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Old 07-17-2009, 05:16 PM   #1
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FLA, LRA, No Load Amps


What kind of relationships is there between these amp draws?

I know LRA may be 5x to 10x FLA, but what about NLA?

Here's one dataset
NLA: 104 FLA: 349 LRA: 2265.
and another
FLA: full-load current 36A LRA: locked rotor current 218A NLA: no-load current 14A

but I'm looking for these numbers for motors less than 2 hp.
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Old 07-17-2009, 05:29 PM   #2
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FLA, LRA, No Load Amps


nla =No load amps?
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Old 07-17-2009, 05:49 PM   #3
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FLA, LRA, No Load Amps


I'm not sure of any relationships to each other but no load amps (nla) is the amps used to determine the rpm of the motor magnetic field and therefore its synchronous rpm.
Full load amps determine rpm under it's rated load before service factors. The difference between these two (synchronous speed and full load speed) is the slip factor of the motor.
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Old 07-17-2009, 07:40 PM   #4
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FLA, LRA, No Load Amps


Yes, no load amps.
I'm trying to tell what a well pump is doing by sensing its current.
Dry = NLA
pumping = FLA
clogged = LRA

So far my ratios for large motors are

LRA/FLA = 6.48, 6.06, avg = 6.3
NLA/FLA = 0.298, 0.389, avg = 0.35
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Old 07-17-2009, 07:59 PM   #5
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FLA, LRA, No Load Amps


NLA; No Load Amps. Just what it says, amps expected to be drawn at nameplate voltage with nothing connected to the shaft. On low HP motors (less than 1/2 HP), this figure is pretty close to FLA. NLA goes up quickly with a rise in voltage, and down just as quick with lower voltage. This figure is useful when choosing overloads, especially if the service factor is more than 1.0

FLA; Current expected to be drawn when the motor is producing its rated HP and operated at nameplate voltage. Any deviation of voltage (either up or down) will cause this to increase. Very useful for choosing overloads. Also useful in determining the actual HP the motor is producing.

LRA; Locked Rotor Amps. This is the current drawn when nameplate voltage is applied, but the shaft is not turning. Normally, this current is seen for less than a second, as it drops off as the shaft begins to turn. This is useful for choosing instantaneous trip breaker sizes or fast-acting fuse sizes. It's also useful for choosing a controller, as it might need to break the LRA occasionally.

SFA; Service Factor Amps. If a motor has a service factor of more than 1.0, it can produce more than its nameplate HP continuously without damage. Suppose a 10 HP motor has a SF of 1.15. (Very common). This motor can be made to produce 11.5 HP continuously if exactly nameplate voltage is applied, and it's operated in an ambient temperature of not more than 40C (104F). The real-life reason for a higher SF is that the motor can produce its nameplate HP in a hotter ambient, or with higher or lower voltage. The reason for the SFA listing is because the current draw isn't linear to the load. If a motor has a SF of 1.2, and a FLA of 10, the SFA isn't 12, it's more like 12.7, or 13. The SFA is the absolute maximum current the motor can draw continuously without damage.

Interesting to note, on refrigeration compressors, the LRA is listed on the compressor nameplate. The FLA is listed on the equipment it's installed in. The reason is because the motor is cooled by the returning refrigerant. The motor can produce more HP (and thus higher FLA) if more cooling is supplied. The same compressor installed in different units can have different FLAs.

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Old 07-17-2009, 08:11 PM   #6
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FLA, LRA, No Load Amps


Relays are made for the purpose of detecting over or under current. Grainger #6C055 is an example.

On just about any size motor, the LRA is about 6X the FLA. The FLA VS NLA gets smaller with smaller motors.

The first example you gave is very likely a 300 HP at 460 volts 3 phase. It'll have a lot of difference between FLA and NLA. The second example is likely a 30 HP 460 volt 3 phase, or a 15 HP 230 volt 3 phase. It'll have a similar ratio.

The NLA catches up to the FLA at around 1 HP. Some small motors (less than 1/4 HP) have no difference at all, thus making a current sensor pretty much useless.

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Old 07-17-2009, 08:23 PM   #7
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FLA, LRA, No Load Amps


For well pumps the formula is quite simple.

Clogged: High current, no water from faucet.
Dry: Low current, no water from faucet.
Normal: Moderate current, water from faucet.
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Old 07-17-2009, 09:37 PM   #8
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FLA, LRA, No Load Amps


So maybe I can remotely detect motor speed by putting a scope on a current shunt and looking for ripple in the current waveform. This is a bit complex but well troubleshooting can be costly.
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Old 07-17-2009, 09:43 PM   #9
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FLA, LRA, No Load Amps


Quote:
Originally Posted by Yoyizit View Post
So maybe I can remotely detect motor speed by putting a scope on a current shunt and looking for ripple in the current waveform. This is a bit complex but well troubleshooting can be costly.
Perhaps. You might also be able to use some kind of transducer on the well casing to determine the state the pump is in. A 60 Hz hum would indicate the rotor was locked. A resonant whir would indicate the motor is running, and a lower frequency damped whir would indicate it is running in water, plus the fact that the well will produce water.
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Old 07-17-2009, 10:17 PM   #10
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FLA, LRA, No Load Amps


Pressure switches are available that will shut off the pump if the pressure falls more than 10 PSI below the cut-in setting.

Obviously, these will only work on a system where the water demand is never greater than the pump capacity.

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Old 07-17-2009, 10:36 PM   #11
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FLA, LRA, No Load Amps


Thanks, folks!
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