Smart Energy User – Calculating What It Costs to Run a Motor
Doing a Rough Calculation –
If the load on a motor is fairly consistent, and if you have access to a voltmeter and clip-on ammeter, it is easy to arrive at rough kVa and kWh figures. For a 3-phase motor the procedure is as follows:
1) Measure all three phase-to-phase voltages and average them. For example, in our 20HP motor, let’s say we measure 226, 230, and 234 volts. This will give you an average of 230 volts.
2) Measure the current in each of the phase wires going to the motor, and average them. We measure 41, 42, and 43 amps, giving you an average of 42 amps.
3) Calculate the kVa:
kVa = 0.00173 x volts x amps
In our example:
kVa = 0.00173 x 230 x 42
kVa = 16.7
4) Calculate the Kw:
Kw = kVa x PF (Power Factor) x 0.01
(where PF is the power factor in %)
Unfortunately, a voltmeter and clip-on ammeter won’t tell us what the PF is. Still, even if we don’t have equipment to measure PF directly, we can estimate it:
First, calculate % Max Amps – Actual motor current as a percentage of rated full load current.
100 x measured motor amps
% Max Amps = (In our example % Max Amps = 100 x 42 / 49 = 86)
Second, estimate the motor PF. Do this by choosing the motor power curve closest in value to the rating of your motor. Then, following along that curve, see what PF is at the % Max Amps value of 86 gives a PF of approx. 76%.
Third, calculate the input power in kW.
kW = kVa x PF x 0.01 (PF in %)
And for our example:
kW = 16.7 x 76 x 0.01 = 12.7
5) Calculate monthly kWh:
kWh = kW x hours of operation per month
Let’s assume that out motor operates 333 hours per month.
KWh = 12.7 x 333 = 4230
6) Calculate the cost.
First, figure out how much each kWh costs – by taking a recent electricity bill and dividing the total dollar amount (including taxes) by the total number of kWh consumed during the billing period.
Second, multiply this number times the kWh value calculated in step 5 above – this will give the approx. monthly cost of running the motor – including that motor’s share of demand charges and taxes as well as its kWh consumption.
In our example, if the average cost of kWh is 11.2 cents, then the cost of running this 20HP motor would be 4230 x 0.112, or $474 per month.
NOTE: If you see a way to reduce the load on the motor or reduce its running time, you may want to know how much you will save. To perform an accurate cost calculation requires taking into account the multiple “block” structure of some electricity rates.
UNDERLOADED AND OVERLOADED MOTORS
Another reason to make motor measurements is to ensure that the motors in your plant are properly sized for the tasks they perform. Motors that are built into commercial equipment tend to be properly sized, but other motors may not be. For one thing, motors are often specified to have more horsepower than is actually required. These underloaded motors cost more to run than properly loaded ones because they operate on a less efficient part of their load curve. At the other extreme, some motors are called upon to deliver more power than they are designed to deliver. They will do this for a while, bt they overheat and eventually fail.
A MORE ACCURATE ASSESSMENT
In some applications, the load on a motor is not constant, but varies greatly (and perhaps unpredictably) from moment to moment and hour to hour. Here, longer-term data gathering is needed, using a device known as a recording wattmeter.
