25hp Electric motor heats up 3

[jonescharlie76]

I have a 25hp 415v delta 2pole motor driving a pump. It is connected to a 415v (as before) on same supply. When running off load it takes only 3.8 Amps per phase, balanced. On load it takes 25amps per phase,balanced, exactly as an adjacent similar pump and motor. Off load it can run for 1.5hrs with the temperature staying as ambient. On load it heats up in an hour, enough to trip a thermal overload in the windings of 120degs. The other motor runs for long time and does not heat up. When running off load the rpm are 3000 and on load it drops by 100 rpm to 2900rpm. I have never met a similar problem. Can any one help please? Thanks in anticipation.

 

[MikeHalloran]

Is the ventilation system working okay?
I.e. not occluded with debris or paint (if open frame)?
I.e. is the internal fan present, rotating, and properly sized (if TEFC)?
I.e. is the external fan present, rotating, and properly sized (if TEFC)?
Are the windings clean, or covered with ... well, crud?

Is the motor properly sized for the hydraulic horsepower that the pump is producing? (Flow and deltaP and viscosity)
Exactly how similar are the adjacent pump and motor that you are using as a standard?



Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

Remember too, the more the pump actually moves the more load the motor sees. If you have any kind of difference that could cause this pump to move more, it will be the more heavily loaded one, possibly even overloaded. For instance this one could have a straight line into it from the source and a straight line out of it while the sister pump is a couple of elbows removed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

I agree with Mike about checking the cooling.

Pump or fluid system differenceswould shore in current .

By the way....120 deg C ? Measured where/how?


=====================================
(2B)+(2B)' ?

 

[waross]

You said that the current was 25 Amps and balanced. You may have a faulty rotor. If the adjacent pump is identical, try switching rotors and see if the problem follows the rotor. Dropping to 2900 RPM under load would tell us more if we new what the rated speed was. 100 RPM slip could be normal or it could be an indication of a problem. Depends on rated RPM.

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

 

[zlatkodo]

When running off load it takes only 3.8 Amps per phase

This is to low.
Do you have any other information: rated current, frequency, efficiency, PF?

 

[jonescharlie76]

Thanks for your replies. There is no nameplate on this motor. I am strictly reliying on my clients words. I have to go and check on site. Well the winding and motor are totaly clean, good fan and good ventilation.The problem is not with the ventilation as there is a difference between the outside temperature and the inner temp. I have not seen the sister pump but according to my client they are totaly similar. The external heat reading was 85 degs c when the thermal sensors switched off. Thermal sensors rated 120 deg C and I replaced them with new ones.But had the same result. I find that the off load running current is very low for a 25hp motor.
The motor has been working in place for a decade or so according to the owner. I assume that if the motor was overloaded the current would rise more than the other sister pump;s current which is working perfectly well.
Regards and thanks once again

 

[jraef]

Are these run from VFDs by any chance?


"Will work for salami"

 

[Skogsgurra]

A few thoughts:


Is this a client that you know well? Or is it one of those that knows everything so well and tells you what the problem is and what it is not?

"Identical" is seldom identical.

No nameplate. Also a problem. And that low idling current, I had expected 30 - 40 % of FLA not 3.8 A. Sounds like the motor is running at too low a voltage. Then slip increases and motor heats without necessarily drawing excessive current from grid.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

It is definitely connected in delta, isn't it? A delta machine connected in star would cause the symptoms you are describing.

 

[jonescharlie76]

On Monday I will go on site myself and check things out. I beleive that if it is ass it seems the rotor is faulty, it may have an open circuit bar butI don't have means to check it.
Thanks.I will get to you with the results. Have a pleasant weekend.

 

[waross]

Good advice from Skoggs and Scotty. Check the connections as they suggest before checking the rotor.

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

 

[Skogsgurra]

I have been in this business for a longer time than the creationists think is possible. And I have only seen two rotor bar problems in all that time.
One was when a Schorch motor with a warning "DO NOT CONNECT TO A VFD!" on it had been run from a VFD and the other one was a suspect rotor bar problem, but never verified. Motor replaced and no one cared too take it apart. It was just scrapped.
In my world, rotor bar problems are an interesting theory - but very seldom the problem. Bring a clamp with analogue display to see if there are slow motor current variations. There should be if you have a rotor bar problem. But there are others causes for slow current variations.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rhatcher]

You can check for a broken rotor bar by attaching a low voltage, single phase, power supply to one pair of motor line leads. Slowly turn the rotor through one complete revolution while watching for variation in the current reading on the power supply. Any significant variation in the current while turning the rotor indicates a broken rotor bar.

Having given this explanation, my belief is that the motor is improperly connected. This is due to the low current reading for no load operation.

 

[LionelHutz]

With the low no-load current I would check if there are power factor capacitors connected between the motor starter and the motor. The real full-load current would be higher if the client is meauring corrected current. The motor actually being overloaded would also explain that rather high slip level.

 

[maxwellch]

The problem can be caused by motor bearing failure or misalignment of the pump which can cause excessive vibration and over heating.