Motors, Part 4

After presenting definitions, Article 430 provides requirements for part-winding motors [430.4]. Why would you install a part-winding motor versus another type?

The simplest way to wire a motor is across the line. This applies full voltage to the motor upon starting, but motor inrush current can be significant.

One way to solve the inrush problem is by installing a device that applies the line voltage over a short time curve rather than instantly. A soft-start or electronic drive will do this.

Another way is to energize one part of the motor windings first. This provides a low starting torque that’s typically insufficient to turn the motor and it will quickly overheat if the other part isn’t energized within 2 or 3 seconds.

Part-winding starting reduces the amount of inrush current (typically by 25 to 40%) and corresponding voltage dip. However, there is a cost – the motor presents a higher initial impedance.

The language in 430.4 may be confusing because the second and third paragraphs refer to each set of windings as “one-half.” This assumes the standard arrangement where each part equals one-half of the windings. Verify that your part-winding motor follows this standard. Sometimes it’s a one-third/two-thirds arrangement and you will need to adjust accordingly.

A part-winding motor must have branch-circuit, short-circuit, and ground-fault protection for each motor-winding connection. Additionally, you might need separate overload devices for each one.

Don’t confuse part-winding with multiple voltage. The latter is for permitting connection to various power supplies.

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