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Tool Talk Discussion Forum

Electric motor: power vs. rpm

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Brad Buchanan

12-24-2013 09:33:58
74.79.218.23



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Hey folks.

Still working on my bandsaw issues and had a question about 3 phase motors.

I need a motor that produces 3 hp at 600 rpm (direct drive).

If I use a variable frequency drive to drive a 3 phase motor at a slower speed than its rating will I get as much power from it as its rating?

Will the motor draw as many watts at the slower speed? Produce as many newton meters?

Would it be better to start with a 3450 rpm motor or a 1770 rpm motor?

Should I start with a higher hp motor and then slow it down?

Thanks in advance,

Brad

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buickanddeere

12-25-2013 08:18:15
209.240.119.130



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
The solution remains simple. A 5 to 10 KW single phase 240/600 transformer which are cheap and plentiful on kijiji. A 600V 5HP VFD. Done.



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Brad Buchanan

12-25-2013 11:34:44
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to buickanddeere, 12-25-2013 08:18:15  
Hey there.

I tried kijiji and could not find the transformers.

It seems that that site is tied to geographical location.

If you could send me a link to one of those transformers offered I would be obliged.

Brad



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buickanddeere

12-25-2013 20:46:19
209.240.119.130



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-25-2013 11:34:44  
More

http://ontario.kijiji.ca/f-transformer-600-three-3-Classifieds-W0QQKeywordZtransformerQ20600Q20Q2dthreeQ20Q2d3QQSortZ3QQisProvinceSearchZtrueQQmaxPriceZ500QQmaxPriceBackendZ100000QQminPriceZ275QQminPriceBackendZ27500
This post was edited by buickanddeere at 20:51:09 12/25/13.



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Brad Buchanan

12-26-2013 07:58:10
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to buickanddeere, 12-25-2013 20:46:19  
Thanks for the link.

Brad



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buickanddeere

12-25-2013 16:24:07
184.151.63.190



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-25-2013 11:34:44  
Quoting Removed, click Modern View to see

http://toronto.kijiji.ca/f-Transformer-single-600-Classifieds-W0QQKeywordZTransformerQ20singleQ20600QQSortZ3



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missouri massey man

12-25-2013 08:01:29
108.90.234.133



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
[URL=http://s741.photobucket.com/user/whpete44/media/Moving%20Dutch%20Oven%20and%20Ship%20Saw/MovingDutchOvenandShipSaw036_zpsb8291cf1.jpg.html][/URL]

Thats a mean looking saw you are powering up...This one was at a farm auction a month or so ago. Laying right where they dropped it unceremoniously off the forks of a skidloader setting up for the sale. It is a "shipsaw" and was bought for junk by a scrapper. Casting was cracked in the fall or the SIL would own it now. We wanted it for resawing 12x12 squares off the circular sawmill.....This is (was) a 36" and wood only.

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Leroy

12-25-2013 05:56:17
69.88.207.185



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
No electricitain but being direct drive from factory does not mean it has to stay that way. Get a shaft machined that would be equivilant to the motor shaft and mount on pillow block bearings and use belts from the motor to that shaft. My friends would do that and instead of a electric motor would drive it either with a diesel engine or a horsepower through a line shaft. That size saw they might put a second Belgin on.

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buickanddeere

12-24-2013 18:32:47
209.240.119.130



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Is there a reason for reducing the rpm? Will finer teeth and slower material feed be sufficient? Some drives will allow "adjustment" of the torque to be either constant or variable with the rpm. Variable will run cooler.



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Brad Buchanan

12-24-2013 19:28:57
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to buickanddeere, 12-24-2013 18:32:47  
This machine is a direct drive. The motor directly drives the lower wheel which is 30"+ in diameter.

I think if the speed was increased much it would have problems with roller guide overheating.



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Indiana Ken

12-24-2013 17:00:27
66.249.234.135



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Quoting Removed, click Modern View to see

I am not a electrical expert so others will need to check my assumptions concerning motor operation with a VFD. However, the relationship between HP and Torque is as follows:

1) HP = (T x RPM) / 5252
And
2) T = (HP x 5252) / RPM

From 2) we see that 3 HP at 600 RPM requires 26.26 ft-lbs of torque ((3 x 5252) / 600 = 26.26).

From 1) we can can plug in the 26.26 ft-lbs of torque to determine what the motor HP would need to be at rated RPM. We can also see it would be better to start with a 1750 RPM motor rather than a 3600 RPM motor. We see that a 8.75 HP, 1750 RPM motor would be required to produce 26.26 ft-lbs of torque at 600 RPM ((26.26 x 1750) / 5252) = 8.75 HP).

Note the above assumes that a 3 phase motor will make full rated torque a 600 RPM when operated with a VFD. Since the motor is producing 3 HP it should draw similar wattage to that of a 3 HP motor running at rated speed. I say similar since there will be some loss in the VFD etc. The motor cannot produce its rated power (HP) at 600 RPM since it is not running at rated RPM, see equation 1).

Those who are more knowledgeable in electric motor operation please add any corrections as required.

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Brad Buchanan

12-24-2013 19:32:48
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to Indiana Ken, 12-24-2013 17:00:27  
I was thinking that I might have to double the HP but you figures may cause me to update that.

Brad



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David G

12-24-2013 16:06:28
205.215.206.18



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
I agree with John T, HP is at rated RPM.

A VFD will increase current to maintain torque within reason, but will not over current the motor.

I would get an 1800 RPM motor and build a mechanical reduction. That configuration will amplify the torque as it reduces the RPM.

I would expect if the unit had 3HP motor at 600 RPM, you would need at least a 7.5HP motor to accomplish the same torque at reduced speed. You could probably get by with a 1HP motor if using mechanical reduction.

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Bob

12-24-2013 15:53:43
64.255.159.192



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Gearmotor. Or motor followed by gear reducer.

Must be a HECK of a bandsaw to need 3 HP!
This post was edited by Bob at 15:57:52 12/24/13.



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JMOR

12-24-2013 16:11:02
72.181.173.171



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 Re: Electric motor: power vs. rpm in reply to Bob, 12-24-2013 15:53:43  
Quoting Removed, click Modern View to seeMore than big enough to cut a finger off.



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Brad Buchanan

12-24-2013 19:30:26
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to JMOR, 12-24-2013 16:11:02  
You are a wascally wabbit for finding and grabbing that jpg!

Brad



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Tramway guy

12-24-2013 14:49:38
198.228.200.22



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Suggest you start with a 4 HP 900 RPM or 6 HP 1200 RPM motor rather than a 1800 one. With a VFD you can vary the frequency down.



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missouri massey man

12-24-2013 14:35:05
108.90.234.133



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
In the rambling of my previous post I failed to address your main concern. Doesn't matter because I don't know the answer to the question you asked anyway, but my two cents is to start with a 1725 rpm motor and either sheave it to achieve your requirements or like others have said, install a set of step pullys. With the steps, when all those horses are not required but a little more blade speed is, you could increase the rpm accordingly.
Contrary to some of the other suggestions, being as how a VFD is a "linear" device means the lower the rpm request, the lower the amp draw which equates to less heat. Of course lower amperage means lower horsepower, so it would be hard to maintain if not impossible to maintain a 3 hp load at 600 rpm on a 1725 rpm 3hp motor with a vfd. Back to the step pullys.......

By the way, what size and brand of bandsaw do you have and what are you cutting to need a 3 hp motor ?? ...just wondering out loud...must be a biggun.

Again, not complete information...as volumns could be written and still not get it all covered.

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Brad Buchanan

12-24-2013 19:40:48
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to missouri massey man, 12-24-2013 14:35:05  
The belt pully reduction is somthing that I considered however it would involve mounting a rather precisely positioned jackshaft and fabricating another motormount which would jut from this side of the machine.

Doable, but not a very elegant solution for the problem.

Thanks for your posts and interest.

Brad



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missouri massey man

12-24-2013 13:52:59
108.90.234.133



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Unless you are rasonably well versed with electricity, I wouldn't recommend you or anyone else to try to build you own rotary phase converter....that being said,

The reason many home made converters seem to have less than a stellar performance is that they usually apply incoming power to the rotary converter with the third leg through a run capacitor (or capacitors in parallel) without first using a "pony" motor and a time delay relay to get the 3 phase motor "up to speed" first.
Sure, it will start on single phase by paralleling one of the legs through the capacitor to the third winding, but it will never attain full speed, consequently not optimum output either. However.... using a pony motor with the same rpm rating of the 3 phase motor you are using and connect them together with a lovejoy coupling or even same size sheaves and a belt you will first get the driven motor to full speed then the time delay relay will drop power off of the pony and simultaneously apply 220v single phase with the third leg going through the capacitor AFTER full rpm is achieved. It still will not supply the rated power, but it will perform remarkably better.
Typically one 20 mfd oil filled run capacitor will suffice on most small 1, 2, 3 and even a 5 hp three phase motor. It's not going to be perfect, and it is considered a pretty dirty three phase power, it will work for a home work shop. With an oscilloscope and a pile of additional capacitors, in theory, you should be able to keep adding capacitance in parallel until you get very near to a 120 degree phase shift. A three phase motor has three identical windings and with correct three phase power applied, has 120 degrees phase shift between the three windings. That is the reason they are referred to as a motor with "high starting torque".

Without the capacitor, you will "single phase" the motor and it will literally smoke in minutes....all the capacitor is doing is creating a little offset in the third leg (ergo: phase shift) to allow rotation to initiate. Not being a perfect 120 degree phase shift is what causes it to be underpowered. Nature of the beast.

That all being said, this description is totally incomplete and not intended to suggest that you should try it at all !!! But it DOES work for me in the way it is described. As a matter of fact you should be able to easily run a total horsepower higher than that of the motor you are using for the rotary converter.
Not for the faint of heart, and NOT RECOMMENDED for a rookie with little or no electrical background.....BE SAFE !!!!! ...and enter John T's normal disclaimer here: "____________________"

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George Marsh

12-24-2013 19:47:05
50.121.6.110



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 Re: Electric motor: power vs. rpm in reply to missouri massey man, 12-24-2013 13:52:59  
From a previous post, Brad said the motor is 560vac 3 phase, 600 rpm. All 3 phase converters my dad made were 220v. I don't know where he is going to get 560 vac 3 phase in this country.



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T in NE

12-25-2013 15:13:33
75.238.142.109



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 Re: Electric motor: power vs. rpm in reply to George Marsh, 12-24-2013 19:47:05  
The volt meters in all the pivot control boxes I have looked in showed around 550 volts.



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John T

12-24-2013 12:06:44
216.249.76.176



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Sorry, its been too long since I was with Century Electric Motor Company (very first EE job out of school) and I done forgot about motors grrrrrrrrrr. However, I can tell you HP is a function of Torque X RPM so if RPM drops that much, and I doubt torque will rise the same degree, you cant achieve the same HP (unless Torque went up as much as RPM went down) and its my best guess the efficieny will be reduced and more heat losses.

Id say there would be less losses and it would be more efficient to use pulley reduction versus that much frequency variation, but Im NOT a mechanical engineer.........

John T Merry Christmas

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Brad Buchanan

12-24-2013 19:46:09
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to John T, 12-24-2013 12:06:44  

thanks, John.

I am considering all options including having the existing motor rewound for 220v.

A bit pricy but I got a rather amazing deal at auction so I can afford a few more dollars.

Brad

P.S. Merry Christmas to you and yours but I imagine that you are somewhere warm with yer RV!



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bill in in

12-24-2013 11:30:22
50.121.113.226



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
You can drive a motor with a variable freq drive to very low RPM's, the problem is cooling and if you put a "stinger" (small dedicated fan motor) on it, they survive well. I had such an application on a conveyor and after the second motor Baldor sent me one rigged as above. It may still be working.

Horsepower would be tricky. There are horsepower and torque curves available for most modern and you could probably interpolate. But is it that critical? Does the saw have a power feed and does it under a load?

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Brad Buchanan

12-24-2013 19:49:21
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to bill in in, 12-24-2013 11:30:22  
Hey Bill.

No power feed but I am planning to do some quartersawing in hardwood so I am going to need the hp to make that happen.

Brad



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George Marsh

12-24-2013 10:42:38
50.121.6.110



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Brad, I live in Terre Haute. 45 years ago my dad built phase converters for farmers using capacitors and another 3 phase motor. It was called a dynamic converter. I have a 3 phase motor, 3 or 5 hp? It was used on a air handler, so it may be 3 hp and they needed 5 hp. It was like new. It's been sitting on my garage floor for about 35 years. I also have buckets full of old run capacitors from air conditioners, caps have PBCs in them. I don't rememeber the rule of thumb, so many MFD/hp for start and some many MFD/hp for run. That said I think I have enough to make a 220v 3 phase converter. However with converters you are lucky to get 70 % efficiency.

Then you will need a 220v 3 phase motor and an electronic device to control it's speed. By the time you get done, you will have a pant load invested.

Where I used to work, they had a 100 year old off set printing press. It was powered by an AC universal motor that rubbed on the flywheel. The foot control moved the burshes which controlled the speed of the motor. I've only seen one motor like it. The motor was so old, it had to be rebuilt, insulation was falling off wires.

It would be cool to find a motor like that. Universal motors produce max torque at any RPM.

Good luck
George

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Brad Buchanan

12-24-2013 19:54:02
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to George Marsh, 12-24-2013 10:42:38  
Hey George.

I considered a rotary phase converter however I think I can accomplish pretty much the same with a vfd.

The attraction of the rotary is that it can handle more that one load up to its rating, somthing the vfd cannot, at least to my knowledge.

Brad



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bill in in

12-24-2013 11:31:55
50.121.113.226



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 Re: Electric motor: power vs. rpm in reply to George Marsh, 12-24-2013 10:42:38  
Speaking of motors in Terre Haute, do you know a guy name of Lankford?



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George Marsh

12-24-2013 13:15:52
50.121.6.110



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 Re: Electric motor: power vs. rpm in reply to bill in in, 12-24-2013 11:31:55  
Bill,
Jerry Lankford is my neighbor for the past 36 years. I think his dad had a motor shop and he played in motor repair for a while. He recently retired.
George



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wisbaker

12-24-2013 10:29:02
173.30.33.15



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
You can turn down a motor with a AC freq drive if you dial down an 1800 rpm motor to 600 rpm it won't last long. They say don't go below 30% (540 rpms) but I haven't had much luck running them less than 1/2. You may do better if you get an inverter duty motor, some even have a little electric fun that runs independent. ALternative would be to freq drive a 1200 RPM motor 600 would be about 1/2. Not sure if the torque is higher at the lower speeds, if it is that may explain the short motor life at lower speeds. Other alternatives use a 1200 TENC (totally enclosed not cooled)motor, a gearmotor or set it up with a DC motor & drive. I've run them off a little drive with AC in/DC out, the few I've dealt with seemed okay running slow.

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Brad Buchanan

12-24-2013 20:04:39
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to wisbaker, 12-24-2013 10:29:02  
A dc motor sounds interesting and I will have to check into that.

Just studying and getting a glimmer on how 3 phase power works and find the whole subject fascinating.



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George Marsh

12-24-2013 10:02:51
50.121.6.110



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Where are you going to get 3 phase from. If you make it with a converter you need to go much bigger. Not sure what a variable speed box for 3 phase costs and not sure if you try to make your own 3 phase if it will even work.

Still think your best bet is belts, pulleys, and making a shaft to use the bearings of your old motor. George



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Zachary Hoyt

12-24-2013 09:59:32
184.8.251.166



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Can you set it up with a multiple V belt drive arrangement? Seems like the bandsaws I have both have a much larger pulley on the saw than on the motor. I have an 8" jointer that was given to me that is all cast iron and pretty old, it has a pulley on the head that had 3 or 4 sheaves on it, can't recall which. I don't have a motor for it yet so I don't know how well that setup works. Surplus center has those kind of multipe pulleys, I think.
Zach

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Brad Buchanan

12-24-2013 20:00:55
74.79.218.23



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 Re: Electric motor: power vs. rpm in reply to Zachary Hoyt, 12-24-2013 09:59:32  
Hey Zachary.

I have considered the pully reduction and may still possibly do that.

I recently bought an 8" sears craftsman jointer at auction and set it up on a bench with a 2hp induction motor.

Howls like a banshee and works great. Still have to fabricate a stand for it.

Brad



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greg k

12-24-2013 09:50:09
70.198.23.251



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 Re: Electric motor: power vs. rpm in reply to Brad Buchanan, 12-24-2013 09:33:58  
Well I can't give you any answers but I can raise some more questions lol. IMHO you should keep the RPM's as close to factory as possible for the simple reason that most motors are cooled by the fan on the back of the motor and if it is slowed down and maybe making more heat that way, AND the fan turns slower it can overheat the motor. I am awaiting the answers so that I know this stuff for future reference.

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Steve@Advance

12-24-2013 19:29:43
107.203.134.67



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 Re: Electric motor: power vs. rpm in reply to greg k, 12-24-2013 09:50:09  
I work in a production machine shop, we have a Cosen automatic saw, came from the factory with an inverter drive. It's about 8 years old, gets run 8 to 10 hours a day, never a problem.

I think it's a 5 HP 1725RPM running through a reduction gear, don't know the ratio off hand.

I would probably go with the 1725 RPM motor, just because of the cost and popularity.



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