Blower Motor Question

[Sliver]

We have several 200hp dust collector fans that start across-the-line.
They start usually about twice a week and run almost continously.
They have dampers for control that are shut when starting
Typical startup times are in the 60 second range (high inertia load)When the air temp drops air becomes denser increasing load and starting time. This is when we have troubles starting without tripping overloads. We have started changing out the thermal o/l with electronic/programmable to allow starts but protect motor when running.
Across-the-line makes drive belts squeal, so mechanical is tempted to tighten belts which has caused a few motor shaft failures.
If I could wave a magic wand we would install softstarts on the works.

Brian.

 

[JesperMP]

An electronic overload relay is good, but a thermistor relay is better (and probably cheaper - well not if there is a long cable to pull).
I should have mentioned before that it is really a good idea (a requirement maybe) with thermistor relays for this kind of application.

 

[JesperMP]

We have several 200hp dust collector fans that start across-the-line.
They start usually about twice a week and run almost continously.
... Typical startup times are in the 60 second range (high inertia load)When the air temp drops air becomes denser increasing load and starting time. This is when we have troubles starting without tripping overloads.

Rereading this post I realise that these motors must be very close to the limit with respect to available torque for accelleration.
200hp direct start - and still 60 sec. startup time.

If you changed to y-d starter then you would only have 33% starting current and the same for the torque.
If you changed to softstarter then you probably would have 50-60% starting current and the same for the torque.
This will increase the starting time with more than 1/0.33 or 1/0.55. This because there must be extra torque available to accellerate.
There is even a risk that the motor torque curve and the fan load curves gets so close or even touches each other before nominal speed, that the fan will never start up.

I believe that the fan manufacturers matches the fan to the available standard motors.
In your case they must have counted on direct start.
Y-D start is so common for fans, that I thought it is more or less standard. 95% of the large fans I have seen starts with Y-D starter.

 

[DickDV]

Jesper's comment about motor torque and horsepower being very closely matched to the fan brake horsepower is certainly true.

But, his statement that the torque curves might overlap and the fan not reach full speed when Y-D or electronically softstarted isn't reasonable. When the Y-D starter switches to the D configuration, you have the same available torque and horsepower as across-the-line starting. The same is true for electronic softstarters--when the voltage ramp gets up to full voltage, you are across-the-line with all of the available torque and horsepower.

Maybe, what Jesper meant was that there would be no real softstarting available (and no reduction in inrush currents) due to the heavy load. This could easily be true for the Y-D method because a large inrush will occur when transition is made to D with the motor lagging way low in speed.

However, an electronic softstarter, at least the voltage ramp types, will proceed gradually up to full voltage regardless of how much the motor is slipping. (Up to the current limit, of course) Because the electronic softstarter makes transition smoothly there is no sudden inrush pulse except at initial start. After that, the softstarter will proceed up to full voltage even driving the motor deep into overload if it has to. From the description of starting across-the-line, it doesn't sound like much overload capacity would be needed.

As Jesper has indicated, it surely will take longer to accelerate the fan up to full speed when softstarting but, in the end, isn't that to be fully expected? And, as long as the process can tolerate longer accels, why not take our sweet old time about getting up to speed. The only thing you have to watch is the softstarter heatsink temperature since they are generally designed for faster ramps (smaller heatsinks). And, the benefit is that very little of the motor torque is diverted for accel purposes. Practically all of the motor torque is available for the fan load which, at top speed, seems necessary.

 

[JesperMP]

Dick is right that an electronic softstarter can do the trick.

sliver said that even at direct start, when conditions are worst, then he had occasional trips with the old overload relay.
He now uses an electronic overload to counter these overload trips.
That tells me that reducing the starting current will mean that the starting time will be more than proportionally longer.

What I thought of (and very probably does not apply to sliver's aplication) is that fans may have mechanical resonances in the speed range between standstill and nominal speed. At least that was a problem with the geared fans that I have worked with.
The problem is that you must pass these resonance points with a certain accelleration. If you dont, you may risk the fan to be "caught" at a resonance point without being able to pass it, this ending with a mechanical breakdown.

 

[DickDV]

The issue of mechanical resonance points is an important one and I missed it completely.

As far as lengthened accel times are concerned, it is generally stated that available motor starting torque goes down as a function of the square of the starting voltage. Since accel time with a fixed mass is an inverse function of torque, one might conclude that the accel time would vary by the inverse of the square of the voltage.

The only problem with the above statement is that the voltage on an electronic softstarter varies from whatever ratio is set for starting up to full voltage at full speed.

The only thing I can deduce from this is that the accel time would be longer that across-the-line and shorter than that time divided by the square of the voltage reduction.

Now, wasn't that a tortuous path to a nearly worthless conclusion!!! I think I might need a second cup of coffee. Sorry!!

 

[JesperMP]

Its the difference between the driving torque provided by the motor and the load torque provided by the fan that makes the torque available for accelleration of the masses of both motor and fan.
If you put these as curves (torque as a function of the speed), then you can see an area between the two curves.

Because it is the difference between the two curves, and not the difference between zero and motor torque, that there is a dependancy on how "high" the load curve is in comparison with the motor curve.

You will see that the area between the two curves gets smaller - much more than just proportionally - if the fan and load is matched very closely and you then begin to decrease the motor torque.

In short I think that slivers fan will have its startup time increased from 1 to 3 minutes or even more if the starting current (and thus torque) is halved
Maybe thats not a problem if the fan can take our sweet old time anyhow. It is probably not a problem.

edit: I rephrased some of the above. It is one of these cases where a picture would say more than a thousand words. But I dont have such a picture at hand right now.

 

[JesperMP]

I couldn't help it, I just had to make a picture:
[attachment]
Notice how the green area (= available torque for accelleration) gets much smaller when reducing the torque.
An Y-D starter would have reduced it even further (33% in stead of 50%) leaving practically no torque for the startup.

 

[elevmike]

More of a question than a suggestion.

So if you were to have a butterfly damper on the exhaust side of the fan chamber, wouldnt that significantly reduce starting load, and eleminate resonance during starting?

On a much smaller scale, I once built a bullit trap that used a high volume fan to create a draft into the trap and catch the lead dust/vapors in a heppa filter. We at first had a problem with the fan not comming up to speed and eventually tripping the OverLoad. the installiation of a motorized damper that was timed a few seconds after motor start solved the problem.

 

[JesperMP]

So if you were to have a butterfly damper on the exhaust side of the fan chamber, wouldnt that significantly reduce starting load, ..

Yes it would. I mostly see dampers on the suction side of the fan, but the effect will be the same.

... and eleminate resonance during starting?

Indirectly yes. Because the fan would move so much faster to speed from standstill.

But notice that sliver said he had 60 sec. startup time and occasional tripping even with closed damper .

 

[Sliver]

There's closed and then there's CLOSED.
Sometimes we find mechanical problems with the dampers or actuators but generally we can verify that the dampers are closing properly.
Start-up time would not be a significant factor as the equipment starts as part of a staged or timed sequential system start that takes about 15 to 20 minutes total anyway.
I have come in on call to help when the motor has been started and tripped so many times that the overloads never get cool enough for a decent start. As soon as they can be reset they try again instead of being patient and letting them really cool down. (that's the old thermal o/l obviously) I must admit that I would sometimes pull out a can of air, invert it and freeze the o/l coils then get a start. Don't recommend it and I'll deny it if you guys tell on me.
You could fry a steak on the poor motor....

Brian

 

[elevmike]

I'm gonna suggest that if you have tripping problens with melting alloy thermol overload, you might want to replace them. Eventually the alloy gets to the point that it seperates, and trips a lower then the rated temp/current levels. In many cases 20+ trips and your done..

 

[shooter]

as this problem has to do with the weight of the **** it has to pull together with the mass of the motor it will never be answered by a motor company.
I always use my experience in timing and as compressors do have mass but not as much as fans i can use 10 seconds.Theory says the contactors can have twice the normal load for short time when starting in delta this will happen with the netcontactor. after switching over same will happen with deltacontactor.

 

[JesperMP]

We have several 200hp dust collector fans that start across-the-line.

Sliver,
the only way to know if it will work is to try.
As you have several (maybe a lot ?), any supplier should gladly lend you a softstarter and even help with an onsite test.
If everything goes well, you can say "Right, let me have 10 of those".
If it doesnt go well, then you didnt loose anything.

Just be ready to hit the stop button in case of funny noises during the extra long starting sequence.

 

[DonsDaMan]

How about this, instead, Jesper?

You need the bypass for when you're across the line after using the softstarter, so why not just do all of them with the same starter (I'd still use a VFD, but you're going to need to do a line synch if you do use a bypass on a VFD, where the thyristor softstarters are inherently in sync), using a panel of contactors for its output to the different systems.

Seems the way to go, to me... Start motor one... bypass it using second contactor, opening the softstarter's motor 1 output contactor... Start two with softstarter's motor 2 output contactor... bypass... etc., until they're all started.