I'm researching the purchase of a 350 hp air compressor.... 1

[DrDarrell]

I'm researching the purchase of a 350 hp air compressor, what is the electrical consumption for this machine?

Motor speed: 1780 rpm
Voltage: 460/3/60
Horsepower: 350
Motor efficientcy: 94.1%
Cost per KWhour $0.072

Thank you for any formulars and explanations.

Darrell Roak

 

[jbartos]

Suggestion: Please, would you add the power factor from the motor nameplate.

 

[DrDarrell]

Unfortunately, I am only in the research and quoting stage and I don't have the motor available to me. There is one more item on my quote sheet that might be of benifit.

Motor: ODP Premeium Efficiency

My conceren is, someone suggested that "...it will probably cost more to run the compressor than to actually purchase it..."

I hope you can help me with some way to estamate the cost to run this machine, if not the actual cost.

Thanks again,

Darrell Roak

 

[MJCronin]

Darrell,

Try the following website:

http://www.cleananddry.com

They have a survey form and software that will calculate the cost of compressed air based on your number of hours/year of operation.

Be aware, it is not unusual to spend as much money on electricity as the cost of the air system in the first year, and at seven cents/kwh you may get there quickly.

Also try

http://www.kaeser.com

for more info.....

MJC

 

[MJCronin]

Try the following website..... it has the formula you need

http://www.cummins-wagner.com/electricpowercost.htm

Based on my calculation, assuming 4000 hrs/year

I get about $77,770 operating cost or about $19.44 per hour

Please post your results and why you are interested in this calculation

Good Luck !!

 

[busbar]

Horsepower: 350
Motor efficiency: 94.1%
Cost per Kwhour: $0.0720

746 w/hp
277 kwatts
8760 hrs/yr
2,430,644 kwh/year
$175,006 $/yr

This is based on continuous operation fully loaded. It would be unusual not to have an additional kW-demand charge and some sort of power factor penalty; i.e., kvarhr/kvar-demand, kVA-demand or Qhour metering. This would significantly raise the calculated figure.

 

[jbartos]

Suggestion: Consider standard motor HPshaft formula and calculation for a typical 350HP motor to match the compressor need.
HPshaft=1.73 x Vline-line x Iline x PowerFactor x Efficiency / 746
Vline-line=460V
Iline=? ....estimated = 394.2A
PowerFactor=? .... estimated = 0.8845
Efficiency=0.941
Consider Baldor Motors and Drive Catalog No. ECP44354TR-4:
350HP
1785RPM
460Volts
95.6% Efficiency
388Full Load Amps
88.45% Power Factor
Power Factor 0.8845 is reasonably good to the Utility. The only caution has to be exercised if any kind of power electronic speed control would be applied because of potential harmonic distortion and associated harmonic mitigation cost/penalty.
Then the cost per year:
Baldor:
Cy=(0.072 $/kWhr)(350HP)(0.746 kW/HP)(8760hrs)/(0.956)=172,260$/yr
Yours:
Cy=(0.072 $/kWhr)(350HP)(0.746 kW/HP)(8760hrs)/(0.941)=175,006$/yr
Also, notice that the motor efficiency decreases, if the the motor shaft horsepower decreases, which implies an increased cost.

 

[BJC]

The motor you are looking at is not the highest efficiency you can get. I think you can get up to 95.8 % on a motor that large, The additional cost for the higher efficiency motor you can easily evaluate.
You can also get the efficienc of motors at 25%, 50%, 75% as well as 100%. If you have a load profile you should calculate the cost over a 24 or 168 hour cycle to account for the periods of reduced load.
A 350 HP motor is beyond the normal size for running at 480 volts. If you have 4160 volt power available seriously consider using it. Generally the break over point is 250 HP, motors larger than that are canidates for medium voltage. Starters are cheaper at 4160 for a motor this size. Your 480 Volt motor starter will probably be some kind of reduced voltage scheme ( like y-delta, part winding, or autotransfromer). At 4160 your full load current will be 48 amps while at 480 it's around 410 amps. Raceway is cheaper because you need smaller sixe. Cable cost may be the same because 5KV shielded cable is higher.

 

[jbartos]

Comments:
1. Reliance Electric produces SXE Plus / XE Premium Energy Efficient Three-Phase motors, e.g.
350HP, 1800RPM, 460V, Model Number P44G5050, Nominal Efficiency 96.2% for about $18,203 in 1994 prices.
2. If the medium voltage poses no problem (original posting does not address it), then it will have some advantages in starting (full voltage across the motor terminals). There may be some drawbacks coming with the medium voltage, e.g. maintenance cost up, safety more strict, insulation trees, surges, damages due to faults if not high-resistance grounded neutral system is involved, etc.