Induction Vs Synchronous motors for paper mill

[polyphase]

What are the decision making advantages / disadvantages for choosing induction or synchronous motors for industrial applications such as paper mill refiners? The sizes are 1,000 to 3,000 HP, and the voltage is 2.4-kV.

The mill is served by a 33 MVA 115/12.47-kV transformer, and the expansion will require the installation of a second 33 MVA transformer and second 12.47-kV bus. The refiners are served through 12.47/2.4-kV stepdown transformers.

 

[electricpete]

Generally for high horsepower and low speed applications, sync motors are sometimes considered due to slightly higher efficiency and a power factor which is higher and controllable. (power factor for induction motors decreases as number of poles increases to decrease speed).

There is a very detailed article on this subject on the GE website if you can find it.

imho, a great disadvantage of sync motors is they are more complex ==>> more stuff to maintain and more stuff which can fail.

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[electricpete]

Here is the GE article

http://www.geindustrial.com/products/reference/GEZ-6209.pdf

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[edison123]

Problem with synch motors is with starting especially with a high torque load. I have seen (and rewound) some synchronous-induction motors for such applications to overcome this problem.

 

[dpc]

In economic evaluations, the motor speed can be the key factor, as electricpete mentioned.

I believe low-speed synchronous motors will provide a signficantly higher efficiency than an equivalent induction motor, or at least that used to be true. If speed is 720 rpm or higher, I don't think there is much difference in efficiency.

The ability of the synchronous motors to operate at a leading power factor to provide vars to your system is also another advantage.

 

[aolalde]

If we consider a 1000 HP, 8 Poles motor working 24 Hrs, 7 days a week:

Typical average efficiencies and then kWH consumptions in one year are:

Induction Motor (EFF =93.8);
kWH= (1000 * .746 / 0.938 )*365*24 = 6,966,908

Synch. @0.8 PF (EFF=94.2);
kWH= (1000*.746/0.942)*365*24 =6,937,325

Synch. @1.0PF (EFF=95.1);
kWH= (1000*.746/0.951)*365*24 =6,871,672

Assuming energy cost $0.05 dollars per kWH, the yearly electric bill becomes:

Induction ---------- $348,345
Synch @ .8PF-------$346,866
Synch @1PF -------$343,583

 

[Marke]

Hi aolalde

In some territories, there could be an additional pf penalty in one form or another that could significantly boost the cost advantage.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

So the bottom line then in deciding is operating speed (lower speed favors synch.) and power factor correction possibility, because the synch. motors can correct poor power factor from other loads in the mill. It should be noted when considering this last issue however, that for this to be of value the motors must be running almost constantly. If the application calls for cycling them on and off repeatedly, synchronous would not have much value there, and may even become a detriment.

"Venditori de oleum-vipera non vigere excordis populi"