Help - Motor HP, RPM, Pully formula

Status
Not open for further replies.

Minuteman

Senior Member
I have an industrial customer, that bought a machine tool (grinder) built in 1941. The drive motor is not the original and seems too small. They asked me if I could increase the HP and I know that I have to have the right formula. The motor on there now is a 5hp, 480V 3 phase 1725 RPM the drive (motor) pulley is 4" diameter, and the work pulley is 23"




(edit to correct rpm)
 

peteo

Senior Member
Location
Los Angeles
HP = force X speed. What, are they trying to dig deeper? A larger HP motor will give more force, and take more electricity. But it will have the same RPM, output shaft, mount, and pulley. Just be smart, and let your customer know that what you would be limited to doing, would be getting him a larger motor. Whether it solves anything, is something he would be more familiar with...

I'm not sure that it's such an easy answer, because of the control; wiring, fuse size, etc. Make an attempt to find some information on the internet, at least; that old of a unit normally wouldn't have data at the end of a call to a manufacturer customer service department (unless it's a Moore, which would be a contradiction.) But there might be some reference to the original, even some knowledge of what the model number means is helpful. Depending on where it has been installed, there may be a set of third party inspection data. In my jurisdiction, the machine would have a very expensive third party inspection sticker and I wouldn't touch it without first contacting them. Asides from that, the question is 'how do you expect to work on this.'

The motor will have some frame size, which establishes the mount, output shaft, etc. One way to do this is to replace the existing motor with a larger HP unit, at the same time replacing the pulley and belt(s) with identical ones. You can work with Grainger, where they would have a new motor, pulley, belts, and mounting plate. Their catalog has very good pages with motor, pulley and belt information. Grainger is a little more expensive than some, but a 'full line' supplier.

It sounds as if you're not completely sure about this. It would probably be your best bet, to contact someone who can fill in all the questions for you. This would take calling people who rewind motors until you find one who's willing to work with you on this. My guess is that a little help from an experienced motor company will make a big difference towards both finishing the job and getting your customer what he/she wants.
 

Minuteman

Senior Member
Thanks Petro.

The machine is a Landis Grinder. Sometime in the late 80's it was fitted with variable frequency drives on the head stock motor, hydraulic pump, and way lubrication pump motors.

I was contracted to install the main power to the machine, and we connected it and checked motor rotations. They were a little unsure about how to operate it when we left. Yesterday, they said that the head stock motor failed while grinding a tool. I went out, checked operation as best as I could, and replaced the motor. (It was faster to replace it then send it out for re-wind, and they were in a hurry.) I connected the new 5hp motor, and it seemed sluggish at first, they used it to grind a tool, and durring the 2nd tool - it failed.

The motor brackets have been modified. the 5hp motor shaft is lower than the original motor and the belt guard has been modified. The is very little literature with the machine, and some of it has the hand written notes showing the VFD's and parameters. The old Landis book does not list the original motor spec's, but the motor in the picture is much larger looking. Therefore, the manager assumes that it must have a larger hp motor. So far, they owe me days labour and the new 5hp motor. I would like to keep their business, by helping them figure out why the machine fried the 2 motors.

Side note. The overload DIDN'T protect the motor. There is overload protection for the motor, consisting of 3 heaters by them selves. (Not part of a motor starter) I cannot identify the brand, nor does the heater number mean anything to me. FWIW., the hydraulic motor is an identical 5hp motor with the same setup. (VFD with an external stand alone overload, and has the same size heaters.)

My thoughts. Given the pulley diameters and the RPM, I should know what size HP is required. Any help would be appreciated.
 

Johnmcca

Senior Member
It sounds like the thermal overloads are improperly sized if you have toasted two motors. Maybe replace the contactor and overloads with a new motor starter and OLs you can properly size for the load if you can't get any info on the OLs you have now.
As for increasing the hp of the motor you have to watch the belt cross section and its ability to transfer the additional hp.
My experience is with older wood working machinery, not machine shop stuff but the basic premise is the same.

John
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Old motors were cast iron and physically very large when compared to newer motors.

For example, an old 5HP 1800RPM motor might have been a frame size 254(about 10" dia.) a newer 5HP might be a 182T frame (about 7.5" dia.). This means a old 5HP might be the same diameter as a new 15HP.
 

Minuteman

Senior Member
Thanks Jim,

I kinda knew that. I called the manager this a.m. He is going a different direction. Seems to him that something is in a bind, maybe not getting lube. So, they are tearing into the head stock mech. I told them that I was cool with that, and to let me know what they find. They bought it from some Machine reconditioning outfit in Wisconsin. They let it set for about 4 months and the 30 day warranty has expired.
 

Minuteman

Senior Member
Update

Update

Well, they did not find a problem with the head drive. They figure the 5hp is not big enough to turn their part. They want me to install a 20 hp. I passed, but they are persisting. I'm not too sure. I know that I would keep the same motor pulley size (4") and would have to find the right overcurrent protection. Also, would have to re-size the VFD. The 20hp motor would have a larger frame, so, I would have to modify the belt guard. Of course, if I chose to take on this project, I will insist on having a disclaimer signed waiving any responsibility to their machine. Oh boy :?
 

Johnmcca

Senior Member
If this is a machine shop, let them be responsible for all the mechanical modifications to the machine, and you just do the electrical as needed to make it electrically safe, and operable.

John
 

peteo

Senior Member
Location
Los Angeles
Good deal! The new vector drives are remarkable, with software which 'learns' the motor/wheel parameters automatically. What I'd suggest is to call a few drive or motor people to bounce it off before committing. Not knowing about grinders, I'd hate to offer any type of applications ideas, but there's no reason to expect less than great results. I'm with John, have the customer in there with the selection and upgrade process, then any 'where's the beef' issues will be a question of did we follow the plan rather than who's responsible. :)
 
Status
Not open for further replies.
Top