Line Frequency Stuff Makes My Head Hertz
While the Motor Doctor can’t clear up that mystery (and later in this article, I’ll even add to the complexity), I can explain some of the physics behind line frequency. While you may not need this information every day, in our increasingly global economy, it may come in handy.
Actually, frequency is one of two potential issues that electric motor manufacturers face when selling to international customers. The second concern is voltage, but this is a relatively simple problem because most electrical equipment is designed to operate between ±10% of its rated voltage. To determine compatibility, you just need to know if the voltage source falls within the voltage range of the equipment in question.
The issue of line frequency (expressed in a unit called a Hertz) can be a bit more perplexing, especially when magnetic devices such as motors, equipment with transformers, or equipment with magnetic ballasts (fluorescent or vapor-type lamps) come into play.
Frequency, EfficiencyOne critical relationship between line frequency and magnetic devices is efficiency. The physics of electric circuits tells us that ac magnetic devices increase in efficiency as line frequency increases. Sounds simple, doesn’t it? Just build your power systems and devices to the highest frequency possible.
Not so fast. Another physical characteristic to keep in mind is that current flow in a conductor tends to be closer to the surface of the conductor as frequency increases. So, as the frequency goes up, solid conductors begin to resemble hollow pipes as the electrons making up the current flow migrate to the outer surfaces of the conductors.
At these higher frequencies, the energy of the electrons has a tendency to actually leave the surface of the conductor. A common example of this principle in action is radio transmission. As the frequencies get higher, all of the energy can be made to leave the conductor in a form of energy called radio waves.
This also helps explain why overhead power lines tend to interfere with radio reception (the annoying 60-cycle hum). What you are hearing is energy loss from the power lines becoming a radio wave that is intercepted by the radio receiver.
Consequently, the designers of electrical devices must strike a balance between the desire to use higher frequencies to improve the efficiency of converting electrical power to mechanical work and the need for lower frequencies to keep power from escaping the conductor as radio waves.
The evolution of 50- and 60-Hz systems developed as a result of this need for balance, with additional influences coming from politics and geographic considerations. North America and other regions struck the balance at 60 Hz, while Europe settled on 50 Hz.
You probably realize by now that frequency is a parameter for motor selection and application. What you may not realize is that a motor designed to operate at the lower, 50-Hz frequency may operate in a satisfactory manner at 60 Hz, at least from an efficiency and heat loss standpoint. This is because a 50-Hz motor contains added material to make up for the modestly decreased efficiency found at the lower line frequency. But keep in mind that when going from 50 to 60 Hz, the motor’s speed will increase proportionately to the increase in frequency (60/50 = 6/5 = 120%).
On the other hand, applying a 60-Hz motor to a 50-Hz line frequency application is more problematic. A motor designed to operate efficiently at 60 Hz may not have enough active material (copper and iron) to sustain efficiency at 50 Hz. Heat loss and dissipation become issues.
Another consideration is that some motors designed to operate on single-phase power may have an internal switch that is speed sensitive. At the lower frequency, the motor may never reach the normal switched operating speed. The internal start switch will not open, and this will lead to burnout of the starting circuit.
ComplexitiesI promised to add a little more complexity to the 50-Hz/60-Hz debate, and you don’t have to look too far from home to find it:
In our increasingly interrelated world, don’t be surprised if you encounter the 50-Hz/60-Hz debate in your work. Now, you’ll have some understanding of how to deal with the issue.
Simon is with A.O. Smith Corp., P.O. Box 245010, Milwaukee, WI 53224-9510.
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