Power Cost Calculation: 600hp 2300V vs. 600hp 460V 1

[hydrogirl]

How do I calculate the cost of power ($0.085/kWh) for a 600hp, 3-phase, 2300V, 1775rpm, 60hz, 81.6% efficient, 95%pf motor? I am also trying to calculate the cost of power for a 600hp, 3-phase, 460V, 1775rpm, 60hz, 81.8% efficient, 95%pf motor. I have calculated the amps as 144.7 and 723.3, respectively. Is that right and how does it figure in to electrical costs? If it costs the same to run both motors, why go with the higher voltage since it costs more up front for the motor and its accessories?
Thank you for your help!
Sherry

 

[jbartos]

Suggestions marked ///\\///$/Hr=($0.085/kWhr)xHPx0.746kW/HPx(1/EFF)xHrs=
=($0.085/kWhr)x600HPx0.746kW/HPx(1/0.816)=$46.625/Hour
As you can see, the voltage does not matter.\\
If it costs the same to run both motors,
///Yes, it does since the voltage does not enter the calculations.\\ why go with the higher voltage since it costs more up front for the motor and its accessories?
///Quite true for this size of the motor. Try 10000HP motor cost at 460V (if physically realizable) and at 13.8kV, for example.\\

 

[busbar]

Based on voltage alone, there will be no significant difference. It is unlikely that the 600hp motor is the only load, so voltage choice would be better based on other electrical equipment associated with the installation.

If there are other local motors in the 1000+hp range, 4kV may be the economical choice. {2300V is less common because it and 4kV both use 5kV-class switchgear.} If there are many other motors 200hp or less, the low-voltage system costs are likely lower.

 

[aolalde]

For the 2300 Volts motor:

Cost per hour = 0.085* 600*.746/.816 = $46.625; Cost 1 year(7/24)= $407,436.36

For the 460 Volts motor:

Cost per hour = 0.085*600*.746/.818 =$46.511; Cost 1 year(7/24)= $407,743.36

The current will produce losses in the installation wires, proportional to the current squared (R*i^2)

@ 2300V, I=600*746/(1.732*2300*.816*.95) = 144.94 Amps

@460 V, I=600*746?(1.732*460*.818*.95)= 722.95 Amps

for 90°C Temperature rated conductors and 250 ft from the substation, the minimum size required are:

1/0 for 144.94 amps---.0255 ohms DC per 250 Ft
1750 kMCM for 722.95 amps --- 0.00154 ohm DC per 250 Ft

The line losses (assuming the DC resistance only)

Line loss=3*.0255*144.94^2 =1607.08 watts $1196.63/year

Line loss=3*.00154*722.95^2=2414.67 watts $1798/year

In spite of the voltage insulation, the initial cost of 1750 kCM line is larger than 1/0 AWG. Handling of elevated currents develops more operation problems as compared to handling of higher voltage (2300V).

 

[charlierod]

As suggested by busbar and aolalde the choice in this case should be based on aspects like voltage rating of existing installations and length of feeder.

 

[hydrogirl]

Thank you all so much for your information and expediant responses. They have been very helpful.
Sherry

 

[wschel]

The cost differences are not so much is actual power used, but in the cost of installation. Do you have sufficient 2.4kV and 480V power close to the motor location? Will the a transformer be required for one or both voltages? You should determine what is required for each voltage, calculate the installed costs and then compare the results.

 

[jraef]

It also depends upon where you are metered. Some utilities offer the best rates when they meter you on the line side if the primary transformer. In that case, YOU will be paying for the transformer losses. If that is true here, the 2300V motor will cost even less over the long run than the 460V because the transformer will be smaller and present lower losses.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Comment: The higher voltage power distribution and load tend to have larger leakages and losses for the same size of load, i.e. HP or kW, than the lower voltage power distribution and load.

 

[edison123]

jb,

That is strange. HV transmission is the accepted way of reducing transmission losses world over.

At the power rating of 600 HP, 2.3 KV is better in terms of cost of installation (cables/bus ducts, breakers etc.)due to overall lower sizes and ease of installation. Also, a 2.3 KV motor insulation (mica based) is more reliable than a 660 V insulation (paper based).

 

[jbartos]

Comment on the previous posting:
Yes, I agree if the amount of power increases. Also, I mentioned in my past posting "for the same HP or kW." Generally, the voltage is kept on the lower side, and it is increased when the hardware for the lower voltage becomes more expensive and the product very big and heavy.
E.g. Ships tend to have as low voltage as possible to propel their power distribution loads since the higher voltage create larger leakages in humid and salty environment. Although, there are other issues involved too. E.g. volume, weight, cost, etc.

 

[peebee]

Realize too, that you may not be able to start the 480-volt motor across the line without substantial derating of the upstream transformer or generator. You might be required to provide a VFD or reduced-voltage starter for that motor to avoid excessive voltage drop, and such starters might add $50,000 - $150,000 to the low voltage motor installation cost.

Of course, that's not an issue if you were planning to use a reduced voltage starter anyway.

Feeder cost (copper) is usually the #1 reason to go to a higher voltage. The more copper you need, the fatter or longer the cables required, the more cost effective it becomes to look at higher voltages.

Rule of thumb: medium voltage usually starts to make sense at about 500 to 600 hp motor size. Anything below that usually wants to be low voltage. Anything over about 1000 hp usually wants to be medium voltage.

 

[jraef]

peebee,
You are off on your cost estimates for low voltage RV starters and drives. A low voltage solid state soft starter, 600HP 460V complete with CB and bypass contactor in a NEMA 12 enclosure is only $21,500 LIST, which is a quick cheat for estimating installed cost. Compared to a NEMA size 7 Across-the-Line starter at $18,200 for the same package, you can see that at WORST, they are priced pretty close, and in some cases the RVSS might actually be cheaper if you can't find an ATL starter ready to go. Even a 600HP 460V VFD is only $49,000 list, a far cry from $150,000.00.

Your numbers were closer to installed costs for Medium Voltage controllers, but still a bit high.

"Venditori de oleum-vipera non vigere excordis populi"

 

[peebee]

jraef -- you obviously don't live in Chicago ;-P

I actually have a quote here from a chiller manufacturer, their adder for an alternate 480-volt solid-state soft-start bypass (around a base-bid VFD) is $50,000, not installed, for a 500hp motor. Yes, that's adder only for bypass soft-start only. I'm not saying that's a good price, but I didn't just pull those figures out of my butt, either! Must be the Chicago premium!!! (Of course, we do have 4 isolation switches & some additional controls included in that price too).

Just curious, what all does your number include? Soft-start & enclosure only? Or full-blown starter complete with disconnects, harmonics correction, all the bells & whistles, etc, etc? Any bypass operation?

Thanks.

 

[jraef]

peebee,
Not to be a master of the obvious, but I did say "600HP 460V complete with CB and bypass contactor in a NEMA 12 enclosure is only $21,500 LIST".

I gotta move to Chicago and sell soft starters! Was that Carrier by chance? If so, they are making a killing on that option. The bare-bones soft starter chassis is only $6700 list, and as a volume OEM believe me they are paying FAR less than that. And 4 isolation switches? Sounds like overkill to me. Someone does not understand soft starters if you ask me. One Line isolation contactor is potentially usefull IF you are subject to frequent lightning hits, but beyond that the rest is superfluous. Sometimes engineers think that the SCRs need to be isolated the same as the transistors of a VFD but such is not the case.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

Oh, I just read your post more carefully and now see that you meant it as a Bypass Softstarter FOR a VFD, not in lieu of the VFD. In that case, you would need 2 additional load isolation contactors. That can be a bit spendy, especially if they want NEMA rate contactors.

"Venditori de oleum-vipera non vigere excordis populi"

 

[peebee]

Ha -- we both need to read more carefully!!!

The vendor will remain unnamed. But it begins with a "C".

Yes, 4 iso switches, 2 for VFD & 2 for soft start, to enable isolation of either while maintaining the other in operation. Not cheap, but my electrical budget is peanuts compared to the rest of the job. At least I thought it was until the "C" prices came in.

"Venditori de oleum-vipera non vigere excordis populi" -- Snake oil salesmen thrive on stupid people? Chiller vendors thrive by overcharging for VFD's? My latin's weak, help!

 

[jraef]

Close. Snake oil salemen cannot thrive without an ignorant populace.

Consider having just the ouptu isolation contactors and buy a standard RVSS combination starter wired in paralell. If the VFD fails, open the VFD breaker and close the RVSS breaker; if the VFD fails you usually have someone decide to manually override with the soft starter anyway don't you? That way the line sides are isolated and a safety lockout can be applied to either. Sounds like it would be a lot less expensive.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Suggestion to peebee (Electrical) May 14, 2004 marked ///\\
Rule of thumb: medium voltage usually starts to make sense at about 500 to 600 hp motor size. Anything below that usually wants to be low voltage. Anything over about 1000 hp usually wants to be medium voltage.
///Reference:
Robert W. Smeaton "Switchgear and Control Handbook," 2nd ed., McGraw-Hill Book Co., 1987,
indicates in Table 3B Standard Voltages for Large Polyphase Induction Motors on page 9-9, e.g. for 60Hz and
Voltage in V HP
460 100-600
575 100-600
2300 200-4000
4000 400-7000
4600 400-7000
etc.
The main reason is the economy. However, there are some technical pluses and minuses involved too, e.g. the medium voltage power supply can start large motors with full voltage across the line, which is a big advantage since the medium power distribution system usually has a smaller Zsys that causes small or negligible voltage dips.\\

 

[dcasto]

I'm not a EE, we have quotes for complete installed cost for an 800 HP motor and VFD in both 480v and 4160v. The 480v is a little cheaper mainly because the difference in VFD costs. The VFD will only be 100 feet from the motor so wire isn't a factor. The motor is WPII and the price in minimal. I just see that the added safety of 480v, the cost to repair the 4160 VFD and harmonics that must be filtered out of the 4160 are not worth going to medium voltage at this location. The operations people just say that they like 4160v because the wire is smaller. Any suggestions or other selling points I'm missing?