Frequent Starting of Low Inertia Loads 2

[gordonl]

I have some 500 & 1000HP water pumps which I am considering changing over to an automated start stop system. These motors pump cooling water which is only required between about 1 minute every 7 minutes, to 1 minute every 18 minutes depending on the process.

The motors start in about a tenth of the allowable locked rotor time. The motors are NEMA design, 5800 series westinghouse frames.

My thought to date is to allow a max 6 starts an hour with the motor running for the required 1 minute if the process is at a slow rate, and the motor running 8 miunutes on 6 minutes off, if the process is paced at 7 minutes. Operation should be considered as 24/7.

I'm already looking at the life of the starters, but I'm not sure what sort of life expactancy I can hope for from the motors starting them this frequently.

 

[electricpete]

Clearly if you put aside thermal concerns than a reduced voltage start is better on the motor than a full voltage start. The forces as I'm sure you know go roughly by current^2 = voltage^2. I am still skeptical of tossing aside thermal effects, but you manufacturer knows best.

 

[gordonl]

Certainly I will consider the thermal effects, but that is much easier to quantify than the benefit of reducted mechanical stress from reduced starting torque.

Can I expect a substantial increase in motor life from the use of the soft starter, or should I be able to expect a long life from the motor with frequent starting that is within the thermal limits of the rotor anyway?

 

[electricpete]

For large fabricated-cage motors subjected to repeated starts, a common suggested mechanism of failure is fatigue failure of the bar-to-ring braze joint due to differential thermal expansion. Yes, a single start is within the thermal capability. But repeated starts will take the toll (fatigue). If we neglect items #1 and #2 as negligible, then each start has the same impact on motor life whether reduced voltage or not.

It is perhaps a little simplistic to assume as I have above that there is only one failure mode/mechanism for all starting-related motor failures. But this is the one that receives a great deal of attention and I believe that economic considerations allow stators to be well-braced and designed to withstand mechanical and thermal starting stresses, but toughening the rotor is a lot more expensive and this typically ends up as the weakest link.

 

[gordonl]

Electricpete,

Thank You for your patience, I would like to make one more kick at the cat and see if your in agreement or if I'm misunderstanding. (The opinion of all others is appreciated as well of course)

Is it reasonable to say that under quick starting times, less then 10 seconds with the softstarter, the softstarter will not provide extended life to the motor, and actually the best I could hope for is the same life as multiple DOL starts?

Is anyone aware of reference material that discusses this issue?

 

[electricpete]

Hi gordon - your 2nd paragraph aptures what I was trying to say.... with the qualification as we discussed there are some ways in which soft start provides some benefit but I don't think those are big effects (spreading out the heat is not a big factor for <10-sec-start motors and decreased magnetic forces are not important because they do not relate to the main failure mode).

I base my understanding mostly on what I have read in EPRI documents, and those provide a lot of discussion without coming out and statating a specific conclusion on this issue. So I am certainly not positive of my statement.

I know the vendors of soft starters do advertise increased motor life in terms of number of starts. Maybe they can provide some more justification.

I'm interested to hear others' comments as well.

 

[Marke]

Hello electricpete et al

What about the reduction in the transient loading when using a soft starter? Peovided that it is working correctly, there will be no transients or steps in current or torque. Does that have any value?
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[GusD]

Hello Gordonl

The allowable # of starts is also dependent on the # of &quot;Hot&quot;starts /versus Cold starts.The manufacturer says that the motor is good for 10 starts,but I don't think those 10 starts could be performed on a drive that had tripped 10 times due to O/Loads.If this motor is protected with a MMR,this protector relay will measure stator TEMP and determines how much Thermal Capacity is available for a safe start.If motor is not protected in this manner,than;
a more cautious approach on the # of starts may prolong the life of your motor.

GusD