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Motors, Part 2

Before we discuss how motor requirements differ from the requirements for all other circuits, let’s look at why they differ. The shaft of an electric motor rotates because of the magnetic fields induced in the motor windings when electric power is applied. When the motor’s running, those windings present impedance (opposition) to current flow because of counter EMF developed during rotation.

When the motor starts, it isn’t yet rotating so there’s no counter EMF. The only impedance to the source current is the impedance of the winding wire. If you look up the ohms per foot of copper or aluminum conductors of the size used for motor windings, you’ll see it’s just about zero.

This means that when a motor starts up, it momentarily presents a dead short to the supply. This moment doesn’t last long because the motor starts rotating almost instantly. However, the motor isn’t instantly up to its running speed. This period between start and running speed is a high current draw period for the motor. The current is typically five or six times what the motor draws while running. We call this the inrush current. And we must allow for it so that our circuit protection devices don’t open during the period between startup and running.

This is why the requirements for motor circuits differ from those of all other circuits. It's also why we have Article 430. In all other circuits, a single device provides overload protection and fault protection. Not normally so with motors.

« Part 1Part 3 » | Source: Mark Lamendola | Mindconnection