Is there any mechanical difference in the following:
A. 2 hp 3450 rpm motor with a 3" pulley driving a 12" pulley
B. 2 hp 1725 rpm motor with a 6" pulley driving a 12" pulley
Would one be more efficient than the other? If so, how come?
[OP]
Member
Is there any mechanical difference in the following:
A. 2 hp 3450 rpm motor with a 3" pulley driving a 12" pulley
B. 2 hp 1725 rpm motor with a 6" pulley driving a 12" pulley
Would one be more efficient than the other? If so, how come?
Member
Mechanically I don't see any advantage either way. Maybe slower speeds would have less windage in the bearings but that should be negligible. I suggest looking at the motor efficiencies to make the decision.
Gary
Contributor
If I had to choose between these, I would go with the slower speed motor. It will have lower bearing speeds (both the motor and pulley bearing) and 'maybe' a longer life. Also, the larger dia pully might put less stress on the belt (again, longer life and possibly quieter running).
The motors themselves are a big consideration. Many times, a particular motor is designed to work on multiple voltages/frequencies/windings. Meaning, they often wind the motor so that you can connect it for different voltages. Although its flexible, it means the motor is NOT wound for the optimum efficiency for the particular power you are using it with.
So if you know what your power source will be, a motor capable of running on that particular power only has a higher chance of being optimised for it. (of course, knowing the actual curves of the motor is the best answer, since efficiencies can be all over the map)
And then the next obvious advice is - they will either one work - do the one that is the EASIEST for you!
Member
From an engineering standpoint regarding the ultimate output to the 12" pully, there will be no difference in speed. If you, or the designer, is trying to create a mass effect (i.e. a flywheel), the 6" pully system would have an advantage. Likely in our applications where these things are so light weight, there is zero noticable difference.Originally Posted by Bill LaPointe
Quis custodiet ipsos custodes?
Member
As far as the efficiency goes, look a the nameplate amperage draw of each motor - there's likely not a lot of difference. The lower draw will be more efficient. The slower motor will be a larger diameter and provide more "Flywheel" mass. But since rotational energy is similar to linear energy in that in is proportional to Rotational Velocity (W or Omega) squared like linear energy is proportional to Velocity squared. The higher RPM motor will very likely take up much less room though.
The opinion of 10,000 men is of no value if none of them know anything about the subject.
- Marcus Aurelius ---------------------------------------- ------------- [SIGPIC][/SIGPIC]
Contributor
I'd want to know what the torque/HP curves look like for the motors if I really had to make a choice and if it really mattered all that much. You'd also need to know what you're turning. For example, if you're spinning a very heavy mass, the higher speed motor may have an advantage on startup.
For most things, it probably doesn't matter. I'd just install whatever's easiest assuming you don't have anything weird going on.
Member
Exactly. In flywheel applications, we're trying to resist the change in omega.
Quis custodiet ipsos custodes?
Member
Just another question...
Wouldn't the 3" pulley have more torque then the 6" at the shaft of the 12" pulley?
In other words wouldn't it be harder to stop the 12" pulley if it was being driven by the 3" pulley?
Just asking....
Contributor
All things being equal, yes, but the ultimate performance depends on the torque curve of the motor. I expect the lower RPM motor to generate more torque than the higher RPM motor, but without the motor's characteristics, there's really no way to know which one will perform better.
For example, you can have a teeny tiny little motor that spins at 30,000 RPM and generates 1HP, but generates practically no torque.
Really, it's only going to matter on startup. You'd expect the two motors to be designed appropriately so that they're generating good power at the design RPM.
[OP]
Member
Thanks for all the good answers. I have a bunch of motors, some 1725 and some 3450. Was curious why make 2 speeds if all else was equal. (Physical size, current draw, etc.) Thought it might have to do with drive ratios, but apparently not.
Contributor
OK...let me give this a try. The things you'd consider in choosing a motor are things like:
pulley sizes, i.e. belt wrap (big pulley with little pulley can cause traction problems on little pulley if they're too close together)
There will be a trade off with current and effeciency, but this is negligable for a tool motor. For example, a faster turning motor will dissapate more heat through the mechanism, so there will be a loss of effeciency there. That's just one example of the tradeoff.
Size differences between the motors/gearbox/pulleys. How much space do you have to mount things?
I think 4 pole motors (your 1800 rpm guy) are generally less picky and potentially more reliable. You're also turning at least a subset of the system slower, so theoretically at least some part of the system will last longer and run cooler.
For our tools, though, we're typically talking pretty minor differences. If I were designing something meant to be run continously, with specific requirements, that would definately skew a decision one way or another.
Generally, I prefer the slower turning motors. Why spin something twice as fast as you have to? Really, though, it probably just doesn't matter for us.
Member
Sometimes you don't want the speed. Say you're driving a pump. If turbulence matters then you would prefer a low speed and a larger pump than vice versa. (This is a more expensive option). I have a project going where a couple of the pumps run at 80rpm max.
The same probably goes for tool grinding. You want to avoid excessive speeds.
Member
But in real life there are minor differences.
1725 RPM motors always seem to be physically larger than 3450 RPM motors with the same HP rating. Maybe this has to do with the 1725 RPM motor needing more windings since both are operating at the same 60 Hz power line. The slower RPM motor has twice the number of windings so it becomes larger. This adds to the cost, but probably also improves the ability to disipate heat so it might last longer. The bearings may even be larger and the RPMs slower so the 1725 RPM bearings will probably last longer.
The larger diameter belt on the 1725 RPM motor should be more efficient because the belt does not need to flex as much. It also has twice the surface contact so it will have less slip.
I think the slower RPM motor will work better if you already have one sitting around. If you need to buy a new one, then I would buy the faster RPM motor because they are easier to find and much cheaper. Cost is the only advantage to the 3450 RPM motor.
Steve
Member
Actually no. The result from either the 3" or the 6" would be a tension result on the belt. Yes the 3" pulley has a shorter "lever" and should be harder to stop...but the faster motor has less torque to pump into it. HP is a function of torque at a given speed - so a motor twice as fast is going to have 1/2 as much torque (give or take). So the tension in the belt from either motor will be roughly the same, so the 12" pulley really will not be able to tell the difference.
The opinion of 10,000 men is of no value if none of them know anything about the subject.
- Marcus Aurelius ---------------------------------------- ------------- [SIGPIC][/SIGPIC]
Contributor
HP is a factor of rotational speed (RPM) and torque. Since the 1725 motor is half the speed of the 3450 motor, it must have twice the torque of the 3450 motor. So let's say the 3450 motor has torque "T", while the 1725 motor has torque "2T".
For the 3450 motor, the gearing is 3:12 or 4 times, while the gearing on the 1725 motor is 6:12, or 2 times.
So the output torque (assuming no losses) is 4T for the 3450 motor and 4T for the 1725 motor.
Tools like the DeWalt 735 planer have universal motors that run very high RPMs, with fairly low torque. But the HP can be significant because of the very high RPM. Of course, those motors have to be geared down significantly to drive the cutter. They also produce a lot of noise because of the high RPMs.
Mike
[Added note: since the 1725 motor has to generate twice the torque of the 3450 motor, it will be physically larger.]
Go into the world and do well. But more importantly, go into the world and do good.
Posting Permissions
Reply With Quote