motor efficiency: single phase vs. 3 phase vfd

I am trying to figure out if an idea I have is feasible or not. A friend of mine has a fairly large greenhouse operation which use fans to cool during the spring and summer. Only single phase power is available. All fan motors are 1 hp and there must be at least 50 fans at this location.

I suggested replacing the single phase motors with 3 phase and using AFD's to run the fan motors. I have found VFD's up to 3 hp that can run off of a 220v single phase input. So, he could run 3 fans off of every drive. The fans would always run at full speed.

My question is: Would this save electricity over the current system? I know that the 3 phase motors would be a lot more efficient than the single phase, but would the AFD's use a lot of additional power?

Thanks!

Feasible yes. Expensive yes.
Allow $200 to 350 per 3 HP drive and around 95% efficiency for each.
you can run multiple motors off one vfd PROVIDED they are closely matched.
BUT each motor must be 3 phase -- ADDED COST

YOU CANNOT DRIVE 3 SINGLE PHASE MOTORS OFF A VFD -- PRACTICALLY

I would evaluate having utility bring in a 3 phase service 208 120 wye. With a total motor load of just fans of 50 HP you are easily in the 3 phase service range.
3 phase will give you lots more versatility and ability to drive larger loads.

I do not think the increase in efficiency would pay off in a 3 year period - OFF TOP OF HEAD ESTIMATE

RH68, I agree with leitmotif. All though it should not be expensive from a electrical load control, which consumes the kVA and MD from your utility supplier, via a simple PLC control system, if the process allows it. Three phase makes sense, then you can also do phase load control, which will directly influence your running cost of the operation on a daily basis, actually hourly basis, because that is how you should be billed.

The initial set-up cost for a three phase system might be high, but if the concern is profitable, then it should be able to pay some dividends over a set period of time. This should be calculated.

Using VSD's on a three phase system and integrate a load control system on top of this, would be ideal, again, depending on the process and quality requirements of the product. If you need good quality, then you should have some form of minimal human interface and a well structured control system, with checks and balances.

Good luck with your project.

Greetings from sunny South Africa.

no go...

Sorry RH, you would be facing an extra energy cost of approx 4%!

Efficiency of a single phase 0.75kW-4p motor is 71% nowadays, efficiency of a three phase 0.75kW-4p motor is (surprise, surprise) 72%. (Check Siemens or MEZ catalogues).

Since you're planning on running at full speed, the additional VFD losses make your savings go up in smoke...

Where did you get the idea that three phase motors are a lot more efficient? Maybe if the existing single phase motors are quite old (= before nineties)...

The existing motors have an efficiency of 67%. There is no 3 phase power available here. It is very rural. It's not a matter of paying to have the utility run it from the pole, it's not on the pole.

Sparkz, 3 phase motors are more efficient than that. I am looking at one right now that is 85.5% (Marathon XRI). Baldor Super-E is 87.5%.

Actually, now that I'm looking at the Baldor Super-E's I see they have a single phase 1 HP motor that is 84% efficient. Didn't know you could get that with a single phase motor. Well, I guess the whole point is moot now.

Hi RH, hopefully all 50 fans don't turn on at the same time. If they do you can still help your friend with some good 'ol relay logic and timers to help releive some of the inrush current.
Sounds like a fun project.

RH68 said:

The existing motors have an efficiency of 67%. There is no 3 phase power available here. It is very rural. It's not a matter of paying to have the utility run it from the pole, it's not on the pole.

Sparkz, 3 phase motors are more efficient than that. I am looking at one right now that is 85.5% (Marathon XRI). Baldor Super-E is 87.5%.

Actually, now that I'm looking at the Baldor Super-E's I see they have a single phase 1 HP motor that is 84% efficient. Didn't know you could get that with a single phase motor. Well, I guess the whole point is moot now.

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Politically you may just be better off not to give your utility more to think about and reclassify your rates.
You may be at the bottom of the commercial structure and doing just fine. You go talking to them about 3 phase or whatever they may look at your consumption and change your rate class.

ALSO if you are single phase you may not (prbably) not pay a demand charge and of course you are not paying for power factor. You go 3 phase they will maybe start charging for these also.

Course you are several miles away from 3 phase so it is definitely cost prohibitive to have utility string it out just for you.

Dan Bentler

people,
The 50 fans are spread out over about 15 different greenhouses on the property. The largest greenhouse has 20 fans in it. No, they don't all come on at once. There is an environmental control computer that turns the fans on and off automatically. I don't know if it can support running the fans at variable speeds or if its just on/off based.
One of the other ideas I liked about using the AFD's is that he could soft-start the motors.

RH68 said:

The fans would always run at full speed.

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RH68,

The full speed issue is where the money is at. In round numbers if you can reduce the speed to 80% the horsepower drops to 1/2. Then the 4% loss in the VFD is offset by the drop in motor amps. This is presuming you can live with 20% less air flow

I would guess that high efficiency blower wheels will help tremendously also. Presuming the units now in place are "low bid" items.

RH68 said:

people,
The 50 fans are spread out over about 15 different greenhouses on the property. The largest greenhouse has 20 fans in it. No, they don't all come on at once. There is an environmental control computer that turns the fans on and off automatically. I don't know if it can support running the fans at variable speeds or if its just on/off based.
One of the other ideas I liked about using the AFD's is that he could soft-start the motors.

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I dont think soft starting these motors is a cost effective idea. Not even sure if you can readily do it on single phase motors. theoretically probably so but in real world probably too much folderall for small single phase motors.

Along that line I WOULD check to ensure I was not overloading the service equipment either during start up or during run.
One quick and easy way is to see if the lights are dimming especially if incandescant.

In your case if you are starting to max out during start then do a stagger start which with multiple buildings I am sure you do anyway just by the walking time between them.

Dan Bentler

leitmotif,
The idea was to do away with the single phase motors and use 3 phase motors and vfd's. That's why softstarting would be an added benefit. All fans turn on via the environmental control computer automatically.

Soft start (actually ramped starting / speed up) in VFD is a good thing for several reasons
1. IF you are near overload on service it will reduce the starting current. If the utility is on your back over high start current VFD will help. I would still get the computer to do staged starts -- not multiple motors all at once and for sure NOT 50. IF you are paying demand charge it will help reduce this - more so if you stagger start.

I think you need to look at the big cost picture
16 VFDs at 2 to 300 is ----------------------- 3200 to 4800
50 3 phase 1 HP motors is 1 to 200 ---------- 5000 to 10,000
Rewire from VFD to the motor 100 or so ------- 5000

Keep VFD to motor distance to minimum FOLLOW FACTORY reccos otherwise you get into more cost from "funny electrical effects" and have to do filtering MORE BUCKS

Darn near forgot - you should have overload protection for each of 3 motors fed off one VFD. More bucks.

A major savings with VFD can be gained by reducing fan speed. Fans blowers and pumps have affinity laws where a doubling of speed results in a doubling of flow a squaring of discharge pressure and a CUBE of power.
Running two pumps at 50% speed gives full flow but at 1/4 the power of one at full speed. MAJOR savings. For sure see if you can run fans at variable speed. This alone may make project pay for itself - it does on larger equipment.

Replacing motors to gain motor efficiency will not really produce big savings and is rarely done for this reason alone. Maybe if you can get one really good deal on a bulk purchase it would work. I think you will be disappointed. HOWEVER there will be savings on maintenance ie replacing capacitors on single phase motors.

Running VFDs will appreciably raise the PF on AC line and will reduce the power factor charge. They run at near unity PF. I doubt you are billed for power factor so this is doesnt do much good for you.

I think you have a good idea but for sure I would start out on a smaller green house and see how it goes and how you want to change your control strategy.

As an aside - are you injecting CO2 into greenhouses? Photosynthesis "runs??" primarily during daylight - am I correct?

Greenhouses get HOT in summer and take bucks to keep cool just with fans. This now wasted heat can be converted to bucks.
Can you use cold irrigation water to help cool greenhouses during summer daytime? Is there a nearby facility that uses bunches of water and have to heat it - you could preheat their cold water and make a buck off your now wasted heat.

Dan Bentler

I'm not very savy with the PID function on drives, but if RH decides to install some could he use PID to keep constant temperature? That way only some fans would run until all are needed. I'm sure you could get the same results with T-stats and relays.

peoplehouse said:

I'm not very savy with the PID function on drives, but if RH decides to install some could he use PID to keep constant temperature? That way only some fans would run until all are needed. I'm sure you could get the same results with T-stats and relays.

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Fully agree. If he goes with VFDs designed for HVAC and pumps he will have off the shelf designed for purpose. I believe he needs to look at the up front costs and see if he is willing to jump that hurdle.

Unless he takes advantage of affinity laws he is going to be sorely disappointed in the saved energy dollars I predict. Reducing fan speed where able especially so with PID will give him best return on invested buck.

Dan Bentler

The real cost savings won't be recouped for a very long time.

If this was simply a matter of trying to use existing three phase fan motors I would suggest that you build a rotary phase converter. They are simple and inexpensive to construct (google rotary phase converter) and more than adequate for driving fans and pumps. They are common in rural American farmlands.

But where you would have to change out existing motors plus add the drives I don't think there is any practical advantage to be gained.

Don't forget that greenhouses are acoustic nightmares, and the sound of a motor on an inverter may be quite uncomfortable in a greenhouse.