2 motors off one vfd?

[396foxRN]

I'm looking to put two modified 18" supervac fans as exhaust fans in my garage. I want them each to be on their own motor, and want to be able to control their speed. I have two 2hp Dayton single phase motors, but from what I understand, they won't really run at variable speeds. So, I was looking into other options.

Question is, can one VFD power two 3-phase motors. If so, how do you figure the ratings needed? Say I get two 1hp 3-phase motors, do I buy a VFD rated for 1hp, or 2hp, or something else? Any other tips?

This link (https://www.automation.com/library/...ives-control/vfds-can-control-multiple-motors) implies that two identical powered fans is a good application for using one vfd, and that to do so requires using a VFD rated for the combined hp (or better to use the combined FLA) of the motors at least. In the above case of using two 1hp motors, I take it that I could use a VFD rated for 2 or 3hp. Am I on the right track?

Thanks!

 

[TurnipTruck]

My employer has two air compressor packages. Each one has a 250 hp compressor AND a 25 hp fan motor. Both motors in each package are run from one large VFD. The full load current of both motors are added together to set the trip current.

 

[mm08822]

I'm looking to put two modified 18" supervac fans as exhaust fans in my garage. I want them each to be on their own motor, and want to be able to control their speed. I have two 2hp Dayton single phase motors, but from what I understand, they won't really run at variable speeds. So, I was looking into other options.

Question is, can one VFD power two 3-phase motors. If so, how do you figure the ratings needed? Say I get two 1hp 3-phase motors, do I buy a VFD rated for 1hp, or 2hp, or something else? Any other tips?

This link (https://www.automation.com/library/...ives-control/vfds-can-control-multiple-motors) implies that two identical powered fans is a good application for using one vfd, and that to do so requires using a VFD rated for the combined hp (or better to use the combined FLA) of the motors at least. In the above case of using two 1hp motors, I take it that I could use a VFD rated for 2 or 3hp. Am I on the right track?

Thanks!

The ISA article does not address the opening of the circuit(s) on the output side of the vfd. While short circuit and overload protection is important, it is risky to implement on a vfd output. Opening of the vfd output circuit while under load can blow the output stage transistors.
You can have 2 simpler circuits by using dedicated vfd’s unless you forgo the individual motor protection which is not code compliant.

My employer has two air compressor packages. Each one has a 250 hp compressor AND a 25 hp fan motor. Both motors in each package are run from one large VFD. The full load current of both motors are added together to set the trip current.

The 250 hp may be protected within its limits of overload as the additional fla’s of the 25 hp motor is within the 125% value of the 250 hp fla’s. Adding the motor flc’s together provides no protection for the 25 hp motor from the drive.

There is also the issue of conductor sizing to each motor. How do you protect smaller size conductors to the 25hp if used or if very large conductors are run to the 25 hp motor. Fusing and overload protection on the output side of the vfd is not the best practice especially with 300hp vfd at risk.

I would be doing all I could to install proper wiring and parameter settings to protect a 250 hp motor and 250 hp vfd. That would include separating the 25 hp motor from the 250 hp drive.

 

[Mr. T]

One hp VFDs can be had on the cheap. I see absolutely not advantage to running them from one drive. And there are a ton of negatives.

Will it work? Yes! But when something goes south, and eventually it always does, you’re loosing two motors and a drive at the least instead of an individual component.

 

[laser3kw]

yes - one vfd can run two motors. Plan on buying a vfd equal to the sum of both motors.
We have been doing it with industrial machines for 12 years (or more). The machines have 2, 7.5hp motors powered by a 15hp vfd. The machines run 8,10,12 hours or more, everyday, for years and we have not experienced any weird failures.
Your application is not demanding, start, run at speed(low power requirement), stop. That will not damage a decent quality vfd.

 

[sberry]

Use the singles and shutter to change draw. Might run one or 2 motors before I would.fuk with all that. If I had to maybe adjustable pulley but would try to get it tuned where I didn't need to mess with all of it. Once there is a sweet spot it's likely that's where it will run.

 

[TurnipTruck]

The ISA article does not address the opening of the circuit(s) on the output side of the vfd. While short circuit and overload protection is important, it is risky to implement on a vfd output. Opening of the vfd output circuit while under load can blow the output stage transistors.
You can have 2 simpler circuits by using dedicated vfd’s unless you forgo the individual motor protection which is not code compliant.



The 250 hp may be protected within its limits of overload as the additional fla’s of the 25 hp motor is within the 125% value of the 250 hp fla’s. Adding the motor flc’s together provides no protection for the 25 hp motor from the drive.
There is also the issue of conductor sizing to each motor. How do you protect smaller size conductors to the 25hp if used or if very large conductors are run to the 25 hp motor. Fusing and overload protection on the output side of the vfd is not the best practice especially with 300hp vfd at risk.

I would be doing all I could to install proper wiring and parameter settings to protect a 250 hp motor and 250 hp vfd. That would include separating the 25 hp motor from the 250 hp drive.

Exactly. The lesser motor had been on it’s own starter, and the wiring still exists so it could be protected again. We’re not sure why it’s wired the way it is, maybe a not-fully-thought-out efficiency program or what.
This air project has seen at least six engineers decide they want to work elsewhere.

 

[Fix Until Broke]

Yes - as mentioned above, size the VFD for the sum of the two motor HP's and/or FLA's. Each motor needs it's own overload protection though so if one motor fails or overloads, just that motor shuts off. Typically the control side of the overloads go in series with the start/stop switch wiring so the drive shuts off if there's an issue with either motor. Have tripped a single motor overload a few times without damaging the drive - this was a lesser known drive as well, not a Baldor, AllenBradley, etc. These were not high inertia loads so that may effect the results.

 

[mm08822]

Yes - as mentioned above, size the VFD for the sum of the two motor HP's and/or FLA's. Each motor needs it's own overload protection though so if one motor fails or overloads, just that motor shuts off. Typically the control side of the overloads go in series with the start/stop switch wiring so the drive shuts off if there's an issue with either motor. Have tripped a single motor overload a few times without damaging the drive - this was a lesser known drive as well, not a Baldor, AllenBradley, etc. These were not high inertia loads so that may effect the results.

The o/l relays could open the stop ckt or enable ckt, but there still needs to be short ckt protection for the motors. Cb's opening on the output side of the drive would create the concern for drive damage.

2 drives each controlling a seperate motor would be the cheapest and most direct solution for the OP assuming he really even needs variable air.

 

[Fix Until Broke]

For the low power motors that the OP is looking at, it is much easier/cheaper to buy 2 separate VFD's.

In our case, we were running 2) 100 HP motors on a single 200HP VFD. Each motor had it's own, independent overload protection which is MUCH cheaper than a separate drive of that size. However short circuit protection was a single 300A breaker upstream of the VFD and of course the VFD itself had layers and layers of protection built in.

If anything after the VFD shorted out, the VFD wouldn't care what it was, it'll shut things down. If the anything ahead of and including the VFD shorted, the upstream 300A breaker was the protection. If for some reason the VFD didn't shut things down if the load had a short, the 300A was the secondary as it had ground fault detection. Short circuit events are typically milliseconds in duration so no significant concerns about wire sizes and such (though it makes a BIG difference in what you need to suit up in to go into one of those panels live!).

 

[laser3kw]

For the low power motors that the OP is looking at, it is much easier/cheaper to buy 2 separate VFD's.

In our case, we were running 2) 100 HP motors on a single 200HP VFD. Each motor had it's own, independent overload protection which is MUCH cheaper than a separate drive of that size. However short circuit protection was a single 300A breaker upstream of the VFD and of course the VFD itself had layers and layers of protection built in.

If anything after the VFD shorted out, the VFD wouldn't care what it was, it'll shut things down. If the anything ahead of and including the VFD shorted, the upstream 300A breaker was the protection. If for some reason the VFD didn't shut things down if the load had a short, the 300A was the secondary as it had ground fault detection. Short circuit events are typically milliseconds in duration so no significant concerns about wire sizes and such (though it makes a BIG difference in what you need to suit up in to go into one of those panels live!).

Kind of timely you mention short circuits. We just had a case where the drive output wire had slight contact with the frame. The VFD has a protection circuit in it that as soon as it detected the short it went into a protection mode and shut down the drive and gave a fault indication. Thank God for modern electronics.

 

[Fix Until Broke]

Yeah - they do a good job with that kind of stuff. It's almost anti-climactic (which is a good thing if you've ever experienced the climactic version!). The machine just turns off like normal, you check the fault codes and find a current imbalance or similar. Once you dig into it you find something slight like you mentioned and only then realize what could have happened!

Not sure if it's all 100% NEC/NFPA compliant as codes tend to rely on electro-thermal-mechanical devices instead of software and embedded controls/circuitry. There's probably advantages/disadvantages to this.

 

[manwithtools]

If the drive has proper input short circuit protection and properly sized overloads for each motor it controls, it's perfectly fine to use in dual motor control. Overloads contact in series back to the drive enable circuit and all motors will stop if any experience a problem. I've had as many as five motors controlled by one drive. This was for a conveyor and it made economic sense to be able vary the speed of the entire conveyor.

If any doubt, consult the drive manufacturer. That's if you are using a VFD that you can call the manufacturers tech support.

It's important to look at the exact application, VFD's are getting cheap enough that it may make more economic sense to have individual ones for each motor.

 

[396foxRN]

I think I'm just going to run one of the fans with an on/off single phase motor, and the other one with a vfd/3 phase motor. That way, I can run them both at full speed, or just the 3 phase one at variable speeds.

The cfm output is crazy high, but I want the ability to work in the garage in the summer with a strong draw-through air current....with the option to just run the vfd powered fan on a low speed on those damp days where I just want to keep the moisture/condensation out of the garage.

 

[sberry]

Is this a common garage? There are a lot less complicated ways to do this, probably a lot cheaper too. This is a lot of power, probably air condition it for that run cost and would make for a real windstorm.

 

[396foxRN]

it's a 24x30 garage, 13' eaves. I got all the stuff at great prices, and it gives me something to tinker with. This ought to make it tolerable to work in the garage in the summer...