Motor Full Load Current Interpolation

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Motor Full Load Current Interpolation

There are times when the horse power for a motor in a calculation is not included in the table. In cases like this, interpolation is needed to find the full load current (flc). This is a linear progression calculation.

An easy way to look at interpolation is as follows:

    ---amps difference between two table values on either side of the motor to be calculated 

    ---divided by the number of even steps between these two given values where the motor to be calculated is on one of the steps

    ---equals the amount of increase for each step.

In the case of full horse power motors, the steps are easy to find. Here is an example.

Say you have a 6 HP 240 volt single phase motor. What is the flc?

Well, you can find the flc for a 5HP motor to be 28 amps and a 7 1/2 HP motor to be 40 amps.

Now think about the difference between these two table values. See that it is 12 amps?

Then how many steps are there between these two motors? Since one ends on a 1/2, the steps will have to be in 1/2 increments so each motor will land on a step.

So they will be starting with 5
then 5 1/2
then 6
then 6 1/2
then 7
then 7 1/2.

Even steps. The two table values are on a step. The motor to be calculated is on a step.

Now the difference in amps divided by the number of steps.

Or 12 divided by 5. That gives 2.4 amps per step.

So 5 HP is table value of 28
5 1/2 HP is an add 2.4 or 30.4
6 HP is an add 2.4 or 32.8
6 1/2 HP is an add 2.4 or 35.2
7 HP is an add 2.4 or 37.6
7 1/2 HP is an add 2.4 or 40 which matches the table value so the step increase checks.

Now just look at the table you have created to see that the flc for a 6HP 240 volt motor is 32.8 amps. You're done.

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