what would be the most accurate method to determine the annual operating costs for three phase and single phase AC motors ranging from 1 to 50 hp. Do you determine kw's and multiple by number of hours in service for kwh and then multiple by cost per kilowatt hours? Thanks for the help
estimating yearly electrical costs for three and single phase motors
- Thread starter pageman
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petersonra
Senior Member
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- Northern illinois
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- engineer
pageman said:
that would be an obvious way.
however, most motors of this size are used in an environment where the actual electricity cost is variable based on the time when the electricity is used.
winnie
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- Electric motor research
Not quite. For three phase, VA = A * V * 1.732, for single phase VA = A * V. In both cases, the product is volt-amperes, which includes both real and reactive power.
In general you only get charged for real power, however large facilities will also be charged if they have poor power factor.
On top of this, you need to figure out the correct amp value to use. For motors, you cannot simply use the nameplate full load amp value, because the current drawn by the motor will change as the mechanical load changes. A 50 hp 480V three phase motor with a mechanical loading of 25hp will draw similar current to a 25hp 480V three phase motor supplying the same mechanical load; there will be differences in efficiency and power factor, but I would expect the real current drawn in both cases to be within about 10% of each other.
You can convert directly from horsepower developed to watts; W = hp * 746 / efficiency. The numbers that you need to find to do your comparison are the efficiency values at the mechanical load. You will find that single phase motors of similar quality and rating will be nearly as efficiency as three phase motors.
-Jon
In general you only get charged for real power, however large facilities will also be charged if they have poor power factor.
On top of this, you need to figure out the correct amp value to use. For motors, you cannot simply use the nameplate full load amp value, because the current drawn by the motor will change as the mechanical load changes. A 50 hp 480V three phase motor with a mechanical loading of 25hp will draw similar current to a 25hp 480V three phase motor supplying the same mechanical load; there will be differences in efficiency and power factor, but I would expect the real current drawn in both cases to be within about 10% of each other.
You can convert directly from horsepower developed to watts; W = hp * 746 / efficiency. The numbers that you need to find to do your comparison are the efficiency values at the mechanical load. You will find that single phase motors of similar quality and rating will be nearly as efficiency as three phase motors.
-Jon
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