Motor Tripping Almost Instantly 7

[ch2mguy]

Can anyone advise me about why a motor can megger perfectly, but yet trip the breaker within seconds after starting?

I have a 75 H.P. supply fan motor, 1750 RPM, 89 Full Load Amps, 460 v. three phase. The 150 Amp breaker trip setting is adjusted to 6 x the frame rating. From my understanding, this breaker can handle an inrush of 900 amps [6 x 150A = 900A] before it will take the motor out of the circuit. The motor pulls 680 amps upon start up, then a second later trips the breaker.

I meggered each winding to ground, and they all had about 500 Megohms to ground. There were no open windings when I meggered from phase-to-phase. Any ideas why this motor would trip? It had been unused since 2004, and spinning backward until recently when we decided to recommission it. We verified that the fan was stopped before we energized it. Any suggestions as to why this fan would trip?

 

[itsmoked]

Was it originally running in the same setup? Or is this all new stuff? Bad breakers aren't unheard of. Can you swap yours out?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

600% is the STARTING current for a typical Design B motor. The INRUSH current, defined as the current that flows in the first few cycles as the windings magnetize, can be theoretically as high as 2000% for newer energy efficient motors. The NEC addresses this in an exception below table 430.52, allowing for the Instantaneous Trips to be set as high as 1700% if you can demonstrate that a breaker will not hold in at a lower setting. I think that you have just accomplished that.

Typically, breakers have an instantaneos trip setting range that goes up to 10X the current rating of the breaker. So a 150A breaker will have an instantaneous setting that will allow 1500A. On a 75HP motor, with a likely FLA of 100A, that is only 15X. Try that, if it doesn't work, you may have to go to a larger breaker.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[dpc]

I suspect it is tripping on inrush, as jraef mentioned. But if this is the case, it can't be running for a full second before tripping, but tripping almost instantaneously. If it is truly running for an actual second, it may be something else.

How are you measuring the motor starting current?

 

[WTRVS]

Have you tried measuring resistance between all phases? I don't mean the resistance of each phase (to check for open windings) but there might be a problem between two phases. Make sure that you remove the starpoint connections when doing so.

Kind regards

 

[bayoubare]

Check for broken rotor bars

 

[ScottyUK]

Specifically what type of breaker and tripping unit do you have? Some breakers designed for have phase imbalance sensitivity. At the moment we're all assuming that it is a simple mag-only tripping unit, but on younger installations that can be a mistaken assumption.

 

[RoyDoug]

Sounds like the fans high inertia is prolonging the startup. At startup with a direct on line starter the inrush current is about 6 x that of full load. If your run up time is longer than say 3-4 seconds this could trip the breaker. Remember the run up time is dependant upon the difference between the load torque and the motor torque, it is called the average accelerating torque.

 

[ch2mguy]

Thank you all for your very helpful comments. The starting components (i.e., 150 amp frame breaker, NEMA Size 4 mag starter) were salvaged from an outside vendor. I originally suspected that the breaker's sensing element (I don't know if it is thermal or magnetic) was weak, and tripping on a lower setting than specified by the breaker trip setting. I know from experience that any manufacturer of circuit breakers will only guarantee ONE trip at the specified amperage rating of the breaker. After the breaker trips once, it is no longer considered "in spec". The salvaged starting components were an unknown quantity in the equation.

However, we changed out the motor, and the problem disappeared. I didn't have a micro-ohm meter to determine if one or more of the windings were damaged. Chalk this up to unexplainable phenomena. The original 75 HP motor had been in service since the 1980's, so perhaps the inrush characteristics were such that it would not start with the used circuit breaker we installed. The original components inside the cubicle of the MCC for this fan motor were gone; this is why we had to purchase salvaged parts. Also, I did not have my power monitoring instrumentation on the circuit during initial start-up, so I have only a rough idea of the actual inrush current value. I used a Fluke 321 amp meter to measure inrush, thinking that this would be a reasonable method of troubleshooting, but this was more complicated than I thought. Worse yet, this was a rush job, so we had to make an executive decision. That's why we opted for another motor.

Thanks again to all the contributors to my questions.

 

[itsmoked]

Thanks for coming back with a result. That helps us all.

We used to have a used electrical supply place in my town. Whenever we picked a breaker the guy would dash in back and test its tripping current. Since breakers don't care about voltage his rig used a very low voltage. He told me a surprising number of used breakers were bad and that was why he always tested them.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DaveMeharg]

ch2mguy,

I'm glad you were able to solve your problem. I have struggled before with tripping issues on motors. Here are some thoughts for future use

1. If the motor is tripping instantly on start then it could be an inrush issue. Especially if the NEMA code letter on your motor is an "H" or higher. When you read the fine print on the time-current curve of many of the circuit breakers they indicate there is a 15 to 25% tolerance. As someone else indicated above the six times rating is not what occurs on the initial inrush which can be much higher in the 12 to 13 times or greater. It also varies with where on the sine wave you happen to pull in the starter. So one day it may hold and the next it could trip. For example the inrush could be say 89 amps x 13 or 1157 amps. If you have a 150 amp thermal mag breaker with a instantaneious of 10 times or 1500 amps the instantaneous could have a 25% tolerance which could make it trip down at (1500x.75)1125 amps which would have it very close to the possible inrush. I have had this situation occur several times and it was resolve by going to the next higher breaker as long as you stayed above the NEC limits in article 430. For a Design B energy efficient motor you are allowed to go to 250% of the full load current for thermal mag circuit breaker which would allow you to go to 225 amps. It may be that when you changed the motor the new motor has a lower inrush (compare the NEMA code letters between the old and new).

2. If the motor is tripping after a secound or two I don't think its a inrush issue which would only occur during the first few cycles.

 

[waross]

You mentioned that you did not know if your breaker had thermal or magnetic trips. It has both. The thermal trip has an inverse time characteristic and will take quite a while to trip at rated current. As the current increases the tripping time shortens.
The adjustable trips are magnetic and are considered instantaneous.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Unless it was a Mag Only breaker of course...


"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

I call those Motor Circuit Protectors Jeff, but your comment is valid.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

"Motor Circuit Protector" and it's acronym "MCP" are both trade marked, originally by Westinghouse, now owned by Eaton / Cutler Hammer. When I worked for Siemens, were were SPECIFICALLY told to never use that term.





"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

I was not aware of that, Jeff. If you ever get up to Alberta, I'd love to buy you a drink and swap stories for a few hours.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I was just there, in 1996. Where were you? Flew into Calgary, drove up to Edmonton, on to Ft. McMurray, back down to Edmonton, and flew home from there, took about a week all total. Caught a Flames / Avalanche game while in Calgary the year the Avalanche won the Stanley cup. I remember the Saddledome stadium had big flame throwers in the ceiling that said Enron on them; they fired off big balls of flame whenever the Flames scored. I wonder if they still say that?

I have gone to Calgary a couple of times for the Oil and Gas show too, but it may have been before that, I can't remember. I like that town, lots of fun stuff to do. It's also close to Lake Louise and Banff. Been there too. Edmonton was more businesslike and bleak, Ft. McMurray was a pit and that was one of the longest loneliest drives I have ever made.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Ditto on the Ft Mac drive. About 470 miles from Calgary. But I was in Central America in 1996.
A lot of contracts in the tar sands are now flying everyone in from Edmonton or Calgary in 737's with connections to almost anywhere. Work 14 days and then spend 7 days at home.
I spent some time in the Yukon years ago and had an out of gas experience on the Campbell Highway that wasted about 8 hours. That's when I started carrying extra gas.
I probably drove some of the same roads as you did in North Central BC. Do you remember the Cassiar Road? That's another long lonely drive.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter