Motor Question 1725 versus 3450 rpm?

[Mark R Webster]

I am replacing the motor on my Parks 12 inch planer. I can go with either a 3450 rpm or a 1725 rpm. I just have to use the right pulley. Is there any reason one should use one motor over the other? Or are they equivalent? I was going to go with a 2 hp and wire for 220. Thoughts?

[Charles Lent]

You need to determine the correct speed of the cutters, and then adjust the pulley sizes to get it right with whichever motor you choose. but be careful. There may not be enough room for the pulley size change needed to use a motor with double the speed of the original motor. I might use a motor with a little more horsepower than the original motor, but I WOULD NOT try to come up with a way to use a motor that runs twice as fast as the original motor. The motor itself isn't the problem. It's what it would take to reduce the speed to the correct speed for the tool that I would try to avoid, because of the need for a very small pulley on the motor and a very large pulley on the tool, which likely won't fit the guards, etc. Change the horsepower a little, but don't try to go with a motor that turns twice as fast as the original motor. It just gets too messy.

Charley

[Mark R Webster]

Here is what the manual says
344486
So I am guessing it doesn't matter between the two RPMs?

[Mark R Webster]

So I suppose my question is.... is there something inherently better about the 1725 over the 3450 (or the 3450 over the 1725) in certain applications or are they equivalent, other than the rpm.

[Mike Henderson]

HP is RPM times torque times a constant. So a 1725 RPM motor has to generate twice the torque to provide the same HP as a 3450 RPM motor, which means that a 1725 motor will be physically larger than a 3450 motor for the same HP.

[A 3450 RPM motor is a 2 pole motor. A 1725 motor is a 4 pole motor. If you had an 8 pole motor, it would run at 862.5 RPM and would be even larger for the same HP.]

Note that all these RPM speeds are under full load. At light load, the 3450 will run close to (less than) 3600 RPM, and the 1725 RPM will run close to 1800 RPM. As you load the motor, the slip increases until you reach full load which is where the specification is given. A 3450 RPM motor can never run 3600 RPM because there must always be some slip for the motor to operate. They make synchronous motors that can run 3600 RPM (for example) but those are special purpose.

Mike

[David L Morse]

... is there something inherently better about the 1725 over the 3450 (or the 3450 over the 1725)...

No. With the obvious exception of direct drive applications the two are equivalent. With the right pulley your planer will never know the difference.

[Bill Dufour]

In general a low speed motor costs more to purchase and is bigger/heaver. But a low speed motor may be a little quieter, not an issue on a planner. The low speed motor will use a smaller pulley which may be easier to fit into the machine. The high speed motor may be smaller and easier to fit into the machine even with it's bigger pulley.
Bill D.

[Mark R Webster]

Thanks for all the info guys!!

[Doug Hepler]

Guys,

It seems to me that a 2 HP 1725 rpm motor operating through the 7" pulley will deliver half the torque of a 2 HP 3450 rpm motor operating through a 3.5" pulley. If a 3450 rpm motor is smaller and cheaper for a given HP it would seem to be the better choice, if maximum torque is an issue in the application.

Doug

[Mike Henderson]

Guys,

It seems to me that a 2 HP 1725 rpm motor operating through the 7" pulley will deliver half the torque of a 2 HP 3450 rpm motor operating through a 3.5" pulley. If a 3450 rpm motor is smaller and cheaper for a given HP it would seem to be the better choice, if maximum torque is an issue in the application.

Doug
No, the 1725 motor will have twice the torque for the same HP as a 3450 motor. So if you use pulleys to double the speed with the 1725 motor, you will cut the torque in half at the tool - it will be the same as the 3450 motor.

There's no free lunch. If the tool is to run at the same RPMs, it doesn't matter which motor you use - you'll have exactly the same HP and torque at the working edge.

The only difference is that the 1725 motor will be physically larger.

Mike

[Mark R Webster]

Good info thanks Mike.

[Doug Hepler]

Mike,

My physics terminology is rusty. I meant the force exerted would be halved. If the force exerted at the end of a 7" lever was the same as the force exerted at the end of a 3.5" lever, there would indeed be a "free lunch".

Doug

[Mark R Webster]

My physics it rusty as well ;-) thanks

[Art Mann]

I agree with Mike. I use Newtonian physics fairly often.

[Mark R Webster]

My physics is about 40+ years rusty and fading fast. ;)

[Doug Hepler]

Guys,

My reasoning was faulty but I think that my conclusion was correct nonetheless. I did not recognize that the HP of a motor is directly proportional to the RPM, so for a given torque doubling the rpm doubles the HP. The formula is approximately HP=(torque x rpm)/5252. So for a given HP (say 2 HP) doubling the rpm would halve the torque. That was my original point, although badly argued.

Doug

[Mark R Webster]

Thanks Doug... I think my brain is hurting ;-)

[Doug Hepler]

Mark,

"I think my brain is hurting" Mine, too. This is pretty much hijacking your thread with an esoteric discussion. Unless you buy a low-powered motor relative to the work you plan to do, I doubt the difference in torque will matter much to you. There are too many other practical variables like price, value for money, availability, etc. I love a good argument but I think I am done.

All best wishes

Doug

[Mark R Webster]

Thanks Doug :)

[Rick Lizek]

Sears Park planers were sold with a 1750 rpm motor and the cutter head speed was listed around 4000 to 4500 rpm.

[lowell holmes]

Intuitively, I always replace the motor with the size and rpm of the motor I am replacing.

That is assuming the old motor has been satisfactory in performance. Why would you change?

[Mark R Webster]

Yes, mine is a Sears Parks planer and the 1 hp on it is a 1750. I have decided to go with the 1750 in a 2 hp. Thanks guys