Column-by-Column: Efficiency %

As we return to page 13 of the AIM Manual this week, this week’s post will focus on the column labeled Efficiency %.

Simply put, electric motors take electrical energy and convert it into mechanical energy. That is, we put electricity into the motor and out comes the rotation of a shaft that powers a pump. However, it’s not a “one-for-one” conversion; we don’t get the same amount of energy out of the motor that we put into it. The energy that is lost in the conversion process gets turned into heat. This happens not just with motors, but with all devices that convert energy from one form to another. Perhaps the most obvious everyday example is a light bulb. Only a small portion of the electricity that goes into a light bulb comes out as light energy. The rest becomes heat, as anyone who has touched a lit bulb knows.

This ratio between the amount of energy that we get out of something versus what we put into it is called efficiency. It’s generally stated as a percentage, but it can also be stated as a decimal.

The efficiencies of Franklin Electric’s single-phase submersible motors are listed on page 13 of the AIM Manual. Like several of the other columns on page 13, there’s a column for full load (FL) and one for maximum load, also called service factor load (SF). [For an explanation of service factor, see the Franklin AID post on Full Load Amps and Max Amps.] Since the motor operates at or near service factor most of the time, we’ll limit our focus to efficiency at service factor, or the SF column in the table above.

Using the 1 hp, 3-wire motor as an example, the table shows that the motor is 65% efficient. Once again, this simply means that 65% of the electrical energy that goes into the motor is available to turn the pump.

We can even calculate this ourselves as follows:

This is a 1 hp motor, but to keep the units consistent, we’ll use the equivalent value in kilowatts, in this case 0.75. We’ll also convert this to watts by multiplying by 1000 (kilo=1000). So, mechanically, this is a 750 watt motor. Since there’s a service factor involved of 1.4, this motor is actually a 1050 watt motor (750 x 1.4 =1050).

Now we know what we get out of the motor in terms of mechanical energy, but how much do we put into it in terms of electricity? That is found in the Maximum Load column under watts. That value is 1600 watts. So, we put 1600 watts of electricity into the motor and get 1050 watts out. That means the efficiency is:

1050 watts / 1600 watts = .65, or 65%

(output     /      input      =  efficiency)

This matches the 65% listed in the efficiency column.

Keep in mind that so far, we’ve just covered motor efficiency. The pump’s not going to be 100% efficient either, but we’ll cover that in another post.

In actual practice, the efficiency of smaller single-phase motors is generally not too critical. Because it actually costs so little to run a residential single-phase motor [refer to the Franklin in the Field post The Deal of a Lifetime], efficiency gains may only result in pennies of savings per day. Efficiency becomes more important, however, in applications with greater power consumption.

Next week we’ll move on to power factor in our attempt to make the numbers make sense, column-by-column.