Constant Torque VFD Application

[sconnol]

I want to evaluate the energy savings for applying a VFD to an agitator (constant torque) currently being controlled by 380V, 40kW motor running approximately 3000 hrs/yr.

I have found some online calculators that will calculate energy savings for variable torque loads but nothing for constant torque.

I'd appreciate if anyone could give me advice on how to proceed.

Thanks

 

[LionelHutz]

Power applied to a constant torque load is directly proportional to the speed. If you half the speed you half the power used.

 

[Skogsgurra]

True, but if you "agitate" less violently, do you not need to agitate for longer time then? You may even come to a point where the agitation doesn't work any more.

And then you can run for 8760 hrs/year with very little result other than getting a huge bill from the electricity company.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

Since you are now running a constant speed motor at a constant torque (it appears) there is no energy savings by using a VFD. The saving that comes from a VFD is if you can, say, throttle pump/fan loads by varying the motor speed.

 

[jraef]

There are two energy usage issues at play here.

1st; energy consumed on a constant torque load through a VFD is directly proportional to speed, as is HP. So IF your load can use less HP to get the job done, a VFD can facilitate that. But is should be pointed out that any other mechanical method of reducing the speed will do exactly the same thing. So for instance, if you use a gearbox or variable sheave drive to lower the workload on the motor, that too will save energy (let's ignore the inefficiencies of varidrives for this argument), as long as the work being done by the motor is less than it was before. For that matter, if you can live with less agitation, you can also achieve the same effect by varying the duty cycle (on-off control) of the agitator. So that means it isn't that a VFD will not save energy by reducing the speed, but that the amount of energy it saves will not be significantly different than any other method of reducing the workload over time.

2nd; the energy savings involved in variable torque loads is different. By using the VFD, again on a load that CAN be varied such as flow control in a centrifugal pump or fan, you save EXTRA energy when compared to other flow control methods by taking advantage of what are called the "affinity laws". When you choke off the flow of a constant speed pump, you do still reduce energy consumption because you reduce the work being done, but when you lower the speed of the motor instead of choking off the flow, you save MORE energy. The difference between the two levels represents the energy savings that we all get so excited about with VFDs. It can be significant.

So to summarize, you can save energy in a constant torque motor application by using a VFD, but no more than you could using far less expensive or less complicated technologies. The extra energy savings you get with variable torque loads is real, but does not translate to constant torque applications.

 

[unpluged]

Is it possible that by dividing the loads in the agitator, you could save more than using the VFD?

In other words as jraef pointed out Quoteas long as the work being done by the motor is less than it was before.) the work would be less by dividing the jobs to more machines that handle the loads with max.use.

However the labor factor would have to be considered also.
Sometimes labor cost can keep utility costs down, if the labor is set up to run smoothly.

Maybe more automation can be applied too.
For instance add an gravity delivery system to the load being transfered from place to place.

You also may be able to save lots of money by getting rid of two cell phone talkers. One person without the need to wait for the cell phone to ring can be a wonderful experience.