AC Electrics -AC Motors — Page 249, Lesson 262

I want to walk you through the concept of slip speed in an AC induction motor, and then we'll look at how single-phase induction motors are started, plus what happens when a three-phase motor loses one phase. Let's start with slip speed. The speed of an induction motor is determined by two things: the frequency of the AC supply, and the mechanical load on the motor. Now, here's a key point — the rotor never quite reaches true synchronous speed. If it did, the squirrel cage rotor bars would not be cut by any lines of magnetic force, and therefore no voltage would be induced in them. Without that induced voltage, there's no current, and no torque to keep the motor running. So the rotor must always lag behind the rotating magnetic field of the stator. The difference between synchronous speed — the speed of the rotating magnetic field — and the actual rotor speed is called the slip speed, or simply rotor slip. A typical value of slip is about 5%. Because of this necessary difference in speed between the stator field and the rotor, the induction motor is sometimes referred to as being asynchronous. That's an important term to remember: asynchronous means not synchronous, not running at the same speed as the field. Now let's move to starting single-phase induction motors. A single-phase induction motor is not self-starting. That's a fundamental limitation. So different methods are used to make it self-starting. The most common method is something called a split phase winding. Here's the principle: if the current in the split phase winding can be made to lead or lag the current in the main winding by exactly 90°, then a rotating magnetic field can be produced. That rotating field is what gives the motor the torque to start turning. The lead or lag can be produced by several methods. I'll list them: resistance starting, inductance starting, resistance/inductance starting combined, and capacitance starting. The application of each method depends on the power output of the motor. For example, capacitance-started motors are usually used for motors of less than 2 horsepower output. Finally, let's talk about fault operation. Occasionally, a three-phase induction motor loses one phase of its supply. If the motor is lightly loaded when this happens, it will probably continue to run — but at about half of its normal speed. This creates a humming noise in the motor. However, because these motors are often mounted in remote locations, that humming noise may not be noticed. The fault usually becomes apparent the next time someone tries to start the motor — because with one phase missing, it will not start at all. Let me show you a diagram that illustrates how a rotating magnetic field is generated, which is the foundation of everything we've just discussed. And here's a diagram of the squirrel cage induction motor itself, so you can see the rotor bars and the construction we've been talking about.