MCP Trips every other time

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edgarp

Member
Location
East Tennessee
We have a weird situation that has us scratching our heads...sorry for the long post and I hope I don't leave anything out.

System: Two 40 hp belt driven fan motors. Each motor has a normal and an emergency supply. Motor #1 starts and runs fine on both feeds - no problems at all.
Normal supply is about 30 feet from transformer. Emergency supply is from 600 feet away, but voltage drop doesn't appear to be a factor. A manual transfer switch changes from normal to emergency - double throw, 3 pole, 600V, 100A non-fusible.
On emergency power, motor #2 starts and runs every time. On normal, it starts every other time. The MCP (Cutler Hammer HMCP100R3) trips every other time, and once it is reset, the motor will start. Settings on the MCP have been adjusted up with no change - currently set at 600A. Everything in the combination starter is common to both feeds from the XFR switch.
We online (Baker) tested the motor with normal results, same on both feeds. MCP has been replaced. The guts of the manual transfer switch have been replaced with no change. Both motors start and run with same current and acceleration time, and voltage is within 5 volts on either supply. The power quality appears to be fine, tested with Fluke equipment.
All branch wiring has been meggered with good results. At one point the motor was reconnected for the wrong rotation and it started every time. That shouldn't significantly affect inrush, just running current I thought.
Every time we come up with an idea, it just doesn't add up since the motor starts and runs every time on one feed, and every other time on the other. Once the motor is running, it runs perfectly, just as motor #1 does. If we had a fault somewhere we believe tripping would be more random, but I have seen multiple starts and it is a definite pattern of every other time.
Has anyone ever seen anything like this? We believe the combination start is ok because it is common to both feeds, and we believe the motor is fine, as well as the transfer switch. Between engineers and electricians we have tons of experience, but like I said we are scratching our heads.
 

rcwilson

Senior Member
Location
Redmond, WA
Initial motor inrush current will vary depending on when the starter contacts close in relation to the voltage sine wave. The instantaneous value of current also depends on the available short circuit at the motor terminals. Said another way, the impedance of the power system limits the inrush current to the motor.

When your motor is on the standby source, the short circuit level is less than your normal source because there is more impedance in that circuit from the utility to the motor. That extra impedance is just enough to keep the peak inrush current below the instantaneous trip point of the Motor Circuit Protector.

When you start it on normal power source, half the time you get lucky and close the contacts when all three phase voltage waveforms are at beneficial point and inrush is less than the trip setting. The other half the time, one of the phase voltages causes a high peak inrush and trips the MCP. (This is the charging current inrush, not the motor acceleration inrush).

Two "identical" motors will probably have different inrush currents. The solution is to raise the trip setting on the MCP to the 700, 800, 900 or 1000 A setting until it doesn't trip. You will still have short circuit protection

This is a very common problem with high efficiency motors.
 

G._S._Ohm

Senior Member
Location
DC area
Two 40 hp belt driven fan motors.
>Steady state VA draw for each motor is slightly more than 40 x 746 = 32000 VA?

Each motor has a normal and an emergency supply.
Motor #1 starts and runs fine on both feeds - no problems at all.
Normal supply is about 30 feet from transformer.
Emergency supply is from 600 feet away, but voltage drop doesn't appear to be a factor.
On emergency power, motor #2 starts and runs every time.

On normal, it starts every other time.

>means the problem is on the verge of happening. 50-50 chance.


Once the motor is running, it runs perfectly, just as motor #1 does.

>so it's an inrush current issue



Normal power @ 30' probably has less source impedance than emergency power. Less source impedance = higher inrush = CBs not happy.
You could confirm this by adding some resistance to the 30' line for that particular motor.
If this is all true then you were just lucky with motor #1 and if you had 10 motors wired like these two then, on average, you'd have half with this problem.


This reminds me of a [supposedly] 15 A chopsaw that tripped the resi. 15 A breaker 1 time in 4.
The fix was to run the saw on an extension cord of whatever gauge and length that would reduce the trip frequency to a much lower level without damaging the motor because of a too-high source impedance.

Good problem.

If I'm right I take MasterCard, Visa and American Express. Operators are standing by! :)
 
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edgarp

Member
Location
East Tennessee
Thank you for the suggestions. An electrician did tell me he adjusted the trip setting on the MCP to the maximum, but I cannot verify since that happened before I was involved. This is usually our first "fix" for a trip when installing new motors or MCPs. I will see if we can adjust it higher as our next step.
The other option we were considering was to bypass the MCP with fuses and test again, to see if we could eliminate the MCP as the culprit.
Thanks again!
 

zog

Senior Member
Location
Charlotte, NC
Thank you for the suggestions. An electrician did tell me he adjusted the trip setting on the MCP to the maximum, but I cannot verify since that happened before I was involved. This is usually our first "fix" for a trip when installing new motors or MCPs. I will see if we can adjust it higher as our next step.
The other option we were considering was to bypass the MCP with fuses and test again, to see if we could eliminate the MCP as the culprit.
Thanks again!

Settings should be determined by a coordination study, not just raised when a breaker trips.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I would think that you would be able to measure the inrush every time and see what the results are. Are you able to graph the inrush for that first second of time or are using min/max or peak readings from a clamp? Do they change? Are they similar to each other? Being able to see how long the current stays high may help more than just knowing the peak. Can you swap the MCPs?
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Have you tested the breaker?
Zog, they replaced the breaker. First line of the 3rd paragraph, post #1.

edgarp,
I'm with Bob Wilson on this (and G.S.Ohm), that this is just a simple inrush problem. You said the electrician turned up the breaker "to the maximum", but he is probably thinking maximum per the NEC, and without reading the exceptions. Because of the issue of new Energy Efficient motors having much higher magnetizing current, the NEC has been allowing settings up to much higher levels than traditionally taught to electricians. In table 430-52 the code allows up to 1100% for energy-efficient motors. But if you read further (and many people don't) an exception says you can set them up to 1700% if you can show that a lower setting will not stay in consistently. A lot of electricians who don't do industrial motor systems every day are unaware of this.

So assuming 480V, 40HP would have an FLA of approx. 52A, 1100% would be 572A, so a 600A setting would be closest on most dials. You most likely have a 100A breaker, which means the mag trip setting will go up to 1000A so now that you know it nuisance trips, you have room to adjust it all the way to 884A (call it 900) and still be OK per code.

430.52.C.3... Instantaneous Trip Circuit Breakers (MCPs) from the 2005 NEC
Exception No. 1: Where the setting specified in Table 430.52
is not suffıcient for the starting current of the motor, the
setting of an instantaneous trip circuit breaker shall be
permitted to be increased but shall in no case exceed
1300 percent of the motor full-load current for other
than Design B energy-effıcient motors and no more than
1700 percent of full-load motor current for Design B
energy-effıcient motors*
. Trip settings above 800 percent
for other than Design B energy-effıcient motors and
above 1100 percent for Design B energy-effıcient motors
shall be permitted where the need has been demonstrated
by engineering evaluation. In such cases, it shall not be
necessary to first apply an instantaneous-trip circuit
breaker at 800 percent or 1100 percent.

* If the motor is new, chances are about 99:1 it's an energy-efficient one, because that's all anyone is supposed to sell now unless you get a variance.
 
Thank you for the suggestions. An electrician did tell me he adjusted the trip setting on the MCP to the maximum, but I cannot verify since that happened before I was involved. This is usually our first "fix" for a trip when installing new motors or MCPs. I will see if we can adjust it higher as our next step.
The other option we were considering was to bypass the MCP with fuses and test again, to see if we could eliminate the MCP as the culprit.
Thanks again!

In your original post you stated "Settings on the MCP have been adjusted up with no change - currently set at 600A.". So now you're saying that you don't know this? 600A should be plenty. Are you also certain that this happens at every second attempt? Is this a definite patetrn?
 
Zog, they replaced the breaker. First line of the 3rd paragraph, post #1.

edgarp,
I'm with Bob Wilson on this (and G.S.Ohm), that this is just a simple inrush problem. You said the electrician turned up the breaker "to the maximum", but he is probably thinking maximum per the NEC, and without reading the exceptions. Because of the issue of new Energy Efficient motors having much higher magnetizing current, the NEC has been allowing settings up to much higher levels than traditionally taught to electricians. In table 430-52 the code allows up to 1100% for energy-efficient motors. But if you read further (and many people don't) an exception says you can set them up to 1700% if you can show that a lower setting will not stay in consistently. A lot of electricians who don't do industrial motor systems every day are unaware of this.

So assuming 480V, 40HP would have an FLA of approx. 52A, 1100% would be 572A, so a 600A setting would be closest on most dials. You most likely have a 100A breaker, which means the mag trip setting will go up to 1000A so now that you know it nuisance trips, you have room to adjust it all the way to 884A (call it 900) and still be OK per code.

430.52.C.3... Instantaneous Trip Circuit Breakers (MCPs) from the 2005 NEC


* If the motor is new, chances are about 99:1 it's an energy-efficient one, because that's all anyone is supposed to sell now unless you get a variance.

To be more in line with all parts of your argument a 40HP high efficiency motor is more likely to be around 42-45A. If this happens every other time, it is possible that on the second attempt to start there is still some residual magnetism is left so the magnetizing peak is not as high.

Baldor data on 841 motors: 45FLA, 286LRA @ 3600rpm; 46FLA, 280LRA @ 1800rpm and 50.5FLA, 355LRA @1200rpm. So all these would be far shy from the 600A the OP indicated the MCP is set.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
To be more in line with all parts of your argument a 40HP high efficiency motor is more likely to be around 42-45A. If this happens every other time, it is possible that on the second attempt to start there is still some residual magnetism is left so the magnetizing peak is not as high.

Baldor data on 841 motors: 45FLA, 286LRA @ 3600rpm; 46FLA, 280LRA @ 1800rpm and 50.5FLA, 355LRA @1200rpm. So all these would be far shy from the 600A the OP indicated the MCP is set.
Good point about the residual magnetism, hadn't thought of that.

My point on the example was only to show that his settings can be moved higher than what his electrician may have tried. It's just based on my experience that a lot of electricians who don't work on industrial applications a lot don't know that the 1700% exception exists, so they default to what they see on the chart, 1100%. There are many white papers out now explaining what this became necessary when motor mfrs began various programs to improve efficiency. I recently read one from Siemens because it became an issue on breaker selection in their MCCs; you can now opt for a larger frame breaker on certain size starters because the mag trip adjustments on their standard sizes don't go high enough for this.
 

G._S._Ohm

Senior Member
Location
DC area
Let's say both motors and breakers 'work' but they don't play well together so it's a tolerance problem.
Motor A pulls current I and breaker A trips at slightly above I
Motor B pulls current 1.1xI and breaker B trips at slightly above 1.1xI

Here's what happens when you mix and match
Motor...Breaker...result
A..........A.............OK
A..........B.............OK
B..........A.............not OK
B..........B.............OK
 
Good point about the residual magnetism, hadn't thought of that.

My point on the example was only to show that his settings can be moved higher than what his electrician may have tried. It's just based on my experience that a lot of electricians who don't work on industrial applications a lot don't know that the 1700% exception exists, so they default to what they see on the chart, 1100%. There are many white papers out now explaining what this became necessary when motor mfrs began various programs to improve efficiency. I recently read one from Siemens because it became an issue on breaker selection in their MCCs; you can now opt for a larger frame breaker on certain size starters because the mag trip adjustments on their standard sizes don't go high enough for this.

The information provided here is an excellent demonstration of how the new motor characteristics - due to the push for ever higher efficiencies - are not harmonized in the MCP setting rules or even UL approved combination starter selection. I wonder why the EU style MCP's, which were true motor protectors, combining short circuit and overload protection tailored to motor characteristics, did not take foothold in the US market. Klockner-Moeller, now owned by Eaton, had an excellent series.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The information provided here is an excellent demonstration of how the new motor characteristics - due to the push for ever higher efficiencies - are not harmonized in the MCP setting rules or even UL approved combination starter selection. I wonder why the EU style MCP's, which were true motor protectors, combining short circuit and overload protection tailored to motor characteristics, did not take foothold in the US market. Klockner-Moeller, now owned by Eaton, had an excellent series.
I use those a lot, I think they are great. Siemens even has a new one out that combines the Motor Protection Switch, disconnect, Solid State OL and contactor all in one unit, with a wide dial adjustment range so one unit covers a lot of HP sizes. Schneider (Sq. D / Telemecanique) has a similar concept that has you plug in little modules to set up different tip ranges. On both of those, the mag trips are designed around EE motor characteristics.

But the resistance I see towards them is mostly a "look and feel" thing. They don't look to be as sturdy as an MCCB and some people are afraid they will self-destruct. Most are rated for 65kAIC now, more than a lot of lower cost MCCBs, but the perception remains. I have proposed the new concept on several projects, most have been shot down without good reason, just "we wanted the regular style". I oblige, the size and cost goes up, they perceive they got more for their money I guess.
 
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