Efficiency of motors (synchronous vs induction) 3

[Zoobie]

How would an induction motor (6000 hp, 6600 rpm, 4160V) compare to a synchronous motor (6000 hp, 1200 rpm, 4160V) in terms of power from the line to shaft power. I understand that there would be some slip with the induction motor but some rough numbers would help. Also, how much, if any, does power factor play into things. Would the answers be any different if their were a VFD in front of this motor?

 

[motorspert]

A 6600rpm motor will be a specially designed motor, the line frequency od 110hz would need special consideration to prevent high eddy currents. But i am not sure why it would need to be 12500hp? The motor will be designed for 6000HP at 6600rpm, the drive would have a contsant V/hz output meaning that at 3600rpm (60hz for a 2 pole) the available output would be 6000HP x (3600/6600)=3273HP. However a compressor power demands as the cube of speed, so would only need 973HP at 3600rpm- no problemo.

The exam question was efficiency. The gearbox effiiency will be high, typically 98.5%, often higher. the drive will be a bit less, depends on the switching technology. However, i would expect a UPF synch motor to be much more efficient than a high speed induction motor.Normally we would expect about 1-1.5% difference between an synch motor and induction at the same speed, the high speed will introduce high windage losses

However, as the guys say, get a firm quote from the manufacturers and work with some real numbers.

 

[aolalde]

Hi Jraef, I disagree with the way you have approached this application.

There is no way to think of buying a standard motor (12 000HP) and applying it separately to the VFD.
For this application both the motor and driver must be custom designed and matched. A few motor manufacturers can design and construct such a motor.

The application is quite simple:

Option 1.- Apply an induction motor 6600 HP, 6600 rpm, 4160V, whatever high frequency to match the speed / poles , DIRECT COUPLED to the compressor.

Option 2.- Apply a synchronous motor 6000 HP, 1200 rpm, 4160 V COUPLED TO A GEAR BOX Ratio 1/5.5 and then coupled to the compressor.

In both cases the load torque is: T = 5250 *6000 / 6600 = 4773 Lb-Ft

In regard to the efficiency, in fact the Synchronous motor could be adjusted to operate at PF=1.0. This condition will reduce the motor primary current and as a consequence the stator winding losses with the squared ratio of the current. That makes a big improvement on the motor efficiency. The gear box will add mechanical losses reducing the gain due to the synchronous motor.
The “special design induction motor-driver” has the advantage of having only one mechanical device (the motor) to transform the energy.

Which one is better? Request for the two options not only price but a guaranteed technical performance quote.

 

[LionelHutz]

Just to point out that Zoobie did post "There are tremendous advantages to having variable speed for this process....this project would not even work without it."

So, according to his statement, he needs variable speed and will be using a VFD no matter what motor is being used. So, consider that fact when you're discussing the motor options.

When I consider using the synchronous motor with a VFD it just makes sense to go with the 6600rpm motor and eliminating the gearbox, synchronous field supply and motor brush maintenance.

I agree 100% with motorspert. This 6600rpm motor will be custom designed specifically to work with a VFD providing a constant V/hz ratio up to 110hz.

If you ever want to overspeed a 240/480V dual voltage motor when using a 480V VFD then connect the motor for 240V and set the motor frequency in the VFD to 100 (or 120)hz.

 

[jraef]

Option 1.- Apply an induction motor 6600 HP, 6600 rpm, 4160V, whatever high frequency to match the speed / poles , DIRECT COUPLED to the compressor.

Option 2.- Apply a synchronous motor 6000 HP, 1200 rpm, 4160 V COUPLED TO A GEAR BOX Ratio 1/5.5 and then coupled to the compressor.

In both cases the load torque is: T = 5250 *6000 / 6600 = 4773 Lb-Ft

Why is that anything different than what I said?

The motor itself would not need to be custom designed, but the mistake I see that I made was in not looking up any MV motors before I posted, because I was thinking about the VFD. I ASSumed that they didn't make dual voltage motors at 6000HP 4000V, but they do (duh, they just connect it in Wye to get the lower voltage rating). So the solution would be what I said in the middle paragraph of my earlier post. Use a dual voltage (2300/4000V) motor, and connect to the 2300V winding pattern so that at 110Hz, you can be at 4160V and will not have lost any appreciable torque at that speed (37.8 V/Hz vs 38.2 V/Hz). The VFD will still need to handle the additional current however, so it will look more like a 10,000HP 4160V VFD (approx. 1440A), regardless of what the "custom" label says on the outside.

So to ammend my earlier statement, assuming the motor design efficiency is close to the same (no way of knowing really)[ol A]
[li]If you use a 10,000HP equivalent VFD to do this job with a 6600RPM induction motor, and will lose about 3% in VFD efficiency, the losses will be about 22kW.[/li][li]If you use a 6000HP VFD on a 1200RPM Synch motor, the losses will be about 13kW in the VFD, plus about 179kW though the gearbox, for a total of 192kW[/li][/ol]

Induction motor is much more efficient of those choices.

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[electricpete]

What about the ability of the motor to mechanically withstand the overspeed 6600/3600? (assuming 2-pole motor is selected).

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[jraef]

Good point, but probably that's where the custom motor comes in.

 

[abduogi]

Can any one give me an idea about the costs involved:
Medium voltage VFD:6000hp
Synchronous motor 6000hp
Induction motor: 6000hp

 

[electmech]

Zoobie,

Perhaps I am missing something, but did you ever get your answer about the 6600RPM motor? If not I think that I can help you if you have the motor FLA data. Oddly enough, simply selecting the correct base motor can get you to 6600RPM with few electrical problems. Depending on the size of the motor, you have only mechanical considerations beyond that since there can be a number of additional stresses placed on the motor frame, bearings, and shaft depending on the characteristics of the load. If this is a VT load, you can integrate it using your affinity laws and estimate power. However, its best to substitute current with torque and base your model on the following:

1) As a speed increases above synchronous speed in a 3 phase induction motor, the output torque is lost as an inverse square.
2) When dealing with a VT load, the torque requirements increase as a square of the speed differential.
3) When dealing with a CT load, the torque requirements are linear.

From there, I just enter the equations into my calculator (or plot them in excel as a speed/torque curve) and the curves are generated automatically. There is a point of intersection between the curves generated by those functions that optimally allows you to size a motor based on FLT (which is directly proportional to FLA...the very element that you need to select your VFD). Now consider duty cycles, etc. compare them against the OL capability of the VFD/motor combination to determine what you can get away with and stick with it.

I have used this method many times in the past, and it has not given me a problem yet.

 

[Zoobie]

electmech and others,

Thanks for all the input and advice. The 6600 rpm motor is off the table. We will be installing a 1200 rpm induction motor and coupling it to the existing gearbox. I won't get into all of the details but this decision makes the most sense for us. We thought that the driveline design would be simpler with the high speed motor but certain details around the existing compressor actually make it more complicated.