pump motor

[BobPE]

I dont know if this was discussed here, I am new to this forum..I am good with pumps but differ to the experts for the motor end...

I have a pump application where a pump was oversized and was replaced. The original motor was a 300 HP syncronus 4160V hollow shaft ODP. It is across the line, no VFD. My question is, the new pump requires ony 200 HP non overloading over the pump curve. I want to reuse the existing motor since its only a few years old.

I know there will be problems with heat and such, but what other problems can I expect?

thanks...

BobPE

 

[AFab]

It Sounds like you have a good situation. If you have any power factor problems you can use the unused HP for power factor correction.

 

[dpc]

As long as the existing hollow-shaft motor will fit up with the new pump, I don't think you will have any problems. Your 300 hp motor will be very happy running a 200 hp load.

The only issue might be the torque imposed on the pump during starting. You may want to verify maximum permissible torque with pump supplier then take a look at the motor speed-torque curve.

 

[electricpete]

I agree there shouldn't be a problem under-loading a motor.

Bob - can you teach us something. I have heard the terms "overloading" and "non-overloading" with reference to pump curves and blade shape. Can you define those terms?
Thx.

 

[jbartos]

Suggestion: The underloaded motor may need a sensitive protection to protect the 200HP pump from mechanical malfunctions, when there is 100HP left to turn the 200HP pump into mechanical catastrophe, unless there is a mechanical protection within the pump itself, e.g. pins.
The motor will be running less efficiently. Motor 100HP leftover may be used to run another load over a pulley and belts or over gears.

 

[CB2]

You described the motor as being a "hollow shaft" type which I suspect means you have a vertical turbine pump and the motor is handling the pump down thrust. It would be prudent to check with the motor manufacturer about lightly loading the thrust bearing.

Also agree with the comments about potential pump overload problems, suggest you invest in a very good overload relay to protect the pump. (Toshiba 2E relay).

CB2

 

[dadfap]

BobPE

The only issue that would concern me is that the motor may be running near the idle amps for the motor and if there is any underload protection on the controller, this may be useless.

All the best

dadfap

 

[d23]

BOBPE:

Both jbartos and dadfap make good points about overload and underload controls on this type application. If this is a stand alone motor (unit) and you can cut the voltage down to about 3600 VAC then the nameplate amps and motor load will be about correct for the 200 HP load. I’m not good at explaining this, but from personal experience if you cut the volts by 5 percent you will increase the amps by 10 percent. The most efficient point for your motor will occur at 100 percent amps, so you can calculate the volts for yourself.

OK “EE” guy’s tear me up on that!!!

There are a lot of products available that can offer both overload and underload protection for this type operation. Dadfap’s company (Baker) offers VORTEX units and my company (SLB.com) offers Comeau type controllers that provide this type control as do many other companies.

I’m not the EE type, but as I understand with soft starter’s (inverter controllers only) you can program them so the amount of voltage allowed to the motor will vary allowing the syencrous motor to maintain a constant power factor on your system as AFAB elided to. This assumes that the power factor is more important that pump flow.

electricpete:

I can send you some centrifugal pump curves that will show you both overload and underload conditions. For both dadfap and me the underload type operation will cause a “down-thrust” type operation which will lead to a catastrophic failure in a very short time frame. We manufacture multi-stage pumps (competitors) that have a min flow before pump damage can occur. In your case you would normally use a split case horizontal that has opposing stages that will reduce the thrust loading or possible down-thrust problems. If you would like some example curves and a better explanation, let me know.

David

 

[Skogsgurra]

BobPE,

You have been running a pump that utilizes the motor one hundred percent. Most installations I know of run at much lower percentage than that - seen loads down to 15 percent, but 60 - 80 percent is normal.

So if you have a 200 hp pump driven from a 300 hp motor then you will have 67 pct loading, which is an ideal situation. Motor efficiency usually is best at 75 pct load, which is very close to your 67. No sweat.


jbartos must be kidding:

The motor will not be running less efficiently at 67 pct load (as stated above).

The idea that "100HP leftover may be used to run another load over a pulley and belts or over gears" can best be described as a Rube Goldberg joke. How does jbartos "suggest" that a belt and pulley be attached to a hollow shaft (probably down-the-hole). And for what good?

d23 also pulls a leg saying "cut the voltage down to about 3600 VAC then the nameplate amps and motor load will be about correct for the 200 HP load". What you get then is an increased slip because your torque is going down with voltage squared resulting in increased rotor losses and temperature. Not good at all.

 

[BobPE]

thanks guys....I too have limited knowledge about electricity...but I get by LOL...plus there is always an electrical engineer in the crowd somewhere LOL.....

electricpete:

Non overloading across the curve means that the motor will handle all loading conditions...nothing more...many engineers miss this and design motor HP for design operating point...not thinking that their design will potentiall travel through all points on the curve at some time....max hp varries depending on the pump you are using...

Let me ask you this, the motor has a soft start...what should I recommend to modify the switch gear for this reduced loading condition, keeping in mind the switchgear was not designed for this duty...but rather a higher load duty. Should I be concerned with the power factor or correcting it?

you all hinted on efficiency, there is a problem I perceive with this as the motor is running hotter than I would expect a correctly specified motor...I prewarned the owner of this and its affect of overall life of the motor, but is there a return on investment I am missing by changing the motor out now rather than waiting?


thanks

BobPE

 

[jbartos]

Suggestion to skogsgurra (Electrical) Jul 4, 2003 marked ///\\BobPE,
You have been running a pump that utilizes the motor one hundred percent. Most installations I know of run at much lower percentage than that - seen loads down to 15 percent, but 60 - 80 percent is normal.
///Normally, 80% up is designed.\\
So if you have a 200 hp pump driven from a 300 hp motor then you will have 67 pct loading, which is an ideal situation. Motor efficiency usually is best at 75 pct load, which is very close to your 67. No sweat.
///Yes, some motors are or may be designed such that their efficiency peak is at about 50%. Visit

http://info.lut.fi/ente/sahko/Hi-motors/Improve.htm

for efficiency plots.
However, normally the motor shall have the high efficiency at close to 100% HP rating with some plateau extending down to 80% or so.\\
jbartos must be kidding:

The motor will not be running less efficiently at 67 pct load (as stated above).
///See the referenced link for some motor efficiency plot.\\The idea that "100HP leftover may be used to run another load over a pulley and belts or over gears" can best be described as a Rube Goldberg joke.
///I have seen various ingenuities around. However, my point is that about 100HP is there spare for what ever reason it may be. Perhaps, the next pump may be rated then at close to 300HP.\\ How does jbartos "suggest" that a belt and pulley be attached to a hollow shaft (probably down-the-hole). And for what good?
///This is a part of mechanical engineering, if found required.\\

 

[buzzp]

Im afraid I have never seen a motor designed to operate at its highest efficiency at 67% of load. Im not saying they are not out there but it is more common to find motors designed to operate at its highest efficiency per the nameplate data.
Im afraid I have to disagree with skogs on the efficiency issue.

 

[electricpete]

buzz - it depends upon the motor:

Here is one example where 50% and 75% load efficiencies are higher than 100% efficiencies.

http://www.reliance.com/pdf/pdf/aced/E08396-A-A001.pdf

If we model losses as no-load losses plus load-square varying losses, then peak motor efficiency occurs at the load where these two types of losses are equal (proof is easy). If full-load load losses are greater than no-load losses, then peak efficiency occurs below 100%.

 

[buzzp]

There is an example of a motor I figured was out there (per my previous post). Thanks for the example electricpete. The efficiency at 100% vs 50% is negligible but nonetheless the efficiency is higher at 75% and 50% of full load vs 100% load. I would like to see the specs for a motor that has a higher percent difference between efficiencies for 50% of full load vs. 100% of full load if anyone can think of one.