Sizing an AC motor

[bbadge]

I'm trying to determine the HP I need for an AC motor that will be driving a rotating load of 500 lbs. The rotational speed of the load is 6 rpm, accelaration can be assumed to be 0-6 in 10 seconds. The load is driven by a pulley & timing belt. The driving pulley is 2" and the driven is 20". The load is on center to the rotating axis. Assume motor to be a standard 1720 rpm with a gear reducer. Please help.

bbadge

 

[electricpete]

This is not a school problem, is it?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[bbadge]

No it's not a school problem. I'm just a little rusty. If the load is (almost) on center to the rotating axis, would the only consideration be the inertia? Here's a sketch.

 

[PeterCharles]

There are two loads to consider -
1) frictional loads in the bearings supporting the rotating table
2) the acceleration of the table and load from rest to full speed

 

[bbadge]

Let me ask it this way. If I select a motor that puts out 500 in-lbs of torque with a 2" drive pulley. Can I assume to be safe driving a 500 lb load (at a low 6 rpm) if the load pulley is 24"? Since torque (in) = torque (out)?

 

[PeterCharles]

Let me answer this way -
(1)
Put your load on the rotating table
Wrap some rope around the table
Pull the rope with a spring balance until the table rotates
The calculated torque (pull x radius) is the torque to overcome friction

You will have to calculate the inertia of the load and table, estimate the required acceleration according to the time to reach full speed, then calculate the torque from
T(torque) = I(inertia) x A(acceleration)

Add the two torques to get the total torque.

 

[electricpete]

Will you are using an induction motor with DOL start?
How often will the cycle be repeated?

Assuming you DOL starts of induction motor, you will grossly oversize the motor if you select a steady state motor steady state horsepower rating based on the average torque for this 6 second acceleration period.

If I were to solve the problem, I would start here:
faq237-1285

If you follow the first link, you will see NEMA limits for repetitive starting of induction motors.

One other piece of info that would be helpful to have is the NEMA MG-1 table of maximum "standard" inertia that can be accelerated by motors. I don't have that handy. Maybe you can find it by google.

The bottom line is that you want to convert all of your load inertia into equivalent inertia seen by the motor (based on square of the speed ratio's of gearbox and belt). Then use the criteria listed to select motor size for which your intended starting duty will not be excessive.

This assumes you are going to use standard NEMA motors. If a gearmotor is something different sold as an integral units with gears and motor as one package, then you need to look at standards or specs for that unit to determine starting limits (how much inertia can it safely accelerate how often).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Actually, I'm not sure about the grossly oversize part. You can try it to see what you get. But the bottom line is that the relevant aspect for this motor seems to be the starting duty, not the steady state horsepower rating.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[PeterCharles]

Always start from first principles, use your basic engineering knowledge. This problem is college engineering standard, it's not degree stuff. Don't make it harder than it need be, don't rely on published data sheets unless you understand where they were derived from.

 

[electricpete]

Very intersting commments PeterCharles.

I would respectfully disagree.

Converting the given data into horsepower delivered by the motor is a trivial textbook problem. (The fact that all the data is conveniently present to compute horsepower led me to question on 13 Jan 09 7:36 whether this was a school problem).

Selecting the minimum sized motor to meet a given starting duty is not a textbook problem and cannot be solved from first principles. (Unless you intend to develop a thermal and mechanical model of the motor to predict rotor bar/end ring cracking over repeated starts... and you're gonna need a heckuva lot more info than was provided in the original post to do that.)

I would also respectfully disagree with your advice "Always start from first principles". I would instead recommend to consider the various approaches available in a given problem (analytical solution, computer solution, reference to standards, review of empircal data, etc) and select the proper approach for the particular problem at hand.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Correction - we do not have enough data to calculate the horsepower - as was mentioned we need the inertia of the cylinder, not the mass (500 lbm)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[PeterCharles]

Firstly you can't select a motor without the characteristics of the driven load. In this case the requirement is to un-stick the load against it's frictional resistance which requires a torque (T1) and then to accelerate the load at the required rate which requires an additional torque (T2).

The addition of these two torques gives the minimum torque required to run the load up to the required speed (T3), in this case 6 rev/min. All basic stuff calculable from first principles by any mechanical engineer.

The drive to the load is via a 10:1 belt drive thus requiring a drive unit of 60 rev/min, typically I'd be looking at a spur geared motor unit from Flender, SEW, et al.

The requirement for this unit would be to provide a starting torque in excess of T3 for the acceleration phase, then in excess of T1 to maintain rotation once acceleration has has been completed. Select from a manufacturers catalogue or discuss with their applications engineer.

Much information was omitted from the original question so the above is is just the generalised solution.

IMO, all of the above is a first principle approach.

Too much reliance charts, graphs, tables, derived formulae frequently leads to ignorance of underlying engineering principles and very quickly the misapplication of data. It's a sad truth that many who read from charts and graphs, put numbers into derived formulae or run computer programmes have no concept of what lies behind the answers they get.

 

[electricpete]

This continued advice about reliance on charts/graphs is rather silly and simply indicates the person making the comments doesn't understand the charts that I referenced.

I never said the chart was going to take the place of determining the characteristics of the driven load. That the give data needs to be converted to torque goes without saying. However I don't think knowing torque alone allows one to select the motor. The link that I posted addresses considerations for motor starting - how many times can I start a given motor based on the driven inertia (inertia is the big factor, load torque is generally relatively minor and not even considered in this tabulation for good reasons). It would be applicable if using NEMA frame motors. I suspect these gearmotors are smaller and may not be covered. In that case I'm pretty sure that the process of going through the application engineer or the catalogue will address repetitive starting somehow. I am 100% positive you will never figure it out from your first principles.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[PeterCharles]

The true experience for a colleague in an associate companies engineering office:

Q, "how did you size this chart?"
A, "if you look in XYZ handbook there's a chart which gives you shaft diameter according to the loads"
Q, "what stress levels does it use?"
A, "no idea, we've always used this chart"
Q, "we use this formula based on a stress level of T ton/in2 and we can change the stress level to suit other materials"
A, "oh"


"inertia is the big factor, load torque is generally relatively minor"
Not so, it depends on the application. Most of my applications are high frictional torque with load inertia maybe 25% of motor rotor inertia.

If you use my method you can compare the two applicable starting torques (friction and inertia) which are seen by the motor. It's also possible to roughly estimate the starting time and the accelerating load in the driven machine which is sometimes quite important.