soft starters on std or high efficiency motors

[littleriver]

I recently installed a 500hp grinder-(hog).I used A SOFT START to bring the 500hp motor up to speed.The motor failed in about a month.The motor mfr. replaced the motor under warranty,but stated that they would not warranty the motor unless we replaced it with a high efficiency type motor.They said because we were using a soft start.I asked the price difference and the high eff motor was less.I asked why,and was told the rotor was aluminum instead of copper.
My question is has anyone heard of this before,and is it the rotors of the motors that make it better for soft start use.Note:I have the same type motor on another grinder with a soft start for several years now without a problem.

 

[jraef]

Sounds fishy to me. Blanket statements like that are usually the fall-back position of people with no clue. I once had a motor manufacturer's rep try to tell me that my soft starter somehow cause his rotor to crash into the stator! In general though, a motor with copper rotor bars usually has better thermal performance than one with aluminum, which tends to expand and contract more.

First off, did you or anyone else identify the failure mode of the motor? That would be the first order of business for me. If it is failing for some other reason (highly likely actually) and that condition still exists, you are going to have a 2nd failure on your hands. Even though the motor mfr. has replaced it under warranty, you still suffer the downtime and replacement labor costs.

2nd, what were your starting conditions, i.e. current limit, ramp time, initial torque settings, duty cycle etc. as well as the power considerations. It is possible to over stress your motor for the application if you don't pay attention to the settings. Most factory default settings on soft starters are for simple common loads such as pumps and compressors. A hog takes some finesse depending on the power availability. People often try to use soft starters to create miracles where the power grid is inadequate to support it.

 

[littleriver]

Thank you for your reply jraef.Iam using a current ramp with a ramp time of 15 sec.A SF-1.15,NEMA DESGN-20.THE MAX CURRENT IS 600% FLA-547.The soft start keeps track of thermal load based on these and other parameters and will shut down at 100%.I do not have all the parameter sets today because I am at another plant at this time.I had installed a new 2000 amp service for this application.The mfr told me that the rotor had moved on the shaft causing enough damage to make the motor beyond repair.I had also beleived that copper rotor was better than aluminum

 

[Marke]

Hello littleriver

High efficiency motors are generally much more difficult to soft start because the LRC tends to be much higher than a standard motor. This results in a higher start current to develop the same torque output.

I see no reason why a high efficiency motor would be more reliable on a soft starter than a standard motor, or why a soft starter would cause less damage to a high efficiency motor.

If the rotor moved on the shaft, I would put that down to a manufacturing problem unless the soft starter was faulty and causing severe torque transients.

The danger with high efficiency motors, is that installers can set the start voltage/current too low and the motor can spend considerable time in the stalled or semi stalled condition before developing sufficient start torque to reach full speed, thereby causing too much power to be dissipated in the rotor and causing a rotor failure. Provided that the starter is set up correctly, this will not be an issue.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

The rotor moved on the shaft??? How on earth would starting it softly cause that to happen? I want some of what he is smoking...

There is another fallacy here as well. An "energy efficient motor" design (which by the way doesn't usually include 500HP) typically results in LESS starting torque than a standard Design B motor, so soft starting is often more problematic in that the starting current generally ends up being slightly higher for a given load. Their energy efficiency comes into play once they are at speed.

Back to the copper rotor issue, if you look at what are often called "crusher duty" motors, they almost always have copper rotor cages, and starting a rock crusher is a lot more difficult than starting a hog.

This article out of Electrical Apparatus magazine should give you some quality insight.

http://findarticles.com/p/articles/mi_qa3726/is_200403/ai_n9361539/pg_1

Here some nice little sales pitch for copper cage construction as well. Keep in mind they are biased opinions, but most of the data can be substantiated.

http://www.copper-motor-rotor.org/pdf/EEMODS-Motor_Test.pdf

http://www.etimotors.com/copper.htm

In my humble opinion, your motor guy is clueless and is just hoping beyond hope that you are less informed than he is.

 

[waross]

Rotor shifted on the shaft? I would be looking around for a large piece of steel such as several feet of 2 inch shafting with signs that it went through the hog.
respectfully

 

[edison123]

jraef

Biased opinion is right. ETI states as advantage for copper rotor

"Higher Melting Point". I guess their motors are apparently designed for volcanic conditions.

*Even inside a hard-boiled egg, there is a golden heart!!!*