Motor Current:
Magnetizing and Load Components
As I mentioned in the previous section, motor current is composed of two parts: "Load Current" and "Magnetizing Current". They add together inside the motor to make the total current delivered to the motor. There are not two separate circuits in the motor, however, you can think of it that way if you wish (in fact, that's how motor current is shown in an electrical model by system engineers - two separate lines). Take a look at the graphic below.
Here are the important features to understand about these two components of motor current (not difficult, but important).
Magnetizing Current:
Magnetizing current establishes the magnetic field so the motor will spin.
Magnetizing current is constant (see the graphic below). The amount of magnetizing current that a motor draws depends on how the motor was made and does not vary with load.
Magnetizing Current uses no energy. What it uses in one half cycle it returns in the next half....more on this later but for now just push the "I believe" button...magnetizing current consumes no power.
When voltage is applied to the motor (remember, its a sine wave), the magnetizing current (also a sine wave) doesn't rise until 90 degrees after the voltage rises. This is referred to as lagging current. Magnetizing Current lags Voltage by 90 degrees. I'll explain why later, but thats not important if you just believe it.
Load Current:
Load Current develops when something tries to prevent the motor from spinning (like a compressor attached to the motor). The resistance to the spinning is the load. The Load Current increases with increased load.
Load Current uses energy. This is what is doing the work (i.e. driving the compressor).
When voltage is applied to the motor, the load current rises and falls perfectly in step with the voltage. It doesn't lag at all. This is referred to as being in phase; a 0 degree difference. Load Current is in phase with Voltage. See the graphic below.
Total Motor Current:
OK, now lets refer to the graphic below. Remember, motor current is the sum of magnetizing current plus load current (A = B + C) and we know that magnetizing current lags the voltage by 90 degrees and is constant, regardless of load. We also know that load current is in phase with the voltage and grows as the load increases. So the sum of those two, which is the motor current delivered by the cable, should be offset from voltage somewhere between 0 and 90 degrees, depending on the amount of load current being drawn. This example shows equal load current and magnetizing current, therefore the offset is 45 degrees (equally between 0 and 90 degrees).
Now look at the next graphic to see that under no-load conditions, the no-load current equals the magnetizing current (important). Also, as the load current grows, the total motor current increases and it gets closer and closer to being in phase with voltage. More on this in the next section.
If you understood this section, you'll definitely get it! Now we are ready for....drum role please...POWER FACTOR!
