Inrush Mystery

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That Man

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A client has a bank of 3 motors that keep tripping on start. We took the trouble to measure the actual values, and it's no wonder why they keep tripping, but we can't understand why the inrush is so high. We were thinking of just adding soft starts and calling it good, but we want to understand what the problem might be first. Please see the attached PDF "inrush". The values in the red squares are nearly 200A for 7.5HP motors, and the running current is only about 4A, which is normal. What could be causing such a high inrush? The motor KVA code is K, not V so this is not a "feature" (see attachhed PDF "motor data").

Any ideas?
 

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Ingenieur

Senior Member
Location
Earth
what is the mechanical load?

edit: pumps
what type
what medium is being pumped
are they sized properly for volume and head?
 

That Man

Member
Starting torque too high?

I don't know. But the locked rotor torque is 37 lb/ft, and this is enough to get the load moving, since we have measured running torque. Am I wrong to assume that since the motor can move the load, that the starting current should be no greeater than the rated starting current of 87A?
 

Ingenieur

Senior Member
Location
Earth
FLA 8.3
your load 3.5 to 4, avg 3.7 +/-
no load 3.2 per spec sheet

so basically no load on it
are they turning the proper direction?
any isolation valves not open?

strange

check valves on each pump discharge?
 
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That Man

Member
what is the mechanical load?

edit: pumps
what type
what medium is being pumped
are they sized properly for volume and head?

These are actually mixers (think industrial sized kitchenaids). We were expecting high inrush, so we sized the breakers to 150A.
 

Ingenieur

Senior Member
Location
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These are actually mixers (think industrial sized kitchenaids). We were expecting high inrush, so we sized the breakers to 150A.

are they geared down a lot?
I'm guessing yes since it is a 3500 rpm motor
so you are producing ALOT more torque than the motor is rated
when starting are they in dough?
 

That Man

Member
are they geared down a lot?
I'm guessing yes since it is a 3500 rpm motor
so you are producing ALOT more torque than the motor is rated
when starting are they in dough?

The client is busy, and can't respond to my questions at this time. However, I would think that the mixers are geared down significantly, as you assume, and generating much higher torque on the mixer end. But the motor will still only see 37 lb/ft or less won't it? And if it can start the load at all, wouldn't that imply that the load is no more than 37 lb/ft, which translates to 87A on the torque curve?

I'll confirm with the client that the motor actually moves the load, and he isn't simply reporting the peak current that the circuit reached before tripping.
 

Phil Corso

Senior Member
Thatman...

Schematic shows thermal Motor feeder Mag-Breaker and thermal OL's, so what is tripping on "inrush"? And, how are inrush-currents being measured or record?

Regards, Phil Corso
 

Ingenieur

Senior Member
Location
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The client is busy, and can't respond to my questions at this time. However, I would think that the mixers are geared down significantly, as you assume, and generating much higher torque on the mixer end. But the motor will still only see 37 lb/ft or less won't it? And if it can start the load at all, wouldn't that imply that the load is no more than 37 lb/ft, which translates to 87A on the torque curve?

I'll confirm with the client that the motor actually moves the load, and he isn't simply reporting the peak current that the circuit reached before tripping.

let's say the mixers turn 2 rev/sec or 120 rpm
so gearing is 30:1 (3600/120)

if motor produces 30 lb-ft mixer shaft output is 900 lb-ft, you figure enough to get anything moving lol
so yes motor torque is the rated, not torque multiplied

but depending on the consistency of the material the shaft output torque could be higher
but what is confusing is starting torque is <90 A ??? but this assumes the motor can turn

LRA 8.99 x 7.5 x 1000 / (460 cx sqrt 3) = 85 A ~ starting current

although they give a line-line R of 0.92 ohm
so if only limited by the winding R you could see >200 A

something weird is going on
is it wired properly? it is a dual voltage motor

but once running <4 A which is ~25% loaded
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
160428-1455 EDT

That Man:

I can not answer your question, but I have questions and comments.

Do you have a digital scope and a Hall device current probe? I would like to see traces for your startup current.

At startup and for a few cycles you have a current that is defined by the winding impedance. If the motor is not rotating, then this impedance is not less than approximatrely the DC tresistance of the coil. Your current level is probably below the DC resistance determined value. Normal unloaded inertia load of a typical motor probably will keep this starting current high for 4 to 6 cycles at 60 Hz. I don't think you will find much magnetic saturation inrush current.

Mechanical steady state loading has nothing to do with initial starting current Initial starting current is simply locked rotor current.

Are the motors really wired for 480 V?

Is your instrumentation correctly calibrated?

See some single phase measurements at http://forums.mikeholt.com/showthread.php?t=174880&highlight=amperage+inrush

A lot of that initial current was the start winding. Relatively should be less on a 3 phase induction motor.

.
 

That Man

Member
Everyone,

you have produced many pertinent questions that I will seek the answers to. It will likely be some time before I can answer them though. I'll get back to you with what I can discover.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Most likely this is simply a "measurement issue" in that you are using a very fast sampling rate on a DMM and have it set to read the highest peak current. What you are encountering is actually not unheard of, it's just rare. There are solutions.

In an induction motor, the 500-600% of FLC is rreally called 'Starting Current", the actual definition of "Inrush" is the MAGNETIC inrush current that happens only for the short instant that it takes to magnetize the windings, BEFORE induction takes place to create impedance and bring it down to the Starting Current level. In that instant, the only thing slowing down the current from being a direct bolted fault is the resistance of the magnet wire in the windings. That instantaneous spike has been know to be as high as 1700% of the motor FLA, depending on design. The duration is very short and most protective devices can't react to it so it is effectively filtered out. But that's not guaranteed, hence the exceptions in 430.52 that allow for setting the instantaneous trips on a circuit breaker that high if demonstrated to be nuisance tripping.

Once we all started going to the newer energy efficient motor designs, it became known that those motors, in the efforts to decrease losses, have even less resistance in the windings and now we are seeing inrush spikes as high as 2200% in certain instances.

So on your pdf, you mention the running amps, but that's irrelevant. A 7-12HP motor will have an FLA of 11A, 1700% of that would be 187A, so seeing 160A or 185A would not be unexpected if your meter can capture a transient that fast. The 200A reading might still be not that uncommon if it is a new motor; 11A x 22 = 242A.

The fact that it is tripping your breakers might be solved by changing the breakers. It's not uncommon that a factory motor starter will use an "MCP", really an Instantaneous Trip (IT) or "Mag-Only" breaker, with the maximum trip setting at 10X the breaker rating (1000%). So if the starter mfr used a IT breaker rated for lets say 15A, the highest setting for the mag trips will be 150A. technically, if you replace those breakers with Thermal-Mag versions of the same make (so the accessories fit), you can go 250% of the motor FLA of 11A, so a max. 25A T-M CB, which will have an adjustable mag setting of 250A. You would not be allowed to set it above 187A, but most likely that will help avoid most of the nuisance trips.

Before going to that extent however, if all 3 motors can start at the same time as mentioned above, that might be a contributing factor and that would be easier and cheaper to fix first. Just make a way to stagger the startup sequence, even if only a fraction of a second apart, i.e. by using an aux of one starter in the Run circuit of another (assuming that works in logic).
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
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Electrical Engineer

kwired

Electron manager
Location
NE Nebraska
What is the source capacity, how close to it are you with this equipment, what size and length of conductors are involved in this circuit and/or feeders up to it?

Large conductors will allow more current to flow during starting, likewise short runs will have less resistance also and will allow more starting current to flow.

If you have a long feeder across the plant then these motors on another long branch circuit you will see less starting current then if the motors were right near the service with the source being fairly close. That said many motor calculators often suggest at least a 20 amp breaker on a 7.5 motor at 480 volts 3 phase, NEC would allow up to 30 amp breaker before you have to see it fail to start before increasing from that. (this for thermal magnetic breakers)

But I have also seen many 10HP motors start fine on a 15 amp breaker - all depends on circuit arrangement, resistance of circuit up to the motor, and characteristics of the motor itself.
 

Ingenieur

Senior Member
Location
Earth
As was just mentioned by jraef
fast meter
catching first 1/2 cycle current limited only by stator and rotor winding R
they list line-line R as ~0.9 ohm
480/0.9 ~ 530 A
probably averaged over a cycle or two
pretty consistent motor to motor
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
160428-1945 EDT

I disagree that there is a large peak inrush current when starting an induction motor, but there is a large sustained starting current. It is hard to drive a motor coil into magnetic saturation because of the large air gap in the magnetic path compared to a transformer where we typically see the possibility (it is random) of a large inrush transient.

In my previously post I referenced a prior post of mine where I showed the starting current of two different induction motors. One was single phase and the other three phase. Neither motor showed any significant inrush current.

Unless you have magnetic core saturation there is no inrush current. The circuit is simply a relatively fixed inductance in series with resistance. The initial starting current is defined by the impedance of that L-R circuit, any external impedance, and the idea source voltage of the loop. As the motor increases in speed there is a counter-emf that effectively reduces the magnitude of the voltage across the L-R circuit.

.
 
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