Motor Running High Amps 1

[matgal1]

Can any one help. I have removed an old motor due to bearing failure and have replaced with a new Leroy Lomer motor. The problem I am having is that when the motor is coupled up the overload is tripping. FLC of motor is 14.5A at 415V, the motor is running at 16.5A(on load) and the motor running light is 9A. I believe that the running of the motor light is a high figure,so I replaced the motor with another Leroy Somer and again the same figures were achieved, on load and run light.

After these probelms the pump has been changed, overload replaced, and numerous tests have been carried out, but everything checks out ok.

Has anyone any suggestions?

 

[ScottyUK]

Matgal,

Sounds like the new motor has a higher magnetising current than the old one. Is it a larger rating? Or perhaps the designer of the new motor has been less generous with the iron, and is closer to the saturation threshold.

What is the FLA for the new motor? How does it compare to the original?

 

[matgal1]

The FLC of the motor is 14.5A. The original motor was a Brook Hansen motor and it was a like for like change. I still have Brook motors next to the new Leroy Somer doing the same task.

But they give completely different amp readings when run light, in the region of 3.4A for the Brook Hasen.

 

[OhioAviator]

Hi Matgal1,

You mentioned that this was a pump application. I suggest making sure that the speed of the pump has not changed (increased) from what it was BEFORE you replaced the motor initially. Remember that motor HP required varies by the SQUARE of the pump speed change. Therefore, a small increase in pump speed will result in a significant increase in motor operating current.

I've run into the same exact problem myself, and found out every time that either the motor sheave was inadvertently changed to a slightly larger one, or the pump sheave was changed to a slightly smaller one. This also applies to fans as well.

Hope this helps.

--John R.

 

[electricpete]

I assume "running light" means running uncoupled?

Under that assumption, I think 9A current uncoupled sounds too high for a 14.5A FLA motor, unless it is very low speed, perhaps 8 poles or more. (what is the speed).

Assuming it is not a low speed motor, you might want to investigate possible voltage imbalance or low voltage.

 

[jbartos]

Suggestion: Please, do you happen to have the motor nameplate data of both motors that might be posted in here?

 

[matgal1]

The Leroy Somer motor details are as follows:
serial no.64601,AC,1450rpm,7.5KW,14.3FLA,415v,ExN.

And the previous motor was a Brook Crompton and details are:
serial no.B662597,AC,1450rpm,7.5KW,14.6FLA,415v,ExN.

As you can see the specifications they are very similiar, as these were specified by our motor supplier

 

[jbartos]

Suggestion: There are several possible causes, namely:
1. The new motor may have a defect. It may need to be checked for its proper functioning.
2. The motor is installed or aligned improperly.
3. The load may experience some problem thus increasing the motor shaft load.
4. Etc.

 

[cbarn24050]

Hi, If your motor is running at 16.5A then it is overloaded which is why it trips.

 

[ScottyUK]

I have to agree with cbarn, your motor is drawing excessive current.

You said your supply is 415V nominal; what is the actual value that you measure at the terminals?

 

[Marke]

Hello matgal1

I agree with Electricpete. The current sounds high. Can you confirm that this is the uncoupled situation. If the open shaft current is 9 amps, then I would suspect a motor problem. If the open shaft current is 4 - 6 amps, then it should be OK.
It is very posible that you have an alignment problem between the motor and the pump.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[matgal1]

I suspected a motor problem, due to the high uncoupled reading of 9 amps. But after changing the motor, like for like I recorded the same readings on the new motor. I can't have a alignment problem due to the equipment being laser aligned.

My main concern is the 9 amps uncoupled reading, why is it so high, when everything on the pumping system is mechanically okay and the electrical tests, are okay as well.

 

[gsimson]

Kindly check the connection of the motor , whether 415 V delta is running at 415 V star. A pump motor cannot get overloaded unless & until the speed is changed.

 

[matgal1]

I am not saying that the motor is getting overloaded when run uncoupled, I just believe that the figure of 9 amps is to high for a uncoupled motor and that when a load is connected, it just compounds the high starting reference, to cause the overload to activate. The motor is connected is delta configuration at 415V.

 

[cbarn24050]

Hi Matgal1, small motors are often star connected for 415v, delta for 240V.

 

[Ahr35181]

I agree with cbarn24050. Many small motors now come dual voltage as marked on the motor nameplate (Star connected at 400V).

In my experience the largest uncoupled motor current I measured on a 3pnase induction motor is half F.L.I.

Being an Exn motor I presume it's located in a zone 2 hazardous area? If so I'd be keen to rectify ASAP.

Alan

 

[jbartos]

Suggestion: The motor problem may also be the aftermath of an above rated shaft load.

 

[buzzp]

What does history tell you the old motor normally ran at for current?
If the power coming in is ok, the motor is too small for the application. It has been my experience that old iron has larger safety margins than new iron (motors) and on a couple of occasions have seen higher current draw on a new motor with the same load vs. an older motor.
Is this one of them efficient motors?

 

[aolalde]

Full load current of an induction motor is a function of; Voltage, Mechanical load on the shaft, Power Factor and efficiency. As you have checked the line voltage to be 415 Volts, analyze the remaining factors.
Low no load current is not only a symptom of low saturation, it could mean a weak magnetic field in your old motor. Then the pump load will increase the slip under load. Pumps change power demand with the “cubic power” of speed change (torque changes with the square power of speed ratio). On the contrary the Leroy Somers motor has a strong magnetic field ( high no load or magnetizing current) and maybe the shaft speed is higher than the nameplate 1450 rpm, then you could be driving more power with the new motor.
Power factor is a function of the motor reactance and change from motor load, design and manufacturer and it is closely related to the magnetizing current. Is your PF lower now?
As for the motor efficiency, it depends on the motor losses under actual load condition. Maybe the Leroy Somers motor has lower efficiency or higher losses.
Add all these factors and you will be driving more amperes with two motors connected to the same pump.

 

[jbartos]

Suggestion: Sometimes, a motor is somewhat damaged by higher motor shaft load. Motor stator interturn shorts may develop due to somewhat higher shaft load.