to move a given quantity of water up a tower regardless of the speed at which you do it. Glad thats cleared up.

 

[electricpete]

fwiw, cbarn’s original comment seemed valid to me.

The statement: “The input power is the output power divided by the efficiency of the pump, motor and VFD combination.” effectively defines that LH considers the pump output to be the boundary between the “system” (whose efficiency varies) and the system output. Increasing flow rate would likely increase total time-integrated piping friction losses external to the system. This means we need higher system output energy (Y) to move the same mass of fluid (X) at the higher flow rate.

With that said, I also understand the intent of selecting the tower example was to focus on the energy associated with height of fluid rather than friction.

So in summary I thought it was a good post from LH and a good clarification from cb.

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[LionelHutz]

Sorry, I guess I didn't explain it correctly. What I really meant is that it takes Y amount of work to move the water from the ground into the tower. To do it quickly requires a lot of power for a short period of time and to do it slowly requires less power for a longer period of time. So, the only variable affecting the input power used is how efficiently the complete system operates. I hope that's got it right.