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Electricity for Permies: Checking out AC Motors

 
pollinator
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I visited the metal scrapyard today. Why? Motors! Specifically AC induction motors, because those are mostly what I spotted out there. I also got two tools that retail for ~$125 CAD for just $6! More on that later, maybe in another post.

Electric motors...they are everywhere. From where I sit I can hear the dryer running. And the bathroom fan. Off the top of my head: in my angle grinders, drills, sawzall, our blender, in our furnace, in the miniature fan I repaired. It’s safe to say that life would not be the same without these gizmos. The thing is, do you know how they work? How DO they convert electricity in motion? What’s the difference between single phase and three phase motors? What about direct current motors? I hope to answer some of the questions in this little series, and we will all end up learning something along the way I think, because I tend to make mistakes when I get overconfident about this stuff. For today, let’s take a gander at some motors in the scrapyard.

Here is my first photo.


This is an old General Electric motor that runs on alternating current. Starting on the second line, you can read that it  is ¼ horsepower, which translates to 186 watts. To convert horsepower to watts you just multiply the number of horsepower by 745.7, or just 746 if you like. The ‘PH 1’ tells you that this motor ran on single phase power - this is a hint that this could have been used in some household item. Not as clearly it says ‘V 110’ which just means it ran on 110V power. It ran at a rate of 1725 revolutions per minute which is pretty darn quick. If I had to venture a guess I’d say this is out of some sort of fan...I’m open to other guesses though! Finally if you look at the very bottom of the plate you’ll see that it says ‘THERMAL ______TION’ which used to say thermal protection. Whenever you see this on a motor plate it means that there is a device inside the motor that will shut things down if it senses things getting too warm. This keeps the motor from burning itself up, which is a great feature.

Onto a bigger motor:


Starting on the third line we read HP 1.25 - 1 ¼ horsepower. Next up is SF 1.00. SF stands for service factor. According to this website it is the percentage of overloading the motor can handle for short periods within the correct voltage tolerances. It says, “I was made to operate at a certain level, but if necessary I can work a little harder for a shorter period of time without burning out”. Next up is PH 3...this means this motor ran on 3 phase power - 3 phase power is most often seen in commercial and industrial settings where a lot of power is needed. 3 phase power is what is generated at most power stations as well. Next line! HZ 60 means this motor ran on a frequency of 60Hz. When discussing AC this means that the waveform completes a full cycle 60 times every second. Or to put it another way, the electricity flow is changing directions 60 times per second. Now how many volts? 575!! Do not lick the terminals. This is another hint to this not being a household electric motor. Or maybe it was…? Just imagine a VERY powerful ceiling fan in your bedroom. Can you imagine how quick you’d cool down? You might die though when it caves your ceiling in. Anyways. This motor pulls two amps of power, which is very low relative to the volts. For perspective, most household breakers max out at 15 amps. This doesn’t mean this motor isn’t powerful though. Want to find out how many watts this motor would use at the rated output? To do this you simply multiply the amps by the voltage. 2 amps x 575 volts = 1150 watts. Now if we convert the 1.25 horsepower into watts we get 1.25 x 745.7 = 932.13 watts. Why the mismatch? Quite honestly I don’t know. Anyone who does know feel free to chip in here. The second last line, consisting of TIME CONT, ENCL and FORM KXM is a complete mystery to me. The last line starts with CODE K. From elongo.com, “A letter code defines the locked rotor kVA on a per-hp basis. Codes are defined in MG 1-10.37.2 by a series of letters from A to V. Generally, the farther the code letter from A, the higher the inrush current per hp.” So K being pretty far from A means this motor would take a large amount of inrush current to get going. Why? Picture yourself on a bike in a high gear at a standstill. Try to pedal...it would be very challenging right? You’d need to push way harder to get the bike rolling than you would to maintain your speed. Motors, especially large ones, are similar to this. INSUL CLASS B. This stands for insulation class, which is, “a letter designation such as "A," "B," or "F," depending on the winding's ability to survive a given operating temperature for a given life.” The further away you get away from A, the better insulated the motor. AMB 60C says that this motor can run at a maximum ambient temperature of 60 degrees celsius before you risk overheating it. If I had to take a guess, I’d say this motor powered some sort of large fan, maybe in a condenser? Lol now if you haven’t already noticed, in the top right corner you can read that this is exactly what this motor did.

Okay now let’s see a real hefty motor.


I didn’t get any info on this one because I couldn’t access the info plate, but just take a look at that thing! Where do you think it was? Maybe some sort of heavy duty ceiling fan? Sorry, that isn’t even a good joke but I do find it funny. Next!



This is not a full motor… in technical terms it is the stator, because it is the part of the motor that remains stationary. The rotor (part that rotates) is missing.

And here are some more bonus photos, with less detailed commentary.



This motor is a little cutie relative to the others. I almost want to snuggle it.



This one someone got kind of frustrated with I guess.

_________

Ever ripped into a microwave? I haven't but when I visited the appliance section of the yard I saw around 12 laying there. Here is a capacitor I found.


0.95 microfarads, but at 2100V. What do we say when we see a high voltage capacitor? Come on, you know it by now. Do not lick!

And another DNL I found in the bottom of a freezer.



And as a teaser for another post: check out my haul in the back of Singer (my car's name that sounds like a sewing machine when running).



More to come.


https://electrical-engineering-portal.com/motor-service-factor-sf-defined-by-nema

https://www.elongo.com/pdfs/MotorNameplate990519.pdf

https://www.priestelectric.com/read-electric-motor-nameplate/
 
gardener & hugelmaster
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Nice write up Cam. Already looking forward to the next installment.

2 amps x 575 volts = 1150 watts. Now if we convert the 1.25 horsepower into watts we get 1.25 x 745.7 = 932.13 watts. Why the mismatch?



A. The true measured current is probably slightly less than the label indicates. To give the person building something with the motor a safety fudge factor in determining an appropriate circuit breaker to use.

B. The horsepower to wattage conversion calculation doesn't take into account losses like friction, eddy currents, skin effect, hysterisis, etc. That conversion is accurate for purely resistive circuits.

If memory serves TIME CONT indicates it can be run continuously.

I think FORM KXM refers to the form factor of the stator & possibly some other parts.

Wondering if that hefty motor is actually a generator? Could be, the 2 things are basically the same. Anytime there is current through a conductor an electromagnetic field is produced & vice versa. In one an electromagnetic field moves magnets to cause rotation. The other uses rotating magnets whose magnetic fields then induce current into conductive wires. The devil is in the details.

Don't lick the terminals. Check.
 
Cam Haslehurst
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Mike Barkley wrote:Nice write up Cam. Already looking forward to the next installment.

2 amps x 575 volts = 1150 watts. Now if we convert the 1.25 horsepower into watts we get 1.25 x 745.7 = 932.13 watts. Why the mismatch?



A. The true measured current is probably slightly less than the label indicates. To give the person building something with the motor a safety fudge factor in determining an appropriate circuit breaker to use.

B. The horsepower to wattage conversion calculation doesn't take into account losses like friction, eddy currents, skin effect, hysterisis, etc. That conversion is accurate for purely resistive circuits.

If memory serves TIME CONT indicates it can be run continuously.

I think FORM KXM refers to the form factor of the stator & possibly some other parts.

Wondering if that hefty motor is actually a generator? Could be, the 2 things are basically the same. Anytime there is current through a conductor an electromagnetic field is produced & vice versa. In one an electromagnetic field moves magnets to cause rotation. The other uses rotating magnets to generate electric fields which then induce current into conductive wires. The devil is in the details.

Don't lick the terminals. Check.



Glad you like it Mike. Thanks for your additional input! Especially about the difference between the HP to watts conversion and the volts x amps calculation. I just learned something new.

And yes it is pretty neat how close motors and generators really are. I took a video recently of me spinning a DC motor from an old drill with my multimeter leads connected, and it was very neat to see the rise in voltage (however small) when I spun the rotor. If I remember that I can include that video in another post.
 
Mike Barkley
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Unless you're designing motors the conversion differences pointed out are insignificant. Was just trying to answer your question. The horsepower ratings are an easy way to compare motors. I don't think it has much real value beyond that.
 
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