Where to get information on motors?

Bodine has been printing the "Small Motor, Gearmotor, and Control Handbook" for many years. It's a pretty good, not too technical overview of many small motor types. It looks like it may be online, but you have to register...

Ned Simmons

Your question is imprecise and unclear. I'm assuming you are asking about AC electric motors for use in a home shop. If you're wondering about retrofitting your ultralight with a diesel, this may not be for you.

My first and often-repeated suggestion is to go visit your local library. Sometimes you will get lucky and find a good book. There are lots and lots of bad books on electric motors, but all of them have many basics.

If you live in the US then the power is 60 cycles per second, or 60 Hz. If you live in Canada, then the power is 50 Hz. Motors made for 50 Hz. have more copper in them than motors made for 60 Hz. and as such are more desirable. Some guys don't know that almost any 50 Hz. motor will run fine at 60 Hz. and most 60 Hz. motors will work on 50 Hz.

Motors run at an approximate RPM. This RPM is related to the power line frequency. For 60 Hz. power, this RPM = (3600 / N) - slip. If the motor has one pole then it is commonly rated at 3450 from which you can easily see it would run at 3600 except for the slip. Less load, less slip; more load, more slip up until stall. If the motor has 2 poles, then it will likely be rated for 1760 rpm or 1730 rpm or 1800 rpm or something like that. If you see a motor that says 1430 rpm then you know it's a 50 Hz

2-pole motor.

Motors come in single phase and three phase. Single-phase motors often have a centrifugal switch that makes a connection until the rotor is spinning fast enough to trip the centrifugal switch. You will hear this switch audibly *click* as a single-phase motor slows. This switch either connects or disconnects start capacitors. Single-phase motors that don't start usually need a new start capacitor. These capacitors are electrolytic capacitors and such capacitors don't last too long in service, maybe 10-15 years, because they dry out and lose their capacity. Three phase motors don't need start caps but they need three phase power.

Motors can be often be run to work at more than one voltage or in either direction. See the reference:

There's a lot more I could write, but that's it for now. - GWE

I picked up a book "Managing Motors" by Richard L. Nailen. Pretty much a text book but really orientated to the 'how to' side rather than the theory side. even has a decent section on phase converters. Out of print and out of stock at Amazon. 1996 copyright.

B.B. wrote:

look at the application notes that the various manufactures put out, for example Minarik. And, there are lots of text books on the subject that are considered of no value by most book sellers, so there's another avenue

Try

as a start.

Ah! Perfect! Thanks.

Grant - are you sure about 50 Hz power in Canada? I spent some time there

30 years ago (no, not avoiding the draft) and seem to recall it was 60 Hz and I have a vague recollection that power can be shared across the border. Could be wrong on both counts, though.

Mike

Yeah, sorry about that. I know there's a ton of info so I should shave it down a bit. AC motors 10hp and smaller. Mostly the stuff found in tools rather than tape decks.

[...]

Thanks. Very helpful. One question for now: What is power factor correction? I've found the term several times today, but not the definition.

BB

I get alot of help with motor design information from a book (paperback) written by Irving M. Gottlieb, titled Electric Motors and Control Techniques.

Jerry

This is a nasty one. If you have an ideal power source providing a sinusoidal voltage, then if you put that in series with a resistor, current will flow, and the current waveform would be a sinewave. If you then add a capacitor across the voltage source, current would also flow through the capacitor. If you looked at the current waveform across the resistor on the same scope trace as the current waveform across the capacitor, they would look very similar (well, one would be bigger than the other) but their peaks and valleys would have shifted relative to each other. In fact, the current waveform through the capacitor is "phase shifted" by +90° relative to the current waveform through the resistor. Similarly, if you apply the sinusoidal power to an inductor, its current waveform would also be a sinewave, but it would be phase shifted by -90° relative to the waveform of the current through the resistor.

OK, now here's the tricky part. Current flowing through a resistor dissipates power, and it is called "real current". The term "real" here is mathematical - it means that when you express the current as a complex number it only has a real part and its imaginary component is zero. However, current through either an ideal capacitor or an ideal inductor does not dissipate power. This current is called "imaginary current" for similar reasons.

OK so far?

Now let's move to a real-world situation. Electric AC motors are magnetic machines. They have windings which "look" very much like an inductor. In reality, any physical component has some resistance and almost certainly some capacitance too, but if you compare the current waveform of an AC motor with that of the current waveform into a resistor, you will see that the motor's current is phase-shifted nearly -90° which is why it is said to "look inductive". The way you can make the current flowing into an AC motor look more like current flowing into a resistor is to add some run capacitors across the motor leads. If you add about the right amount of capacitors, then the power flowing will look like "real power".

The power company cares about this passionately. Why should you? Here's the deal - if you are consuming 1 hp of power for your bench grinder, for example, then current is flowing sufficient to provide 1 hp of real power. But current is also flowing because the grinder motor looks inductive. This current doesn't show up on your power bill but it surely will burn up a wire or melt heaters in your motor control circuit. The way to minimize the total (real + imaginary) current in your motor wiring is to add the right amount of capacitance. This is called "power factor correction."

OK, I have never been a practicing electrical engineer -- my thing was semiconductors and software. There are real EEs on this NG who will almost certainly take exception to some of my wording, especially because it's all off the top of my head, from memory, which memory all comes from college which was more than 2 decades ago. However, I believe it's sufficient to impart the essentials.

GWE

Nope, I'm *not* sure. But I think so. I also live in NW Washington, right up next to BC, and I see a lot of 50Hz salvage motors that the salvage guys describe as Canadian. That works to my advantage if they don't know the motors work on US power.

Anyway, I'm really often wrong. But 50Hz motors certainly exist, and is certainly the standard in England.

Grant

Electric companies do indeed care passionately about the power factor. Those big grey cylinders you see up on power poles are capacitors used to correct the power factor in household electricity. (Most of a household's load is electric motors, hence inductive.)

--RC

"Sometimes history doesn't repeat itself. It just yells 'can't you remember anything I've told you?' and lets fly with a club. -- John W. Cambell Jr.

If anyone is interested in basic DC motor theory and transfer functions, I recently posted some information in response to a question in the LabView newsgroup. I'll leave it up for a few more days-it's in JPG format...

What part of Canada did you get this from? Canada has been running 60Hz for, well, about forever.

European stuff is more likely to be 50Hz (and 220v, vice 110).

Cheers Trevor Jones

I believe that a two pole AC motor typically runs around 3450 RPM at 60 HZ, and a four pole AC motor runs around 1750 RPM at 60 HZ.

So I'd get what looks like a plot of sin(x) and sin(x+pi/2) {radians} if I compared the power supply with the power through a capacitor. Likewise I'd get what looks like a plot of sin(x) and sin(x-pi/2) if I compared the power supply with the power through an inductor. Correct?

I believe so.

I think I get it. If you don't have a power factor correction the imaginary part of the current gets out of phase with the real current. I take it total current(T)^2 = real current(R)^2 + imaginary current(I)^2. Or T=sqrt(R^2+I^2). As long as R = I, T is equal to R, but if you get 90 degrees out of phase T is always 1. Or whatever your current is limited to by the wire resistance and Ohm's law. I think. So, by adding the right capacitance you wind up with a wave of R=sin(x) and I=sin(x+pi/2-pi/2). T stays little, and nothing fries unexpectedly. Am I close enough for horseshoes?

Many years ago (1950 ?) the last ares of 25Hz power were switched over to 60Hz. I have never seen 50Hz although the odd 50 Hz motor does show up at yard sales. Gerry :-)} London, Canada

Europe and Japan. - Maybe elsewhere.

Older English areas - South East Asia places...

When I was with an ATE company - automated Test Equipment - we had to have

50 and 60 hz and many voltage types of inputs.

Tape drives had to have new wheels, pulleys, ..... Fans had to be different.

Motors are maybe 70% strength if 50hz on a 60 hz line. They also tend to heat more.

I know a large German handler - about the size of a good living room - maybe larger - many parts in parallel - the machine was something else, but didn't function just like they liked and they decided to switch out the 50 hz motors to 60 and the machine came up to speed and quality.

I bet they came off machinery (asia based or eu based) and were swapped out to work. Then dump the motors on the local market.

Martin