Centrifuge AC Induction Motor Requireing 30 Starts Per Hour

[rglassburn]

Hello All,

I have been asked to specify a motor for a new centrifuge. It needs to be able to handle 9.4 lb-ft^2 of inertia and spin at 3425 rpm. In order to cycle the number of parts that I need, I will need to be capable of cycling the centrifuge from 0 to 3425 rpm up to 30 times per hour. The centrifuge will be driven by a VFD. My plan is to direct drive the table using a flexible coupling.

As I have been studying the requirements, I have found the MG 1-40.40 table that seems to indicate 10 HP motor. Since a VFD will be needed to control the speeds, I will pick an inverter duty motor. However, looking around a bit more, I found MG 10-2001 which has table 7 "Allowable number of starts and minimum time between starts for Design A and B motors" which indicates that a 10HP 2 pole motor would not be able to start that many times per hour. It shows a little more than 1 start per hour?

Can anyone provide some clarity and/or point me to a source that can help me with this spec?


Thank you in advance for your insight!

Thanks,
Rob

 

[rglassburn]

We are using the mass moment of inertia (I: I = m x r^2) and not the polar mass moment of inertia (J). 'J' deals with torsion, 'I' with angular motion. Easily confused. There is even another inertia that is also called 'I'(second moment of inertia). Wikipedia does a pretty good job of explaining.

You would think that mass(inertia)would be simpler


Thanks,
Rob

 

[sreid]

The reason we are asking repeatedly is because we have been "Burned" by inertia units more times than we like to think about.

You quote (correctly) an equation for time to speed. The wr^2 in this equation has units of lbf-ft^2 because the 308 factor is 32.2x60/6.28 (converts force to mass and RPM to radians/second).

 

[waross]

A suggestion on motor speed. When a speed of 6000 RPM is contemplated, it would probably be based on a 3600 RPM motor wired for 230 volts. It would be supplied with almost 460 volts at almost 120 HZ. At almost double the frequency and almost double the voltage you will also have almost double the HP. If you use a 5HP motor it will develop almost 10 HP at 6000 RPM.

 

[cswilson]

Rob:

Both sreid and I are talking about mass moment of inertia, which the Wikipedia article you cite states goes by either I or J. (In my experience, "I" is used in science texts, "J" in engineering texts.)

The fundamental equation all of us are using is

T = J * alpha

As sreid points out, many, many people have gotten burned by unit confusion, starting from the basic problem that kilograms are units of mass, and pounds are units of force. IMHO, attempts to get around this by using units such as "pounds-mass" create more problems than they solve.

I plugged your numbers in above using the fundamental units and got an acceleration time of a minute. Do you see anything wrong with my calculations?

Curt Wilson
Delta Tau Data Systems

 

[rglassburn]

cswilson, sreid,

I do not see any error in your calculation and it does square with where I am now in my calculations. During the course of this discussion, I have been reducing my table size and increasing the speed required.

Currently I have reduced the inertia down to 2.7 lb-ft^2 (without the motors rotor inertia). The speed I need is ~4200 RPM and the time to spin up to that is around 30 seconds. The average torque required for this is ~1.2 lb-ft.

I spend a couple of days early on worrying about the mass conversion issue. It is amazing how many sources of information have the information incorrectly stated (or poorly explained). The metric system is certainly the easiest to understand.

Thanks,
Rob

 

[cswilson]

Just as a wrap-up for others who may be reading this, because Rob's experience in flailing around with MOI calculations is, unfortunately, pretty typical.

Yes, metric (particularly SI) is best. Use it if you have the information given to you in these units.

If you only have the information available to you in English units, use real English units (pounds-force, MOIs in lb(f)-ft-s^2).

Avoid "crossover units" like "pounds-mass" (lbm) and "kilograms-force" (kgf). It is way too easy to make a mistake and be off by a factor of "g" in a numerical answer.

Avoid formulas that incorporate all conversion factors into a single value. Again, it is way too easy to make a mistake due to unit confusion. Write equations out using the fundamental units, then go through cancelling out units to make sure that your answer is in the appropriate units. I emphasize this in the university classes I teach, and I still do it myself.

Curt Wilson
Delta Tau Data Systems

 

[sreid]

To quote an old NASA joke (slightly modified)

"Don't spec acceleration in Furlongs per Fortnight Squared."