I have an electric motor that originally drove a good-sized squirrel cage blower in a home AC unit. I plan to use it to drive the mandrel on the new sharpening station I am building. I know, of course, that I can lower the speed proportionately by using larger pulleys on the mandrel shaft and smaller on the motor shaft — but does anyone have an idea of the likely speed of the motor? There is no nameplate with specifications.
Joe
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Most standard motors that you would buy are either 1725 or 3450 rpm. But motors used for blowers are not necessarily standard. It could easily be 1140 rpm or some other designated slower speed. You are probably either stuck with trial and error or getting a tachometer to measure the speed.
That's what I was afraid of. I poking around, though, it looks like blower motors tend to be 1725 or lower (although they can be anything). Is this true?Thanks for the post.J
Edited 1/27/2009 9:36 am ET by Joe Sullivan
How fast do you want your mandrel to run. As you said, you can change the speed by the sizes of the pulleys you use. The ratios are linear. If you have pulleys on them now, why don't you try it out. Grinding wheels have a limit to how fast they can be turned. Often grinders turn 3450 rpm and I wouldn't turn one much faster for safety's sake. The maximum speed should be listed on the wheel. My grinder runs in water and turns about 80 rpm so there is quite a variation of speeds they run. Much faster than 80 rpm in water would sling the water all over the place. I fear that you know most of this all ready so please excuse the elemental information.
I don't yet know just how fast I'd like it to go, but certainly not at high speed. I will use it for chisels and planes and such.J
Unless there is a variable frequency control, synchronous ac motors run at predictable speeds based on the number of poles:
Calculating Motor Speed:
A squirrel cage induction motor is a constant speed device. It cannot operate for any length of time at speeds below those shown on the nameplate without danger of burning out.
To Calculate the speed of a induction motor, apply this formula:
Srpm = 120 x F
P
Srpm = synchronous revolutions per minute.
120 = constant [supply voltage]
F = supply frequency (in cycles/sec)
P = number of motor winding poles
Example: What is the synchronous of a motor having 4 poles connected to a 60 hz power supply?
Srpm = 120 x F
P
Srpm = 120 x 60
4
Srpm = 7200
4
Srpm = 1800 rpm
http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm
Edited 1/28/2009 10:55 am by observer
Very useful, thanks. I'll study this.Joe
This may seem simple minded, but how do you tell how many winding poles are in the motor?Joe
Tough to do without disassembly and counting the bundles of wire in the field winding.Coming from a squirrel cage blower it is a pretty safe bet that it is a 1/4 hp 1725 rpm 4 pole motor.
That's what I suspected. Thanks for confirming.J
If you have a truck repair shop that does PTO work or a trucking company that runs a lot of PTOs near you they probably have a tachometer that you push on to what ever you want to measure, really just a calibrated version of the old style bike speedometer but they work well.
It seems you are proficient in the different types of motors. I have a squirrel cage fan in my air cleaner that has four speeds. You mentioned synchronous. What type of motor is it likely to be.
I wouldn't suggest that I am well versed in motors, just enough knowledge to be dangerous.I would guess that your aircleaner uses a universal motor. Stepped speed control can be obtained from AC versions of them by using taps on the field coil. As I understand it, the taps allow changes in the effective length of the coil winding which changes the magnetic field strength of the field coil and results in different speeds and torque levels.
Thanks for your thoughts. A lot better versed than most. I have wondered what it was. Was pretty sure it wasn't an induction motor.
Hey Joe,
I have one of these blower motors in my shop... When i go out there tomorrow I will look into it and get back to you. Not sure if it has a id plate on it or not or if it will help ...
John
Thanks.J
Hi Joe,
Might want to have a look at http://www.pennstateind.com/store/TCLVSKIT.html
With a couple of step pulleys you could vary the speed all you want to. I bought one of these for an old lathe and as soon as Mother Nature gets out of her winter curse I hope to try it out.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Good stuff, Bob. Thanks.J
Hey Joe,
Sorry the motor on my squirrle cage has no tag either (or I cant see it) Guess the don't put tags on them.
Good luck, John
Oh well. Thanks for looking. In researching this, I find that most of them are indeed 1700, although some are slower and a very few are faster.
Induction motors usually are 1725 or 3450 - theoretically 1800 or 3600 rpm. Like you said they would generally use the smaller speed as squirrel cage fans just don't do well at higher speeds. I understand piston airplanes generally use the radial engines because they can run slower than the inline type. Ever watch the props on a lightly loaded airplane? You can almost follow the prop as it turns.
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