Power Cost Calculation: 600hp 2300V vs. 600hp 460V
Power Cost Calculation: 600hp 2300V vs. 600hp 460V
(OP)
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
Thank you for your help!
Sherry





RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
///$/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.\\\
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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.
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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).
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
Sherry
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
"Venditori de oleum-vipera non vigere excordis populi"
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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).
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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.
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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.
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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"
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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.
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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"
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
"Venditori de oleum-vipera non vigere excordis populi"
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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!
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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"
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
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.\\\
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
VFD application makes a big difference, the reasons that you gave are valid.
For traditional supply of motors, it is my opinion that handling high currents results in Voltage dip and more probability for failure in the elements supplying the motor. Electrical joints and breakers will be weak links to handle the motor current. For 800 HP, it will have around 860 Amperes full load and 4300 Amperes at start. Compare to 99 Amperes at full load and 500 Amperes inrush for 4160 volts.
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
You may be missing some important issue here.
You stated "The VFD will only be 100 feet from the motor so wire isn't a factor." You are only considering the load conductors. To do that job at 460V, you must supply that 960 Amperes (probably a typo from aolalde) with cables, no small feat. 960A x 1.25 load factor = 1200 Ampere rating on the cables. Assuming 75degC cables, you are looking at 3 x 600MCM cables per phase. 9 cables x 600MCM is no trifling matter to manipulate. At minimum, you are looking at 3 x 3" rigid conduits both in to and out of the controller (assuming of course that you can find an electrician willing to try pulling 3x600MCM cables in a 3" conduit).
You also must consider losses in the transformers. It is likely that your distribution voltage is something higher than 4160V, probably 12.47kV, so it must be stepped down to feed that motor. Although the kVA size will be the same for either voltage, the transformer to feed the 460V motor will have more losses due to the higher current going through it.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
Everyone keeps saying the same thing "thats a lot of wire in a 480 volt system", but wire is cheap compared to the VFD's. I had one pump station where we had a custom designed motor that had to be 2300 volts, we still used a stepdown transformed to 480 from 12K through the VFD and a step up transformer to 2300v and that was still cheaper than getting a 2300v VFD. The system worked great, that electrical engineer was my hero because he cut costs and improved delivery for me. Wished I had him on this job.
RE: Power Cost Calculation: 600hp 2300V vs. 600hp 460V
Similarly, if you want solid numbers on the cost of power to run this thing, well, the only way to get a definite answer is to bust out the utility company contracts, make some guesses at how long the motor will run each year, and figure out what it will cost.
You could then compare total installation + energy costs over, say, a 30 year period, or do a present-value analysis to see which one's cheaper.
Keep in mind, your motor is right at the borderline of where the rule of thumb says you'd want to use one voltage or another -- so you can't be too surprised if the specifics of your situation go against the rule of thumb. That doesn't mean you've "broken the conventional wisdom", it just means that when you're working right at the borderline you can't be too surprised if the results go either way.