Column-by-Column: Full Load Amps and Max Amps

In the last post we explained service factor: that electric motors aren’t necessarily what is on the nameplate in terms of horsepower, they’re actually more. This difference is service factor. Read more about service factor in the post Column-by-Column: Motor Model and Rating.

Service Factor comes into play in the four columns in table 13 of the AIM Manual labeled Full Load and Maximum Load. These columns list current draw in amps and power consumption in watts.

Let’s begin with amps. Once again, notice there are two sets of columns: Full Load and Maximum Load. Full Load is the expected performance of the motor at the rated or nameplate hp. For example, if a 1/2 hp motor were delivering exactly 1/2 hp of power to the pump, it would be operating at Full Load amps. Maximum Load is what the motor would be delivering when service factor is included. As a result, Maximum Load is often also referred to as service factor amps or max amps.

Taking the 1/2 hp, 230 V example, when delivering 1/2 hp to the pump it draws 5.0 amps. However, if that same motor operates at its service factor of 0.8 hp (1/2 x 1.6) it draws 6.0 amps. In most installations, motors operate near the Maximum Load. Also note that these values assume the voltage at the motor is the nameplate voltage, in this case 230 V. If the voltage at the motor differs from this, the amperage will be slightly different.

The usefulness in having these numbers is that by measuring the current with an ammeter, we can determine how hard the motor is working. The more water we’re moving, the more electricity (current) the motor will need. For example, if we measure the current draw of the motor and it’s less than the Full Load value, that tells us the motor isn’t working very hard. Meaning, we’re pumping on the left side of the curve, we have a loose impeller, or possibly a pump “gulping” water.

If a motor’s amperage is measured and it’s over Maximum Load (service factor amps), it indicates that the motor is working too hard. This could be an indication of a binding pump or a case where we’re pumping on the right side of the pump curve.

Note: 2-wire motors only have a single row of figures. However, the 3-wire motors have 3 rows of figures representing the yellow, black, and red leads of a 3-wire motor. This points out an important difference between 3-wire motors without and with run capacitors. In the models without run capacitors, the red lead shows zero. This is because the start winding in these motors is only used for starting the motor. It comes out of the circuit as soon as the motor comes up to speed.

However, the remaining 3-wire motors with run capacitors all list current in the red lead. This is because in these motors, the start winding stays in the circuit after the motor has started. So, no run capacitor in the control box means zero current in the red lead. A run capacitor means there should be current in the red lead.

Power in electrical watts is listed next to amps and the same rules from above hold true here. The difference is, from a troubleshooting standpoint, we generally don’t have a way to measure power in the field directly. But, where those numbers are required is when calculating the cost to operate a submersible pump. For an explanation of how to do just that, see the Franklin in the Field post:

Next week, we will take a look at the column headed Winding Resistance In OHMS. We’ll see that it contains some especially valuable troubleshooting information.

4 thoughts on “Column-by-Column: Full Load Amps and Max Amps

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