VFD Application?

[Fnewman]

In an industrial plant there is an existing conveyor transporting solid material at a constant rate 24/7, 350 days per year. Motor is 100 HP, 460 v, IEEE 841 design. Based on the quantity of material required by the process, the motor load is only approx. 45% of FLA. The system was designed like this because on rare occasions for fairly short periods the conveyor needs to transport twice as much material. So now the possibility of installing a VFD has been raised, along with the question of how power consumption would, or would not, vary. So, how would the total input power change between the two arrangements given that the amount and rate of material transported will be the same.

 

[Ingenieur]

They are very efficient and improve pf
at 45% pf is low

how do they double the speed now?
or speed is constant and they just load it up more?

 

[Fnewman]

They are very efficient and improve pf
at 45% pf is low

how do they double the speed now?

They don't - the existing arrangement is constant speed - when they need to double throughout, they just put twice as much on the belt.

 

[kwired]

They don't - the existing arrangement is constant speed - when they need to double throughout, they just put twice as much on the belt.

That is where I see a problem. To maintain desired speed you still need to run at full voltage and frequency, little will change when it comes to energy usage, you can always put on capacitors to correct power factor.

You might be better off running say a 60 or 75 HP motor and possibly run it into it's service factor during those infrequent periods of higher loading.

 

[Cow]

If you're not varying the speed, why would you want a VFD?

 

[GoldDigger]

If you're not varying the speed, why would you want a VFD?

For starting, and maybe also to run the motor more efficiently at full speed but partial load?

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[kwired]

For starting, and maybe also to run the motor more efficiently at full speed but partial load?

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Motor efficiency is probably nearly unchanged though if still running at same frequency and voltage isn't it? Power factor improvement on the supply side of the drive is about all that you are gaining, and you could have done that with much less expensive capacitors.

Now if they want to put more material on the conveyor and control the volume delivered to whatever this feeds by varying the speed of the conveyor it may be more efficient that way, but the overall process may or may not allow that.

 

[Besoeker]

If you're not varying the speed, why would you want a VFD?

If it ain't broke, don't fix it?

 

[petersonra]

For starting, and maybe also to run the motor more efficiently at full speed but partial load?

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the VFD does not change how much energy is used to move the load other than by changing the PF to near unity on the line side. but that does not amount to much.

as for starting, it may well prolong the life of the motor and the conveyor. but you can get most of the same benefits with a soft starter.

 

[Besoeker]

In an industrial plant there is an existing conveyor transporting solid material at a constant rate 24/7, 350 days per year. Motor is 100 HP, 460 v, IEEE 841 design. Based on the quantity of material required by the process, the motor load is only approx. 45% of FLA. The system was designed like this because on rare occasions for fairly short periods the conveyor needs to transport twice as much material. So now the possibility of installing a VFD has been raised, along with the question of how power consumption would, or would not, vary. So, how would the total input power change between the two arrangements given that the amount and rate of material transported will be the same.

I thought about this for a bit. The conveyor can clearly transport the heavier load you sometimes put on it. I suppose you could put the same load on it for normal operation and run it at half speed with a VSD to achieve the required material. But then you would have higher motor current than you do now and that means greater losses.

So, I'm not sure that installing a VFD would be the right way to go for this application. Sure, 45% of FLC is very lightly loaded and the motor would be fairly low on its efficiency curve. Loading the motor more heavily, as you do on occasion, will move it up the efficiency curve but with greater losses. Then you'd have to add in the losses in the VFD which are typically around 2-3% at rated load a couple of kW.

It wouldn't get my vote...........

 

[mike_kilroy]

If it ain't broke, don't fix it?

Yep!

Same speed, same load, out in VFD and you REDUCE EFFICIENCY BY VFD EFFICIECY!!!

2-5% more cost!

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[Fnewman]

Pretty sure the VFD option will get approved based on lower mechanical maintenance cost running the conveyor at half speed almost all the time. Less friction loss as well according to the conveyor vendor. It is really a two speed application - about 1/2 speed most of the time there near 100% speed when they need to move twice as much. I had reached pretty much the same conclusion with respect to power consumption.

 

[Besoeker]

Pretty sure the VFD option will get approved based on lower mechanical maintenance cost running the conveyor at half speed almost all the time.

Might be interesting to compare the expected reduction in maintenance costs with the additional operating cost of greater energy consumption.

 

[mike_kilroy]

Might be interesting to compare the expected reduction in maintenance costs with the additional operating cost of greater energy consumption.

There is as good a chance that this scheme will INCREASE maintenance!

Which is worse? 2x speed at 1/2 load or 1/2 speed at 2x load?

In my book, things will wear out 4x FASTER at 2x the load!!

Then you also add another 5% or so heat to the motor running on VFD instead across the line... Remember every 10C cuts life in half... Bet the VFD adds that 10C!

Then you now have the VFD, contactor, maybe other added components that will eventual brake and need maintenance.

KISS. Think this through a bit more would be my advice!

 

[kwired]

Pretty sure the VFD option will get approved based on lower mechanical maintenance cost running the conveyor at half speed almost all the time. Less friction loss as well according to the conveyor vendor. It is really a two speed application - about 1/2 speed most of the time there near 100% speed when they need to move twice as much. I had reached pretty much the same conclusion with respect to power consumption.

Is that going to impact whatever the conveyor is feeding? Or are you going to put more material on the conveyor but run it slower?

What about speed of the machine fed by the conveyor? If you are feeding it at half the rate is possible it can be slowed down as well.

 

[kwired]

the VFD does not change how much energy is used to move the load other than by changing the PF to near unity on the line side. but that does not amount to much.

as for starting, it may well prolong the life of the motor and the conveyor. but you can get most of the same benefits with a soft starter.

Sounds like this machine only starts just a few times a year otherwise runs nearly all the time.

 

[Russs57]

A conveyer VFD will have to be operated in "constant torque" mode. Energy consumption will be proportional to speed. That means running at 50% speed consumes 50% power. Other loads, like fans, go by the cube of the speed so energy savings are much more dramatic at reduced speeds.

Right now you are operating the motor at full speed but consuming half the rated amps. That sounds like a roughly 50% reduction right there. Hard to see any ROI on going with a VFD (that will cost more than a "normal" VFD as it has to be rated for "constant torque").

 

[mike_kilroy]

A conveyer VFD will have to be operated in "constant torque" mode. ...

rest of post ok, but I strongly disagree with this comment and suggest it be removed so the rest of the post is good?

 

[Besoeker]

A conveyer VFD will have to be operated in "constant torque" mode. Energy consumption will be proportional to speed. That means running at 50% speed consumes 50% power.

Don't think so in this application. At 50% speed, the conveyor will have be loaded twice as heavily to deliver the same amount of material as it did at full speed.

 

[Fnewman]

Since some things have wandered off the facts a bit, here is an expanded restatement.
There are actually two identical, parallel conveyors presently equipped with constant speed motors each operating at about 45% FLA. The process runs 24/7, so start frequency may be 1-2 times a month at the most. The design is such that if one conveyor is out of service the other can carry the total load. Typically that would occur only 1-2 weeks per year at most. So, the proposed VFDs would operate at approx 50% speed nearly all the time. The conveyor vendor says that slower speed = longer conveyor belt and mechanical component life as well as reduced friction.