How to spec 3PH AC Motor

[ihopeitworks]

I currently have a 2HP, 3PH, 60Hz AC motor,

that is connected directly to a 12" dia drum. In the past, I would pull a cable that is wrapped around it with 2 lbs of tension (torque of 1 ft-lb). I have a new project that I am working on that requires a 10.75" dia drum pulling a cable that will have 25 lbs of tension (torque of 11.2 ft-lb). My current motor is not adequate to handle this task so I am now looking for a new motor and VFD to swap out. The kicker is I'll need to be able to apply 11.2 ft-lb of torque at both 120 RPM and 1400 RPM. The reason for the different RPM is that I need to pull this cable at two different speeds, but at the same tension.

How would I go about spec'ing in a motor for this sort of application? I would hope I can get to both RPM within 10 seconds or so. I have a panel wired to step up 120 VAC to run at 220 VAC. It seems very difficult to find a motor's torque curve for the entire RPM range, it seems that they're only included for the high RPM end.

Looking for any guidance. Thanks.

 

[LittleInch]

I'm no sparky, but more than 10:1 turndown for an electric motor is not feasible.

Most VFDs go from 40 to 70 htz and that is relatable to speed.

Torque suffers at the low frequency and the motors can stall.

also time taken as most of these motors are air cooled and air flow varies with speed^2 so cooling is impacted.

I think you need to look at maybe a hydraulic motor for your design. they are much better at that level of variable speed but constant torque.

Don't know how many sparkies are going to see this post on the mechanical page....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SwinnyGG]

I agree with LittleInch that this would be better for a sparky forum.

If this were my problem to solve I'd be investigating how to size the motor for the highest load, and provide a drive that allowed a gear change to drop the output RPM, or I'd want to just use a servo motor.

 

[ihopeitworks]

Thanks LittleInch and jgKRI,

I posted in the electrical board.

My existing VFD was able to get down to 40 RPM (I believe this would be ~1.5 Hz... if this is correct: 40 RPM / 1750 * 60), I had the display as RPM and not Hz.