why use NEMA design A motor for centrifugal pump? 1

[electricpete]

We have hundreds and hundreds of centrifugal pumps at our power station and all driven by Nema design B motors.

All except our stator cooling water pumps (75hp 3650 rpm motor driving overhung centrifugal pump) - these use Nema design A motors. As you probably know design A provides higher starting torque and breakdown torque.

I don't know why design A was selected for these pumps. Can anyone make any guess why a designer would do this?

I do have one theory. There are two pumps per generator with only one pump normally operating and the other pump in standby. Upon loss of the running pump the standby pump starts automatically. It occurs to me maybe they selected design A to provide quicker start of the standby pump to restore water flow as fast as possible in this critical system? Surely it wouldn't be more than a second to start even with a design B pump... I will have to check with our generator engineer to see if starting time is really that critical.

Any thoughts on whether my theory is feasible or there might be some other reason to use design A?

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[don01]

electricpete
a stab in the dark. What if we are considering a pipe cicuit that has an inherent run back flow when the pump shuts in or that the circuit is pressurised and so put some extra load on the pump when we change over.

would the water pressure rise with temp quickly if flow was disrupted?

Now thats not scientific but a "gut feel"

Don

 

[electricpete]

Good thoughts.
We do have check valves in the discharge which should prevent backflow and a pressurized expansion tank which keeps pressure somewhat stable.

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[electricpete]

I should clarify the expansion tank is pressurized with a trapped gas blanket.

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[DickDV]

electricpete, in my experience this is more often than not simply a mistake on the part of the equipment supplier. I just recently had to deal with a similar situation where there were NEMA Design A motors on centrifugal fans.

The customer was having problems with starting inrush currents from these motors and, when I contacted the fan manufacturer's engineering dept, they didn't know what the differences were between the various NEMA Designs!

After a little research and explanation, even the manufacturer's engineers agreed that Design A was not an optimal choice, for sure. (sigh!!!!)

But they got a good price on the motors!!!!! (grimace!!)

 

[jraef]

electricpete,
I have had the exact same experience as DickDV, and sometimes they will even substitue a different HP than originally designed because they "got a good deal", but they fail to pass that information on to the electrical engineer who was specifying the controls!

However, isn't your factility a nuke plant? I would hope that every little detail like that would have been carefully considered in the original design and double checked at commissioning. The problem I percieve you having is in trying to second guess that decision by the original designer, especially if it was not well documented. I did some work at the old Hanford plant before it was decommissioned, and somewhere, buried deep in the archives, was the answer to every possible question we could come up with when trying to reverse engineer or trouble shoot a system. But I empathize with you, sometimes it was an arduous task to find it. Given your situation I would hazzard a guess that it was done with some purpose in mind.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

Epete:

If you check the NEMA STD MG1 the design A motor does not necessarily has more torque than a design B motor. The requirements of minimum Starting, Pull-up and Breakdown torques are the same. The only difference as compared to design B is “with locked rotor current higher than the values shown in 12.35 for 60 Hz and 12.36 for 50 Hz”.

It seems like a design B motor without restrictions for the starting current.

Do you have the manufacturers speed-torque curves for the motor and pump?

 

[electricpete]

Thanks for all the comments. Yes, it is possible there was some mistake. Yes, jraef, it is a nuke and we do things strange ways sometimes.

aolalde- I checked and your are absolutely correct about the requirements for NEMA design A. However there is still something mysterious about it that leads me to believe higher torque. I will dig up some references.

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[electricpete]

Here is an excerpt from EASA Principles of Large Motors (2000):
page 2-16, 2-17
#1 - "Typically design A motors are referred to as a high flux motors..."

#2 - "The original intent of the Design A motor was to meet abnormal load requirements when the higher starting currents could be handled with adequate power systems or reduced current starting methods. The design B motor could meet many of these same abnormal load requirements with special designs but with cost, delivery and performance penalities."

#3 - Figure 14 shows typical curves with design A and E much higher than design B.

So I look closely at #2 and the following scenario emerges: A purchaser has a requirement for a special-order high-torque motor with higher torque requirement higher than B which cannot easily be met while still maintaining design B locked rotor current. The purchaser special orders by identifying the exact torque requirements.

Now the motor manufacturer builds the special order motor. Perhaps he codes his model number so that he knows the exact torque associated with this motor. NEMA still requires him to put a NEMA design letter on the labelplate. What letter does he put? He cannot put B because it does not meet locked rotor requirements. So he calls it A. Yes, the torque requirements of his motor are far more than NEMA A but at least he meets it.

So in summary my belief is that A would be the design letter for a motor which was special-ordered as high torque. However specifying design A does not guarantee high torque.

Now back to my motor. It is kva code H, 75hp, 3600 460v. I am not sure if this translates into starting current exceeding NEMA B (will have to check). Also now I know from aolalde's comments there is no guarantee this is a high torque motor but I believe there is no other likely reason we would end up with design A. But you never know.

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[electricpete]

In spite of jraef's glowing comments about nuclear documentation, I have no torque speed curves ;-(

If these were safety related motors surely I would have the papework but they are not. Westinghouse provided them as a skid for generator auxiliaries. They are very well engineered by Westinghouse I'm sure but they didn't share the engineering with us.

It is Westinghouse style 79B6275 for both motors in one plant and Westinghouse 77B16718 in the other plant. Those numbers mean nothing to me but I write them here on the longshot chance they mean something to someone else out there.

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[electricpete]

I did confirm that kva/hp code H places this motor beyond the locked rotor current range allowed for NEMA design B.

Code letter H means 6.3 - 7.1 kva/horspower

Using the lowest value 6.3 for 75hp 460v motor:
I=6.3*75*1000/sqrt(3)/460 = 593A.

Max starting current for design B,C,D 75hp is 1085*230/460=542 (per MG1 12.35)

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[aolalde]

Epete:

I have seen some design A motors manufactured to provide High Break-down torque and low slip. The idea is to handle sporadic peak loads in excess of the nominal full load torque. Has this pump that kind of requirements?. Is the nameplate full load speed very close to the synchronous speed?

 

[electricpete]

I feel confident there is no abnormal load requirement - it is a centrifugal water pump in a closed-loop pumping system (generator stator cooling water). But I agree sporadic high peak loads would be a typical application of high-torque motor.

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