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Thread: Motor speed: # of poles = rpm?
11-19-2009, 07:37 PM #1
Motor speed: # of poles = rpm?
My Whitcomb Blaisdell planer came with a wonderful old 1919 time frame *"Kelley Electric Machine Company" "Powerfloater" motor. The front end of the motor incorporates a planetary gear (reduction) drive on which is hung a huge triple cone for 4" wide flat belts.
The planer was run on the highest belt speed, and apparently that might still have been a little slow according to the guy who last ran it.
I have the (non drive) end bell off to bring out new leads due to the insulation crumbling off the old ones where they came through the motor side. The motor did run fine, until I moved the mass of friction taped wire to re-locate the greensleeve it was taped and routed into. Once the ball of tape was moved over, it crumbled all the insulation at the motor entry point.
*Kelley Electric Machine Company out of Buffalo mostly made woodworking machines, polishers and routers. It is not likely the motor was provided by the planer manufacturer, and was probably added on somewhere early in the machine's history by a previous owner. Possibly even by the US Govt since it has war tags & the work is neatly done.
My plan is to bring out new leads and get the planer operational for the time being. Then I want to plan to add a larger motor if necessary, and am trying to figure out what speed to work with. I can probaby machine a plate & the motor shaft, and make a new 5HP motor fit the planetary drive. But I am not yet clear how fast the old motor is turning.
So, if it has 36 poles, and was rated 50Hz but is running on 60Hz, (wired 220 volts, 8 amps) How fast should it be spinning?
Yeah, I know I can get it fired back up and count, but it would be easier to know if poles x Hz = predictible rpm?
11-19-2009, 07:43 PM #2
If my numbers are right, 200 rpm - any slip you might have so approx 190 rpm
Revised my post because I calculated for 50hz, not 60hz. My bad
11-19-2009, 07:52 PM #3
Wow! It obviously turns slow, you could probably visually count the rpm of the cone on the planetary drive with a dab of white paint. But ~160 rpm?
I'm calling each obvious laminated metallic segment wrapped with a winding, a pole. Is that correct?
11-19-2009, 07:58 PM #4
I agree with your understanding that the number of poles an electric motor has has a direct relationship with its speed of rotation.
In Australia where we operate on 50HZ a 4 pole electric motor rotates at about 1415 rpm while not under load and a 2 pole rotates at 2800 rpm. In your neck of the woods the values are more like 1750 and 3000 or more.
In the earlier days when electric motors must have scarce and expensive. My father's workshop was powered by overhead shafting driving leather flat belts to power individual machines. His sole power source was a large framed three phase motor maybe no more than 5hp and if memory is accurate the motor ran at 815rpm and this related to maybe 16 poles.
In spite of the horsepower rating which also has a direct relationship with maximum amperage draw I believe that the number of poles equates to the torque capacity, meaning the greater number of poles the greater the torque. I'm happy to be corrected by those with more than my scant understanding on this topic. But that is my input so far.
11-19-2009, 08:02 PM #5
Steven, The last guy though the planer table cut speed might be a ittle slow. I suggest you work backwards from the table speed through the various ratios until you get to the motor, work the gear reduction by chalk and counting turns, do some calculation to work out the speeds starting from min to max via the steps in the cone pulley. This mass of figures will confirm the needed motor RPM.
As for your motor, it's a bit hazardous if the lead insulation is crumbling the leads should be re-laced. This means digging back through the lacing to find the connection of the leads to the windings. This is motor surgery I won't di and I'm a reckless fellow. If the motor is otherwise OK I suggest you take the stator to a good motor shop and have them re-lead it.
That way the motor and the gearbox stays on the old timey planer to lend antiquarian authenticity to the image. If the ratios aren't quite right maybe you can rustle up a suitable pulley.
Good luck with it Steven. Holler if I can help.
11-19-2009, 08:04 PM #6
Just remember that 2 poles 60 Hz is 3600 nominal, and @50 Hz is 3000 nominal. 4 poles @ 60 is 1800 nominal, 6 poles 1200 nominal, etc.
Figure about 4% slip, so most 2 pole 60Hz are 3450 rpm...... 50hz maybe 2880.
You apparently have 18X more poles, so divide by 18. 2880 / 18 = 160
but you use 60, so multiply by 6/5, which gives about 192 rpm.
I don't know that I believe that.
Are those poles really poles? Just because a motor has a coil of wire doesn't mean that's a "pole".
If you take it apart, pulling the rotor, then if you run a small DC current through the windings, you can count poles with a small compass. Each "pole" will reverse the compass.
3 phase motors have 1/3 the number of actual "poles" as it looks, since there are 3 windings. Might be a 6 pole motor, which would run faster...... 1150 rpm @ 60 or 960 rpm @ 50. Then the gears make more sense also.
11-20-2009, 04:34 AM #7
My 10 HP Westinghouse 865 RPM motor from 1917 has 72 coils, or slots for windings. I.E., nine coils for each of the eight poles. Its three phase.
11-20-2009, 05:23 AM #8
I have read all of the previous posts and haven't seen the pole-rpm formula, so I thought I'd add it. It goes like this: P=120f/S or, S-120f/P, where S=RPM, f=frequency, P=number of poles. Most induction motors have 36 coil slots (not necessarily poles) in the stator. They are made that way because it provides the greatest advantage in designing the number of poles to adapt a specific motor frame to different speed requirements. In the case of motors, the faster it goes, the more power you get out of a specific amount of copper and iron. The most you can get at 60 hz is 3600 rpm. That's a two pole machine. Power drops off as you increase the number of poles. That's why slower speed motors are bigger. You should be able to determine the number of poles through the formula, but if you are accessing the windings to add new leads, look for the largest coil. A three phase motor is usually wound with one of two different winding configurations, either lap or basket. With Lap windings, all the coils are the same size and span a number of coil slots. The coil slots spanned by the lap coil is the size of the pole. Basket windings are a little different, however, the largest coil determines the size of the pole. Maybe this will help. WWQ
11-20-2009, 05:57 AM #9
Well you guys have helped my understanding quite a bit. There were a number of "AHA!" moments, as well as a few "Duuhs"....Yeah, I just realized from reading above that coils lap more than one metallic segment. I'm a little more awake this morning than last night!
Rotor was pulled last night, so I can probably count the coils and see if any look bigger.
Stopped off at my motor rewind guy this morning to get some wire to bring out from the internal connections. I've done this part before. Can't say I "know" what I'm doing, but replacing old stuff (carefully) is more mechanical work than technical brain work. I've even done it in the past to reverse, non-reversable single phase motors. So 3ph is not difficult.
Most likely I'll have the motor running again later today and can count the revs at that point.
But I very much appreciate the addition to my education.