400 hp hammermill motor common failure once a year 3

[jnorris04]

I have a 400 hp motor on a hammermill that has failed every year for the past seven years.

The motor is equipped with a telamecanique altistart 48 soft start and is 3 phase 480 volt. This hammermill is used to grind corn for a feed mill.

When the motors are taken to a motor shop, it always reveals that it appears to be an overload. After studying how the hammermill is operated, it is never overloaded nor exceeds its nominal nameplate current. The softstart, I believe, has been programmed correctly.

We have tried several different brands of motors and all have had the same result. The insulation is deterioted and goes to ground.

I have had several people to look at this application and I still have no answers.

Any help would be greatly appreciated.

 

[waross]

Corn dust builds up in the windings and inhibits cooling. Even though the motor does not exceed full load current, when the heat cannot escape from the windings fast enough the temperature goes up. Back in my sawmill days we had similar issues with fine saw dust causing motors to overheat. Most weekends were spent blowing dust out of motors.

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

 

[jnorris04]

Motor is cleaned atleast once a day to remove buildup. I did a thermal study on this motor and the temperature of the exterior of the motor never got above 105 degrees.

 

[itsmoked]

105F?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Has it always been the same phase failing grounded?

How close are the phase voltages while it's running?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rhatcher]

Is the motor TEFC (totally enclosed fan cooled) or ODP (open drip proof)? For a TEFC motor it would be sufficient to blow the dust off of the motor exterior to ensure that the cooling fins and cooling fan are clear of dust build up.

For a ODP motor, the problem is more difficult to deal with. The dust will get inside the motor, clogging the stator core cooling vents and the stator-rotor air gap. This will dramatically reduce the motor's ability to cool itself and lead to winding failures that have a uniform dark appearance similar to an overload. Unfortunately, it may be difficult or impossible to adequately blow the dust out of the motor interior. This will be especially true if the motor interior is contaminated with grease or oil that will cause the dust to coagulate into a solid mass.

If you have a ODP motor and are unable to swap to a TEFC motor due to space limitations or budget, you should consider removing the ODP motor every six months for cleaning. The cost to clean a motor is only a fraction of the cost to rewind one and the time required for cleaning is also much shorter than rewinding. It sounds like you should have several of these motors laying around by now so you may be able to swap, or rotate, motors every six months with minimum downtime.

 

[jraef]

Is the soft starter bypassed with a contactor once the motor is at speed, or has someone sold you on the "energy saver" feature that leaves it on the SCRs continuously?

If it is bypassed, do you have an OL relay in the circuit so that the motor is still being protected once the bypass contactor closes? I've seen that one time and time again. The problem is, the diagrams that Telemecanique puts into their manuals do NOT show a thermal OL relay in the circuit! They simply add this statement as a footnote:
"External differential thermal protection will need to be added."
So people who don't fully read all instruction details (translate to mean MOST MEN) fail to notice and don't put in an OL relay at all. Then once the bypass contactor pulls in, the motor is running completely unprotected and nobody realizes it!

Worst possible scenario: are you using the dreaded "inside the delta" connection of the soft starter to save money on the soft start size? If so, that is a quick way to damage motors! If you have one shorted SCR or one welded contactor, you have power flowing through one winding at all times, with nothing but the fuses or breaker to protect it from damage. Usually, the current flow is enough to cause something to clear, but that is not guaranteed.

"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

 

[Chevin]

Agree with most of the comments, I would only add that a thermistor relay would be able to detect if the motor windings became too hot.

A conventional overload relay would not detect over temperature, only over current!

 

[bearsy87]

have your motor shop install rtds. You will be able to monitor each phase's temperature in real-time and also trend. btw, easa states that 1/32" of dust on the stator of a motor can raise the operating temperature by 10*C

 

[automatic2]

with such inertia, you likely have an extended start. If, as suggested, your starting part winding with softstart in open Delta, you'll hammer the one winding until your up to speed, hence the absolute requirement for O/L on both windings. I'd be looking at shortening your start ramp as much as possible while monitoring voltage dip. I just had a situation where a much smaller softstarter was single phasing during the ramp until it went DOL on the relatively small bypass contactors, without the starter reporting any errors.

 

[Tmoose]

OP JNorris has not logged in since Sept 9. So the type of motor, the starting method and the location of the insulation failures is likely to remain unknown.

 

[jnorris04]

It is a TEFC motor. It uses a inside the delta connection. It uses a soft start to ramp up then switches to bypass contractors.


UPDATE: I did a vibration analysis on the machine, and the result was the base was flexing and that the anchor system was not Sufficient. After reanchoring and grouting the machine, also found that the rotor inside the machine was out of tolerance by an eight of an inch causing an imbalance. Replaced rotor, vibration has disappeared. Thank you all for your responses.

 

[waross]

Thank you for the update, jnorris04. We appreciate the closure.
Yours
Bill

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

 

[electricpete]

Do you suspect this vibration is related to the stator failures that had been diagnosed as caused by overload?

Or are these just additional facts to fill in the case study ?

=====================================
(2B)+(2B)' ?

 

[jnorris04]

Motor shop confirmed, after going through the motor, that the losses were contributed by vibration. I will not be certain that this fixes the problem until after a year of running the new motor because failure usually occurs 10-12 months from installation. I am pretty confident this fixes the problem because everything from the peckerhead back to the motor control panel and components inside motor control cabinet have been replaced. Vibration on the horizontal plane of the motor was .933 in/sec, which is double the permissible limit.
After repairs were made, vibration was .185 in/sec.

 

[gernsperger]

How many starts per hour,
Had a similar problem with a pellet mill in a feed mill,
found the operators would plug up the pellet mill, instead of
cleaning it out they would "plug the motor" to try and free it up,
was also through a Benshaw soft start, number of starts per hour
for a 400 hp motor is very low, one or two per hour recommended, a
softstart may help that, but if your exeeding this number of starts/hour
by a large amount, the heat build up in the motor is significant.

 

[mikekilroy]

Please come back in 1 yr with update. I for one can not understand at all how vibration can cause motor overheat in this situation based on your previous facts. It does sound more like just closure for closure sake. Am I missing something? Can someone explain to me how a vibration can cause a motor not pulling excess power to overheat?

 

[automatic2]

Motor shop confirmed, after going through the motor, that the losses were contributed by vibration. I will not be certain that this fixes the problem until after a year of running the new motor because failure usually occurs 10-12 months from installation. I am pretty confident this fixes the problem because everything from the peckerhead back to the motor control panel and components inside motor control cabinet have been replaced. Vibration on the horizontal plane of the motor was .933 in/sec, which is double the permissible limit.[/qoute]

you are likely seeing the cause of overheating. First thing would be to check which class of O/L your set up for. Bearings will dump thier grease at elevated temps, leading to shortened cycles. Exceeding belt tensions on overhung loads are a potential from maintenance. Do you monitor the frequency of starts? Are you regulating the mill's rotor current, possibly via a feed system?