AC & DC MOTOR AND DRIVES

It's difficult to fully assess the best solution, but in GENERAL, steel rolling mills are still being done with DC drives. AC dives and motors are now viable rivals, but not as many people have used them yet and there is a wealth of knowledge available in DC rolling mill drive systems to draw upon. From a technology standpoint,AC drives and motors need less maintenance, but cost more up front. DC drives and motors are less expensive, but there will be brushes to wear out.

Guest #1

This question came into consideration due to under standing because of :-

considering power loss in Rectifier plus rectifier cost should have much better advantages over same rating of AC motor plus drive ?

It all depends on the voltage of the motor in other words your supply voltage.

If you have a 550 or 660v supply votage then an AC motor with a vfd is a very cost effective option.

If you want to go meduim voltage, i.e 2kV to 11kV the AC motor option become very exspensive, as the technology on the vfd is still fairly new and not all the suppliers, at this stage, has this option available yet.

As far as I'm concerned the AC option is technically the better option than the DC:

Less maintenance on the AC motor compared to a DC motor, (do not have to keep spare brushes and clean the motor once a month)

Better speed control at lower speed

The input power factor on a vfd is 0,97, which gives you a saving on the input power to the application (higher power factor = less current from electrical supplier authority).

If the motor/vfd is sized correctly, you will not require any additional cooling on the motor. DC motor has force cooling system which requires additional electrical supply, cabling etc.

Selecting AC or DC motor will depend upon your rolling mill requirement Most of the rolling mill has many stands with cantilevered rolls and roller bearings throughout. In most of the rolling mill plant, DC motor is used due to its excellent of speed control and the torque requirement.

On other hand, AC motor may be cheaper than DC motor but for speed and torque control, it looses to the DC motor operation. When AC motor running at low speed it will looses out its torque.

All the speed drives are design using AC supply for the input and output will be DC supply.

I have never seen or heard of a VFD that can handle 800 HP . . .

Is there such a thing?

Guest #1

Please visit at this website for power rating of drives available --

http://www2.sea.siemens.com/Products/Drives/Low+Voltage+AC+Drives/SINAMICS-S150.htm

Well, now I've heard of VFD drives large enough for 800 HP!

I guess I need to try to "keep up" a little harder . . .