Inrush Mystery

[templdl]

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?

May I assume that you are tripping the MCP? If so has the MCP been sized and set per NEC art 430-152? K3p in mind that the term " inrush" should not be confused with the motors locker rotor current but as one of the other posters brought up is the peak current the occurs when the motor is first energized which most meters are incapable of capturing the value. This current may reach 13x the FLAs for energy efficient motors. Motors commonly today are energy efficient which equates to a much higher than the average motors in the past.
MCPs are mag (instantaneous) only devices that protect the motor circuit should the motor fail, a failure cause by a winding to ground fault and winding to winding fault. Should the mag setting of the MCP be set to low the inrush (magnetizing) current most likely will nuisance trip the on start up. My recommendation is to become familiar with NEC art 430-152 and set the MCP at the highest setting that is allowed. Remember that if you feel uncomfortable with that setting you can keep tweeking the setting down to a point just above where you would get nuisance trips.
You will note the NEC art 430-152 refers to the MCP as ground fault protection.
What hasn't been included by the OP is what the MCP is and what available settings are available and where the MCP is set.
MCPs come from the factory set in the lowest position and the tech. must then readjust that setting per NEC art 430-152. While as an applications engineer I had received countless question regarding nuisance tripping MCPs when they were not set properly.
Now, it may be that the OP is referring to an OLR trip which is thermal. Then you take s look at what may me causing excessive current during start up after assuring that the proper heaters have been installed or if adjustable adjusted correctly. Should there be something during startup that caused the LRA to bwe long enough the OLR would trip to protect the motor winding from being damaged by excessive heating. If this thern is the issue it would now be appropriate to investigate the driven load.

 

[milldrone]

The motor PDF states the motors are NEMA A.

This PDF https://www.google.com/url?sa=t&rct...JeGJMhqxKTrLk1M7SbP4iA&bvm=bv.121070826,d.cGc States that "Design A Normal locked rotor torque and high locked rotor current". Is a design feature of this type of motor. Design B are "Design B Normal locked rotor torque and normal locked rotor current.

 

[gar]

160429-0921 EDT

Can anyone show me a plot of instantaneous current while starting an induction motor that has an inrush current, if any, that is substantially greater than the peak value of the starting current. Assume: normal nominal voltage is applied, worst case turn on point in the voltage cycle can be used, the mechanical load can be anything that is not initially rotating and is not an energy source, but is simply an energy load, and locked rotor can be used.

Locked rotor and a very high inertia load are essentially the same thing, and cause the longest duration of startup current. No external load on a motor produces the shortest startup time. If a motor is initially rotating in reverse and power is applied for forward rotation, then startup current can be greater than locked rotor current.

A very high overvoltage might show some inrush current.

I contend that there is no appreciable inrush current when starting an induction motor, but there is starting current. iwire wants to call starting current and inrush current the same thing because electricians have used the terms interchangeably, but these are different animals. Look at the waveforms for transformer inrush current and compare these with the waveforms for motor starting current. Inrush current is primarily a half cycle event while startup current of an induction motor is typically many half cycles.

A circuit breaker design should be different for a transformer load compared to a motor load, but in many applications the same breaker is used for both types of loads.

When That Man gets more information we can make better judgements on what is happening.

The data in milldrone's reference largely shows how speed-torque curves vary as the rotor resistance is changed.

.

 

[Ingenieur]

This is because peak inrush can be as high as 2.8 times the RMS locked rotor current and may exceed the circuit protector current setting.
http://energy.gov/sites/prod/files/2014/04/f15/avoid_nuisance_motorsys_ts6.pdf

so if we have a LRC of 85 A it would appear an inrush of 200 is possible

http://www.cooperindustries.com/con.../BUS_Ele_Tech_Lib_Motor_Starting_Currents.pdf




When an AC motor is energized, a high inrush current occurs. Typically, during

the initial half cycle, the inrush current is often higher than 20 times the normalfull load current. After the first half-cycle the motor begins to rotate and the

starting current subsides to 4 to 8 times the normal current for several

seconds.


20 x 8.5 ~ 170 A

 

[templdl]

160429-0921 EDT

Can anyone show me a plot of instantaneous current while starting an induction motor that has an inrush current, if any, that is substantially greater than the peak value of the starting current. Assume: normal nominal voltage is applied, worst case turn on point in the voltage cycle can be used, the mechanical load can be anything that is not initially rotating and is not an energy source, but is simply an energy load, and locked rotor can be used.

Locked rotor and a very high inertia load are essentially the same thing, and cause the longest duration of startup current. No external load on a motor produces the shortest startup time. If a motor is initially rotating in reverse and power is applied for forward rotation, then startup current can be greater than locked rotor current.

A very high overvoltage might show some inrush current.

I contend that there is no appreciable inrush current when starting an induction motor, but there is starting current. iwire wants to call starting current and inrush current the same thing because electricians have used the terms interchangeably, but these are different animals. Look at the waveforms for transformer inrush current and compare these with the waveforms for motor starting current. Inrush current is primarily a half cycle event while startup current of an induction motor is typically many half cycles.

A circuit breaker design should be different for a transformer load compared to a motor load, but in many applications the same breaker is used for both types of loads.

When That Man gets more information we can make better judgements on what is happening.

The data in milldrone's reference largely shows how speed-torque curves vary as the rotor resistance is changed.

.

Intersting that locked rotor current and inrush current are considered to to the same? Oh well. Then you may not be aware of trama that NEC art 432-152 caused during the time in the 1980s when energy efficient motors became very common and then required. You masy not have had the privilage of having to deal with the nuisance tripping MCP while still trying to comply with the code requirement ast that time. This made it necessary for a code change to increase the allowable instantaneous adjustment setting of the motor circuit protector which had nothing to do what ever with LRA. Because the stator windings were optimized and better iron was used to improve the magnetics of the motor the first cycle or two of current that is required to magnetize increased greatly which conflicted with the allowable maximum setting of the MCP at that timer.
But still being avoided here is what is tripping, it is obvious that if the MCP is tripping that the questions that I had asked in my previous post have been avoided. I have been quite intregued by some of the responses to the OP. Is it a thermal or an instantaneous current that may be causing the trip and it it the MCP or OLR tripping?
Regarding my previos post you can lead a horse to water but you can't make him drink. By elimination the possibilities for nuisance tripping by first assuring that the MCP is correct and correctly adjusted and if the OLR the same. Has this even been established yet?
If what I have outlined in my previous post has been covered and confirmed as being correct? If so it is at that time that you cvoinsider the motor application and loiad otherwise yu OU asre just chasing you tail.

 

[GoldDigger]

It seems to me that, just as with a transformer, the only plausible mechanism for high initial cycle inrush current is a motor is the residual magnetism and hysteresis that occurs in the iron parts of the magnetic circuit. We do not typically think of this for some reason.
This would also mean that the inrush current can vary greatly depending on the polarity of the residual magnetism compared to the voltage phase at the moment the contacts close.

 

[ptonsparky]

trying and have explained several times that inrush is for transformers. Starting current is for motors. I will not argue either way. Ignorance is bliss. Here is one of the reasons we have problems understanding View attachment 14892 I

Older meter but a good one. Has an Inrush button in the Center.
I tried it on small transformer we have in the shop. 3-5KVA 1PH Dry
I expected to get the magnetizing current as you state. Nothing.

Primary Inrush used in conjunction with a start of a motor load on the secondary was, for APP, identical to Max current readings

 

[gar]

160429-1255 EDT

In a motor the large air gap in the magnetic circuit path greatly flattens the apparent hysteresis curve, thus reducing any inrush current from core saturation relative to startup current. This air gap results from mechanical clearance needs.

In transformers the goal is to make the apparent air gap as small as possible to minimize leakage inductance. By using tape wound cores of relatively square-loop materials the leakage inductance can be made small. These transformers if driven near their saturation point can produce very large inrush currents at times.

.

 

[templdl]

It seems to me that, just as with a transformer, the only plausible mechanism for high initial cycle inrush current is a motor is the residual magnetism and hysteresis that occurs in the iron parts of the magnetic circuit. We do not typically think of this for some reason.
This would also mean that the inrush current can vary greatly depending on the polarity of the residual magnetism compared to the voltage phase at the moment the contacts close.

Yes, induction motors and transformers are similar. When a wire is coiled around a magnetic core and the coil is initially energized with an alternating current the first 1/2 cycle the core is magnetized in one direction. The current we which is allowed to flow in the coil is limited by the resistance of the wire in that coil which can be extremely high. Then after the 1st 1/2 cycle the current which is basically a DC current to that point reverses reversing the magnetic field in the core, the collapsing and reversing magnetic field in the core induced a counter EMF in the coil which opposes the current flow in the coil, hysteresis.
With energy efficient motors the coil windings are optimized along with a more magnetically efficient steel in the core results in less opposition of current flow to magnetic the core resulting is an increased currect "inrush" when the motor is initially energized. Depending upon and residual magnetism is the core as well as the point of the cycle that energization takes place that peak current can vary which can result in one getting lucky when starting a motor. The nuisance tripping of MCPs was a problem until the time the NEC recognized this issue and was changed in the mid to late '80s?
Energizing a transformer can have the same issies by tripping the primary OCPD magnetically.
It is prudent to first assure that the combination motor starter is correct both MCP and OLR and both correctly set for the motor being controlled first before proceeding which the OP has yet to confirm, in trying to diagnose other issues that you may think could be causing the tripping issues.

 

[Jraef]

No plot, but the phenomenon is discussed at length in these papers.

http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/2100-wp005_-en-p.pdf

http://apps.geindustrial.com/publibrary/checkout/DER-019?TNR=Application and Technical|DER-019|PDF

I got my info from an article in the EASA magazine "Electrical Apparatus" by Richard Nailen called "Motor inrush current: what does it really mean?”, but now I can't find it on the web, only lots of references to it. I have a copy of it on a hard drive somewhere and if I find it, I'll send it via PM, but I can't post it here without getting the site in trouble.

 

[templdl]

No plot, but the phenomenon is discussed at length in these papers.

http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/2100-wp005_-en-p.pdf

http://apps.geindustrial.com/publibrary/checkout/DER-019?TNR=Application and Technical|DER-019|PDF

I got my info from an article in the EASA magazine "Electrical Apparatus" by Richard Nailen called "Motor inrush current: what does it really mean?”, but now I can't find it on the web, only lots of references to it. I have a copy of it on a hard drive somewhere and if I find it, I'll send it via PM, but I can't post it here without getting the site in trouble.

The Rockwell document explains it well. Excellent.
While as a leading manufacture's sales and applications engineer I went through a lot of agony addressing nuisance tripping issues in regard to the introduction of energy efficient efficient t motor as they were introduced and then mandated it the 1980s. Until the NEC recognized the new dynamics of the design E motors we were left with our hands tied regarding the NEC motor protection requirements up until the code was changed to accomodate the new motors. When replacing a standard efficiency motor with a EE the settings of the motor control must be re-evaluated.
It is to be recognized that an MCP, should it trip when it has been set correctly, will trip as a result of a motor failure in order to protect the motor control and upstream circuit. MCPs were invented by Westinghouse to reduce the possibility of castistophic motor failure which often resulted in fires because fuse protection wasn't sufficient enough to respond to such failures quickly enough. You have the ability to adjust the instasntaneous pick up to that which is allowed by the NEC even reducing the pick up setting to a point just above the point of nuisance tripping upon motor start up. Should the MCP trip there should be a very strong suspicion there has been a winding failure. MCPs when set correctly should limit motor damage which would allow the motor to be remound and repaired and not destroyed.

 

[templdl]

No plot, but the phenomenon is discussed at length in these papers.

http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/2100-wp005_-en-p.pdf

http://apps.geindustrial.com/publibrary/checkout/DER-019?TNR=Application and Technical|DER-019|PDF

I got my info from an article in the EASA magazine "Electrical Apparatus" by Richard Nailen called "Motor inrush current: what does it really mean?”, but now I can't find it on the web, only lots of references to it. I have a copy of it on a hard drive somewhere and if I find it, I'll send it via PM, but I can't post it here without getting the site in trouble.

The Rockwell document explains it well. Excellent. For some reason I double posted.

 

[Cow]

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

Is that the breaker size or the adjustable instantaneous trip setting?

 

[Ingenieur]

Is that the breaker size or the adjustable instantaneous trip setting?

that may be the solution
a cb with adjustable instant. trip setting and/or a small delay
Electronic trip unit or something

 

[gar]

160429-1530 EDT

From the several references supplied above there is some useful information.

Ingenieur: your reference
http://energy.gov/sites/prod/files/2014/04/f15/avoid_nuisance_motorsys_ts6.pdf

is very good in describing the difference between inrush current and starting current for a motor. This discussion providres a figure of 2.8. But to compare apples with apples a figure of 2 should be used. The 2.8 comes from comparing a peak current with an RMS current. If a sine wave RMS current is represented by its peak value, then the ratio is 2.0 . A factor of 2 is not large compared to the steady state starting current. This means the moderating effects of the motor air gap are very important.

I may have made too tight a description of motor inrush current to starting current, but they are in the same ballpark. Obviously running current is considerably lower than both of these.

In a transformer, and much more so in a high efficiency unit, the ratio of peak possible inrush current to full load steady-state peak current is greater than in a motor. In some ways a motor provides a greater problem than a transformer because of long startup time, more energy is dissipated during the initial starting period.

If an instantaneous trip point is set just above the peak starting current for a motor, then if there is inrush current just above the instantaneous trip pont, then tripping will occur. Obviously you raise the trip point.

In an RL circuit, where L is constant (no core saturation), with an applied AC voltage turned on at a zero crossing there is a DC current transient added to the steady state AC current that is not related to core saturation. This DC offset increases as R is reduced if L and frequency are constant. If the trip mechanism is DC sensitive rather than energy sensitive, then this DC transient needs attention.

In a transformer one is concerned with core saturation as the primary cause of inrush current.

.

 

[Ingenieur]

I think since we are in general precise technical sorts we get lost in nomenclature and semantics

inrush is as good as any term for the iniial 'charging' or unloaded, stationry let through
starting for when it begins to rotate
and locked rotor for stalled against a load > peak starting torque

From all the good literature posted an 'inrush' is 2+ times locked rotor (or starting which are similar in magnitude)

for this motor at 460 vac ~ 85 A
inrush ~ 170+ A which is inline with his measurements
or ~20+ times FLA, this motor has a starting/fla ratio of 10 already!
many cb's are not fast enough to react
in this case possibly load inertia drags it out and it is randomly of long enough duration to trip

he has options to deal with it
and I know I learned something
so a worthwhile discussion was (is being) had

 

[Jraef]

... This means the moderating effects of the motor air gap are very important...

I think that may be another factor in the nuisance tripping on the new EE motor designs. I went to a class on EE motor implementation at the EPRI (Electric Power Research Institute) down the road from me here in Palo Alto about 20 years ago, because I was doing a lot of work retrofitting old motors with new EE motors (to take advantage of PoCo rebates) and we had a LOT of troubles with tripping MCPs too. One of the other techniques they described in that course as being used to increase the efficiency in these newer motor designs is to decrease the air gap as well.

Going from memory, the main design changes were, in no particular order:
Lower winding resistance by using magnet wire with a larger gauge
Higher insulation temperature ratings so a higher rise could be allowed
Smaller air gap
Better bearings (partially because of the smaller air gap)
Reduced fan cooling by increased convection cooling in the frame design
Better grades of grain oriented silicon steel for lamination of the stator and rotor cores to reduce magnetic stray losses

 

[Phil Corso]

That Man...

1) Have you tried starting just one motor?

2) Have you tried starting a motor of like size, but of different mfg?

Regards, Phil Corso

 

[Phil Corso]

Gentlepeople...

My reason for the previous suggestion is because I just completed an analysis of a Soft-Start problem involving a major, well-know, well-liked VFD supplier!

All four units had like-faults, which weren't picked up until it was noticed that their factory assigned serial numbers were sequential!

Replacement of those components solved the problem!

Phil Corso

 

[That Man]

Everyone,

OP here. I have read through the forum, and come to the conclusion that more than likely, this behavior I am witnessing isn't an anomaly; I just haven't gathered field data from installations enough times to recognize that these numbers are actually quite normal, especially for brand new energy efficient motors, which these are. By looking at the trip curve for a 15A breaker, it is plain to see that the starting current will likely engage the instantaneous portion of the curve, and that's ok.

NEC article 430.52 states that for an inverse time circuit breaker, the maximum permissible size of the breaker should be 250% of the rated FLA, upsized to the nearest standard size. Our FLA is 8.3A, x 2.5 = 20.75A, upsized to the nearest standard size, this would bring our circuit breaker to 25A. A 25A CB will not instantly trip at the measured currents, so we are probably OK by simply upsizing the breakers. That's what the client has elected to do instead of further analysis.

This matter has likely been solved, but I'll post more if further issues arise. Thank you all for your valuable input; it has been educational.