×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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

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

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.\\\
 

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

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).

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

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.

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

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

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

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.

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

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"


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

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.

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

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).

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

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.

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

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.

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

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"


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

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.

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

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"


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

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"


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

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!

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

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"


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

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.\\\

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

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?

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

dcasto (Chemical);
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

Thanks.  So, would it be fair to say: Conventional motor and starter have a better life cycle cost at higher voltages (in the 300 to 1000 HP range), but whe an VFD is introduced, the life cycle cost may favor a lower voltage sytem in that same range.

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

dcasto,
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

Sorry for the confusion about my statement of 100' being no problem. I am trying break the conventional wisdom says always use 4160v on a 800 HP motor here. We have sized the wire and have recieved cost estimates for a complete set up, transformers, VFDs, wire, conduit, labor, motors and the 480v system was slightly lower because the 4160 VFD is so much more expensive than a 480 VFD.  

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

dcasto, if you have specific installation cost data, then you have much better information than any rule of thumb will ever provide.

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources