Async vd Sync Motor Evaulation` 1

[7carisfast]

Just need to ask if I'm missing anything here in this analysis:
Motor Async Sync
Load 1169 1169 KW
Motor Eff 0.96 0.96 %
Gear Box Eff 0.985 0.985 %
Power Factor 0.88 1
hours/day 24 24 Hours
days/year 350 350 Day
$/KWH $0.09 $0.09

Cost Per Year
to Operate $934,607 $934,607

KVAR of Mtr 631 0
KVAR Charge $1.50 $1.50 Demand Charge
Yearly KVAR
Demand Charge $11,357 $0 Cost per year per motor


Motor Cost $55,000 $320,000

Am I correct in seeing that this is a no brainer to go with the async motor? I'm also assuming that the MV motor starters are the same price (when the sync starter is probably 1.3x the cost). In addition, if I'm looking at 5 of these units, and keeping an ROI of 15%, I'd have about $500,000 for capacitors

 

[aolalde]

Since the synchonous motor has PF=1.0, the FL current is smaller. Then the copper losses are lower and the efficiency should be better around 2 points (0.98?), what is the motor speed?. That could provide some operating cost reduction, still the initial investment looks too big.

 

[dpc]

Synchronous motor benefits are more pronounced for slower speed motors (and larger motors). So the induction motor may be lower cost. I have no idea if you've calculated your power factor charges correctly or not, but I will point out that the synchronous motor can run at a leading power factor to provide vars to your system.

 

[Laplacian]

The cost you have for the induction motor is suspect unless you are buying an "as-is" used motor with no warranty. If that is the true quoted cost, you need to pay attention to temperature rise, sound pressure, rotor construction, vibration, and testing. Something is being left out.

I'm assuming this is a 4 or 6 pole machine based on the power factor quoted. Synchronous motors don't offer any real advantages until at least 10,000KW at this speed (for us anyway). Capacitors are cheaper, easier to control, and more reliable than field excitation systems for synchronous motors at this power level. What is your starting requirement?

Also, you need to consider your system constraints such as available fault (starting) MVA, utility requirements for power quality, site maintenance issues, etc.

 

[7carisfast]

Thanks for all the input. Believe it or not, the $55k is a real number quoted to us by WEG (Don't start please, we've found them to be a decent motor).

1250KW
4P/1500 RPM (Using in Portugal)
Rated Speed 1486
Design N
F Insulation
Temp Rise = 80 Deg C
SF 1
IP55/TEFC
Noise 88dB
100% Load PF = 0.87
100% Load Eff = 96.7
Thermistors and space heaters

Nothing elaborate, 24/7 operation.

 

[Laplacian]

That is an unbelievable price for that size motor. You should buy a few extra and sell them on the surplus market. Every large motor manufacturer I contact in North America is jammed up with business. Prices are high and deliveries are long across the board.

 

[jraef]

As previously stated, you don't mention operating speed. Synch. motors are typically better for lower speeds. Also, don't discount the fact that if you have other induction loads in your facility, you can run the synch motor in leading power factor mode, correcting the overall pf to avoid utility penalties and/or improve your on-site generator capacity and lower losses in any T&D gear that you own. That can add up to a lot of overall operating cost savings. In short, the evaluation shouldn't be that simple unless you are looking at this as the only significant motor in the installation.

A problem I have had with Weg motors is that they tend to produce less torque per amp than other designs. In other words your starting torque may be significantly lower for the amount of power you can give it, which becomes important if you plan to use reduced voltage starting of any kind. It can also mean that if you are running the motor at less that full load, your losses may be a slightly higher percentage of the energy input. You can tell by comparing the efficiency at 75% and 50% load, something any decent motor manufacturer should be able to provide you with.

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[7carisfast]

The motor speed will be 1500RPM with a gearbox/sheave reducer to ~600RPM.

50% 75% 100%
PF 95.5 96 96.1
Eff 0.81 0.84 0.87

Does anyone have an idea (or even if it possible which I am sure it is) if they even make a 12 pole motor? I can't imagine the cost. BTW, you are correct on leadtime.....52 weeks.

I'd share the quote if anyone is interested. Again, thank you all for your input and advice.

 

[waross]

Note that when using a synchronous motor for plant power factor correction, the motor horse-power must be derated so that the combined load current and reactive current don't exceed the rated current of the motor. The derating will progress until the reactive current equals the rated current and the HP derating is 100%. At this point the motor has become a synchronous condensor and the output shaft is superfluous.
respectfully

 

[davidbeach]

Compare a 600rpm synchronous motor, without gearbox, against the induction motor. General rule of thumb is consider a synchronous motor when the HP exceeds the rpm and it looks like you are there.

 

[electricpete]

There's a good white paper on the subject somewhere on

http://www.geindustrial.com

If anyone can find it, it deserves to be a FAQ link IMO

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[dpc]

Maybe

http://www.geindustrial.com/publibrary/checkout/38652.30055.22461.64532/PDF/GEZ-6209.pdf

 

[electricpete]

Yup, that's the one I was looking for. Star for you.

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[LionelHutz]

Like davidbeach said, compare the 1500rpm asych motor and gearbox to a 10 pole, 600rpm synchronous motor direct coupled without the gearbox.

 

[VxA]

7carisfast,

I'm guessing you've got your row labels mixed up in your last post. Otherwise, the Eff seems quite low, and in conflict with your first post.

I didn't notice the motor voltage stated anywhere. What voltage is the motor going to run at?

For every % more efficient one motor is versus the other, you will save on the order of $10G a year, but even considering that, I suspect you would need to see a difference of 5% of so for a business case to suggest selecting the more efficient motor (at the costs you've quoted). That much difference is unlikely.

 

[swn1]

The usual justification for a sync motor is based on their stiffness with respect to cyclic torque variations. Reciprocating compressors and certain kinds of mills are the usual applications we see.

Back in the day they used sync motors as something akin to a virtual gear train to maintain syncronization among stages of a production line, but modern controls have obsoleted that justification.

In addition to the first cost, sync motors have higher rebuild costs and suffer from the disadvantage of relative obscurity -- few service companies have experience with them.

A machine with a rotating exciter will minimize the starter cost penalty -- 30% is excessive. Not surprising, but excessive.

Finally, sync motors are hard to start. If your application requires full load torque while starting an induction motor may well be your best choice.

We are not seeing new applications of sync motors in this size range. Not enough benefit to justify the hassle.

But if you need caps, don't discount the cost of protection and control equipment for the caps. They require a severely derated breaker and instrumentation / relaying that roughly equal the exciter. None of that is significant for a 1MW motor, though.

Basler sells their generator exciters configured for motors, with protection, control, and power supply for rotating exciter machines all in one package. Don't let anyone sell you an old-school exciter package.

Regarding the motor power derating when "making vars" for plant power factor correction: when the sync motors are a significant fraction of the total load, the leading power factor required is small. 95% leading PF is making a lot of vars. Some sync motors are rated for non-unity power factors to begin with. You can run with the stator electrically overloaded by a few percent safely as long as you are monitoring the RTDs.

Finally, take a good hard look at using a sync motor with no gearbox. Lower maintenance, less foundation cost and space, less high-frequency noise. Less capital cost. Less coast-down time. No gear losses = higher efficiency.

None of these considerations will make up for the loss-leader pricing you're getting for the induction motors.

 

[electricpete]

loss leader = a comment on the efficiency of motors from that OEM?

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[swn1]

No, I'm just suspicious. Why are they selling those motors at that price to this customer at this time? Probably trying to get the design-in, might be dumping, who knows? But the engineering decision is being biased toward the induction motor because of the below-market price offered for those motors. Once the initial design is completed will you be able to buy more of the same motor at the same price?

 

[7carisfast]

Thanks for all of the help guys.