Popular Woodworking 2000-11 № 118, страница 6
An employee at the Kosta Plant in Taiwan balances the armature on a universal motor.The plant assembles tools and tests motors for Delta Manufacturing. way that the motor is built. Here's the inside story: Induction Motors The reason they are called "induction" motors is the way they convert electricity into a spinning rotor. To understand how induction motors work, let's say you've got one of these puppies in your table saw and you're about to turn it on. As you flip the switch, power flows into what's called the "stator" and magnetizes it. The sta-tor is a mass of copper windings that surround the rotor in the center, which is what spins the saw's blade through a series of belts and pulleys. Inside the stator are two or four "poles" that become magnetically charged because of the electricity running through the wires. When the electricity changes direction or cycles, as it does 60 times a second in the United States (hence the term 60 cycles), each pole changes its magnetic strength, from a positive to a negative value or from a negative to a positive value. The induced poles in the rotor are then attracted and repulsed by these ever-changing electromagnets in the surrounding sta-tor. The motor isn't running, but the rotor is excited. What this hulk of iron and copper now needs is a shot of power from another copper winding (called a "starting winding") that is out of phase physically and electrically with the main winding. And that's where the capacitor comes in. In most modern tools a capacitor (which is in series with the "starting winding") helps with the starting torque. Then, when the motor reaches 85 percent of its speed, the capacitor and the starting winding drop out of the circuit and the motor runs on its main winding. Whew. So, this is the long way to explain why these are called induction motors. As you can see, the rotor spins because it is "induced" by the electromagnets in the stator. Induction motors are large and heavy because the induction process takes a lot of iron and copper (a V2 hp induction motor weighs about 25 pounds; a V2 hp universal motor weighs 212 pounds). Induction motors are reliable because they're simple, their parts are built for long life and they run at slow speeds (so they don't generate as much motor-damaging heat). In fact, a well-built induction motor won't heat up more than 40 degrees centi- So now you can understand why you wouldn't want your router powered by an induction motor — you could barely lift it, and it probably would be too slow and not have enough torque. Universal Motors Universal motors get their name from the fact that many of them can operate on both alternating current (from an outlet) or direct current. The way that universal motors work is a little more complicated than their induction cousins, but there are similarities. Instead of a rotor, universal motors have what's called an armature that spins in the center. Instead of a stator, universal motors have what's called a field, usually consisting of two coils surrounding the armature. Universal motors also have some parts that induction motors don't. On one end of the armature is a part called the commutator. This part is round like the armature, but it is usually smaller in diameter and is made of small bars of copper. It's through these bars that the armature winding is energized. Universal motors also have what are called "brushes." Brushes |
