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Electric motor: power vs. rpm

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Brad Buchanan
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PostPosted: Tue Dec 24, 2013 9:33 am    Post subject: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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Greg K
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PostPosted: Tue Dec 24, 2013 9:50 am    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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Zachary Hoyt
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PostPosted: Tue Dec 24, 2013 9:59 am    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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Geo-TH,In
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PostPosted: Tue Dec 24, 2013 10:02 am    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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wisbaker
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PostPosted: Tue Dec 24, 2013 10:29 am    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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Geo-TH,In
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PostPosted: Tue Dec 24, 2013 10:42 am    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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Bill in IN
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PostPosted: Tue Dec 24, 2013 11:30 am    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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Bill in IN
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PostPosted: Tue Dec 24, 2013 11:31 am    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

Speaking of motors in Terre Haute, do you know a guy name of Lankford?
 
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John T
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PostPosted: Tue Dec 24, 2013 12:06 pm    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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Geo-TH,In
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PostPosted: Tue Dec 24, 2013 1:15 pm    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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missouri massey man
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PostPosted: Tue Dec 24, 2013 1:52 pm    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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missouri massey man
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PostPosted: Tue Dec 24, 2013 2:35 pm    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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Tramway Guy
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PostPosted: Tue Dec 24, 2013 2:49 pm    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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Bob
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PostPosted: Tue Dec 24, 2013 3:53 pm    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

Gearmotor. Or motor followed by gear reducer.

Must be a HECK of a bandsaw to need 3 HP!
 


Last edited by Bob on Tue Dec 24, 2013 3:57 pm; edited 1 time in total
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David G
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PostPosted: Tue Dec 24, 2013 4:06 pm    Post subject: Re: Electric motor: power vs. rpm Reply to specific post Reply with quote

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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