Wednesday, May 2, 2012

The Least You Should Know - AC Motors

You thought the last topics were big, so I'm going to narrow it down a bit.

I like to think of the electrical world as being composed of a few "continents".  One continent is certainly electronics: computers, calculators, TV, radios, audio amplifiers, electronic music, and the many electrical toys we're surrounded by.  Another continent, certainly a very important one, is power: generation and transmission.  For the most part, these continents don't touch - except where your electronic device connects into the power generation and transmission system.  There are not many components that these two continents share. I can think of only a handful: wires and connectors, sure, but also motors.  From the giant electric motors in diesel electric locomotives to the tiny "vibrator" in your cellphone - the silent ringer - motors occupy big places in both continents.

The basic reason motors work is because electricity and magnetism are one and the same thing - electromagnetism.  This stuff gets mathematically complicated: if there's one area that the vast majority of other engineers I've known hated in school, or had a wicked hard time with, it was electromagnetics.  I'll leave out the math, but just know that an electric field creates a magnetic field, and likewise, that magnetic field will create another electric field. 

The most basic motors, the ones in widest use for the last hundred years or so, are AC induction motors.  AC motors, like all motors, are built from a combination of coils of wire (which we know are inductors) and magnets.  They have a rotating piece (not very useful without that!) called the rotor and a piece that's not moving (stationary) called the stator.  Something like this (source -and more in depth tutorial than this):
These motors ordinarily rotate at a speed related to the AC frequency.  60 Hz AC (60 cycles per second) is 3600 in a minute, and motors rated at 3600 RPM or 1800 RPM are very common.  Under load, that frequency tends to go down from an effect called slip.  The dependence on the AC frequency to set the motor speed in RPMs makes them hard to use if you need variable speeds.  A variable frequency drive is needed (generally created by electronics). These motors are available in a very wide range of torque and power, from small fractions of a horsepower (1/4HP electric drills used to be very common) up to behemoths you won't have in your house or on your farm.

Electric motors are capable of very high efficiency, over 90%, one the reason they're attractive to "alternative energy" folks.  As you might expect, weighing against this advantage are some disadvantages. One is that being connected across the power line lets AC motors draw high peak currents, as much as seven to 10 times normal operating currents, at start up. This is one of the reasons for the starting capacitor many motors have, and large motors (like central air conditioners) will sometimes have a "soft start" kit.  Obviously, these current surges can affect other devices on the same circuit. 

Much of the power loss in an AC motor is in the rotor, which typically relies on air for cooling, as shown. The act of moving air through the motor housing for cooling leads to power loss overcoming air resistance. Other losses  include resistance in the copper wire as current flows through the stator winding, magnetic losses through stray magnetic paths, and magnetic losses in the motor iron.

AC motors are the workhorse of the shop.  Chances are you have several around your house right now: in power tools, an air conditioner, fans and other things.  They are very limited, though, by the dependence on line frequency.  Many more motor applications are available for motors that can rotate at variable speeds, or that can move a fixed number of turns, or simply move an accurate portion of a revolution.  Those are DC motors of various kinds, and that's the topic for next time. 


  1. Starter and run capacitors... is bigger better.. how to wire it in?

    1. I see two questions here, but maybe I'm reading you wrong. The first one, "is bigger better?" - ordinarily, you'll replace the one that's on the motor with the same value. In theory, there's a calculated value that works with that motor. In practice, those parts are not usually extremely precise values, and within a loose tolerance. A bit bigger is usually better than too small. I wouldn't go to twice the size of the cap I needed to replace, but 50% bigger? I'd probably go there.

      This is actually a fairly involved question. Motors are a big topic area.

      As for how to wire it; again, replace it where the old one was. In general, the cap is across a centrifugal switch that throws open as the motor gets up to speed. It should be marked on a little circuit card or something.

      There's a fair amount of detail on Wikipedia

  2. Thank you, Nikola Tesla.
    When I was in College, and leaning strongly towards RF (hey, I had my General at age 12!), I always shook my head when a classmate was asked if he was going into Electrical Engineering vs Electronics Engineering, and they'd always reply "What do you think I am....Power People???"
    WELLLLL....if it wasn't for the "Power People", we wouldn't have anything to plug our fancy schmancy stuff into, would we?

  3. somewhat related? this is just too extreme DIY to not share - from a link - DIY hydropower complete with self built power controller, transformer & more:

    thought it might be of interest


    1. Itor - that's an incredibly impressive project!

      Gang - check out that site if you're even remotely interested in generating your own power.

  4. Never did care for single-phase motors. Most of my stuff that I worked on was 3-phase; more power for the size than single phase.
    Worked on a lot of traction motors also, I imagine you'll be mentioning them in the next post.