Motor / Pump Question

[leeds25]

I got a Motor Starter board for Motor. based on the liquid level switches (High/Low) in a tank it starts/stops the Motor.

As per design , Motor will be started/Stopped 24 times a day.

~Could this lead to any problem to Motor in long run.

( As the high level switch in switched , Motor starts , as the liquid falls to low level switch , motor stops)

 

[tygerdawg]

Probably not.

Every time a motor starts there is essentially a short circuit situation until the magnetic coils build up a mag field. So there are heat issues to think about. Probably not in your case.

You need to check with your motor manufacturer for "maxium starts per hour" to be sure. 24 / day is typically nothing to worry about. What most likely will fail eventually is the starter, not the motor. But only after a very long time.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[electricpete]

What is the voltage, speed, and horsepower?

NEMA limits for repetitive starting are given here:

http://www.joliettech.com/allowable_starts-intervals.htm

Also load inertia is needed as a check, but if single stage centrifugal pump it is usually not large enough to affect the results in this chart.



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[vpl]

It may depend on when the motor starts and stops. One sentence says 24 times a day, but your last sentence indicates that it is level dependent -- which makes me think of a sump pump.

My experience with sump pumps, admittedly more from home than work, is that they turn on and off multiple times a day (been about every 15 minutes the last two days) and will last for a very long time (20 + years). But while there are periods where the motor starts and stops frequently, there are also long periods where it doesn't operate at all. In other words the average starts and stops might be 24 times a day, while on any given day it might range from 0 to 100.

Larger pumps & motors may have limitations on how many times they can be started and stopped in short succession due to heat build up (as mentioned by tygerdog) and may then require a longer period before the next start. (Memory says 5 - 6 minutes for the first three starts, then a half-hour before the 4th for a 600 gpm pump -- I don't remember the required dP.) Twenty-four starts a day would probably not be a problem -- if they were each an hour apart. However, if they aren't evenly spaced out, you could run into difficulties.

Patricia Lougheed

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[electricpete]

In the above link there are two different types of limits:
1 - limit on the starts per hour
2 - limit for minimum off-time between starts
The limits depend upon the size of the motor, which is one of the paramters used to enter the table.

It is not a perfect table. For one thing the time between starts should be longer if you start and immediately stop than if you start, run for awhile, then stop. Because heat removal capability (cooling air) decreases when stopped. It's easier to remove the initial slug of heat from starting if the motor is running for 5-10 minutes after start. You reach a lower final end temperature with motor secured than running, but the initial cooling rate is much lower with motor secured than running.

Here is a FAQ from the motor forum:

http://www.eng-tips.com/faqs.cfm?fid=1285

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