Single VFD controlling two motors

[ChefTurnedSparky]

Two large AHU units are being replaced, moving from a single fan unit to quad fans. Two VFDs will control two each of the four fans, each fan having a disconnect at the unit. VFDs are being provided by HVAC contractor, I don’t have any specs yet. I am assuming that the two fans controlled by a single VFD will be controlled identically? I’ve never seen it set up this way. If that is the case, would it be acceptable to run both feeders from the VFD in the same pipe and then split before the disconnects at the fans? I assume if they are controlled seperately then they should be run separate as well, due to the frequency differences?

 

[ptonsparky]

The disconnects will need an interlock to stop the VFD prior to opening the motor load. A Control pole in each.

 

[ChefTurnedSparky]

Yes, there will be an interlock on each fan disconnect. I don’t believe I’m responsible for that path, I’m not doing controls on this project. Assuming though, that the control wiring would need its own pipe, regardless of voltage(24/120)?

 

[petersonra]

The disconnects will need an interlock to stop the VFD prior to opening the motor load. A Control pole in each.

It's not required, just a good idea. But if they are so cheap they won't supply a vfd for each fan what makes you think they want to supply an interlock from each disconnect?

You will need to have a separate branch to each motor from the VFD because you will need a separate overload device for each motor tied into the enable line of the VFD.

 

[ChefTurnedSparky]

It's not required, just a good idea. But if they are so cheap they won't supply a vfd for each fan what makes you think they want to supply an interlock from each disconnect?

You will need to have a separate branch to each motor from the VFD because you will need a separate overload device for each motor tied into the enable line of the VFD.

Each disconnect will have an interlock, specs for the project. Also calls for a branch for each motor. My question is do I need to run them in individual pipes or can I run them together?

 

[petersonra]

Each disconnect will have an interlock, specs for the project. Also calls for a branch for each motor. My question is do I need to run them in individual pipes or can I run them together?

I don't see any reason they can't be in the same pipe.

 

[Jraef]

You need separate overload protection for each motor, the VFD cannot differentiate. How you do that requires some thought, partly because not all OL relays are suitable to be used on the output of a VFD. What size are the motors?

Since both sets of motor leads are from the same drive and thus will be at the same frequency and phase relationship, they can be in the same conduit. Normally you don’t want to do that.

 

[ChefTurnedSparky]

You need separate overload protection for each motor, the VFD cannot differentiate. How you do that requires some thought, partly because not all OL relays are suitable to be used on the output of a VFD. What size are the motors?

Don’t have specs on that either, and equipment is not set. This is, overall, a very large project. Gubment money. All engineered and laid out for me, mostly. All other disconnects are fused, there’s a total of 18 I believe. But the 4 for fans are not fused. Maybe the VFDs are built for controlling multiple fans, with multiple OL relays? Otherwise, I dunno. They arrive this week.

Since both sets of motor leads are from the same drive and thus will be at the same frequency and phase relationship, they can be in the same conduit. Normally you don’t want to do that.

Even if the VFDs were to have multiple OL relays, they’d still operate at the same frequency, correct?


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[ChefTurnedSparky]

Correction- the fan disconnects will be fused at 30 amps.

 

[ptonsparky]

Correction- the fan disconnects will be fused at 30 amps.

The fused disconnects are on the load side of the VFD? You might rethink that.

 

[ChefTurnedSparky]

The fused disconnects are on the load side of the VFD? You might rethink that.

Like I said, it’s all engineered, laid out for me. County job, super spec’d out, right down to the pipe size. Single 2/0 feeder for both VFDs, tapped in a trough (size specd) to 60a fused discos, to each VFD, feeding two each NEMA 12 fusible discos for the fans. Like I said - super specd.

 

[Jraef]

Even if the VFDs were to have multiple OL relays, they’d still operate at the same frequency, correct?

Yes, same issue in that the VFD is just putting out a frequency. It has no way of knowing how many motors that goes to.

Just a word to the wise, even if it is already spec’s out by someone else, you as the installing electrician are responsible for making sure that the NEC is followed, and the NEC requires separate OL protection for each motor. This is a detail often missed by people writing specs. I urge you to make sure in advance as to what is coming.

Case in point:

…feeding two each NEMA 12 fusible discos for the fans.

You do NOT want fused disconnects on the OUTPUT side of the VFD, because a fuse blowing on the output side could cause damage to the VFD transistors. So if the “super spec” engineer did not understand that, they likely have no idea what they were doing, so may not be aware of the very specific requirements in the NEC regarding this too, which as I mentioned, may fall upon you to try to correct in the field. I always like to know what’s coming rather than try to scramble when it arrives. But you do you…

 

[ChefTurnedSparky]

You do NOT want fused disconnects on the OUTPUT side of the VFD, because a fuse blowing on the output side could cause damage to the VFD transistors. …very specific requirements in the NEC regarding this

Mind sharing the code, so when I make my argument I have something to go on?


I’m still fairly new to all this, just finished my apprenticeship. Now, when people hear that, they think I’m some zit-faced kid, but this was a career change later in life. I rose thru the ranks of the restaurant industry. So leadership, problem-solving, critical thinking, time management, all skills I possess. I’m just applying them to a new skill.

 

[petersonra]

There is no code. It is a design choice. Incidentally, my understanding is that more modern designs of vfds are not afflicted with this problem.

Personally, I think it is unnecessary. Put a sign up that says "This is not an emergency disconnect. Do not use under load."

If people actually use it in an emergency and fry an old VFD that will need replacing in the not too distant future anyway, nothing much is lost.

 

[ptonsparky]

What would the response of a newer VFD be if one of the fuses blew? An orderly shut down with a fault code of some sort?

 

[garbo]

We have only used a single VFD to feed two to five packing line conveyors. Much prefer to use a drive for each motor 1HP & larger. At the large hospital that I retired from when they remodeled OR'S they had to increase air volume and ended up replacing the AHU's with units that had 4 to 6 supply fans each on its own drive. At a remote site they installed a roof top AHU with only one drive to feed 4 supply motors. This feed two OR'S and maybe other area. Anyway one motor grounded out and our senior electrician was not smart enough to just remove the wires from the combination breaker/ over load device that feed the bad motor so they had to close this location for a day until contractor came out with a replacement motor that was under warranty. These OR'S only did minor surgeries and location did not have provisions for over night stays. Do not think you save that much money using a single drive to feed multiple motors when you add up the cost of individual overload, disconnect devices & labor. Goss newspaper presses had a great design years ago where 10 or more 60HP motors each had thier own drive but were tied to a common line shaft. If a drive went down you only turned off that drive and could keep printing papers. Allen Bradley then came out with drives that guess had hi tech encoders that no longer required a line shaft and each motor always ran at the same speed. At shows Allen Bradley would have two motors each on thier own drive. They would place a $100 bill between motor shafts and have people ramp up speeds, perform E stops etc. If $100 bill ripped they would give it to you.

 

[Jraef]

There is a direct Code citation requiring individual overload protection for each motor, that’s what I meant. 430.53.B. It requires that each individual motor in a multi motor branch circuit (which is what this is) be properly protected, both for short circuit and for overload. Sometimes you can get away with the upstream branch SCPD being within the 250% of the smallest downstream motor FLC, but you ALWAYS need OL protection.

The issue of not putting fuses on the output of the VFD is not a direct Code issue, other than 110.3 requires that you always follow manufacturer’s instructions, and most drive manufacturers will instruct you to not put fuses on the output side of a drive.

The best practice on this subject is to use Motor Protection Circuit Breaker (MPCB) devices on the output of the VFD. These are essentially Manual Motor Starters that also act as SCPDs and disconnect devices all in one. Some are specifically designed to be used behind VFDs so that they don’t nuisance trip due to the harmonic heating effects of the PWM output of the drive and suffer accelerated aging due to reflected wave phenomena.
https://www.rockwellautomation.com/...controlling-multiple-motors-with-one-vfd.html

If you don’t know what they are doing, but you are responsible for installing it, the onus is on you to meet Code requirements. Forewarned is for-armed.

 

[petersonra]

What would the response of a newer VFD be if one of the fuses blew? An orderly shut down with a fault code of some sort?

Probably nothing. The VFD does not care how little current is flowing. It only trips on overloads and short circuits.

 

[LarryFine]

If it's bad to disconnect a running motor from a VFD, why isn't it risky to have overload protection between them?

 

[ptonsparky]

So it won't recognize that one Set of windings is open or the current imbalance?