FLA interpratation in pump applications

[Wedoca]

I am new at this so please pardon me, if I ask something that might be trival to alot of people ....

I need some clearification on this .....in pump applications... on the name plate aways states what the FLA is for the motor and lets say if on the name plate it says FLA is 25A, I mean what should I be expecting to see when I take a current clamp and clamp one of the legs while the pump is in operation???? and is there a relation between the FLA and locking Amp?? I understand that it all depending on the size of motor but is there a way to estameting its value ??? and last , is FLA ideally the max. amount of current a motor should be pulling (disregard the service factor)?????
I talk to many electricans and factory engineers and them all got thier own explaination on things ....so ...
is there anyone can help clear this up???

 

[dpc]

The motor has no idea what is connected to it. The FLA is the amperage drawn by the motor when it is producing the nameplate shaft horsepower. The actual amps can be much less than this depending the power needed by the driven equipment.

Locked rotor amps are related to full load amps by a factor that is generally provided on the nameplate, at least for NEMA motors.

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[Kiljoy]

There is a direct relation to HP and FLA assuming RPM and voltage stays constant. So, if you have a 20HP motor with a nameplate FLA of 25A, and your meter is reading 25A, then you are pulling 20HP. If your meter were to show 12.5A then you are utilizing 10HP.
Here’s the tricky part. Horsepower is related to pump GPM and pressure. You need to know what the maximum pressure at the motor/pump’s RPM to calculate maximum horsepower required. Your system could be rated for 15HP, but they put a 20HP motor on. You should never see FLA.
I’m going to go out on a limb here and guess that your motor is stalling at some point. Hydraulic pumps often have multiple pumps in one unit. Lets say both a high volume (low pressure) and a high pressure (low volume). The high pressure may use 20HP at 3,000PSI and the low pressure 20HP at 1500PSI. If something in the system is not unloading then you could be trying to drive high pressure with a low pressure pump. Just a guess.

 

[jraef]

So, if you have a 20HP motor with a nameplate FLA of 25A, and your meter is reading 25A, then you are pulling 20HP. If your meter were to show 12.5A then you are utilizing 10HP.

Well, it's not quite so exactly linear like that but pretty close.

Wedoca,
An AC motor is like a "dumb animal". Putting a load on it causes it to slow down slightly (increases the "slip") and the motor will pull more current from the line to try to regain full speed. The more load you put on it, the more current it will pull, even to the point of self destruction. The FLA (FLC) on the nameplate is a maximum current value that the motor can SAFELY pull without risk of damage to the winding insulation. Even if you run a motor into the posted Service Factor, there is an understanding that normal motor performance is now compromised and a shorter insulation life can be expected.

Locked Rotor Current (LRC)is the Maximum current the motor can pull at all, i.e. you have put so much load on it that the rotor has stopped turning. At that point the only thing resisting the flow of current is the motor winding resistance alone; no inductance to speak of. This also takes place when you first start the motor since at that very first instance, the rotor is not turning as well.

LRC is determined by the motor design and a set of "code letters" on the nameplate as mentioned, but for estimation purposes we generally use 600% of FLC.


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[DickDV]

Wedoca, to be a bit more detailed, motor amps is actually the vector sum of magnetizing and torque-producing amps. The result of that is that under about 80% load on the motor, amps is not a very accurate indicator of motor loading. In fact, with the motor totally unloaded, it will still draw about 25-30% of the nameplate FLA. All of this current is magnetizing the motor as there is no torque in the shaft.

Assuming full rated voltage and frequency on the motor, slip is a better indicator of shaft torque but you will need a tach or strobe light to measure it.