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Need help selecting an electric motor
2

Need help selecting an electric motor

Need help selecting an electric motor

(OP)
Hello,

I have no experience working with electric motors, and I need some help selecting a motor to meet my requirements. Any help would be greatly appreciated.

See the attached JPEG for my simplified setup.

Requirements:
The motor must pull the load at a constant designated velocity of 100 ft/min throughout the lift.
The motor will be expected to pull a load that starts out at 700lbs, but quickly increases to 3000lbs, and then almost instantly drops back to 700lbs.
The load only needs to be lifted approx 15 inches.
Assume a 0.2 coefficient of friction.

Design inputs:
I'm assuming a 12 inch diameter wheel fixed to my motor, but this can change if needed.

Also, I'm not sure if I can simply use a motor, or if I'll also need to consider a gear reduction box and or switch to control the velocity. Thanks again!

RE: Need help selecting an electric motor

A simple question but the answer may make a big difference in cost. What percentage of speed regulation do you need?
A basic design with a design B motor and DOL starting should be adequate for almost all real world applications.
RMS loading calculations will let you use 150% loading for that short time. Ref. Cowern Papers Link
You will need 31.8 RPM on the final drum.
With a motor speed of 1760 RPM you will need a gear reduction of 55.3:1
Hint: Find a gear reduction that is close and adjust the radius of the drum to get a match. (or the diameter if yoy prefer. grin)
You can do the torque calcs.
That will probably give you about a 5% speed drop when the heavy load comes on.
If you need better than that go to a design A motor if you can find one and get the spec sheet from the supplier if you can.
If your instructor is a stickler and wants exact speed control, add a sensor to the motor in the basic design and use a better quality VFD drive to control the speed exactly.
You can undersize the motor about 30% to 35%. It may be prudent to oversize the motor by the same ratio.
Given the comparative rarity of design a motors the VFD solution may be more economical than the design A motor option.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Need help selecting an electric motor

Your best bet is to access catalogs / engineering guides from any of a few gearmotor manufacturers and educate yourself on how to size a gearmotor. SEW Eurodrive, Dodge, Falk, Nord, others.

The variable loading matters not, you must size for the peak load always.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com

RE: Need help selecting an electric motor

Recognizing that links don't always work in all geographic locations:

Ed knows more about this than I do.

ABOUT THE AUTHOR
Edward H. Cowern, P.E.
Ed Cowern was Baldor’s District Manager in New England, U.S.A. from 1977
to 1999. Prior to joining Baldor he was employed by another motor company
where he gained experience with diversified motors and related products.
He is a graduate of the University of Massachusetts where he obtained
a BS degree in Electrical Engineering. He is also a registered Professional
Engineer in the state of Connecticut, a member of the Institute of Electrical and
Electronic Engineers (IEEE), and a member of the Engineering Society of Western
Massachusetts.
Ed is an excellent and well-known technical writer, having been published many
times in technical trade journals such as Machine Design, Design News, Power
Transmission Design, Plant Engineering, Plant Services and Control Engineering.
He has also been quoted in Fortune Magazine. In addition, he has authored many
valuable technical papers for Baldor, used repeatedly by sales and marketing
personnel throughout our company.
Ed lives in North Haven, Connecticut with his wife, Irene.

RMS HORSEPOWER LOADING
There are a great many applications especially in hydraulics and hydraulically-driven machines that have
greatly fluctuating load requirements. In some cases, the peak loads last for relatively short periods
during the normal cycle of the machine. At first glance, it might seem that a motor would have to
be sized to handle the worst part of the load cycle. For example, if a cycle included a period of time
where 18 HP is required, then the natural approach would be to utilize a 20 HP motor. A more practical
approach to these types of “duty cycle loads” takes advantage of an electric motor’s ability to handle
substantial overload conditions as long as the period of overload is relatively short compared to the total
time involved in the cycle.
The method of calculating whether or not the motor will be suitable for a particular cycling application is
called the RMS (root mean squared) horsepower loading method. The calculations required to properly
size a motor for this type of application are relatively simple and are presented in this paper.
The RMS calculations take into account the fact that heat buildup within the motor is very much greater
at a 50% overload than it is under normal operating conditions. Thus, the weighted average horsepower
is what is significant. RMS calculations determine the weighted average horsepower.
In addition to reducing the size and cost of a motor for a particular application, RMS loading also offers
the advantage of being able to improve the overall efficiency and power factor on a duty cycle type of
load. For example, when an oversized motor is operated on a light load, the efficiency is generally fairly
low, so working the motor harder (with a higher average horsepower), will generally result in improved
overall efficiency and reduced operating cost............
If the link still doesn't work I can post the balance of the article.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Need help selecting an electric motor

"The load only needs to be lifted approx 15 inches."
1) I am surprised that you are considering a hoisting solution for this application. Did you consider a hydraulic solution? After all 15 inches is not a big lift (and could be easily be handled with hydraulic cylinders).

2) BTW, what is the duty cycle for this application? If too frequent, this might eliminate a DOL type motor starter (at least for the hositing solution). However a PWM type ASD could be made to work.

GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Need help selecting an electric motor

(OP)
A hydraulic solution would still require the use of a motor and a pump though, correct?
Might be a more logical solution, but not sure if it would be cheaper.

The duty cycle would only be a few seconds, and the plan is to perform around 20 cycles (probably 30 min between cycles).

Looking at the original motor/ pulley concept would my RPMs remain constant throughout the duration of the lift? My main concern is the load building up and then almost instantly dropping off. Looksing at the torque/speed diagram gives me confidence as it looks like I’ll have plenty of torque. Thanks a lot for your input on this. It’s been immensely helpful

RE: Need help selecting an electric motor

What about the return? Idling?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Need help selecting an electric motor

Quote (OP)


Looking at the original motor/ pulley concept would my RPMs remain constant throughout the duration of the lift?
What do you mean by "CONSTANT"?
You can expect a standard induction motor to drop about 1% or 2% speed as the load increases from zero to full load.
If you need more constant than that it just takes more money.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Need help selecting an electric motor

How springy is your hoist line? Could be interesting when it contracts again after having been stretched by the 3000lb load.

RE: Need help selecting an electric motor

(OP)
Good consideration about the hoist line. I plan to use 7/8” 6x36 class wire rope (79600lb) breaking strength.

I think up to a 5% off nominal velocity will suffice. If however I did need more accuracy with speed control, I could introduce a VFD?

I’m assuming the use of a limit switch to stop the motor. Is there a switch to return to home? I might need to consider some options for the return

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