Soft start motor starts in 1 hour
Soft start motor starts in 1 hour
(OP)
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?





RE: Soft start motor starts in 1 hour
RE: Soft start motor starts in 1 hour
RE: Soft start motor starts in 1 hour
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.
RE: Soft start motor starts in 1 hour
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!
RE: Soft start motor starts in 1 hour
So to answer your question - the problem you can expect is replacing the motor every few weeks or months at best.
RE: Soft start motor starts in 1 hour
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
RE: Soft start motor starts in 1 hour
There is a table giving recommended max starting duty based on NEMA MG-10 (not MG1)
Page 19 or 30 here:ht
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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RE: Soft start motor starts in 1 hour
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RE: Soft start motor starts in 1 hour
Bill
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Jimmy Carter
RE: Soft start motor starts in 1 hour
RE: Soft start motor starts in 1 hour
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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