Single phase backup for VFD.

[charlie b]

We are upgrading an existing (very old) facility to add a smoke control system. Building codes require the fan to have a backup power source. The existing backup source is a battery system with an inverter that supplies 120 volt, single phase power. The mechanical engineer wants to use a VFD to drive the smoke control fan, because that simplifies the process of balancing pressures and air flow rates. But I can?t give a VFD a backup source from a 120 volt single phase supply. The fan will be between 5 and 10 HP. I can only think of two possible solutions.

1. Run the single phase power through an electronics gizmo that puts out three phase power.
2. Use a single phase motor, and make the ME do more work maintaining flow balances.
   

Is solution 1 even viable? Can I get a single phase to three phase converter capable of supplying a 10 HP motor, and listed for use in supporting a life-safety function? If so, does anyone know how big this thing would be?

 

[steve66]

I think the VFD takes the 3 phase input and converts it to DC before converting it back to 3 phase AC.

Given that, I think there are VFD's available with a single phase input. Or if this is a small motor, you might even be able to run a 3 phase VFD off single phase power.

You could probably contact a VFD rep and find out if I'm right, or if I'm full of hot air.

Steve

 

[skeshesh]

I think Steve's approach is a reasonable one but for a 10HP motor I think you will be forced to use a VFD that's something like twice the size. Obviously one of the downsides of this approach is that you will have to size your conductors based on the VFD input, so using a significantly larger rated VFD will force your client to take a hit on Copper costs.

 

[Cow]

Jraef would probably be the guy to ask, but I'll mention a couple things.

I've seen 120v single phase input VFD's for small 3 phase motors. I don't know the max HP though. I think the real killer here is the fact you want to run a 10HP off 120v single phase. I don't see that happening. Is it possible for them to use 2-3 smaller fans rather than one large 10HP? Then you might have a shot at making this work.

One other option, can you put a step up transformer on the 120v to bump it up to 240 single phase? You can go with a much larger VFD using 240 single phase input and outputting to a 3 phase motor. Most VFD's have the single phase input option from what I've seen, you just need to read the fine print to be sure.

 

[Jraef]

There are VFDs that take in 120V 1 phase and put out 230V 3 phase. But there are no 120V input VFDs that will handle more than a 1HP motor.

Now this point: 10HP at 120V single phase? Did you look at the amount of power that is? (7500W / .8pf) / 120V = 78.5A at 120V!!!!! I would SERIOUSLY doubt your little power supply unit is that big...

But if it is, you can step it up to 240V and buy a 20HP 230V VFD (which is what you will be needing anyway).

 

[charlie b]

Thanks for the replies. I had not done any math, or I would have told the ME that a 10 HP was not going to work. :ashamed1:


I am curious about one aspect of this option. How does the 120V get connected to the VFD? Recall that the 120V is just the backup supply. I have 208V three phase available for the normal supply. But I can?t imagine taking a three phase, 4-pole transfer switch, connecting A-B-C to one side, connecting single phase Hot & Neutral to the other side, and taking three phase wires (that may only be carrying single phase current) to the VFD.
:?

 

[Jraef]

Thanks for the replies. I had not done any math, or I would have told the ME that a 10 HP was not going to work. :ashamed1:


I am curious about one aspect of this option. How does the 120V get connected to the VFD? Recall that the 120V is just the backup supply. I have 208V three phase available for the normal supply. But I can?t imagine taking a three phase, 4-pole transfer switch, connecting A-B-C to one side, connecting single phase Hot & Neutral to the other side, and taking three phase wires (that may only be carrying single phase current) to the VFD.
:?

On the VFD versions that accept 120VAC, they have a special device on the front-end called a "voltage doubler". It rectifies the AC to DC, uses caps to double the DC voltage (you can Google a "voltage doubler circuit" to see how, it's a common circuit), then feed the DC straight into the VFD DC bus where it is used by the transistors to recreate a pseudo 3 phase AC output.

 

[charlie b]

I see that part, so perhaps my question wasn't clear. I am confused about supplying both a 3-phase and a single phase to the same VFD. On loss of the normal (3-phase) utility source, can I have an ATS device switch over to the backup (single phase) source? This would be using the same wires from the transfer switch to the VFD. Somehow, I don't see this working, but then I don't know how the VFD is internally wired.

 

[Besoeker]

Thanks for the replies. I had not done any math, or I would have told the ME that a 10 HP was not going to work. :ashamed1:

I am curious about one aspect of this option. How does the 120V get connected to the VFD? Recall that the 120V is just the backup supply. I have 208V three phase available for the normal supply. But I can?t imagine taking a three phase, 4-pole transfer switch, connecting A-B-C to one side, connecting single phase Hot & Neutral to the other side, and taking three phase wires (that may only be carrying single phase current) to the VFD.

There are ways of doing this. How long do the fan have to run in the absence of the normal 208V supply?
One option would be to have a battery across the DC link. It could be charged either by the 208V supply or the 120V supply.
We have done such things. This short description from a list of projects:
"180 kVA water-cooled sealed hose-proof Variable Frequency Inverter, powered by a 400V traction battery, operating cable reeler, emergency pumps...."
It was for a tunnel application where, as the boring machine got deeper into the tunnel, a flexible section of supply cable was unreeled. When that had all been unreeled, it was disconnected and a permanent section installed. While the new length of supply cable was being installed, there was no supply to the boring machine and the flexible cable had to be re-reeled and there had to be provision for emergency pumps to be deployed - the tunnel was under the sea.

Obviously this was custom made stuff but a standard off-the-shelf VFD could be adapted to do this. The operation would be seamless - a bit like a UPS. As it happens, we were on site today in the last stages of testing a standard 200 kW DC fed inverter installation.

So, it can be done. Whether it's worth the effort/cost of going for an engineered solution even if it uses standard parts would depend on the application on a case by case basis.

 

[Besoeker]

Somehow, I don't see this working, but then I don't know how the VFD is internally wired.

Probably you know this already but I'll add it anyway. It might make my previous post clearer.
Most variable frequency drives first convert AC to DC via a rectifier bridge, usually 3-phase but on some smaller units, it may be single-phase. Assuming 3-phase, the rectified voltage is 1.35 times the 3-phase supply voltage. About 280Vdc from your 208Vac 3-ph. The 120Vac won't produce anything like that so changing over supply from 208V 3-ph to 120V 1-p isn't on. Even if the 120V is capable of supplying the 10HP plus losses.

That's why I was suggesting an alternative means of sourcing the DC if you lose the 208Vac.

 

[Jraef]

I see that part, so perhaps my question wasn't clear. I am confused about supplying both a 3-phase and a single phase to the same VFD. On loss of the normal (3-phase) utility source, can I have an ATS device switch over to the backup (single phase) source? This would be using the same wires from the transfer switch to the VFD. Somehow, I don't see this working, but then I don't know how the VFD is internally wired.

If your VFD were big enough (i.e. the 20HP for a 10HP motor I mentioned earlier), then what you could do is use a transformer to boost the 120V to 240V single phase and have your ATS feed the VFD with either source. The VFD would not know the difference. On normal 3 phase 208V power, you just have a grossly oversized VFD, it doesn't know or care. Then if you lose power and you have to run on backup, the ATS switches sources to come from the 240V 1 phase and feed the VFD with only 2 of the 3 wires. Again, it wouldn't know or care (*see below) and just go along blissfully serving up 3 phase power to your 10HP exhaust fan. The only caveat, as I said earlier, would be a 120V 1 phase load that big. You would also have a little bit of losses in the transformer, so assume 80A at 120V. That's a LOT of power from a battery bank. Make sure you calculate the Amp-hrs even if the inverter is capable, a 7500W draw for 8 hours would be a LOT of batteries! The same would hold true for Besoeker's direct DC backup idea, although honestly I like his way better for other reasons. If puts the worst drain on your backup system as a separate load that doesn't take down everything else when someone leaves the fan on in the bathroom...

* The caveat I mentioned is that there are a few VFDs that will have phase loss protection in them that cannot be turned off. You would not be able to use those, so make sure the one you select would be capable of getting 1 phase power without tripping off.

 

[kwired]

Even if you use transformer to derive 208 or 240 volts you do realize that your 120 volt source will need to be able to deliver 80-100 amps if you are powering a 10 hp motor that is a pretty stout supply at 120 volts.

Do you need an instant transfer to backup supply or can you handle the delay for a generator startup and automatic transfer switch? That will likely be easiest and less costly route to take if it is acceptable.


Another idea is to use a smaller fan if reduced airflow is acceptable as the backup, or set the VFD to only run up to a specific maximum speed during backup mode if reduced airflow is acceptable when running in backup mode. I have seen this done for VFD running an irrigation water pump before, the POCO usually shuts down irrigation equipment when necessary during peak demand periods via radio controlled equipment. I have seen cases where they allowed the pump to still run during those periods if the controller was capable of limiting the maximum load when called for. We just used the radio controller to fire an input and tell the VFD it can only run at XXX speed maximum when this input is active.

 

[hurk27]

I have not seen a single phase motor over 7.5hp in a long time, much less one that will run on 120 volts:?, I think if your going to want a speed control system and between a 5-10 hp motor you might want to check out a 3-phase UPS system, then use the VFD, or at least a 240 volt UPS system, but at 7.5kw thats not going to be cheep by no means, and depending upon the required operation time of this smoke control blower, you might be looking at extra battery banks or a stand by generator with an ATS<<< which might be the answer for this whole idea?

P.S. An AB Powerflex-70 is what we used on our 10hp lift station pumps to run them on single phase 240 volts they are rated at 20 HP and accept single phase input, single phase input load current will be around 49 amps.

 

[hurk27]

Here is two in operation at a location where we only had a single phase supply from the POCO:

 

[Besoeker]

Here is two in operation at a location where we only had a single phase supply from the POCO:

Hurk, I'm sure those VFDs work just fine but there are a few things in there that we wouldn't get by with.
The control section at the top right is comparable to what we do, but if you look at the terminal block to the right of VFD1 there are no idents on the conductors and they have been put into the terminals without cable terminations lugs i.e. as bare conductors. The exposed terminals on the fuses etc. would also not pass inspection by our customers.

This a 45kW unit we did recently prior to it being tested. Top cable entry which is an unusual requirement. The untidy conductors at the top are a temporary test supply.

It's fairly neat but will that make it work any better? Probably not but, the labeling and conductor identification make testing, installation, and subsequent fault finding all a bit easier.

 

[Besoeker]

That's a LOT of power from a battery bank. Make sure you calculate the Amp-hrs even if the inverter is capable, a 7500W draw for 8 hours would be a LOT of batteries! The same would hold true for Besoeker's direct DC backup idea, although honestly I like his way better for other reasons. If puts the worst drain on your backup system as a separate load that doesn't take down everything else when someone leaves the fan on in the bathroom...

TY Jraef.
The batteries could be charged from the main source - that's how we arranged it for the tunnel inverter - so there would be no drain at all on the 120V.
A lot of batteries? Depends on the autonomy required. We do a fair number UPS systems mainly for lighting in public buildings. Typically for a cinema, these would be 30kVA for three hours*. The battery racks are quite manageable and that would more than do the eight for the 10hp motor for eight hours. Not sure where that eight came from BTW.

*The three hours comes from a Cinematographic Code of Practice. I've often wondered about this. In the event of a supply failure, the cinema would be evacuated. I can't believe that would take anything like three hours. But, if it's in the spec, that's what you provide.

 

[Jraef]

TY Jraef.
The batteries could be charged from the main source - that's how we arranged it for the tunnel inverter - so there would be no drain at all on the 120V.

I meant the battery drain when on emergency power. A 120V battery backup is most likely for a life/fire/saftey alarm monitoring system. Having that 10HP motor drain that same battery down faster presents additional risk. Having separate batteries just for the fan means that even if the fan dies, the other systems stay active.

A... Not sure where that eight came from BTW.

The 8 hrs came from a tunnel ventilation fan system I worked on once. We had to have generator fuel storage for 8 hours of operation because the fans would potentially be supplying air to firefighters. We don't know what the actual application is, I was pulling that out of my hat as a potential worst case possibility to help the OP remember that critical aspect of the design.

 

[charlie b]

Again, thanks for the information. The project involves three hydro-electric dams along the Columbia river separating Washington from Oregon. They have been in service from 50 to 100 years. There are work spaces well below grade level, making the facility technically a "high rise," from the building code perspective. That brings the requirement of a smoke control system to the party, at least by modern codes. So we are retrofitting the systems. Generators are not an option for reasons I have not been given. I am not the designer. I just happen to be the closest EE to the ME working on the project, so I got asked the question. I am told that the fans have to be running for only 30 minutes, in the event of a fire. Since the normal source of power comes from the hydro plant itself, and since a fire within the plant could disable the generating equipment, the battery system could be the only power source available to the smoke control fans.

I have learned, from a more senior ME, that a motor that almost never runs, and only operates in the event of an emergency, is not a good application for a VFD. So I think the question is about to become purely academic.

 

[Besoeker]

I meant the battery drain when on emergency power. A 120V battery backup is most likely for a life/fire/saftey alarm monitoring system. Having that 10HP motor drain that same battery down faster presents additional risk.

For the system I meant, the battery voltage would be that of the DC link voltage - about 280Vdc in this case with the battery connected directly across the DC link.
That's we did on the tunnel system and a few other projects.
I simply wouldn't have considered a 120V back up.
Academic now anyway, it would seem.

 

[Besoeker]

I have learned, from a more senior ME, that a motor that almost never runs, and only operates in the event of an emergency, is not a good application for a VFD. So I think the question is about to become purely academic.

Many, if not most, VFDs are used for energy savings. For infrequent operation of short duration, then I agree, it isn't a good application on that basis.
If there are sound operational/set up reasons for requiring a different speed to direct on line operation, that might be a different story.