In this segment of our AC motor training course, David discusses the mechanism that causes three-phase electric motors to function. He also discusses differences between three-phase AC motors and other electric motors. This portion of the video might seem overly technical or unnecessary, but when attempting a repair, it is vitally important to understand how a machine works. If you lack understanding of the basic mechanism of a motor, how can you hope to repair it properly? Or more aptly put by the course instructor, “When you understand how stuff works, it’s a lot easier to fix!”
Video Synopsis
Q & A: The first section of this video is a short question and answer session. David explains why it takes more electricity to start a motor than it does to keep one running under its workload. David also gives a short discussion about soft starting a motor.
Energy Flow / Circuits: For an electric motor (or any electrical appliance) to function, electricity must flow. Electricity enters the system, does work, and then exits the system, using energy in the process. If this circuit is not complete, electricity cannot flow and the motor will not function. David makes an analogy to the flow of water through a treatment plant, with electrical switches likened to water valves (this course was intended for water company employees). If something stops the flow of water/electricity, the system stops working.
Three-Phase Diagram: The three phases in a three-phase motor are referred to as A, B, and C phase. Each coil is wired in such a way that the three phases form a full circuit. A phase has leads 1 and 4; B phase has leads 2 and 5; C phase has leads 3 and 6. The two leads in each phase are wired to each other, and each phase is also wired to the other two phases. Lead 1 is wired to lead 6, lead 4 is wired to lead 2, and lead 3 is wired to lead 5. A diagram of this can be found eleven minutes into the video.
Efficiency Question: A question was asked about the difference in efficiency between the different designs of motors. In theory, there is no difference between the efficiency of a single-phase and a three-phase motor. An electric motor should require the same wattage for the same job regardless of its construction. One of the participants of the class noted that three-phases motors use more volts but fewer amps. In theory, wattage is equal to Volts × Amps, so if volts decrease, amps must increase, and vice versa. In practice, construction costs prevent some motor designs from being constructed.
Two-Phase Electricity: Almost all electricity in the United States is either single-phase or three-phase. However, there are two “pockets” of two-phase electricity left in the country. One is in Litchfield, Kentucky, and another is in the northeast (New Jersey or Rhode Island, according to the instructor). Even though it’s very unlikely to encounter a two-phase electric motor, they are considerably different from both single-phase and three-phase motors, so troubleshooting them and reconstructing them can be difficult for an unaware repair technician.