Motor Upgrade 1

[rockman7892]

I have a 480V 200hp motor used on a screw pump application that I am looking to upgrade. This motor is currenlty being overloaded by the process and management wants to upgrade it.

This particular motor is constantly overloaded and as a result we keep tripping on overload. Production folks simply go and reset the overloads (against my reccomendation) and keep restarting the motor after trips. We have smoked this motor three times in the last month. We keep sending it out for repair put it back in service and the the same thing happens. Mechanics say that they have gone through all mechanical part of the application and there is nothing wrong, so it simply leds to the process loads.

We have taken several amp readings on the motor and see that it is pulling about 300A when the rated FLA is only 226A.

Management has asked me how we can get about 10% more capacity out of this motor. They have asked me weather we had to buy a larger motor (next standard size is 250hp) or if we could have this one rewound to a larger hp. Is it possible to have this motor rewound to give a larger hp maybe an additional 10% or so? Motor data is:

3-Phase
6 Poles
1190 rpm
60hz
FLA = 226A
S.F. =1.15
Alt S.F. = 1.0
Ambient = 40deg C
PF = 86.5
NEMA B

The one constraint we have is that if we upgrade to a 250hp motor then we will need to upgrade the starter size to a size 6 for the size 5 that we have is only rated up to 200hp. I'm assuming that this would be the same case even if we rewound the motor and upgraded the hp some?

I'm going to put a power logger on this motor. Other than power kW is there anything in particular I can look for as a red flag to say that there is a problem other than the process overload. Could power factor be a problem? Starting current?

 

[edison123]

I doubt if you can upgrade this motor to suit your load, which seems to require 270 hp. Buying a new motor to suit the load is more economical than rewinding/upgrading the old motor.

 

[rockman7892]

Thanks Edison, I'm not seeing how you arrived at the 270hp though. I'm guessing you calculated this based of the 300A current measurement that I provided you with?

 

[edison123]

Yes. 226 A = 200 HP. 300 A = 265 HP.

 

[DickDV]

The 300amp loading indicates more than 250hp and certainly more than 10% upgrade.

To cover the load represented by 300amps you will need a 300hp motor. Keep in mind that the motor frame size is going to change and the shaft elevation will be greater. That will require that the motor mounting be lowered or the pump mounting be raised if this is a coupled power train. Not so much an issue if the power train is belted.

If belted, you will need to look at the belting for 300hp rather than 200hp.

 

[Laplacian]

You might be able to cheat a little bit depending on what your motor terminal voltage is. If it's a little low (450-460V), and if you have taps available on the upstream distribution, you can adjust the taps to raise motor voltage (480-500). You may still pursue this option as an interim solution until a longer term approach could be implemented if applicable.

 

[Laplacian]

Sorry, I neglected to explain my response.

The motor's current doesn't really tell the whole story in the context you posted because you don't mention the motor's actual operating voltage, or the HP (KW) it's operating at. If the operating voltage is low, then the motor is pulling extra current drive the load. Raise the voltage, and current will go down. How much depends on where it is operating. If it is below nameplate voltage, then you can feasibly increase to around 485V and have operating current in the range of its service factor rating. Hope that helps.

 

[ScottyUK]

Ultimately the rating is determined by the motor's ability to reject heat to the environment. If you have an environment cooler than the temperature the motor is rated at then you will be able to squeeze a little more than nameplate rating out of it. I guess this is probably the reason why you have been able to overload your motor and get away with it.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[jraef]

Don't know this for sure, but can a 200HP motor be rebuilt to have a 1.25 service factor without changing the frame size? That's about what you need here.

I know the use of SF as a continuous HP increase is controversial, but is the overloading situation continuous or occasional?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[waross]

Can you reduce the load? I know that the first answer is going to be no way!
Then after adding up the price of a 300 HP motor, a larger starter, possibly new cabling, new mounts to accommodate the higher shaft, and labor to put it all together, ask again if the load may be reduced.
On the other hand, how many rewinds will it take to equal the cost of a 300 Hp installation.
Of course if it's belted, a sheave change will be quick and cheap.

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

 

[itsmoked]

Could a multi-HP blower be put on the motor to force down the temperature?

Can an additional motor (75hp) be coupled onto the drive system and actually share the load correctly?
If it could a much less expensive addition of switch gear could be added.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I would check the torque speed curves on both motors before coupling them. A bad mismatch in curves may result in the smaller motor hogging the load and overloading badly at slight overloads or not picking up its share of overloads. If the curves are matched, the motors will probably share the load correctly. Beware if the smaller motor has a slightly different rated speed than the larger motor.

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

 

[zlatkodo]

If the motor is so acted from the beginning when it is installed, then you definitely need to get a new one with more power.
But if it is not the case, then cause is in motor. What is the state of the motor, it can say only in rewind shop. You need to find the most important examination-detail:
“ no load“ current with run at full voltage.
In this case the current should not be greater than 70 -80A , if motor is intended for S1
(S1 - continuous duty: the motor works at a constant load for enough time to reach temperature equilibrium).
If “ no load“current is significantly higher, can be two causes:
- inadequate winding-details or
- core-iron insulation damage, due to frequent burns of motor and (maybe) inappropriate heating the motor before removing the old windings (see attach.)
Power of the motor you can not increase just like that. This can be done only in case the motor works, for example, in S2 – duty (S2 - short-time duty: the motor works at a constant load, but not long enough to reach temperature equilibrium, and the rest periods are long enough for the motor to reach ambient temperature.)
Power of the new motor you can not specify only from the current that old motor now has, because you dont know „no load“ current. What to do if the „no load“ current is 150 or 200 A? The only correct way is to calculate the necessary power, torque etc, by professional or the manufacturer.
Zlatkodo

 

[ischgl99]

Increasing the motor size may only be pushing the problem elsewhere. If the blower cannot reliably handle 300 hp, then increasing the motor HP might just start causing failures of the blower instead of the motor. Before changing motors, I would check and see what the blower was designed for to see if the shafts, bearings, etc can handle the increased torque.

 

[itsmoked]

Excellent point ischgl99! I've seen that phenomena carried out up thru 5 steps.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Why would a higher rated motor damage the blower, pump or whatever is the load ?

 

[ischgl99]

because a 300 hp motor produces 50% more torque then the 200 hp motor running at the same speed. If the shaft was designed for 200 hp, adding that much more torque could reduce the safety factor enough that the shaft fails prematurely.

The bearings are also selected for a given speed and load. Since the torque has increased, the load on the bearings could also increase resulting in reduced life of the bearings. Increasing the load 50% on the bearings results in a reduction in life of about 70%. Another issue is the lubricant was selected for a certain loading on the bearings, a higher load may require a different lubricant.

 

[rockman7892]

Just trying to think outside the box, is there anything that can be done with a VFD to provide the required torque but limit the current to the motor thus preventing it from overheating? Can you possibly use a VFD to drive a motor over base speed and provide more hp for a given touque? Can any kind of torque control be used?

I should have all the power readings tomorrow from a 3-day data log to look at.

 

[itsmoked]

Yes but then you will be increasing the actual load at possibly a faster rate. You would have to increase the voltage over your base and reduce the gearing.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

ischgl

A 300 HP motor is capable of producing 50% more torque only if the load requires it. It won't dump the torque on the load just because it can. The motor is reactive component not a proactive one.

rockman

Why would you want to run your pump above the base speed, which would not only increase the load on the motor but affect the pump flow as well ? The bottom-line here is that if a load requires a certain torque and power at a given speed, your motor must match it. In your case, the motor getting burnt thrice within a short period indicates the existing motor is undersized.

A 300 HP motor doesn't necessarily mean an increase of shaft center height of the motor frame. It all depends on what is the frame size of the existing motor. Normally motor OEM's use the a variation of basically the same frame (with same shaft center height and across bolt center holes) for different outputs by varying the length i.e. along the axis. This is done so that the same diametered core can be used to produce a wide range of motors since core punching tools are expensive.