Recommendation? Motor RPM for VFD

H

HWooldridge

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Jan 1, 2017
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I am going to build another grinder or two with VFD's and was curious what most people prefer for basic RPM. I can get a little bit of a discount at WW Grainger and depending on HP, the prices vary for 3450 RPM vs. 1740 RPM. I have read that some people can use the doubling characteristic of a VFD on a 1740 motor.

Of course, if you come across a lower rpm motor then the ability to increase the speed would be helpful but I see no need for that if the 3450 motor will still slow down when you want it to.

Any thoughts are appreciated.
 
It’s been debated at length, I think the consensus was 1725 rpm motor has a better torque curve and it’s a more robust motor. 2 pole vs 4 pole motor.
 
The 1750 rpm motor with doubling jumper will turn the required 3500 rpm, and have a tad more torque at the low end. The 3500 rpm motor won't need the doubling jumper, and will still have more torque than required at slow speeds. When running motor slow (300 rpm?) you're doing light touch grinding, not hogging where you need the torque. The 3500 rpm motor will have a tad less weight because it's got only 2 coils vs the 4 coils for the 1750 rpm motor, and is a bit cheaper than the 1750 rpm motor.

I used a 3500 (3450?) rpm motor because it was a tad less weight (I move my grinder outside for some grinding) and was about $40 cheaper than the 1750 rpm motor. So far, I've never had any problems due to lack of torque at low rpms.

Either motor will give you good service.
 
Thanks for the explanation, Ken. I can get a new 3450 Dayton motor for about $140 while the same motor in 1750 is about $220. Considerable savings if it's not needed on a regular basis.
 
I always looked at it like this. Like having a crotch rocket but only going to the local store for groceries on it. I have a 1hp leeson 1725 doubled with a vfd, and I rarely go above 50-60%, however when finishing I spend a lot of time at 10-20%. And if I need to go slower I have the option of removing the 2x jumper and if I need faster I can increase the drive wheel size but even heavy hogging I haven’t felt the need to.
 
I'm with Tin.Man, I use a 1hp 1750 with 7" drive wheel. most profiling is done at 60-90HZ, beveling around 50hz and fine touches at about 15-30hz. I have hogged material at 100-120hz (doubling) but never really find I need to go that fast and when I'm working at 20HZ I never have bogging issues or even come close as its light to medium touches to the belt.
 
3500 RPM motors are two pole and 1750 RPM motors are 4 pole.
I agree with ken's info, but come to a different recommendation from it. The four pole motor has heavier windings and is generally more durable. It cost more to make and thus has a higher price. I feel the additional life and torque are worth the extra cost. As far as the "jumper", most VFDs are set up to run at 10% to 200% rated speed, so the 1750 motor is just right.
 
I also prefer the 1750 motor. There are things I do at low Hz that require high torque, whether it's moderate pressure on a flat platen which is more torque intensive than on a contact wheel, or when using a radius tool on the flat platen which requires quite a bit of torque since you're pinching the belt over a non-rolling corner.

I believe they run smoother (in particular at low Hz where it becomes more noticeable) than 2 pole motors as well but that is by no means conclusive as different VFDs and motors behave differently. But having 4 pulses per revolution vs 2 pulses per revolution simply means less radial travel between pulses and therefore less loss of rotational speed per pulse.
 
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