Column-by-Column: Winding Resistance in Ohms

When we left Single-Phase Motor Specifications on page 13 of Franklin Electric’s Application, Installation, and Maintenance (AIM) Manual at the last post, we were ready for the column titled Winding Resistance in Ohms.

Single-phase motors have two windings, a start winding and a run winding. Technically, when the motor is a capacitor start/capacitor run type, these are called the auxiliary winding and the main winding respectively. However, we’ll just use the terminology of start and run windings to keep things simple.

Measuring winding resistance is a power-off check. Power must be disconnected and locked out.

Starting with 2-wire motors, winding resistance is measured between the two black motor leads. This can be done at the motor, at the well head, or even at the pressure switch. Referencing Table 13, the first thing noticed is the single row of values listed for 2-wire motors. For example, the ½ hp, 230 V 2-wire motor lists the winding resistance as 4.2 to 5.2 Ohms. This is the resistance of the run winding. But wait, where’s the value for the start winding? The answer is that because these are 2-wire motors, we don’t have access to the start winding. It’s there, but since this is a 2-wire motor, we can only measure the winding resistance between the two black leads; only one reading can be taken.

Notice that a range of values is provided, not exact numbers. And in reality, if you get something close to these numbers, the winding resistance is probably good.

In the case of 3-wire motors, winding resistance is measured for both the start and run windings. The main winding resistance is measured between the black and yellow leads. The yellow lead is the common here, and the start winding resistance is measured between the yellow and red lead. There’s no need to memorize this, since it’s in Footnote 1 at the bottom of the page.

Using a ½ hp, 230 V 3-wire motor as an example this time, we see that the main winding resistance is 4.2 to 5.2 ohms (same as the 2-wire motor). The start winding resistance is 16.7 to 20.5 ohms.

Winding resistance for 3-wire motors can be measured at the motor itself if it’s out of the well, at the well head, or at the control box.

Regardless of 2-wire or 3-wire, what does winding resistance tell us? When troubleshooting and measuring winding resistance, we’ll generally get one of three readings: zero, infinity, or a value close to what’s listed in the table. If the reading is zero, which indicates the winding is shorted. If the reading is infinity, which indicates the winding is open. In either case, the motor will need to be replaced. If the measurements are being taken at the well head we’ll also want to check the drop cable.

Winding resistance is one of two electrical checks, insulation resistance being the other, that tell us the electrical condition of the motor. If both the insulation resistance and the winding resistance are good, our motor is good from an electrical standpoint. It tells us that in terms of troubleshooting to look other places. By the way, we’ll cover insulation resistance in another post.

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