Electric conveyor drives 1

[CuriousJohn]

Can someone give a comparitive analysis between Slip Ring Induction motor drives c/w resistors and Squiral Cage motors drives with VFDs applied to a i.8 kM long 1.8 m wide conveyor with four 2000 kW drive motors. What are the advantages and disadvantages of each system?

 

[dpc]

Wound-rotor induction motor - old way of controlling starting torque to avoid belt damage during starting. Limited speed control capability with high losses.

VFD - new, better way of reducing starting torque, while offering wide speed control range, using standard induction motor.

You might also consider solid-state soft starters.

For a 2000 kW motor, I'm not sure what will be less expensive these days. Certainly the soft starter with a standard induction motor is the least costly option, provided it can do the job. Probably the WRIM will be less expensive than the VFD, but I'm not 100% sure on that anymore.

If you can provide more details on the requirements, someone smarter than me can probably give you a more quantitative comparison.

Cheers,

Dave



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[jraef]

That's a deep subject, something that people get PhDs for. The Reader's Digest version is that long overland conveyors end up acting kind of like rubber bands, and there are all kinds of methods and approaches to dealing with the problems. Besides WRIMs and VFDs, there is also a camp that insists on CSTs as the only viable solution (I am NOT in that camp).

In general through, VFDs are probably the best way to go as far as control, but are likely the most expensive. WRIMs are less expensive, but not as infinitely adjustable (unless you use Liquid Resistance, which gets expensive again). The smartest guy I ever met on the subject is a strong proponent of VFDs as the best solution. He doesn't sell them, but he runs a company that specializes in the complex control algorithms that deal with the wave dynamics involved in overland conveyors. His name is Larry Nordell and has a PhD in Wave Dynamics. He runs a company called Conveyor Dynamics in NW Washington State. He has some White Papers on the subject available off of his website. They are a great resource for overland conveyor projects. I haven't spoken with him in probably close to 20 years now, but I still hear of his work all over the world.




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[dpc]

CST? Central Standard Time?

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[jraef]

Controlled Start Transmission.

http://www.dodge-pt.com/products/gearing/cst/cst.html

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[roydm]

I worked on a similar overland conveyor, it had wound rotor MV motors.
I think your MV VFDs would be more expensive, If you don't need speed control why go that way?
I may be wrong but I think wound rotor has more starting torque.
Roy

 

[LionelHutz]

We have had good luck with wound rotor motors connected to a soft-starter with a single step resistor sized for about 10% slip. This gives lots of starting torque yet the soft-starter allows the starting torque to be easily adjusted.

Wound rotor motors work well to get starting torque but are wasteful if you use them for speed control. So, they are a good option if they will run full speed. Also, wound rotor motors will easily load share if you just leave a small resistance connected to the rotor at all times. VFD's can load share too but you need to get the right VFD with the right settings to have the torque controlled. If the settings are wrong, you can get torque oscillations or unequal sharing.

About the only other thing I can add is to also consider the location and personel. A wound rotor motor and resistor step contactors is brush and contact tip maintenance. A VFD is electronics maintenance. In some places, the maintenance staff can handle brushes and contactors, but not electronics.

 

[7anoter4]

I agree with LionelHutz.
My opinion, but it is only my personal opinion, that as VFD would maintain a constant torque it could be very good for conveyor starting with empty belt. Soft starter reducing supply voltage may produce a similar effect. If the belt is full- and 1.8 km is a huge amount of material- only slip
ring wound- rotor may move the belt since it will reduce the starting current but will enhance the starting torque close to maximum torque. Of course, the belt may be exposed to such a stress that could break it sometime.

 

[CuriousJohn]

Thank you all for your input. You have at least pointed me in a direction that will increase my knowledge