Synchronous vs Induction on a centrifugal compressor
Synchronous vs Induction on a centrifugal compressor
(OP)
I have a 1000 Hp, 1800 RPM, synchronous ODP motor driving a centrifugal compressor through a 1:2 speed increaser. This motor because of its location needs to be removed and cleaned approximately every two to three years. The speed increaser has recently developed some strange vibration spectrums on our PDM analyzer and it appears the gearbox is going to need a rebuild. Due to the combined costs of having to rebuild the gearbox as well as the periodic cleaning costs I am contemplating changing the motor to a 2 pole TEFC design and driving it direct to eliminate the speed increaser as well as the need for periodic cleaning.
My question is: How do I calculate any energy savings, if there are any, between my current set up with the synchronous motor and speed increaser and a straight induction motor? There must be some losses in the speed increaser and I'm just not sure whether the synchronous motor is more expensive to operate than an induction motor, especially when you consider the exciter costs. I'd appreciate any thoughts you guys might have, Thanks, Lektrikman
My question is: How do I calculate any energy savings, if there are any, between my current set up with the synchronous motor and speed increaser and a straight induction motor? There must be some losses in the speed increaser and I'm just not sure whether the synchronous motor is more expensive to operate than an induction motor, especially when you consider the exciter costs. I'd appreciate any thoughts you guys might have, Thanks, Lektrikman





RE: Synchronous vs Induction on a centrifugal compressor
RE: Synchronous vs Induction on a centrifugal compressor
The benefits of an induction motor directly coupled are the initial investment savings and service expenses for the gear box. As Davidbeach said check if your compressor accepts the speed reduction due to the slip in an induction motor.I assume this is a centrifugal compressor since reciprocant type could require an additional flywheel.
May be the overall expense after several (10) years will be similar for both options.
RE: Synchronous vs Induction on a centrifugal compressor
Re the speed loss with an induction motor, I believe that the Horspower speed curve for a centrifugal water pump is a third power function. A speed reduction of 2% on a water pump may cause a 6% drop in work done. There are other factors such as change in dynamic head with a change in speed which tends to increase loading etc.
Can any-one out there tell us what the Speed-Horsepower relation-ship is for a centrifugal compressor?
RE: Synchronous vs Induction on a centrifugal compressor
So should you switch to induction it may not be just a matter of adding a PFC capacitor bank to this motor in order to maintain efficientcy, you may be required to add capacitors to ALL of the motors in the plant!
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RE: Synchronous vs Induction on a centrifugal compressor
The VFD is expensive, but can incorporate power factor correction - if that is an issue, there may be other synchronous motor that can take up the burden.
As the correspondents state above, to make a decision you need to determine:
Is the load constant?
Is power factor an issue?
Waross
the speed HP relationship is a cube law for a centrif compressor
RE: Synchronous vs Induction on a centrifugal compressor
Two pole induction motors are problematic as ball bearings fail frequently, rotors are long with respect to diameter and tend to flex.
All things being equal and ignoring down time, I would go for induction at this HP and speed.
RE: Synchronous vs Induction on a centrifugal compressor
Thanks for the information.
Lektrikman
Comparing a 3600 RPM synchronous motor with a 3500 RPM induction motor, the slip is 2.8%. When this is cubed, your loss of capacity will be over 8% {1-(3500^3/3600^3)}
If you have lots of capacity then there is no problem, but be aware that a small drop in speed is significant.
If you can withstand the capacity loss, you can make up the power factor with capactors. If you don't add capacitors, everything will work fine but you will probably get an expensive penalty on your next power bill.
RE: Synchronous vs Induction on a centrifugal compressor
In the typical induction vs syncronous debate, I like to throw in my favorite overlooked aspect: sync motors are much more complex, more equipment to maintain, more equipment to possibly fail. In contrast the rotor of a squirrel cage motor does it's job reliably forever with no external support (excepting rare rotor bar problems).
My vote on that basis alone is to go induction. Also if you have a chance to get rid of a gearbox by putting in a higher speed motor, another great opportunity to reduce maintenance requirements and improve relaibility.
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RE: Synchronous vs Induction on a centrifugal compressor
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RE: Synchronous vs Induction on a centrifugal compressor
Because of the high inertia the "star punching" type design would probably not have enough thermal capacity to dissipate the heat during start up and so solid pole rotors are often offered for compressor duties.
electricpete - i agree that the ocilllating torques seen during start-up need addressing - but the existing drive is synchronous, so the mechanical system would be able to withstand the oscillating torques.
There are many pole induction machines in service with sleeve bearings which have no problems with shaft flexing, so this should not be too much of a concern
RE: Synchronous vs Induction on a centrifugal compressor
I am not saying by any means saying we have enough info to draw any conclusions or to say torsional resonances are at fault.. but conversely, we also can't use the existing installation to prove the system is working properly.
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RE: Synchronous vs Induction on a centrifugal compressor
RE: Synchronous vs Induction on a centrifugal compressor
With a VFD you could run a 4-pole motor at 122 or 124 Hertz - a 4-pole or 6-pole induction motor usually has optimal design and usually has the best efficiency and mechanical ruggedness. However, a 2-pole motor started across the line has simplicity going for it and if your compressor can tolerated slightly less than 3,600 RPM that would be the best way.
RE: Synchronous vs Induction on a centrifugal compressor