Wound Rotor vs Squirrel Cage 14

[OhioAviator]

Hi All,

I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+) automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?

Thanks!

 

[OhioAviator]

The screwdriver (if you don't mind the broken handle or broken shaft), and the adjustable wrench (if you don't mind the finishing nail sticking out of where the knurled shift adjuster lockscrew is supposed to be), and the duct tape (after you warm it up and peel it off the old furnaces and ductwork) are all three easily obtained from the shredder feed pile. The Simpson 260 will probably need to be purchased new (I probably wouldn't trust a meter that I dug out of the shredder pile; at least not more than once!). As DougMSOE says, we are VERY cost conscious!

In all serious folks, I had no idea that this thread would be so popular. Thank you for all the great and valuable input. I learned a great deal from this thread.

Best Regards,
John Ruble

BTW... I'm thoroughly convinced that a WRIM is all-round the best way to go for a scrap yard shredder application.

 

[jraef]

Spoken like a man forced to improvise on the job trying to get something operating before he is allowed to go home...

Been there, done that, got the T-shirt and the hat.

 

[jbartos]

Questions: How much time/downtime does it take to replace worn out slip rings of WRIM, how soon do they get worn out, and how much does it cost?

 

[DougMSOE]

jbartos,
That depends on;
The design of the slip ring assembly.
I have seen these motors in this duty last 20+ years with only cleaning and a air dry insulating varnish and no ring change outs.
Other machines with a poorly designed slip ring assembly and poor maintenance 5 years.
Some designs can be changed out inplace in about 4 hours, others inplace, 2 to 4 days.

 

[FACB25]

One of the more informative and entertaining threads I've seen. Thanks to all.
Smaller scale similar problem is the chipper in a sawmill, high shock loads when a chunk of oak or hard maple goes through. We used a slightly oversize SCIM with a Benshaw SS. The chipper itself includes a large inertia mass which helps. I guess my rambling point is that while a WRIM is probably best, look to increase inertia if possible.

 

[aolalde]

FACB25:

I am glad that you mentioned INERTIA, which is a resource that has been forgotten lately. It is interesting the way it works on heavy peak load applications.

 

[jbartos]

Comment on DougMSOE (Electrical) Feb 13, 2004 marked ///\\jbartos,
That depends on;
The design of the slip ring assembly.
I have seen these motors in this duty last 20+ years with only cleaning and a air dry insulating varnish and no ring change outs.
///This 20+ years life-expectancy is probably what the automobile shredders WRIM need.\\Other machines with a poorly designed slip ring assembly and poor maintenance 5 years.
///The frequent current transients due to variable and impulsing automobile shredder loads reduces the slip ring life expectancy.\\Some designs can be changed out inplace in about 4 hours,
///4 hours for the slip ring replacement seems to be optimistically short time; especially, if the rotor balancing is included.\\ others inplace, 2 to 4 days.
///This is more like it, if the good unionized workmanship is in place.\\\

 

[gsimson]

Some of the requirements which are required to be met as indicated are better met by SqIM along with VFD.
Easy to fix- No rotor resistance, Rotor cables , Rotor contactors . Only a Sq. cage motor with 3 leads connected to VFD.
Simple parts & simple equipment - What is nmore simpler than SQIM?
Tough beyond belief ( Shock , Vibration) - Sliprings & rotor resistances are certainly not tough.
Cost - A slipring motor with rotor resistance / rotor panel is certainly costlier than a Sq. Cage motor & VFD.
Easy to understand - The requirement is only for maintenance. When SqIM & VFD do not need any maintenance why every one need to understanmd ?
ALL repeat ALL the applications can be met by SQIM & VFD at a cheaper cost both capital & running, with min. maintenace and much better speed /Torque control. Both Dc motor and WRIM will become museum pieces & for academic interests as stated earlier.
Only the cost of VFD is abnormally high for MV applications. Over a period of time this also will come down.

 

[electricpete]

gsimson - I appreciate all your expert advice and opinions.
I have to take exception with one thought process:

"Easy to understand - The requirement is only for maintenance. When SqIM & VFD do not need any maintenance why every one need to understanmd ?"

I wouldn't buy from anyone that told me I don't need to understand it because it's never gonna break ;-)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[jbartos]

Suggestion to gsimson (Electrical) Feb 16, 2004 marked ///\\Only the cost of VFD is abnormally high for MV applications. Over a period of time this also will come down.
///ABB rep mentioned sometimes ago that the cost MV VFD for large HP application is coming down so that the LV VFD is not that much better for such applications.\\\

 

[jraef]

gsimon,
You made some good points. I am a big promoter of VFDs, but I beg to differ with you here. As mentioned earlier in this thread, the APPLICATION is not appropriate for a MV VFD. I have been on startups and retrofit projects for plenty of MV VFDs; AB, ABB, Robicon and Ross Hill (now owned by Robicon), and I can report that my experiences lead me to believe that conditions need to be near prefect for them to operate reliably. By that I mean clean, air conditioned spaces, clean, reliable power, clean, knowledgeable operators and clean. If any of those conditions are less than perfect, the equipment tends to cause moe headaches that it is worth. Did I mention it needs to be clean?

An Auto Shredder operation meets NONE of these requirements, and in fact fails them in with extreme prejudice. I have put low voltage VFDs in rock crusher applications for yerars, and only after 10+ years of exposure to them are they finally gaining acceptance by users. That said, I just had an "electrician" at a quarry connect 120VAC to the 4-20ma input on one of mine yesterday, blew up a 250HP VFD. He didn't know exactly what "speed reference signal" meant, he thought it was the Start button! Auto shredder operations are the same if not worse.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

FACB25
Your point about inertia is correct, except that most auto shredders were not designed around a flywheel effect as were chippers. You can't add significant inertia just by increasing the motor size, you need to add mass to the machinery. A wood chipper is inherently a high mass machine. You may have needed to increase HP a bit because in the old days, chippers handled occasional loads of scrap material. When a load came into them, inertia allowed them to chew it up, and the motor simply re-accelerated it after it was gone. Now chippers are generating revenue and are fed a continuous stream of material. The extra HP is sometimes needed on older machines to maintain speed of the mass (intertia) under that more constant loading.

"Venditori de oleum-vipera non vigere excordis populi"

 

[gsimson]

Thank you every one for your valuable comments.
Certainly VFDs are understood by engineers. When we are in the age of "Prevent Maintenance" VFDs are preferred as their reliability is very high with minimum or rather no maintenance. As on date the electricians may not understand and considering the reliabilty their understanding is not a must.
MV VFD is a general comment and not relevent to this thread.
As I mentioned 'all' applications, the cost factor was posted for information.
For isolated drives located far away with 1 electrician I agree that the SRIM with resistor is better option.