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

 

[aolalde]

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.

 

[dcasto]

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.

 

[jraef]

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"

 

[dcasto]

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.

 

[peebee]

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.