This topic is so vast that I can't squeeze it into one post, so this is part one of a two-part blog post that I'd like to write on the subject. If your attention span is anything like mine, I'll try to keep this short! You're welcome.
One of the toughest things when discussing the real-life benefits of Electric Motor Testing (EMT) is overcoming the gap in most folks’ understanding of motors. Motors are everywhere. Take a look around wherever you’re reading this blog. I would wager that there are half a dozen electric motors of some variety within your field of vision. There’s at least one in your computer, running the cooling fan. Electric motors are a big part of our everyday lives, yet many people involved in maintenance and reliability aren’t really sure how they work, much less how the technology behind EMT works to discover motor failures in their infancy.
So for now, let’s start with how electric motors work. I’m going to simplify it here and discuss one of the most common AC motor designs, the squirrel cage motor. Its name comes from how the rotor (that’s the part in the middle that turns the shaft) looks if you could see only the rotor bars. A simple squirrel cage motor consists of a rotor and a stator. The stator is made of thin steel laminations, around which wires are wound in a particular pattern. These groups of wires are called “windings”, you know, since they’re wound around the stator. The windings consist of numerous layers (referred to in motor talk as “turns”) of wire called magnet wire. The winding turns are insulated from one another by a very thin layer of insulation, essentially like a veneer.& This insulation is so thin that many mistakenly believe the wires to be bare. It’s important to note though, the wires are in fact not bare, but are insulated from one another. If there was a conductive path through the individual wires, the motor wouldn’t operate properly. The breakdown of this insulation is one of the most common modes of failure in an electric motor, giving us what motor folks call “turn-to-turn shorts”.
The windings are wound in a particular arrangement to produce a rotating electromagnetic field when electric current is applied to the motor. When AC is applied to the motor stator, the electromagnetic field generated by the current passing through the windings induces electrical current in the rotor portion of the motor. The lines of magnetic flux (a topic for a much deeper discussion later) that emanate from the stator windings cut across the rotor bars, which is what induces the rotor current. As current is induced in the rotor, it flows through the rotor in a path that is created by the connection of the individual rotor bars and “end rings” on each end of the rotor. Many people don’t understand that the electric current on the rotor is induced by the stator, not supplied by motor wiring. Many have asked me how a motor turns while not damaging wires, believing the rotor to be wired also, which it isn’t. The current in the rotor is electromagnetically induced by the electromagnetic field produced in the stator.
Ok, stick with me here!
So, we have a rotating electromagnetic field in the motor stator that induces current in the motor rotor and the rotor in turn develops its own electromagnetic field that follows the field in the stator. The effect is much like what you see when playing with refrigerator magnets. You know how you can push one magnet around with another one? The only difference here is that the polarity is what causes that. When you try to push the positive pole of a magnet into another positive pole, the opposition in their fields pushes them apart. This is essentially what a simple electric motor does. The stator current “chases” the rotor current, causing the rotor to turn.
Chew on that for a while. Stay tuned for the next installment where we will talk about the relationship between the rotor and the stator, and how we are able to make determinations about motor health and operation by examining that relationship.