VFD for Multiple Drives

Yes, but I think you need to maintain overload protection for each individual motor. One way to do this is to keep the existing starters in place, and put the VFD in the feeder to all the starters. Keep full voltage and frequency to the starter control circuit(s).

Yes if the drive is sufficiently rated to the total load you can use one drive, however the beauty of the VFD is the accurate motor protection set up, for example if one fan is to fail it has an overload setting 22 times greater than its rating. I have included thermal over loads down stream of the single VFD at each motor to overcome this problem.

The VFD market is very competitive now days, and if one VFD fails out of 22, it wont stop your process, hindsight is always a wonderful thing. The wrong brand will prove troublesome, and finally for reliable harmonic distortion elimination, the VFD's are best placed close to the motors, with earthed screened cable between VFD and motor , or power filtering a must.

Not sure how old your process is, but be sure you have Inverter Duty rated motors on the fans.

And follow the advice provided regarding OL, start-up, setup and manuf.

Sir,

With the output of the VFD on a reactor, as suggested by Jraef, the motors do not need to be inverter duty rated. All the suggestions by Jraef are valid.

--JMM

Yes Sir, you are correct that reactors or filters, some sort of conditioner, does lessen the need for inverter duty ratings. But, it does not mean that they alone should be used in every case. In fact, if you check NEMA MG 1-2011, Part 30, they too recommend that both may be necessary, under certain circumstances. While the application in question may not need them, it is not something he should ignore looking into.

Yes , You can use a single VFD of suitable rating to feed all the motors. But Please check whether the motors are suitable for VFD application(Inverter duty) or not. At reduced speed the motor cooling system may be affected & as Output of VFD is not purely sinusoidal it will create more heating in motors.

Lastly if the VFD fails due to some reason , then your total system will be affected. Pls check with process point of view also before going for a VFD

Maybe.. on paper.... however look at the practical application...

How would you run your fans IF the VFD failed? What would happen to the 24hour process? How much would you loose if these fans/drive failed.

Why not look at how many fans you don't need, and the 24 hour process would still carry on! You could just remove those fans!!

Then you need to study the ROI, plus the harmonics you will no doubt generate.

How much do you expect to save over what period of time (months & years), and how much does it cost to run a 2HP motor for 24 hours at the moment?

Brich,

Good point about the what-if and VFD failure. One good way to keep this from being a problem would be to put in a "reversing" contactor, wired so it can bypass the VFD (with appropriate interlocking, etc.).

--JMM

Some VFD's can be ordered with a bypass contactor.

I know..... however, would not be a requirement, or require thinking about if the OP had suggested numerous drives instead one!

I think on a personal note, it will all end in tears!

After reading all of the replies and your original question, I am not quite sure you are seeking the right information. The answers that have been given are good for the question you ask. But, it sounds like your asking to lower the wattage use per fan section. I don't see how we could possibly answer that question given the limited information stated. I would suggest that you seek the input from the manufacture or an engineering firm who specializes in this type of oven. Knowing the process and how the design of current system works, make it much easier to suggest.

Good Luck

Hi,

After reading your question, I have 2 things in mind what you really wanted to achieve in your case. You want to reduce the power consumption, meaning the cost (current bill) associated with your current set-up which eventually lowering your operation costs, thus increasing you profit margin? Second, in reducing the consumption, you want to lessen the power of the fans by 20% and use only the 80% of their maximum speed load because you think you are wasting power you don't really need? I would suggest before you make a move, try to review your current design, installations and set-up because you did not mentioned here. Remember, your oven process needs a controlled temperature to maintain during the entire operation.Your decision may affect the entire process if in case you will encounter unforeseen event in the future.

Reducing the SPEED to 80% could reduce the power requirement almost 50%. Since 22 x 2 HP fans is about 33kw, reducing this to 17 kw could potentially see a savings of 16kw, or operating 24/7/365days at .10$/kwh could potentially save $14k per year. Reducing the airflow may potentially save additional $ in heat loss from the ovens, and this could be significantly much greater savings.

The system needs to be modelled and trial runs done. All the existing motor protection should already be in place. A 50Hp VFD is inexpensive - it could be worth the expenditure just to prove the theory.

VFD output reactors can definitely reduce the harmonics to the fans. I have even seen simple LC filters that provide almost pure sine waves to the motor (and that was on 800HP motors).

Discuss the matter with the VFD manufacturer's technical helpline.

That the power consumption of the fans is under consideration, and not the better methods of insulating and regulating the temperature inside the oven, is currently abstruse.

Could you damper the airflow to each fan? This would also reduce the power consumption without needing to change the power distribution system.

Fans are dampered all the time to reduce airflow.

Nvirciglio--

Yes, you can reduce the air flow by putting a damper on its duct. However, the fan is still running at full speed (but with a slightly lower current load). A greater reduction in energy use is to run the fan at a slower speed through a VFD, so no damper is needed.

Note--depending on the type of fan or blower used (forward curved, backward curved, propeller, etc.), the amount of energy saved with a damper is difficult to measure. Some of this difficulty is because as the air flow is reduced and the motor load is reduced, then the efficiency and power factor change. With a VFD, the motor's power factor stays high.

--JMM

I agree. Just thought it was an easy solution that could be implemented without having to spend too much money upfront. The VFD will have a final more efficient outcome, but at a higher initial cost. Thanks

Hi,

To rectify my own personal opinion(above statement) in this case of reducing the power consumption and speed of the fans, you also mentioned that it will not affect the operation of the oven process, I will reduce the number of fans operating at full speed by 20%. In other words, I will shut down 4-5 fans and observe the controlled temperature of the oven if it could still deliver the desired heat needed.From that observation, you can estimate how many fans is needed to operate. Considering the 24 hours operation of the fans even with regulated power will most likely shorten their life than with intermittent operation.If only familiar with the set-up and the air circulation, I would rather maximize the fans speed with less numbers and operate them alternately within 24 hrs., so other fans may work every 12 hrs or 6hrs or depending at what schedule you desire. Other than VFD, you may consider timer switch. My consideration of this set-up is to prolong the life of the fans and peace of mind of possible breakdown and operation of the oven process.

Sir,

If the fans are pushing air past heating elements, it is very likely that each fan has a dedicated number of elements it is blowing air over. If so, then turning some fans off will result in some elements overheating and burning-out in a fairly short time-frame. Plese study the physical layout of the equipment--the ducts, the fan locations, the heating method and location--before arbitrarily deciding you can reduce air flow just by turning-off fans.

--JMM