1170 rpm motor

[Mark Gibney]

I’m looking at a 5hp motor that is 1170 rpm, 3 phase.
I can’t find any information online about 1170 rpm motors compared to the typical 1725 or 3450.

Can anyone enlighten me on the pros / cons and otherwise of these motors?

Thanks, Mark

[Mike Henderson]

In general, for equal HP, the slower a motor runs, the larger (physically) it is.

HP is torque times RPM, times a constant. Slower RPM means larger torque needed and that means larger physical size.

A 3450 RPM motor is 2-pole, a 1725 RPM motor is 4-pole, and a 1170 RPM motor is 6-pole. Torque is force times length of the lever, so to increase the torque, the rotor is made larger around. The bars of the rotor are on the outside of the rotor so the force is at that location, times the distance to the center of the rotor.

So, same HP, slower RPM, means larger rotor and that means a larger motor.

Mike

[stephen thomas]

The slower the motor, the more poles.
I've always related that to smoother, torque-ier. Maybe an EE will chime in.
The idler motor in my metal shop is 1200 rpm, and since i never graduated that one from pull-start, it is nice to have the slower speed for starting it & more poles for electrical stability.
(to make 3ph for the rest of the machines)

[Bill Dufour]

Or buy a 7.5 horsepower 1750 one and run it with a VFD at the desired rpm with reduced horsepower. get variable speed, power brakes, slow start, etc. A motor on a vfd the horsepower varies about equal to percent of hz away from 60.
Bill d.

1150/1750 = 0.66%
7.5hp x 0.66%= 4.95hp

[stephen thomas]

We need an EE.

I think, (regarding the more poles) that buying a 1200 rpm motor and goosing it up with a VFD is better than underdriving a higher rpm motor. You don't gain a lot of power, but you don't lose it.
My 6 ft. metal planer has a 19-teens 800 rpm motor (further geared down with an integral planetary box) that i over drive a lot - up to about 95 Hz. Between there and 100 hz, something quits; never determined if it was a setting in the (huge, ancient, 1970's) VFD, or if it sensed a weakness at that point in the motor insulation.

[Jerry Bruette]

What are you driving with the motor? Could you use a 1750 rpm motor and change the pulley or sprocket sizes to get the desired final rpm? It would be cheaper than messing with a VFD, unless that's how you're driving it now.

[Steven O Smith]

I've sold a couple of those used, but not recently. They are useful primarily in slower rpm use; in a blacksmith power hammer, with a 1750 rpm motor you have to do a two step speed reduction. With 1150 rpm you can get the speed right in a single stage reduction.

I suspect that there is little output difference at the same rpm.

[Rod Sheridan]

I’m looking at a 5hp motor that is 1170 rpm, 3 phase.
I can’t find any information online about 1170 rpm motors compared to the typical 1725 or 3450.

Can anyone enlighten me on the pros / cons and otherwise of these motors?

Thanks, Mark

Hi Mark, unless the application specifically requires the slower speed, there are no upsides to selecting it over a 4 pole motor.

The lower speed motor will be larger and more expensive than a 4 pole motor.

Regards, Rod

[Bradley Gray]

If you've found a good deal on this motor ( I have several 3phase motors purchased for under $50) and the physical size works, pulleys can get you to proper RPM.

[Mark Gibney]

I'd use this motor on a 20" band saw.
Thanks for the insights, people.