Rule of Thumb
Rule of Thumb
(OP)
Guys
Is there a rule of thumb? (and if not let's create one)
Starting a motor on a pump load takes as much energy as running for x mins
any advance on 5 mins?
Is there a rule of thumb? (and if not let's create one)
Starting a motor on a pump load takes as much energy as running for x mins
any advance on 5 mins?





RE: Rule of Thumb
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Rule of Thumb
RE: Rule of Thumb
Aside from the starts-per-hour issue, the ENERGY consumed when starting is infinitely variable as well. It all depends on the load, not the motor. If you have a 5HP motor on a deep well pump, it will take a LOT more energy to accelerate to full flow than a 5HP motor on an end suction tank transfer pump.
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Rule of Thumb
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RE: Rule of Thumb
Starting a loaded motor requires an amount of energy equal to Eunloaded * x where x is the weighted average of Te/(Te-Tm) from 0 to full speed with the weighting factor being s (low-speed torque differential causes more heating than high speed torque differential.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Rule of Thumb
That didn't come out right. Low margin between Te and Tm causes more heating at low speed than at high speed.
BTW Te is motor torque and Tm is load torque from their respective torque-speed curves.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Rule of Thumb
electricpete;
In your first post, should the kinetic energy of the rotor be added to the kinetic energy of the load?
Also, let's make an allowance for all work done during the acceleration period.
Then we have the phenomena of the stator being more effective at rejecting excess heat than the rotor.
For a rule of thumb, I like the manufacturers who recommend maximum seconds of starting duty per hour.
Btw pete. Look for a connection to ground instead of to neutral for your 5 lights. On the other light, does the dimming correlate with the acceleration of the exhaust fan?
respectfully
RE: Rule of Thumb
RE: Rule of Thumb
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Rule of Thumb
If you are concerned about power demand charges created by motor starting, you can almost always not worry about it. The kw demand from the running motors will be what determines your utility demand charge. Demand is averaged over 15 to 60 minutes, so a few seconds of increased demand for starting a motor generally has little impact.
I've had a lot of clients who expected their power demand charges to decrease if they installed VFDs and/or RVSS starters on their frequently started motors. They are always disappointed.
RE: Rule of Thumb
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Rule of Thumb
RE: Rule of Thumb
That is starting the motor every 50secs for 10 minutes probably won't get you the same result as running it for ten minutes.
But I am probably picking at your comparison a little too hard sreid..
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Rule of Thumb
You're neglecting the fact that for ac circuits, only the portion of current in phase with the voltage contributes to watt consumption. Power is proportional to Isquared only for the same power factor. The motor starting current is at a very low power factor, so only a fraction of that current is developing power. For a power factor of 0.2, only 20% of the current is providing metered power. The running power factor is much higher - more like .80 or better.
RE: Rule of Thumb
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