alternating a drive on multiple motors

DRThorne · May 23, 2007

I have been using a variable speed drive for every pump for our machines and had the idea of using 1 drive to alternate between 3 pumps, the other 2 would run start-stop from contactors if demand increased. I've asked drive manufacturers about doing this and they keep telling me there's no easy way to do it (I'm assume they're telling me this to sell more drives). thinking I would have to use single drive, starter for each pump and interlock relays to prevent the starter and drive from powering the motor at he same time and also to switch the drive power to each motor. Any other things I should watch out for?

Andybr · May 23, 2007

We have systems which work like this and there is one potentially serious drawback. When the demand on the system increases to the point where you want to start an additional pump the VFD will already be running at full speed. When the next pump starts this will run up to full capacity very quickly if it is DOL started and you will see a sudden large increase in flow/pressure until the VFD has ramped back down to the required speed. The reverse will happen when the demand is falling. On our systems this is an annoyance rather than a real problem but I can see it being a real issue for many applications.
Andybr

cntrlfrk · May 23, 2007

I have also done something similar to this. To compensate for the "bump" when a pump goes on-line or off-line, you need to do some 'feedforward' adjustment to the drive speed. For example, if the VFD is at 100% and you start an additional pump, you may need to throw the VFD speed into manual, drop the speed by 70%, then throw it back into 'auto' control and do the reverse when taking a pump offline. You may also need to add delay timers in when you make this change. This, of course will depend on what control system you are using.

Good Luck.

theDave2 · May 23, 2007

From an electrical view, you should check the rating of the motors to see if the FLA & HP is different for VFD or across the line line running.

If so (and it is mostly so) then you'll have to size wires to the motor for the larger value, the starter larger and the branch circuit feeder wires larger and fuses correctly. Maybe even have two sets of fuses, one feeding the vfd and one feeding the starter.

I'm sure DickDV will chime in about this and correct me if I've misspoken.

504bloke · May 23, 2007

I have in the past used a 690+ SSD Drive on one pump and had the other two pumps on siemens 3RW soft starts Used the 690 to control from an analogue and also used the 690 to switch the other 2 pumps in and out on demand and it worked very well.

DRThorne · May 23, 2007

so would it be a good idea to have voltage from the motor starter applied to the output power terminals of the drive when the starter is energized?... or should I have a contactor to isolate the drive output from the starter

theDave2 · May 23, 2007

DRThorne said:
... should I have a contactor to isolate the drive output from the starter

Yep. Not good to put current into a drive from that end of it.

Something like a AB 700N series contactor that should have NC/NO break before make contacts.

504bloke · May 23, 2007

If you are going to do that then use a pair of contactors that are mechanically interlocked and also use an electrical interlock from the drive as well.

DickDV · May 23, 2007

ABB provides a Pump/Fan Macro with both their 550 and 800 series drives. It handles the start/stop of up to five additional motors plus the variable speed unit. If you provide dual contactors for each motor, the software will also provide for periodic rotation of the variable speed pump so each unit, over time, gets the same running hours.

Nice stuff, really. The drive output relays pilot the various contactors. You just add the contactors.

DickDV · May 23, 2007

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

Norml · May 23, 2007

A better installation would be to also put a contactor on the drive input to isolate it. I've seen drives with shorted inputs that make the bypass feature useless.

DickDV · May 24, 2007

Actually, a manually operated drive-isolation switch is preferred. That way, when servicing the drive with the control power on, there is no way for the contactor to close and re-energize the drive.

Tom Jenkins · May 24, 2007

I used to see this sort of thing in the "old days" when VFDs were very expensive and reliability was questioned. Nowadays, by the time you get done with the engineering, the extra hardware, the extra wiring, the testing, and so on I seriously question if this scheme is worthwhile economically. The cost of drives has dropped in recent years, making the economics questionable unless you are talking high horespower medium voltage.

The performance benefits of all pumps VFD operation is also a consideration. You can't usually turn a pump down to much less than 50% flow on a VFD. With the across the line scheme you go from 100% flow to 150% flow in one step. With VFDs on all pumps you can go from 100% flow to 101% flow if you need it.

cntrlfrk · May 24, 2007

Tom Jenkins said:
I used to see this sort of thing in the "old days" when VFDs were very expensive and reliability was questioned. Nowadays, by the time you get done with the engineering, the extra hardware, the extra wiring, the testing, and so on I seriously question if this scheme is worthwhile economically. The cost of drives has dropped in recent years, making the economics questionable unless you are talking high horespower medium voltage.

I have done a couple projects in reverse of this. Installed two drives to supply one motor. The customer seemed somewhat scared of VFD's and so was his staff, so they wanted the ability for me to connect from anywhere in the world, and switch over to a 'stand-by' drive until I could arrive on-site and repair the failed VFD. We had seperate fusing on the supply side with electrically operated 'break before make' contactors on the supply and load side, as well as manual disconnects so maintenance could safely be performed.

DickDV · May 24, 2007

Some years ago I was involved in a Ford Motor Company project where there was 23 machines all with the same drive motor.

We supplied 25 identical pre-programmed drives for these machines and all drive input and output connections were on plug connectors. If a drive failed, pull the plugs and insert the spare. And they're off!

As I recall, the hp was around 30.

I don't think I would do this today. The flash hazard from pulling the motor lead plug under full load would be unacceptable. I suppose we could have used an XP type connector which disconnects before it opens which would have reduced the flash exposure but the drive IGBT's in those days would not have survived the loaded disconnect either way.

One for the history books, I guess!

alternating a drive on multiple motors

DRThorne

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