alternating a drive on multiple motors

[DRThorne]

so this is what I'm planning on trying, this is just a single line diagram. 3 pumps with 1 running from the vfd, alternate a pump on the vfd every hour, start the second and third pump by contactors if demand increases. A plc will control the vsd start/stop and contactors

 

[DickDV]

That looks fine to me as long as the PLC does the switching and the contactor pairs are mechanically and electrically interlocked so power can not backfeed to the VFD.

Rotating the variable speed pump every hour is much more frequent than is normal. I see 200 to 500 hours between rotation, as a general rule.

 

[theDave2]

theDave2, the motor FLA and running current will not usually vary from drive to DOL given the same speed and load.

No disagreement about the running current at the same speed and load. Just wanting to mention that the wire and OCP *MAY* need adjusting.

Most of the time we will supply a motor & VFD as that's how they are to be run. However, the motors we supply are mostly dual rated. In front of me (a Reliance 184TC) is one that is rated 2HP 3.4 FLA for inverter duty and 5HP 6.1 FLA DOL, right on the name plate.

So when the customer decides not to use the 2 HP VFD we sent, we tell him to check the motor wire size and increase it if needed. Ditto for the branch circuit wire & OCP now supplying the motor starter (vs the VFD).

Granted that for smaller sizes of motors the wire to the motor is generally a minimum of 12 or 10 awg, which is going to be just fine for the above situation, as long as they OCP for a (now) 5HP motor. When getting into larger (30HP and up) motors the change in wire size & branch circuit OCP is very much on my mind.

 

[milldrone]

Lockout tagout?

 

[Tom Jenkins]

Back to economics.

For the "less expensive" system you need 1 VFD, six contactors mechanically and electrically interlocked, 4 CB disconnects, two sets of leads to each motor, and wiring six contactors. You need two sets of I/O for each motor (fault oon bypass, running on bypass) plus fault on VFD, and running on VFD. You do save on analog I/O, since you only need one set. You also need to do more programming in your PLC, and a lot more testing and de-bugging. (We just got done with a project in Spain that had existing constant speed, and converted only one to VFDs - believe me, the programming and testing are not inconsequential!)

For the other system you need three VFDs, three CB disconnects, and wiring for three motors and VFDs.

How much are you really going to save? What is the motor hp and supply voltage?

 

[DRThorne]

yes, I still need to look into the economics of it. Drives are inexpensive these days and so are contactors. I only use IEC contactors with motor starter protectors for each motor which are relatively cheap. Voltage is typically 600Vac, with hp ranges from 3hp to 15hp. I've been using the ABB ACS550 drive because of their legendary reliability . I guess with the drive for each pump I still have the softstart option which reduces motor startup current and stress on the coupling

 

[TimWilborne]

Back to economics.

For the "less expensive" system you need 1 VFD, six contactors mechanically and electrically interlocked, 4 CB disconnects, two sets of leads to each motor, and wiring six contactors. You need two sets of I/O for each motor (fault oon bypass, running on bypass) plus fault on VFD, and running on VFD. You do save on analog I/O, since you only need one set. You also need to do more programming in your PLC, and a lot more testing and de-bugging. (We just got done with a project in Spain that had existing constant speed, and converted only one to VFDs - believe me, the programming and testing are not inconsequential!)

For the other system you need three VFDs, three CB disconnects, and wiring for three motors and VFDs.

How much are you really going to save? What is the motor hp and supply voltage?

I'm with Tom on this one. Some VFDs are about as cheap as a a reversing contactor now.

I saw this done on a boring mill one time. It was awful. A bunch of contactors and relays to make that drive run 4 different motors. The PLC would have simplified it but still, the cost difference was marginal and it didn't take but a few times of having to troubleshoot it when it was down to make it cost us more money in time than we saved in parts

 

[DickDV]

Aw shucks guys! Take a look at the Pump/Fan macro in the ACS550 drive instruction manual. The logic, sequencing, and rotation is already done for you.

You need one drive and two contactors to do it without rotation and one drive and six contactors to do it with rotation. I'll bet the existing motors already have starters so there are the first three contactors already.

You are making this too complicated with a PLC and all!

 

[Tom Jenkins]

You are making this too complicated....

I resemble that remark!

 

[DickDV]

OK, then, I repolymerize!

 

[ahmad_marmoush]

Abb

Dear ,

you can easily use ABB Drives for this case cuz it's very simple adn effiecient, one VFD for 3 pumps wihtout PLC all preprogrammed inside the unit.only you will need to know how to use PFA ( pump and fan alternation) and it's explained good in Firmware.

BR,
Marmoush

 

[rsdoran]

I like Tom's approach the best, just because you can use 1 VFD for multiple motors does not mean you should. I think doing that complicates the issue.

I will assume that a "system" is in place that already has a PLC that turns on the "pumps" as demand requires.

I do not understand the wiring thing either, whether VFD or DOL the wiring should be the same.

If you need to interlock the starters then they should be mechanically interlocked too, electrically, alone, does not offer good safety; therefore you would not be able to use any existing contactors.

Technically I have never found a "bad" brand of VFD, some just may not have the features that others may offer. I was capable of buying 5HP drives for under $300 US and in a situation like this could have all 3 installed and ready to run in less than half a day....depending on how involved the wire run to motors may be, if I have someone running the wiring I stick with the original half day.

The thing about having 3 VFD's is that you can alternate as desire and run at the necessary speed for the flow required i.e. no line surges and possibly less power being used in the long run; which relates to less costs for the user.

We have the technology but it appears that many do their best to go back to what was, naturally some manufacturers offer options so they can do this. It may be my opinion but I do not see any advantage, at least when the motors are low horsepower.

 

[jraef]

I'm jumping in late, but I see that this discussion has a missing piece: transfer surges. If you are running Pump A with the VFD and get it to 100% speed, you want to switch Pump A to being Across-the-Line right? Not switch Pump B on while A is still running. That would never work for the reasons stated earlier in this thread.

But you CANNOT have Pump A's VFD isolation contactor and the X-Line starter energized simultaneously, so the only way to do it is an open transition. In most cases, opening the VFD isolation contactor while the VFD is under load is NOT recommended; it can damage the transistors. So you must technically turn OFF the VFD, open the isolation contactor, then close the X-Line contactor. Even if that took only a second, the pump will be unpowered and slow down significantly during that open transition, but the new power source will bring it back on with a vengence X-Line, causing a torque / pressure surge. Then if by chance you transfer TOO quickly, the motor field may not have collapsed yet and it is still a generator, but out of synch with the line frequency. That causes a HUGE voltage and torque spike that can seriously damage motors, pumps, breakers etc. etc.

Robicon has what they call a "synchronous transfer" option which puts a large inductor into the circuit during transition and actually has both sources on-line at the same time by matching the VFD output frequency to the line frequency, but it's a very expensive option and makes the entire system only viable (economically) on Medium Voltage drives where the VFD cost is so extreme.

DickDV, how does the ABB macro deal with that issue? Or do they? I know they have been selling this feature right and left in the market place, and now my own company (Siemens) is apparently going to do it too, but I have never used it because it scares me. I have seen motor shafts twisted off by open transition spikes in Y-Delta starters, I don't see the difference here.

 

[[email protected]]

What if we needed to switchover one motor to another without cutting off load completely, as and when required - like maybe only for repair purpose, is this possible using just one ACS550 ?

 

[Robobob]

You may want to start a new tread. This one is 10 tears old.