Minimum HP

[ptonsparky]

I want to use a single VFD to run multiple motors. We have a 30 hp drive with nine 3hp motors. Each motor runs a 30ft section of conveyor that is used to fill a large warehouse. As the warehouse is filled a section is removed. Is there a minimum motor load that should be connected to this drive?

 

[barbeer]

I would DEFINITELY!! check with the manufacturer of the drive and let them answer that question, they are not cheap.

 

[ptonsparky]

The drive and conveyors were originally set up some years ago but they suffered from to many motor burnouts and overheating of the individual motor overloads so the drive was removed. No load reactor was installed at that time and page ___ of the installation manual warned of the exact problem so we are going to try again with a load reactor. We did not do the original install.

 

[noonan]

I agree this sounds like something to consult with the manaufacturer.

Have you thought about splitting it up to individual drives, or a group
of say 3?

 

[augie47]

the idea may be commonplace, but foreign to me.
How do you provide proper individual overcurrent protection for seperate motors with this arrangement ?

 

[hillbilly]

If this is one long conveyor with a seperate motor powering each 30' long section....I'd be worried about speed control and synchronization of each motor.

In a perfect world all of the motors will run the same speed with the same hertz and voltage applied.
In the real world, there's too many factors that will affect the individual performance (load speed) of each motor.

If the following conveyor doesn't deliver as much (or more) product as the one that feeds it....you've got problems.

This problem could possibly be overcome with the gearing of each conveyor, such as each conveyor running at a slightly higher speed than the one preceding it.

Just a few ideas
steve

 

[iwire]

I want to use a single VFD to run multiple motors.

I don't believe you will find an NEC compliant way to do that.

At the least you would have to provide separate OCPD for each individual motor.

 

[weressl]

I want to use a single VFD to run multiple motors. We have a 30 hp drive with nine 3hp motors. Each motor runs a 30ft section of conveyor that is used to fill a large warehouse. As the warehouse is filled a section is removed. Is there a minimum motor load that should be connected to this drive?

Sounds like a constant torque application and that is why you have a problem with it.

This calls for individual ASD's for each motor. There are different ways to assure that all runs at the same, or near the same speed with an analog or digital communications between the drives. Talk to the manufacturer of the drive, they will help you out. This also seem like a perfect application for the pigyback drives where the drive is actually mounted on the motor.

 

[Jraef]

It's done all the time, there is no minimum load requirement for VFDs (unless you program one in for some special purpose), in fact they will still "think" they are running with NO motor connected. You are right on about adding the reactor; multiple motors creates added load harmonics that can wreak havoc on the motors. The other rule of thumb is to oversize the VFD by 10%, something else working in your favor here, but make sure they understand that it would not be a good idea to add a 10th motor some day without bumping up the VFD size.

As others mentioned, you WILL be required to have not only individual OL protection for each motor, but also individual Short Circuit protection for each as well, because there is no way that the SCPD for a 30HP VFD is going to provide protection for a 3HP motor*. So that means fuses, circuit breakers or other means of providing that.

What OEMs do in this case is use the little IEC "Motor Starter Protectors" which are a combined SCPD and OLR in one device. They will open the individual load circuit if it overloads, so you will also need NC aux. contacts on each one, wired in series back to the VFD to disable the VFD if any one unit trips. That also makes the removal of one section a bit trickier because you would have to re-establish that series string. If that's too much of a problem you could alternatively use N.O. aux contacts wired in parallel, but that would mean reversing the logic in the VFD to shut down on a contact CLOSURE, something that is not fail safe.

* I know that from an electrical standpoint, the VFD is like it's own power source and the theoretical SC current is limited by what the VFD can put out, but I don't see where the NEC makes any allowances for that. I'm open to being taught differently though if anyone else knows better.

 

[Besoeker]

I want to use a single VFD to run multiple motors. We have a 30 hp drive with nine 3hp motors. Each motor runs a 30ft section of conveyor that is used to fill a large warehouse. As the warehouse is filled a section is removed. Is there a minimum motor load that should be connected to this drive?

Fundamentally, there is no problem running many motors from one VSD.
It's something we have done for roller tables in steel mills and on fiber spinning lines.
There are a few caveats, of course.
The VSD rating obviously needs to be at least the aggregate of the connected load.
If the motors are all driving a common load (like a conveyor of roller table) they need to be the same motors.
Each motor needs its own overcurrent protection.
Single motors can't be started independently unless each is small in relation to the VSD rating.
And all the motors will run at the same speed (for the same number of poles).

There are probably other constraints too but, provided you can live with those constraints, driving multiple motors from one VSD is perfectly feasible. It has been done. That's a pretty fair indication that it can be done.:smile:

But I don't know whether, for this application, it is the best option either technically or commercially.
And I don't know whether there are NEC or other regulations that would exclude the use of such a system in USA.

On your other point about minimum load.
For functional testing of VSD systems in our works, we generally use a motor that is much lower rated than the VSD. That works just fine. Running just one out of your nine shouldn't be a problem.

Apologies for such a long post.

 

[ptonsparky]

We service a system of fans for combustion air that uses five motors and one drive. Each motor has individual protection for SC & OC with aux contacts that are seriesed with the stop of the drive. Very much as Jraef describes. When a motor fails the drive usually shuts down on overload before the individual motor protection trips. We won't be so lucky on the conveyors.

Manager says whatever we do to reduce produce brusing will easily pay for itself. Just keep eating tater chips and I thank you.

 

[weressl]

It's done all the time, there is no minimum load requirement for VFDs (unless you program one in for some special purpose), in fact they will still "think" they are running with NO motor connected. You are right on about adding the reactor; multiple motors creates added load harmonics that can wreak havoc on the motors. The other rule of thumb is to oversize the VFD by 10%, something else working in your favor here, but make sure they understand that it would not be a good idea to add a 10th motor some day without bumping up the VFD size.

As others mentioned, you WILL be required to have not only individual OL protection for each motor, but also individual Short Circuit protection for each as well, because there is no way that the SCPD for a 30HP VFD is going to provide protection for a 3HP motor*. So that means fuses, circuit breakers or other means of providing that.

What OEMs do in this case is use the little IEC "Motor Starter Protectors" which are a combined SCPD and OLR in one device. They will open the individual load circuit if it overloads, so you will also need NC aux. contacts on each one, wired in series back to the VFD to disable the VFD if any one unit trips. That also makes the removal of one section a bit trickier because you would have to re-establish that series string. If that's too much of a problem you could alternatively use N.O. aux contacts wired in parallel, but that would mean reversing the logic in the VFD to shut down on a contact CLOSURE, something that is not fail safe.

* I know that from an electrical standpoint, the VFD is like it's own power source and the theoretical SC current is limited by what the VFD can put out, but I don't see where the NEC makes any allowances for that. I'm open to being taught differently though if anyone else knows better.

How much short circuit can an ASD deliver? I bet no properly sized OCPD would trip before the ASD's own overcurrent limit would cut the power supply.

I disagree that one can connect any number of motors on a single drive. Drive characterization portion of the software will try to calculate the motor parameters, including the cable parameters and optimizes the power supply parameters accordingly. Different cable lengths to the individual motors will result in overdriving certain motors and underdriving others in the same group while everything is running. When you start taking motors out the situation gets even more complex, especially if it is desirable to have the motors running in synch.

 

[Besoeker]

It's done all the time, there is no minimum load requirement for VFDs (unless you program one in for some special purpose), in fact they will still "think" they are running with NO motor connected. You are right on about adding the reactor; multiple motors creates added load harmonics that can wreak havoc on the motors. The other rule of thumb is to oversize the VFD by 10%, something else working in your favor here, but make sure they understand that it would not be a good idea to add a 10th motor some day without bumping up the VFD size.

Jraef, I missed your response before I posted mine.
I started mine but didn't complete it before the witch of the kitchen served dinner - it was excellent as always (red, orange and green peppers, stuffed chicken wrapped in Panchetta, and saute potatoes - am I lucky or am I lucky?):grin:

Digression over.
I agree with most of what you posted. The multiple motor scenario isn't new and a 10% capacity margin in the VFD is prudent.
But why would multiple motors create additional load side harmonics?

 

[Besoeker]

I disagree that one can connect any number of motors on a single drive.

We have done so on a number of projects. Not just a few motors but some tens of them. It has been done so it can be done.

Drive characterization portion of the software will try to calculate the motor parameters, including the cable parameters and optimizes the power supply parameters accordingly.

That's a fair point.
In my experience, default parameters are set for open loop control and simple constant V/f ratio. Sure, if you need optimum dynamic performance and you have a VFD that allows it, you can enter motor equivalent circuit values but many, probably most, applications don't need that.

 

[weressl]

We have done so on a number of projects. Not just a few motors but some tens of them. It has been done so it can be done.

That's a fair point.
In my experience, default parameters are set for open loop control and simple constant V/f ratio. Sure, if you need optimum dynamic performance and you have a VFD that allows it, you can enter motor equivalent circuit values but many, probably most, applications don't need that.

Read my motto: 'just because it works, it does not mean it's right.'

In this application the motors will have different cable lengths that further aggravates the flux optimization profile, especially when you start switching motors in and out. The normal impedance tolerance difference between individual motors will further aggravate the condition that you will overdirve some and underdrive other motors. Not to mention if you happen to have different manufacturer's motors.

The premature failure of some motors will likely to occur. Individual thermal OL protection is pretty much useless. If you want to thermally protect the motors in this scenario you should employ individual thermistors and relays with contactors to trip. If you do that, you need to look the real cost comparison of individual drives vs. single drive.

Individual OCPD's are also pretty much useless since the drive's own electronic overcurrent protection would never allow the drive to deliver sufficient current for long enough time to activate the individual OCPD's.

 

[Besoeker]

Read my motto: 'just because it works, it does not mean it's right.'

I didn't say it was right. Just that it works.

In this application the motors will have different cable lengths that further aggravates the flux optimization profile,

In basic V/f control there is no flux optimization profile for most VFDs.

Individual OCPD's are also pretty much useless since the drive's own electronic overcurrent protection would never allow the drive to deliver sufficient current for long enough time to activate the individual OCPD's.

That would depend on how many motors there were and the settings of the overcurrent protection.

 

[weressl]

I didn't say it was right. Just that it works.

Works until something goes wrong, like in the OP.

In basic V/f control there is no flux optimization profile for most VFDs.

Yes, there is, but it is a constant in the algorithm and it is designed for a machine corresponding to the electromagnetic properties of the nameplate kVA and NOT 10+ individual motors. It is also engineered for the impedances of a single cable, not many parallel cables which may be the equivalent of cable length that exceeds the recomended maximum length.

That would depend on how many motors there were and the settings of the overcurrent protection.

No, it would not. OL relays are designed to simulate the thermal conditions of a single motor at full frequency. Everything else is a misapplication of the device.

 

[Besoeker]

Yes, there is, but it is a constant in the algorithm and it is designed for a machine corresponding to the electromagnetic properties of the nameplate kVA and NOT 10+ individual motors. It is also engineered for the impedances of a single cable, not many parallel cables which may be the equivalent of cable length that exceeds the recomended maximum length.

I'm not going to get into a protracted discussion.
We have systems operating with 80+ motors from a single VSD.
Some were put into operation 20 years ago.
It can be done and has been done.

 

[weressl]

I'm not going to get into a protracted discussion.
We have systems operating with 80+ motors from a single VSD.
Some were put into operation 20 years ago.
It can be done and has been done.

...and you will keep doing it until something goes wrong and somebody sues your ar$e off

 

[Strahan]

This is just totally nuts to me!! Obviously not a critical operation. How do you provide feed back a control when they are all controlled from the same VFD? Obviously timing is not an issue unless you plan on changing gear boxes everytime some new face enters the picture. From a money standpoint yea saves money from a control standpoint absolutly crazy?