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?

 

[rockman7892]

I just retrieved the information from a 3-day power log on this motor.

The voltage which was logged stayed at about 265V L-G. This is about 4% lower than the nominal 277L-G voltage. Does this 4% seem o.k. I dont see from the results that increasing this voltage would give enough headroom to reduce the current on the motor.

I looked at all three line currents and they average about a 3-4% current imbalance between phases. I dont think this is much cause for alarm.

Attached you will see the power log for this motor. The first sheet shows the motor kW on the bottom and the PF at the top. PF seems to stay above .8 which seems o.k. The second plot shows the individual phase kW plots with the total kW plot at the end.

As you can see from the plot the motor kW is well over 150kW which corrosponds to a 200hp motor. For the first 18hrs the motor seems to average about 180kW which corrosponds to about 241hp. There is also a timeframe of about 9hrs where the motor appears to average about 200kw which is equivelent to about 268hp.

The highest peak that was recorded was 226kw or about 302hp. Based off all of this information I am thinking that we need somewhere between a 250hp to 300hp for this application.

We never had any problems with this 200hp motor in the past, however due to more efficient productin increases we are now putting more of a load on this pump and thus obviously overloading it.

 

[edison123]

Yup. You need a higher rated motor. May be you can put some caps while you are at it to improve pf closer to 0.95.

You also need to look at switchgear, protection, CT's, cables etc.

 

[ischgl99]

edison, I understand that completely, and if you read the first post again, you will see that the motor is drawing more amps then the 200 hp motor is designed for indicating the plant is already overloading the pump and it could be operating at or near 300 hp if the plant installed that motor. My experience is if there is available HP, someone will try to use it all.

My point was and still is, don't install a larger motor if the driven equipment is not designed to handle that load, the motor might survive, but the pump most likely will not. I work in the reliability field and see this all the time, someone who does not understand the whole picture trys to "fix" the problem and creates a new one.

 

[edison123]

No, the motor wouldn't ... survive. Often, the motor is the weakest link due to that damn insulation.

 

[waross]

I suppose that you have checked and determined that the pump is capable of the increased through-put.
I would consider a 300 Hp motor with a 1.15 service factor. I would also evaluate the pump capacity and consider a 350 HP motor if there is any possibility that the load may increase in the future. If you have any problems with a 300 HP motor, the extra cost of a 350 may be small compared to possible plant down time.
But, check the pump. If the pump is maxed out, consider either a larger pump and motor or a second pump, if that is feasible in your layout.

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

 

[rockman7892]

O.k. so it sounds as if I would need at least a 300hp motor (Assuming pump is capable of handeling)

As I stated earlier this motor is fed from a 480V size 5 starter. My question now becomes if I upgrade this motor weather or not to leave it at 480V or increase the motor to a 4.16kV. All other motors in the plant are 480V for 200hp and below and all motor above 200hp seem to be at 4.16kV. This may be because above a 200hp requires a size 6 starter.

If I install a new motor at 300hp at 480V then I have to install a new size 6 motor starter in the MCC which we do not use elsewhere in the plant. I would also have to run a new branch circuit to the motor due to the fact the the existing one is only 350MCM and would not be large enough for a 300hp motor. I would probably need 750MCM or pull a parallel pair of 350's.

In the same electrical room there is a spare 5kV starter that is no longer in use. If I were able to utilize this starter (verifying CT's etc..) I would probably only need to pull a #2 branch circuit out to the motor which would be much cheaper than the 480V size cable.

What do you guys think about 480V vs 4.16kV? Any other things to consider?

 

[waross]

There are a lot of safety and training issues to be addressed before installing the first MV motor in the plant. If you already have MV motors installed in the plant, those issues will have been dealt with, (hopefully).

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

 

[rockman7892]

Waross

We have several other MV motors in the plant, so this would not be the first. These training issues have been adressed and are done so on a continuing basis.

 

[DickDV]

The part that seems to be a bit hidden here is that, at the overload conditions on the 200hp motor, there already is 300hp being drawn from the electrical system and poured into the pump. It's really too late to ask if the pump can handle it. It already is!

Asking for 300hp from the electricals involves overloading most of the components obviously so they would have to be changed out to get back inside their design limits. If MV is an option, I think I would take it.

It's worth observing that a NEMA B motor's nameplate rating is really set by thermal conditions. At peak overload, that same 200hp motor is somewhere just over 400hp. If that motor was located in a deep-freeze it probably could go on making 400hp all day long but certainly not at its rated ambient temp of 40 degrees C.

My point is that today's NEMA motors are amazing machines, capable of huge overloads and lots of abuse.

 

[flexoprinting]

Rockman

What about the pump, has someone adjusted the butterfly valve on pump head to tweak more vacuum out of the pump causing it to overload?

Are there leaks in the system, requiring the pump to move more atmosphere than necessary, or are too many machines down on set up leaking vacuum?

We had the same problem with our pump and after vibration analysis, several tune ups, KW logs, and system audit basically were going to need an additional pump in the system.

In my experience if the system requires a lot of flow, the blower type pumps work better, but have a history of double the maintenance such as bearing replacement. Screw pumps are good in closed systems.

I'm just a Tech and these things were not mentioned, so I am curious about your findings on the pump side of the problem.

 

[rockman7892]

I made a mistake and had my power logger set up for a 3-phase wye configuration when the motor is wired in a Delta configuration.

I'm not sure exactly what this would do to my kW results but I'm sure it would skew them. I'm going to set up the meter for a three element delta configuration and take another set of measurements.

If my new measurments are fairly close to the ones posted above (not sure how far off they will be) then I think i am more inclinded to lean towards a MV setup.

 

[edison123]

The part that seems to be a bit hidden here is that, at the overload conditions on the 200hp motor, there already is 300hp being drawn from the electrical system and poured into the pump. It's really too late to ask if the pump can handle it. It already is!

Bingo !

 

[rockman7892]

I went back and set the meter up for the correct configuratin. I did not see much of a change (only a few kw) between the wye and delta configuratin methods for measuremnt. I'm thinking that my previous measurments will then be o.k. to use.


Will the actual kW calculated to the input of the motor be greater than that calcuated on the output of the motor. I belive it will be because of motor efficiency.

In other words if I calculate the ouput kW by using motor hp * .746 this will differ from a kw calculation using motor V * FLA * 1.73 * PF. This difference will be the efficieny of the motor?

I'm assuming that the cauculation on the elctrical motor input will be larger that that used for the motor output. However I guess motor hp * .746 is a good aproximation to use when doing comparisons as I have been doing in this thread?

 

[edison123]

Supply to the motor does not care whether the motor is wye or delta connected. Power input to the motor is Sqrt (3) x Line to Line Voltage x Line Current x pf x Efficiency.

So I do not undertsand what you mean by setting your power analyzer to correct configuration.

 

[rockman7892]

Edison

You are correct. I was just getting confused with some information that someone had given me in regards to my data logger. Power is power so the measurements that I have are valid.

Going back to the 480V vs. 4.16kV discussion, I see that the maximum size motor a size 5 starter can handle is a 200hp. If I go to a 250hp motor can I push the limitations of a size 5 starter or must I go to a size 6?

 

[waross]

No, go to a size 6.
I saw a 400 HP motor running on a starter rated for 250 HP. I made a lot of money changing out a 400 HP motor with a bent shaft. The contactor flashed phase to phase trying to open under full load current. Faced with a phase to phase short on the supply the motor became an induction generator with a short on the terminals. The motor slowed down quicker than the fan and the coupling exploded. That's probably when the motor shaft bent. There are reasons for contactor ratings.

300 HP, 4160V is better.

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