Electronic Thermal Overload

[gogo]

Greetings all,

I need some help from the forum experts, I'm not an engineer just a field tech, so begins my problem; I had 2 60hp Franklin submersibles fail due to supply valves being shut, basically ran dry! Now I need to investigate so this can be prevented from happening again. These 2 motors are being controlled by 2 Hitachi Drives What I could use is some insight to setting the overload parameters in the drive, because IMO the drives should have tripped before the motor failed, am I correct in this assumption? Here's a link to the parameter set-up

http://www.hitachi.com/supportingdocs/indu...als/NB604XA.pdf

 

Parameters in question start on 3-29 "Electronic Thermal Overload Alarm Setting". I would be curiuos as to how some of you would set these up. Not that I don't stand behind our engineer 100%

;c;

 

THNK YOU.

[marke]

Hello gogo

 

Welcome to the forums.

I had 2 60hp Franklin submersibles fail due to supply valves being shut, basically ran dry!

Submersible pumps rely on the fluid being pumped to keep them cool also. It is therefore very important that there be a) a continuous flow of fluid through the pump / motor assembly and that the flow rate is sufficient to provide the cooling required.

 

With fixed speed operation, it is recommended that you apply under current protection as well as over current protection. If the pump runs dry, or the flow rate drops too low, the current will fall and this can be detected and used to shut the pump down. If you supplier can not supply you with the low current set point, then run the pump, throttle the flow back to the lowest acceptable and measure the current. This is the point to set your under current protection at. You should allow some safety margin as well!

 

If you are using a variable speed controller, you should determine a minimum setting in consultation with the pump supplier. Additionally, you may be able to apply either under current protection or minimum flow protection using a flow transducer.

Best regards,

[gogo]

If you are using a variable speed controller, you should determine a minimum setting in consultation with the pump supplier.  

 

Thank you Mark, Which minimum setting are you refering to? Frequency, current setting ?? These pumps respond to PID settings and with the minimum run times there are times when they are spinning at minimum frequency under a no flow condition at this point drawing very little current. Would you still suggest undercurrent fault.

[marke]

You need to be sure that the power dissipated in the motor is less than can be cooled by the coolant.

With submersible pumps, the iron loss and copper loss at no load are considerably higher than a standard motor and so you need active cooling even under no load conditions. Depending on how you system is being controlled, there are a number of things that you can do. If you are using a PID controller based on pressure, then rather than keep the drive running at a minimum frequency of say 30 Hz, you may be better to let it run right back to zero. At this point, there should be no current flowing at all. If the drive continues to deliver current, then I would suggest that you need to monitor flow as well as pressure.

I think that you really need to find out the recommendation of the pump manufacturer on this one as they do have varying ideas and application rules. If you don't comply with their ideas, you will get no support.

Best regards,

[marke]

Just to clarify, I see two scenarios where you could have problems. One is where the pump runs dry due to lack of input fluids, (possibly a closed valve, and the other is where the pump is cavitating du to closed valve on output or excess head.

 

In the first situation, the chances are that the VVVF would run the pump at maximum speed and undercurrent protection would normally protect it.

 

The second situation is more difficult and a flow transducer may be needed.

Best regards,

[theDOG]

It could also be because of the temperature of the fluid. All submersible pumps have a defined temperature limit of the fluid that is being pumped, because the fluid is required as the coolant (as Marke suggests).

The only true way to protect a submersible pump (IMHO), is to have RTD's installed in the windings, in conjuction with a thermal O/L. The RTD's will provide protection against high ambient temperatures, as well as lack or loss of cooling.

[TheOne]

Seems like you have several flow situations. A different approach may be more attractive. You may want to consider variable position sensors or switches on the valves. If the valves are not at a minimum setting or greater, then have an interlock set off an alarm or stop the pumps.;f;