VFD to 2 motors. Alternating run time

You don't need an O/L is they are both the same (simply set drive to the correct FLA). Assuming they are not than you need an O/L for each contactor output. I would also suggest interlocking the Starters so only one can run at a time.

Nick

I have used three motors with one VFD with no problems
All motors were 2,2Kw as well as the VFD.
you will need physical interlocking
the VFD can be set to 'ON THE FLY' mode should you want it to catch a running load.
The Interlock you will need, availabe in some VFD's, is a 'ZERO SPEED OUTPUT'
you need to confirm the motor has stopped before you drop the power of the motor and before you change over to the other motor.
the Most damage can be caused by Back EMF

Most contemporary VFD's offer settings to suit your needs

Like,

Multi motor set up, You can select from more than one output "profile" from a single VFD, the motors need not be the same.

Most "better quality" VFD's are recognized as safety devices. Check the specs.

I would say that if the drive required is less than 15kw, the cost of a second drive might off set the control installation costs for a single VFD split between two pumps. Bigger drives start running into real money;

I don't know why you need to switch between two motors on a "runs sometime" application, but there must be a reason. ;-)

CalG said:

Like,

Multi motor set up, You can select from more than one output "profile" from a single VFD, the motors need not be the same.

Most "better quality" VFD's are recognized as safety devices. Check the specs.

I would say that if the drive required is less than 15kw, the cost of a second drive might off set the control installation costs for a single VFD split between two pumps. Bigger drives start running into real money;

I don't know why you need to switch between two motors on a "runs sometime" application, but there must be a reason. ;-)

Click to expand...

Two reasons to switch between the two.
1. In case of problem i want to be able to isolate/not use 1 set
2. For maintenance reasons keep the run times about the same.

I really don't need to run them both, but since there are 2 sets of pumps on the skid, and I already have 2 contactors I may as well incorporate them.

g.mccormick said:

1. In case of problem i want to be able to isolate/not use 1 set

Click to expand...

What if you have a problem with the common VFD ?

If this is a critical application, and you need full time automatic switchover redundancy, why only get 1½ redundant systems ?

if we talk about 2 vfd then it comes to my mind the 2 can be set to same frequency thus reducing energy consumption, since pumps have square load curve.

JesperMP said:

What if you have a problem with the common VFD ?

If this is a critical application, and you need full time automatic switchover redundancy, why only get 1½ redundant systems ?

Click to expand...

You are right, I could put in a second vfd to make it more redundant, but I have to draw the line. The main problem I am trying to solve is water supply issue. I need to slow the pump to get around this issue for now. Once the water supply is updated, I can still utilize the vfd to better match need, save power, etc. The wanting to alternate motors comes at little cost to me. I already have the two motors/pumps, I will only ever need to use 1 motor/pump at any given time, I already have the 2 contactors. When we got this equipment used, the controls were shot. We put on a new enclosure with hardware in it, this included 2 contactors and overloads. It was our intent to run the motors at line frequency. The only thing contactor related that i will be buying for the vfd upfit is a reversing interlock ($15), perhaps some relays for interlocking (say $50 total), some time to figure out interlocking and wiring.

Thank you everyone for the help

Think about it.
There are 2 pumps in a system that could normally do with only 1 pump.
What is the purpose of having 2 pumps ?
VFDs arent proof against problems. 9 out 10 may run for 20 years with no problem. But the 10th may get a problem any time.

JesperMP said:

Think about it.
There are 2 pumps in a system that could normally do with only 1 pump.
What is the purpose of having 2 pumps ?
VFDs arent proof against problems. 9 out 10 may run for 20 years with no problem. But the 10th may get a problem any time.

Click to expand...

You're correct. I'm hedging against that happening.

If I were you I would inform your boss/customer in writing that the system should have 2 VFDs in order to achieve full redundancy.
Having 1 common VFD will jeopardize the redundancy. The risk may not be big, but it is there.

JesperMP said:

If I were you I would inform your boss/customer in writing that the system should have 2 VFDs in order to achieve full redundancy.
Having 1 common VFD will jeopardize the redundancy. The risk may not be big, but it is there.

Click to expand...

It is for use internally. Thank you for the ideas, but again the supply problem is why I'm going to put on a vfd. The partial redundancy is secondary.

Another possibility should be looked at also: If you take two identical pumps with motors, start at the same time, and run for equal times, then theoretically they both will fail at the same time on the same day. That could leave you in a real bind. Most of us would rather the pumps fail at different times, so that one could be fixed while the other takes over the load.

Lancie1 said:

Another possibility should be looked at also: If you take two identical pumps with motors, start at the same time, and run for equal times, then theoretically they both will fail at the same time on the same day. That could leave you in a real bind. Most of us would rather the pumps fail at different times, so that one could be fixed while the other takes over the load.

Click to expand...

True that is a possibility. Like I have said though, primary purpose for the this change is needing vfd to control speed. Having the contactors that are already there, just needing moved and wired differently, is a basically free addon.

Its sounds to me like you had your mind made up from the beginning on what you wanted to do and weren't seeking options, just validation. So if a single VFD is what you really want to do, then just do it.

That said, I'm wondering about a few things:

Why pump from the utility feed to fill the cold well? Is the cold well elevated or at the top of a tall building so that you have insufficient head to fill it using line pressure so you need to add additional lift?

How will upgrading the building water source solve the problem? Will the upgrade bring supply water with a higher static head pressure? (This will generally mean a new water line from a different utility mainline).

If the upgrade doesn't supply a higher static head pressure but is just a larger service line then are you up-$$izing the line all the way through the building to the make up pumps?

You need more flow at the pump inlet, and that means either more pressure or a bigger pipe all the way to the pump. Otherwise you'll still have pump starvation problems, which might be a factor in the previous pump failure. In that case the drive will end up being permanent so you might want to reconsider redundancy or another option.

These are all engineering issues that should be addressed. It really seems like yet another case of the controls guy being tasked with compensating for someone's poor engineering.

I would install a bypass valve to devert some of the flow back to the source. Just install a tee a valve and a pipe back to the supply tank trim the flow as needed for now K.I.S.S.