Column-by-Column: Understanding Three-Phase Motors

Over the last several weeks, we’ve examined each column in the single-phase motor specification table on pages 13 and 14 of the Franklin Electric AIM Manual. This week, we’ll take a quick look at the equivalent three-phase information found on pages 22 – 28.

With so many pages of three-phase motor specifications, at first glance it may look as if three-phase motors must have more going on than their single-phase counterparts. However, the “more” that’s going on here is because three-phase motors come in far more ratings than single-phase. Single-phase power has some limitations, and as a result, Franklin single-phase motors are only offered from ½ to 15 hp. Franklin three-phase motors are available in ratings all the way from ½ to 200 hp.

In addition, there are more three-phase voltages. Whereas single-phase motors are either 115 or 230 V, three-phase motors can have five voltages: 200, 230, 380, 460, and 575 V. So, more hp and voltage ratings mean more models. In fact, Franklin offers three-phase submersible motors for just about every application.

However, although the tables are larger, looking at the column headings you can see that it’s the same information that we already covered for single-phase. Specifications such as maximum load, line (winding) resistance, and locked rotor amps all have the same meaning whether we’re talking single- or three-phase.

There is one difference to note. Notice that all of the three-phase motors only have a single line of data, whereas most of the single-phase motors have 2 or 3 lines of data. This is because single-phase motors have two different windings, a start (auxiliary) winding and a main (run) winding, and there are physical and electrical differences between them. So, when measuring maximum load, for example, in a single-phase motor, there are three readings to take (run, start, and common), and each of these measurements will be different. However, three-phase motors have three identical windings. Therefore, current and winding resistance of each will be the same. So, although there are still three readings to take with a three-phase motor, the expected value is the same for each one since each winding is the same. Therefore, a single line in the table applies to all three windings.

Another difference between single- and three-phase motors is that 3-wire single-phase motors require a control box. Although a panel of some type is generally used, three-phase motors do not require a control box. As a result, we don’t need to make a distinction between the standard control box and a CRC control box.

All of this makes three-phase motors actually simpler than single-phase motors. If you know the information from our series on single-phase motors, then you already know three-phase motors. The differences in the tables don’t actually complicate things, but simplify the system and may even offer you new business and product opportunities.

No matter what kind of Franklin motor you are working with, if you have application, installation, or troubleshooting questions, contact our Technical Service Hotline at 800.348.2420 or email at hotline@fele.com.

2 thoughts on “Column-by-Column: Understanding Three-Phase Motors

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