VFD Application?

[Besoeker]

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.

But.........
At 50% speed the conveyor would have to be loaded twice as heavily to deliver the same quantity of material. Did the conveyor vendor take account of that for friction and belt life? Is conveyor belt life a problem currently anyway? Is it an expensive replacement cost and, if so, how frequently?

And why would the conveyor vendor recommend something that could, if he is right, reduce his potential sales revenue?
Something doesn't add up here.

 

[Russs57]

There are many solutions. To pick the best one depends on the goals.

In general, the biggest advantages to adding a drive are speed control (which you seem to have no need of) and control of inrush current (which again you seem to have no need of). There are other advantages but one must be willing to pay for a drive with the right control logic and someone to set it all up. You would want to read up on 4 quadrant operation and open/closed loop vector controls.

But really a lot more information is needed....things like does the conveyer move product up or down an incline.....would it ever need to start up fully loaded....would it ever need to hold a load in place....and tons more. Now if everything is level, rate of delivery doesn't matter, and you don't start/stop/reverse/hold the load or vary the speed of it, then I would find it hard to justify the cost of the drive.

Heck, ever thought of getting a 50 HP motor and another gear box?

 

[kwired]

But.........
At 50% speed the conveyor would have to be loaded twice as heavily to deliver the same quantity of material. Did the conveyor vendor take account of that for friction and belt life? Is conveyor belt life a problem currently anyway? Is it an expensive replacement cost and, if so, how frequently?

And why would the conveyor vendor recommend something that could, if he is right, reduce his potential sales revenue?
Something doesn't add up here.

Kind of what I was trying to say from the beginning. Just because the thing is only about half loaded now, doesn't mean you can just run the motor at half the speed it currently runs at. If it were a fan or pump you could slow it down if not using all the potential capacity, but say conveyor were currently dumping 100 pounds per minute of material into whatever the next stage of the process is. If only thing you change were to cut the speed of conveyor in half, you now are only dumping 50 pounds per minute. To get back to 100 pounds per minute you need twice as much material on the conveyor. Next question is can the conveyor handle twice the weight even though it is running at half the speed? Since they currently do run more material at times the answer is probably yes, but still all you did was spend $$$ on a VFD but still are using close enough to same amount of energy that it makes the investment questionable, unless you have gained some other benefit by slowing it down.

 

[Besoeker]

Kind of what I was trying to say from the beginning. Just because the thing is only about half loaded now, doesn't mean you can just run the motor at half the speed it currently runs at. If it were a fan or pump you could slow it down if not using all the potential capacity, but say conveyor were currently dumping 100 pounds per minute of material into whatever the next stage of the process is. If only thing you change were to cut the speed of conveyor in half, you now are only dumping 50 pounds per minute. To get back to 100 pounds per minute you need twice as much material on the conveyor. Next question is can the conveyor handle twice the weight even though it is running at half the speed? Since they currently do run more material at times the answer is probably yes, but still all you did was spend $$$ on a VFD but still are using close enough to same amount of energy that it makes the investment questionable, unless you have gained some other benefit by slowing it down.

Like I said, on the basis of the information we have, there is no case for spending money to install a VSD. And some points against doing so.

 

[Russs57]

Also begs the question of why not run one conveyer close to 100% loaded and let the other one sit idle? Alternate on whatever schedule you like. Seems like running half as many hours would extend life and save energy and maintenance costs.

Still trying to wrap my head around a process where delivery speed doesn't matter, just share volume.

 

[kwired]

Also begs the question of why not run one conveyer close to 100% loaded and let the other one sit idle? Alternate on whatever schedule you like. Seems like running half as many hours would extend life and save energy and maintenance costs.

Still trying to wrap my head around a process where delivery speed doesn't matter, just share volume.

I like that idea, probably does save some energy costs.

 

[RumRunner]

Notwithstanding all insights regarding the need or lack of it, there are benefits from deploying VFDs.

The mfg entity must have been mulling about this, not just because the idea came out of thin air.

I'm sure they have consulted several experts (or are still) getting inputs from several engineering consultants whether deploying VFDs would be beneficial.

This is something that a scope of the NEC forum cannot very will serve the need. This would involve sizable investment and cannot be easily carried out with a “shooting from the hip” decision making process.

I suggest consulting an Industrial Engineer with a cadre of mechanical and electrical engineers getting their heads together and not arm-wrestling of individuals which is really nothing but offer confusing and unsupported “do this-do that” that don't really offer a discernible solution.

One member even suggested that one poster be “shut out” to arrive a consensus.

Is this really how to foster exchange of ideas?

 

[kwired]

Notwithstanding all insights regarding the need or lack of it, there are benefits from deploying VFDs.

The mfg entity must have been mulling about this, not just because the idea came out of thin air.

I'm sure they have consulted several experts (or are still) getting inputs from several engineering consultants whether deploying VFDs would be beneficial.

This is something that a scope of the NEC forum cannot very will serve the need. This would involve sizable investment and cannot be easily carried out with a “shooting from the hip” decision making process.

I suggest consulting an Industrial Engineer with a cadre of mechanical and electrical engineers getting their heads together and not arm-wrestling of individuals which is really nothing but offer confusing and unsupported “do this-do that” that don't really offer a discernible solution.

One member even suggested that one poster be “shut out” to arrive a consensus.

Is this really how to foster exchange of ideas?

Some of the people responding have quite a bit of experience in the type of thing being discussed here.

Who suggested shutting out someone else? There may have been replies that disprove or at least oppose an idea here or there, but that is a little of what a forum is about isn't it?

OP can take the good and the bad and make a better informed decision then he had before posting here, things come up he maybe never thought about.

 

[Ingenieur]

Too little info
need info on the process
what feeds the belt, what the belt feeds, it must be coordinated
is the material like coal/gravel or discrete like boxes
what kind of power are we talking about

P = work/time = Weight x dist / time, in this case
so if time gets bigger, ie, slowed down, and weight gets larger, ie, more loaded, They offset and P ~ the same

 

[Russs57]

Myspark, good post and I agree 100%.

I jumped to a conclusion that this was to be an "in-house" solution carried out by maintenance techs and that was why he was asking questions here.

 

[Fnewman]

In an attempt to clarify further: there are two convyors each conveying similar, but not identical, bulk material up an incline which is constant. Conveyors may sometimes have to start in loaded condition - no reversing ever required. Under normal operations, the load on each constant speed motor is approx. 45% which is adequate to deliver the required quantity of product. Since equipment at both the feed and discharge locations can accommodate either product, if one conveyor is forced out of service they would alternate the two products on the operable conveyor at twice the normal rate. This is not an in-house project - rather a questions asked as a part of a much larger project current in design. Very interesting discussion. Thanks

 

[Jraef]

Ok then. In YOUR CASE, sizing both motors for the worst case scenario and using VFDs to make them universally applicable for either load profile MIGHT make sense, but only IF those two load profiles operate at different motor speeds. If the conveyor speed is the same either way, but the HP requirement is different, just use the larger HP motor on each conveyor, no VFD. An AC induction motor only uses the amount of power that the load demands of it, plus whatever internal losses it has. So if in your example your heavier load requires 15HP and the lighter one requires 10HP, you would use a 15HP motor on both of them. When used on the lighter one, the 15HP motor will only draw power required to produce 10HP at the motor shaft. Although the losses of a 15HP will be slightly higher, it's not enough to get excited about in order to gain the universal swap out capability. Adding a soft starter would help with wear and tear issues, but must be done carefully if loaded restart is a possibility. Not all soft starters are created equal and for this, you will NOT want the cheap ones.

Yes, a VFD does reduce the HP at the shaft, but only because mechanical HP is a function of torque at a given speed, so at reduced speed, you have reduced HP but you are also reducing the work it does over time because it is slower. So because ENERGY is work across time, there is no net energy savings and if the speed doesn't change, you don't change anything.

VFDs only save energy when used to REPLACE some other less efficient means of changing speed or flow (mostly flow). So in a conveyor, the only time they might save energy is if you use a VFD to replace a mechanical Varidrive unit that had a lot of mechanical friction losses.

 

[retirede]

Ok then. In YOUR CASE, sizing both motors for the worst case scenario and using VFDs to make them universally applicable for either load profile MIGHT make sense, but only IF those two load profiles operate at different motor speeds. If the conveyor speed is the same either way, but the HP requirement is different, just use the larger HP motor on each conveyor, no VFD. An AC induction motor only uses the amount of power that the load demands of it, plus whatever internal losses it has. So if in your example your heavier load requires 15HP and the lighter one requires 10HP, you would use a 15HP motor on both of them. When used on the lighter one, the 15HP motor will only draw power required to produce 10HP at the motor shaft. Although the losses of a 15HP will be slightly higher, it's not enough to get excited about in order to gain the universal swap out capability. Adding a soft starter would help with wear and tear issues, but must be done carefully if loaded restart is a possibility. Not all soft starters are created equal and for this, you will NOT want the cheap ones.

Yes, a VFD does reduce the HP at the shaft, but only because mechanical HP is a function of torque at a given speed, so at reduced speed, you have reduced HP but you are also reducing the work it does over time because it is slower. So because ENERGY is work across time, there is no net energy savings and if the speed doesn't change, you don't change anything.

VFDs only save energy when used to REPLACE some other less efficient means of changing speed or flow (mostly flow). So in a conveyor, the only time they might save energy is if you use a VFD to replace a mechanical Varidrive unit that had a lot of mechanical friction losses.

Exactly

The current setup's only inefficiency is the efficiency difference between full and half load of the induction motors - not much.

There could be power factor considerations, but there are other ways to deal with them.

 

[Besoeker]

Yes, a VFD does reduce the HP at the shaft, but only because mechanical HP is a function of torque at a given speed, so at reduced speed, you have reduced HP .....

Yes, but to deliver the same amount of material you need to load the belt more heavily at lower speeds.
A VFD does not seem to me to be appropriate for this application.

 

[Jraef]

Yes, but to deliver the same amount of material you need to load the belt more heavily at lower speeds.
A VFD does not seem to me to be appropriate for this application.

I thought I said that... With the caveat that IF the two conveyors run at different speeds, it might make sense since what he is after is to be able to use either conveyor in either application.

 

[Besoeker]

I thought I said that... With the caveat that IF the two conveyors run at different speeds, it might make sense since what he is after is to be able to use either conveyor in either application.

Doesn't that mean that they would have to operate identically?

 

[Fnewman]

I would normally say we have beat this one to death, but the interesting thing now is how the discussions points out the difficulty of describing relatively simple operating conditions so that multiple people understand the same way.
So, with the constant speed motor configuration the conveyor delivers the desired amount of product/hr at about 45% FLA. When the conveyor is infrequently asked to deliver twice as much product/hr. the load just goes up. In The VFD scenario the conveyor would operate about 50% speed most of the time, then speed up to around 100% when infrequently asked to deliver twice as much product / hr. This condition only occurs when one of the conveyors is out of service.
Many thanks for all the interesting discussion. I promised not to respond again

 

[Besoeker]

In The VFD scenario the conveyor would operate about 50% speed most of the time

At twice the full speed load. So no energy saving and the additional energy losses associated with the VFD.
It's not the way to go. IMHO.

 

[kwired]

I would normally say we have beat this one to death, but the interesting thing now is how the discussions points out the difficulty of describing relatively simple operating conditions so that multiple people understand the same way.
So, with the constant speed motor configuration the conveyor delivers the desired amount of product/hr at about 45% FLA. When the conveyor is infrequently asked to deliver twice as much product/hr. the load just goes up. In The VFD scenario the conveyor would operate about 50% speed most of the time, then speed up to around 100% when infrequently asked to deliver twice as much product / hr. This condition only occurs when one of the conveyors is out of service.
Many thanks for all the interesting discussion. I promised not to respond again

I agree with Besoker. Total work being done is same, likely with some variances in efficiency at either speed, but those variances on that type of a load are probably not that significant to justify the expense of the drive, it still takes same overall horsepower to deliver x amount of material in y amount of time regardless of what speed the conveyor is running.

If you measured the motor current at at time when it is carrying the highest amount of material and found the current only increased to maybe 60% of FLA - then you simply have a larger motor then the application calls for, you won't save all that much energy, if any slowing it down, reduced friction is going to be what you do save the most on. Changing gears/pulleys will do that at a much less cost.

 

[Ingenieur]

In an attempt to clarify further: there are two convyors each conveying similar, but not identical, bulk material up an incline which is constant. Conveyors may sometimes have to start in loaded condition - no reversing ever required. Under normal operations, the load on each constant speed motor is approx. 45% which is adequate to deliver the required quantity of product. Since equipment at both the feed and discharge locations can accommodate either product, if one conveyor is forced out of service they would alternate the two products on the operable conveyor at twice the normal rate. This is not an in-house project - rather a questions asked as a part of a much larger project current in design. Very interesting discussion. Thanks

so no speed change is required?

normally 2 belts at ~50% load and full speed (each a different material)
or when 1 is removed from service
1 belt at 100% load and full speed (alternating product at twice the density)

seems like a VFD is not required?

with vfd's
under normal conditions you could slow the belt down and load it to 100%
no advantage
again
Power = work/time = weight x distance / time
if time goes up, ie, slower at 1/2 speed
and weight goes up, twice the material
they offset, the P is the same
work does increase but so does the time