Motor application for a reversing mill 2

[mpqai]

What is a prefered solution (AC vs DC) for a 4000HP, 90RPM motor used in a reversing rolling mill application in terms of dynamic response and generation of harmonics on the supply line?

 

[Skogsgurra]

If money isn't an objective, I would use a synchronous motor and a vector drive with an active front end. It will give you the best dynamic performance, good efficiency and low distortion. LV or MV is not entirely clear. But I would say 3 kV. It all depends on what your supplier can offer.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[DickDV]

Even if money is an object and your customer does life cycle costing, go with Skog's recommendation.

If only initial purchase price matters, a reversing, regenerative DC drive would be less money but motor maintenance will eat up the savings over time. You very well might need a gearbox to get that 90rpm and that's another maintenance item.

 

[jraef]

Anyone concerned about the torque transient response capabilities of a synchronous motor for a rolling mill application? Seems to me that this should be considered. I'll still defer to Skogs' wisdom on that however, my experience with synch motors is limited to higher speed applications. I just remember that being an issue.

My thought would have been a 1200RPM SCIM (with fan forced cooling), a good closed loop vector drive and a gearbox with maybe a 10:1 ratio so that the working speed range at the motor was in the sweet spot of the motor and drive control range.

 

[Skogsgurra]

Jeff,

Put an encoder and a vector drive on a sync motor and it will be better than any DC system, torque-wise and also torque step response can be better than a DC system. New rolling mills I see are built that way.

The continous reversing and the sync motor go very well together.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[DaveScott]

Just involved in a contract, right now, for a new reversing rougher, and spent some time on the drive specs:

I agree with the synchronous machine comments. Active Front End has unity pf, harmonic content is very low, and the basic speed control of the system will be extremely good (except the little oscillations around the poles). It is a standard solution in the industry. Cycloconverters are now superseded by inverter solutions.

I think direct drive is standard for the size you are talking about. Watch the rpm rating; lower rpm will give more torque, and rpm is often increased slightly to give the appropriate power rating. Torque = money.

There is a big difference in cost between, say, a 6000kW 45/90rpm motor and a 50/100rpm motor (and about 20t weight difference, too). But there is no need for more torque if the bite angle is too much - check the rolling schedule carefully.

Recently, I have been offered an alternative, because some manufacturers have very long delivery times for Synchronous machines. The alternative was a squirrel cage induction motor. This solution by that particular manufacturer has been used in some reversing roughers, but not as large as the 6000kW x 2 machines that I am looking for, and the larger ones only in the 1980s. I wonder why it was dropped? I think that the speed synchronisation between upper and lower drives would have been difficult (especially on head end impact), possibly leading to turn up or down in the slab. I would not accept this kind of alternative solution.