Soft start motor starts in 1 hour

[quist]

We have a 75hp ieee841 siemens 480v motor driving a hydraulic pump. We need to start the motor and run it for 3 minutes, shut it off for 1 minute and then start it again for 3 minutes and shut it off for 1 minute and continue this cycle for 90 minutes. What issues are we going to have?

 

[davidbeach]

Motor probably isn't good for that many starts per hour. Why not just bleed the unneeded hydraulic pressure back into the reservoir and leave the motor running continuously?

 

[quist]

energy savings over the years of having to do this along with the more critical issue of noise. we have to operate in a noise sensative area and cannot get enough sound suppression so they want us to shut it off when not in use.

 

[davidbeach]

Fine, what ever. But all the motors you won't have to replace will more than pay for the slightly extra amount of energy (you won't save 25% or any where near that much). The embedded energy in the production of the motor will never be saved if you are changing out motors several times per year.

I can't believe that lack of noise for 1 minute out of 4 will be noticeably better than the same noise continuously. In fact, of those two choices I'd prefer the continuous noise as one can get used to it, but not if it stops and starts.

 

[ScottyUK]

When you start the motor you put a large amount of energy into the rotor as slip losses while the motor accelerates. You should include this when evaluating the apparent savings you will make. Your control gear will be a maintenance nightmare, figure on either putting in a grossly oversized contactor or replacing it fairly frequently. Your proposal is frankly a terrible one from the electrical point of view.

A motor running a pump dumping back to the reservoir won't consume that much energy: are you measuring true power, or just current? Current reading does not tell the whole story.


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If we learn from our mistakes I'm getting a great education!

 

[dpc]

The soft starter is not going to make much difference in terms of # of starts and motor heating. A VFD would make a big difference. But I agree with previous comments - you need to re-think the design and control philosophy.

So to answer your question - the problem you can expect is replacing the motor every few weeks or months at best.

 

[itsmoked]

The last 50HP severe service 50HP TEFC Baldor I controlled categorically stated in its literature that no more than 4 starts an hour were allowed. This also happened to be the same starts per hour allowed for the soft starter.

You are cruising for a bruising with that duty cycle.

You should consider doing the job a better way.

You want to conserve energy? Use a smaller motor and a hydraulic accumulator. That way the motor runs continually to provide the intermittent hydraulic needs. An accumulator would also allow higher peak loads if you need them.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

What is the speed and what is the inertia of the driven load?

There is a table giving recommended max starting duty based on NEMA MG-10 (not MG1)
Page 19 or 30 here:

http://www.joliet-equipment.com/pdf/easahandbook.pdf

Let's say it's a 4-pole 75hp motor driving a low inertia load, then according to the document you should be able to start it 5.8 times per hour (24/7) as long as there is a minimum off time between starts of 90 sec. I gather the purpose of the "minimum off-time between starts" is to avoid rapidfire starting like 4 times in 4 minutes, then 1 time during remainder of the hour (still within the 5.8 starts per hour but much more likely to melt an aluminum rotor). It leaves the impression that the motor cools faster when secured, but actually the heat from starting goes away much faster while running.... while secured the motor cools to a lower endpoint temperature obviously but initial cooling rate is much slower.

Soft start should be same as DOL start IF the motor is unloaded. I suspect it is not to much different for typical poump/fan load. If there is a very high torque load on the motor during start, then soft start is much WORSE on the motor than DOL start. The extreme case is obvious when motor torque falls below looad torque and fails to accelerate. But even when it gets close there ends up being much more total heat created during the reduced voltage start than the full voltage start (if motor torque drops down not far from load torque). You can mathematically derive the relationships using the equivalent circuit.

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[electricpete]

I guess there can be other heating effects due to harmonic content for solid state soft start - those may affect comparison to DOL as well. I don't know how significant that factor is.

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[waross]

I have worked on a few hydraulic power packs. All of them dumped excess flow back to the reservoir. Many had the accumulators that itsmoked suggested. As has been stated, expect your 75 HP motor to fail frequently. The soft starter will not reduce heating compared to DOL during motor starting and may increase heating if not properly tuned.

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

 

[ozmosis]

I would say, expect your soft start to fail too with that type of duty.

 

[jraef]

My experience should give you an idea of what to expect.

Hydraulic Elevator Controls are very similar to what you are doing here; not in practicality, but in regulation compliance. In order to get a soft starter CSA* listed for "Elevator Duty", the tests involve a duty cycle almost identical to what you are describing. Nobody really expected an elevator to cycle that fast or that often, but CSA insists on that type of over kill because it will be directly moving human beings. In order to pass, we had to over size the soft starter components at 300% of what we would normally use. I believe that the AC motors used are also similarly over built / de-rated for the same reasons.

*CSA is used throughout the elevator industry because their test standards are far more stringent than UL, so if you are going to pay for testing, you do it only once.


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