What size motor to use

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mull982

Senior Member
Hello All

I was recently asked in our plant to look into upgrading a motor that drives a conveyor belt. The current motor driving the belt is a 25 h.p. motor and is to small for the application because it keeps tripping out because of the load. (The belt is being used to load railcars from a cement silo) We must keep the same load (tons per hour) in order to meet time requirements, therefore we need a new motor that will be able to handle this load.

I was wondering what type of calculation I could do, or what method to use in determining what size motor would be suffiecient to handle this load on the belt. Someone had mentioned upgrading to a 40 h.p. motor, but I wanted to find a way to be sure that a particular h.p. will handle the load. It there a way I can convert the tons per hour across the belt to a torque or some other measurement and then size the motor based off of that?
(The motor is 480V at 29 FLA)

I would greatly appreciate any suggestions. Thanks for the help!

Mull982
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
You need to know what the initial torque required to overcome the inertia of the belt and at what portion of the curve do you want to run on. That will need to be compared to motor capability curves. Oversizing could be a problem if you don't put it on a VFD.

Contact the conveyor manufacturer, they should be able to determine the proper size, after all thats' what they do.
 

sc57ford

Member
Location
South Carolina
As noted by kingpb, check with your manufacturer first. If thats not possible, then there are some other engineering forums on the internet where an equipment engineer might help. Electrical engineers or installers are not required to size motors. We only have to make it run properly. Your present motor probably has the wiring, overloads and OCPD properly sized for the FLA of your motor. All of these will need to be reviewed/changed when you make the upgrade to a larger motor. VFD's, should you go that way, may need shielded cable to the motor, and maybe other considerations.
Just a few things to think about.
 

hillbilly

Senior Member
I used to design and build asphalt mixing/handling equipment, so I can give you some help.

To give you some food for thought...
1 HP will lift 33,000 lbs... 1 foot... in one minute (33,000 ft/lb. per minute)
or
550 lbs...1 foot...in one second (550 ft./lb. per second).
Figure the total tonnage in 1 hour (or 1 minute) and the height that you are lifting it.
Reduce the number to pounds per minute and feet of lift.
Calculate the Horsepower.

Example...If you're lifting (conveying) 1000 tons per hour and you have to lift it 20 feet to dump it into a car...1000 (tons) per hour = 2,000,000 lbs per hour = 33,333 lbs per minute...
33,333 lbs per minute x 20 ft. lift = 666,660 ft/lb per minute....
666,660 ft./lb. per minute divided by 33,000 ft/lb. per minute (1 HP)= 20.2 HP.

At this point, you must add a safety factor for friction loss in the conveyor (bearings, belts, gearbox, drag, etc.), overloading the conveyor (larger than designed load), starting the conveyor if it's loaded when it starts, plus any other incidental loads that might come up during operation.

The company that I worked for used a factor of 2 on all of the calculated HP for the drives. If a drive calculated at 10 HP, it got a 20. This usually covered all of the drive and other losses. Your factor might need to be larger, which will result in even more required HP. The calculated HP is just a starting point. Technically it's possible but difficult to determine the exact safety factor for the drive. Usually it's determined by on the job experience (school of hard knocks).

Remember, if you increase the motor HP, you will probably have to increase the size (HP rating) of the gear box (if used) and the belt drive (sheaves and belts). Plus the driveshaft on the conveyor may not stand the increased torque.
Check with the manufacturer if unsure.
A soft start is a good idea. If you accelerate the load slowly, it will remove some of the starting (inertia) load on the drive and torque on the conveyor shaft when starting.
Hope this helps.
steve
 

boater bill

Senior Member
Location
Cape Coral, Fl.
Great info Hillbilly!!

The only thing that I might add is how to account for the increased load on the moving belt as material is added to it. Is it a continuous feeder for the conveyer belt or buckets or dump trucks? I guess that is where your safety factor would help.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Rules of thumb are OK, and the method suggested by another poster will give you a minimum size. But you might want to look at why it is tripping, rather than just deciding it is a smallish motor. Could be something is sticking somewhere now and then causing the motor to have to generate a lot more torque than it ought to. if that is the case then a bigger motor will not solve the problem.
 

megloff11x

Senior Member
It's good to look at inertia mismatch and consider gearing too. You want your load inertia, as seen by the motor, to match its inertia, or at least be no more than 10x as a rule of thumb. I give the bicycle example of why we use different gears uphill, downhill, and on the flat. Many servo motor web sites will have the gearing calculations summarized. Galil motion controllers sells a book on sizing motors as well.

Matt
 

hillbilly

Senior Member
megloff11x said:
"It's good to look at inertia mismatch and consider gearing too. You want your load inertia, as seen by the motor, to match its inertia, or at least be no more than 10x as a rule of thumb."

Sounds interesting, would you expound a little more?
steve
 
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