To calculate motor operating costs, the math is pretty straight forward, but accuracy due to application and motor manufacturing variables may require gathering information and some thought. Here's the calculation...
In the above equation, (load) in red, probably isn't the nameplate value. For example, if we're calculating a 100 HP electric motor, and the equipment we're driving requires 75 HP (or 75% of 100 HP) the HP load applied in the equation should be 75HP.
In the adjacent table, a 15 HP motor's efficiency, typical of many motors under 200 HP, is relatively flat to about 40% of load. However, using amperage measurements, if a little less than 20% of full-load, would result in noticeable inaccuracies as load is reduced.
The following calculation for three-phase motors will provide reasonably accurate part-load results if motor loading is between 40 and 100%.
Applied voltage must be within 5% of nameplate, be 71/2 HP or larger, and a 2, 4, or 6 pole motor.
To obtain no-load amperage one might check the motor manufacturer's website, looking for your particular motor's operation characteristics. If the motor has been into a motor service center for repair, it's likely a no-load test occurred. Test results, including no-load amperage, should have been documented and available from the service center. Of course, when a motor is installed, prior to connecting the load and all safety measures taken, would be a time to acquire and document line voltages and no-load amperages.
While considering operation costs, many electric utilities have an additional charge for low "Power Factor". You can see from the table above, as load decreases power factor also decreases. Power factors lower than 80 to 90% may trigger an additional power factor utility charge. Good news, it can be corrected, which is a topic for another blog.
Efficiencies often vary from one motor to another, and not as straight forward as one might think. A number of motor efficiencies of horsepower's from various manufacturers were measured in a 2003 Electrical Apparatus article, the results are reproduced below.
In the operating cost calculation, because of efficiency variations as above, it is suggested "Minimum" efficiency be used in the denominator rather than "Nominal" efficiency. Of course, depending on your calculation goal, you may prefer the higher "Nominal" efficiency value or maybe someplace in-between. Your motor nameplate usually displays "Nominal" efficiency. Click the below link for NEMA Premium® efficiency Table 12-12, providing both "Nominal" and "Minimum" three phase motor efficiencies. large.stanford.edu/courses/2010/ph240/veltman2/docs/NEMAefficiency.pdf
General (handy) North American Hours:
Average hours per month, 730.
Average hours per year 8,760.
Approximate average hours per month (single shift) 200.