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October 1996 Volume 2 Issue 17

Calculating What it Costs to Run a Motor

Much of the electrical energy consumed in a typical production operation flows through large-horsepowerelectric motors. Because of this, it makes sense to gather data on your large motors and figure out how much it

costs to run each one. The data gathered is valuable for other purposes too. It allows you to identify motors thaare underloaded (and thus costly to operate) or overloaded (and at risk of burnout). This data is also extremelyvaluable to have on hand when a motor fails and you must quickly decide whether to repair it or replace it.

With some types of electrical loadlighting loads for examplecalculating the monthly kWhconsumption is straightforward. Whenever these loads are on, they draw a constant amount of power, andnameplate amps and watts normally reflect actual operating conditions. The monthly kWh consumption issimply the units rated power in kW times the number of hours per month that the load is on.

With motors, the situation is more complex. The motor nameplate may say Horsepower: 20, Volts: 230, andAmps: 49.0, but theHorsepowerandAmpsfigures on the nameplate are just the motors maximum ratings.They dont indicate what the operating values really are in your situation. The actual current that a motor drawsfrom the line, the power it demands, and the energy it consumes, all relate to the amount ofmechanical power

that the motor is called upon to deliver through its output shaft. If the motor actually delivers 20 horsepower(hp) of mechanical power, then the motor will draw 49 amps. But under light load, the current drawn by such amotor will be significantly less.

DOING A ROUGH CALCULATION

If the load on a motor is fairly constant, and if you have access to a voltmeter and clip-on ammeter, it is easyto arrive at rough kVA and kWh figures. For a 3-phase motor the procedure is:

1. Measure all three phase-to-phase voltages, and average them. In our 20 hp motor example, lets say wemeasure 226, 230 and 234 volts, giving an average of 230 volts.

2. Measure the current in each of the phase wires going to the motor, and average them. We measure 4142, and 43 amps, giving an average of 42 amps.

3. Calculate the kVA:kVA = 0.00173 volts ampsIn our example:kVA = 0.00173 230 42kVA = 16.7

4. Calculate the kW:

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kW = kVAPF0.01(where PF is the power factor in %)

Unfortunately, a voltmeter and clip-on ammeter wont tell us what the PF is. Still, even if we dont haveequipment to measure PF directly, we can estimate it:

First, calculate % Max Amps

actual motor current as a percentage of rated full load current.

100 measured motor amps% Max Amps =

max. rated motor ampsIn our example,% Max Amps = 100 42 49 = 86

Approximate Motor Power Factorat Various Input Currents

% Max Amps

(Input Amps as % of Max Rated Amps)

Second, from the graph above, estimate the motor PF. Do this by choosing the motor power curveclosest in value to the rating of your motor. Then, following along that curve, see what the PF is at the %Max Amps value you calculated above. In our example, following the 20 hp curve to a % Max Amps value of86 gives a PF of approximately 76 per cent.

Third, calculate the input power in kW.

kW = kVAPF0.01 (PF in %)And for our example:kW = 16.7 76 0.01 = 12.7

5. Calculate monthly kWh:kWh = kW hours of operation per monthLets assume that our motor operates 333 hours per month. kWh = 12.7 333 = 4230

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6. Calculate the cost.

First, figure out how much each kWh costs your firm by taking a recent electricity bill and dividing thetotal dollar amount (including taxes) by the total number of kWh consumed during the billing period.

Second, multiply this number times the kWh valuecalculated in Step 5 above. This will give theapproximate monthly cost of running the motorincluding that motors share of demand charges and taxesas well as its kWh consumption.In our example, if the average cost of a kWh is 11.2 cents, then the cost of running this 20 hp motor wouldbe 4230 0.112, or $474 a month

Note: If you see a way to reduce the load on the motor or reduce its running time, you may want to knowhow much you will save. To perform an accurate cost calculation requires taking into account the multipleblock structure of some electricity rates. For help with this, contact Mike Proud or Ron Estabrooks at368-5010 (toll free).

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 othermotors may not be. For one thing, motors are often specified to have more horsepower than is actuallyrequired. These underloaded motors cost more to run than properly loaded ones because they operate on a lessefficient part of their load curve. At the other extreme, some motors are called upon to deliver more power thanthey are designed to deliver. They will do this for a while, but 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) frommoment to moment and hour to hour. Here, longer-term data gathering is needed, using a device known as arecording wattmeter. The Energy and Minerals Section has one of these units, and if you think it might shedlight on your situation, call Mike Proud or Ron Estabrooks at 368-5010 (toll free).