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daddydog (Electrical)
3 Apr 08 13:32
Hello does anyone have or know where I might find some studies of the advantages to running an 100-200Hp AC motor at low speed versus making frequent starting and stopping? And what energy savings there might or might not be associated with it?
Helpful Member!  jraef (Electrical)
3 Apr 08 13:55
That would be completely useless without specific information about the application. In some circumstances running a motor slowly will be worse, in others better. Energy savings can only be determined in comparison to something else. Please describe as best you can what you are planning to do and why.
daddydog (Electrical)
3 Apr 08 17:50
Maybe it is completely useless that is what I need to find out. I have heard that it will take more energy to start a large motor over and over again then to just keep it running at a low rpm because of the inrush current it takes to get the motor going. Now I may have gotten some bad information i don't know. Here is a senario.
I have a suction fan that pulls material at full speed 60hz it may be used consecitivly for a few hours and then not be used for 5-10 min during which time I want to reduce energy usage. Also when the fan is needed to be used again at full speed I need a quick ramp-up so there is not a long wait time. So would it be more efficent to reduce to speed or to stop the fan? And what if it goes thru this cycle 30 to 40 times an hour? lets say it a 200hp 460vac fan motor.  
Helpful Member!(2)  itsmoked (Electrical)
3 Apr 08 18:22
No can do.  You can't cycle a multi hundred horsepower motor more than 4 times an hour typically or it's toasty critters!

The fast ramp up can heat a motor too.  Less than a start but still it can have thermal consequences.  Perhaps you can block the outlet so the work(mass flow) drops greatly and save that way.

Otherwise is it possible to alter the process so it gets done in batches?  Then you could shut down and start up maybe once or twice an hour?

Keith Cress
kcress - http://www.flaminsystems.com

Helpful Member!  edison123 (Electrical)
4 Apr 08 13:29
keith

OP is talking about running the motor at low speeds.
How does he plan to do that ? A VFD. I don't see any other method.

If he is using (or plans to use) a VFD, I don't see any problem with frequent starts.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

daddydog (Electrical)
4 Apr 08 13:51
Edison123

Most likly a VFD maybe a soft start so this would eliminate the in rush current and reduce temp. Also it is possiable to block the mass flow but not normally done!
edison123 (Electrical)
4 Apr 08 13:56
daddydog

A soft start will not change the motor speed, which depends on the supply frequency. VFD is the only energy efficient way to change to the speed electrically by changing the supply frequency.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

Helpful Member!  Skogsgurra (Electrical)
4 Apr 08 14:06
daddydog,

It is perfectly legitimate to start an induction motor many, many times an hour with a VFD. The VFD does not cause you any extra losses during start/stop. That is because you never go beyond nominal torque and slip if you keep ramp time reasonable - I think that a fan may need something like ten seconds. It depends, a high speed fan needs a longer ramp.

Even the cooling at low speeds may be acceptable since a fan needs very little torque down there. If you notice that the motor needs more cooling, just take the idling speed up somewhat until cool again.

I would say that this is a good application for a VFD.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

jraef (Electrical)
4 Apr 08 15:10
Let's break down your issues:

Quote:

I have heard that it will take more energy to start a large motor over and over again then to just keep it running at a low rpm because of the inrush current it takes to get the motor going.
Bad information. There is no better energy saver than the Off button. High starting current is not a "waste" of energy if it is a result of needing to turn something on. Running a motor that is doing no useful work IS a waste of energy, regardless of speed. But it also depends on the process involved.

Quote:

I have a suction fan that pulls material at full speed 60hz it may be used consecitivly for a few hours and then not be used for 5-10 min during which time I want to reduce energy usage.   
OK, then why would running at a slower speed help? If you can continuously run at slower variable speeds to match your process work flow rather than at full speed in batches, then you can probably save energy. If the batch process is integral to what you do, then running at slow speed in between batches is a waste. The small amount of extra energy expended in re-starting will not be more than leaving it running at a low speed. In fact, if the low speed operation is only 10 minutes out of 120, then the VFD it will take to run at slow speed for those 10 minutes will end up wasting more than anything you could save when it is running at full speed. A VFD running at full speed will waste about 3% energy.

Quote:

Also when the fan is needed to be used again at full speed I need a quick ramp-up so there is not a long wait time. So would it be more efficent to reduce to speed or to stop the fan?
You would need to define "quick" as it is a relative term. Quick to you might mean 10 seconds, it might mean 1 second or 1 minute to someone else. But as to efficiency, see above.

Quote:

And what if it goes thru this cycle 30 to 40 times an hour?
This is a much bigger "what if" than you may realize. As has been said, starting a 200HP motor every 90 seconds is something you CANNOT do with anything other than a VFD, but even then, the acceleration time would be a big factor because if you must pull more than full load amps to accelerate it fast enough to get useful work from it, then you are still likely to overheat the motor. Either you are incorrect as to that duty cycle or there is something else you are not telling us, such as the fact that the work performed could be done by a 50HP motor but because of this duty cycle, they have put in a 200HP so that it can survive. If THAT is the case, then you can almost assuredly save energy using a VFD because the motor is oversized to begin with.
Skogsgurra (Electrical)
5 Apr 08 5:43
There seems to be two scenarios here. One where the fan runs at full speed for "a few hours" and then isn't needed for 5-10 minutes.

Then, there's a scenario where the fan is needed to do its work 30 - 40 times/hour and idle in-between.

The first situation would probably not  be worth trying to reduce the energy. if "a few" means anything between 3 and 5 hours, then the 5-10 minutes energy savings are not worth the investment.

The second situation is what I referred to in my previous post. If there are 30 start-work-stop cycles per hour and the "work" part is, like, 10 or 20 seconds. Then starting the motor using a VFD will save a lot of energy.

Letting the fan run continuously will consume around 150 kWh per hour while stopping in-between will make the application consume anything from - in rough numbers - 25 kWh to just below 150 kWh, depending on duty cycle. 25 kWh being 10 s ramp plus 10 s work.

So, it is a very good idea to use a VFD to let the fan work "on demand". And it does not hurt the motor. There are no extra losses when you start an induction motor with a VFD and using a ramp.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

waross (Electrical)
6 Apr 08 19:39
Another option may be to use an air clutch or magnetic clutch to drive the fan and let the motor run continously.

Bill
--------------------
"Why not the best?"
Jimmy Carter

daddydog (Electrical)
7 Apr 08 19:02
thank you all for your input this has given me some valuable information.

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