Cooling fan overload 2

[gra09]

Currently commissioning a compressor skid, however having trouble with the motor running the cooling fan.

Basically the motor is rated for 15kw / 33.7amps OPERATING BETWEEN 380 - 415v however during the test the amp reading is 36amps. Have reduced the angle of the blades to reduce the effort and its now down to 14.3kw but still pulling 36amps. I was told by the lead electrical test to change the ABB overload to a higher rating, but I feel this only masks the problem as the problem seems to lie within the motor set up.

Any ideas or experience of similar, I am wandering if it could be a waiting issue.

Many Thanks,

B

 

[waross]

What are the phase to phase voltages?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[gra09]

Hi Bill, I don't have a direct answer to your question at the moment but the supply to the motor is 415v

*RE above, last line. It is meant to say wiring issue, not waiting issue

 

[ScottyUK]

The current required to deliver 15kW at the shaft will vary through the operating range. Normally it is stated at the upper and lower limits.

 

[waross]

If the measured line to line voltage as opposed to the nominal line to line voltage is unbalanced, the current will be much more unbalanced.
eg: It is possible for 1 4% line to line voltage unbalance to cause an additional 15% current on the worst phase. This may cause overheating in the rotor as well as in the stator.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Your instinct about not just getting a different overload relay is well founded, your colleague is a nincompoop. Changing to a higher trip setting will simply allow your motor to cook itself from the inside and fail prematurely. The old rule of thumb on that is that for every 10 degrees C you allow the motor temperature to increase above it's rated temperature, you cut the motor life in half. Allowing the motor to run at above the FLA setting is the quickest way to get it to run hotter.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[BJC]

Sounds like the motor is working to hard. Does the machine have the correct fan? Does the fan have adjustable inlet or outlet campers? IS it direct or a belt drive? If so are the drive pulleys correctly sized?

 

[jraef]

I think he means "dampers", not "campers". Although, little critters camping on the fan blades might be increasing the load...

On a centrifugal fan, load = flow. Over load then generally means one of two things: mechanical issues such as bearing friction, or more air flow than the fan motor was designed for. You were attempting to deal with that by adjusting the blade pitch, but maybe it wasn't enough, because someone went way overboard on the system design. One thing I see happen too often is that someone wants more flow, sees an inlet guide vane or outlet damper, and simply removes it or adjusts it beyond the fan's design capacity. There ain't no free lunch as they say, so the dumb animal that is the fan and motor is trying its hardest to satisfy the new system flow requirements, but is overloading the motor in the attempt.



"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[gra09]

Checked the loads across the fans and everything was balanced, so came to the conclusion it wasnt a fault with the motor or supply per'say.......lets look at the design

In line with what jraef has said the pitch of the blades had been designed to operate in low humidity / High temperature situations (26deg c to 46deg c stick it in the desert......) so changing the blades to suit a new set of calculations based on North Sea ambient conditions (-4 to +17) led us to reduce the blade pitch from 29 degrees to 23 degrees, reducing the load from 36amps to 29.8amps, below the 33.7amp the motor is designed for and still provides sufficient cfm of air to cool the main motor.

Thanks all, appreciate your comments

 

[itsmoked]

Yay! Great demonstration of air density actually mattering, "heavy air" in sailing parley.

Thanks for coming back with the solution, as we love that around here.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

North Sea ambient conditions (-4 to +17)

Must be summer North Sea conditions.

 

[proelect8717]

Dear Gra 09,

We had previously faced same problem some time ago. I would suggest that you read the name plate of cooling fan motor very carefully and ensure that whatever connections they say (Star or detla), you are connecting them in same way.

Also, I would suggest that to rule out if fault is inside motor, Remove the cooling fan from motor shaft (makes motor carry no load) and perform no load test by allowing motor to run on no - load. The no - load current should be between 30 - 40 % of the rated current for an induction motor. Then with the help of Clamp on Meter, measure current on all three phase leads. Are all three currents balanced? Also measure voltage on all three phase leads, they should also be equal. Also, check voltages at the motor terminal end when motor is running, because if cable if of not sufficient cross sectional area, then voltages shall be reduced at motor end and it will draw more current. Also, if one phase misses out of three phases, then current also increase (but it is not what seems in your case).

I totally agree with Jraef, don't increase the settings of Overload relay or you will burn the wnding.

My advice is to share the name plate data of motor, and remove all the load from motor shaft (remove fan), when you have only motor with you, switch it on, allowing motor to run without load, hear the sound of motor, check vibrations & temperature, monitor current on all three phases and voltages of all three phases both at MCC end and motor terminal end.

Share these data and we will soon resolve your problem. THanks,

 

[itsmoked]

Umm proelect8717; The Original Poster (OP) describes the found solution to his original problem about 3 posts up..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[flexoprinting]

Proelect


" if cable if of not sufficient cross sectional area, then voltages shall be reduced at motor end and it will draw more current. "

Huh? The voltage will be there but motor will be starved for enough current and things start to get interesting.

Chuck

 

[ScottyUK]

No, he's right. To a first approximation a motor is constant power device, so a slight reduction in voltage results in a corresponding slight increase in current. If the voltage drop is severe then the way the load changes with speed complicates matters due to the increase in slip at reduced voltage and it gets more complex to analyse.

 

[flexoprinting]

Scotty, So torque is proportional to Sq(v) and when torque decreases, slip is increased and motor draws more current?

Chuck

 

[Strong]

"when torque decreases, slip is increased and motor draws more current?"

When torque decreases, slip is decreased and motor draws less current!

Walt

 

[BrianPetersen]

No, you are thinking about it wrong. When the torque is held constant and the voltage decreases, the slip increases and the motor draws more current.

 

[jraef]

This is kind of putting the cart before the horse here. The LOAD is what demands the torque from the motor. You can't "make" the motor demand less torque from the load. But if you decrease the VOLTAGE to a motor at a fixed frequency, the motor becomes less CAPABLE of producing torque. If however the LOAD is at the same time not DEMANDING more torque than the motor is capable of delivering, you may never know this is taking place.

Will the slip increase in that case? Maybe, maybe not. If you decrease the voltage, and thus the torque CAPABILITY of the motor enough, the mass of its own rotor will at some point be enough of a load to make the slip increase. But if you simply place an unloaded motor on a bench and begin to give it less and less voltage than it was designed for, it will appear to be operating normally for quite a while before you will see the slip decrease significantly.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington