AC Motor Rotor Lam Design

[HomeMadeSin]

I've got a potential design that requires a motor to have a "hollow" rotor design. I've seen rotors that have "spoked" lams that lead me to believe that the inner area is not necessary from an efficiency of the motor standpoint. I'd like to find an existing design that has can accomodate a 4" ID hole reamed through it. OD of the rotor is not critical. I only need about 3" oc stack height and 3/4HP @ 3450rpm, so it would be a low L (length), high D (diameter) ratio. Pipe dream?

 

[DougMSOE]

While your proposed L/D rstio is slightly different than most hollow ahaft designs it is very do-able. In fact this hollow shaft design is very common in the chem/petro industry in the US.
Not a pipe dream at all!

 

[electricpete]

I agree with Doug.

I have seen many large motors where the rotor laminations more or less form a hollow cylinder which is supported on the shaft by a "spider" structural support.

I have also seen many smaller motors where the rotor appears solid. Not sure if it is for mechanical considerations.

If you think about the flux path for a 4-pole or higher motor, the flux would travel primarily along the O.D. of the rotor to complete the loop between two points 90 degrees apart (4-pole) where it crosses the air-gap. Removing iron in the middle has no effect.

For 2-pole motor it seems like the shortest-path distance between airgap crossings would be through center of the rotor....but maybe the extra distance traveled through iron around circumference of the rotor is insignificant compared to large reluctance associated with flux flowing through the airgap between rotor and stator.

 

[acmotorengineer]

to Homemadesin

In Fact the inner rotor core is quite essential to the operation of an AC Induction Motor - punching holes reduces the torque and increases the slip - until the motor cannot produce sufficient torque to carry any load.

IN addition, a typical rotor OD for a 3/4HP 2Pole motor is only 3.25 inches - stator OD typically 6.25 inches. Difficult to come up with a 4 inch hole.

The motors refered by doug and pete are normally larger than 440 NEMA (250HP) frames and 4 pole and slower.

The vertical hollow shaft motor has a hollow shaft, however in operation, there is a pump headshaft thru it and it carries some of the magnetic field.

If you are going to have a magnetic shaft inside the hollow shaft you could possibly come up with something - HOWEVER,
I would think you will have extreme difficulty getting anyone to supply a 3/4 HP motor with a 4 in hollow shaft - unless you come up with development money and an expected large annual volume in an established market.

 

[electricpete]

good point acmotorengineer about the physcial dimensions of typical 2-pole motors at this small horsepower make it impossible.

I guess HMS recognized that part because he mentioned it would have to me a large diameter. I didn't think about that aspect.

One quick comment, I have seen the spider rotor design on a 7000hp 2-pole as well as many other 4-pole and slower motors. The next smallest 2-pole rotor I can remember seeing was 100hp and it was cast aluminum construction which appeared solid in the middle.

 

[jbartos]

Suggestion: Visit

http://owe.starforze.net/ca-owee/indexpages/Offshore_technology.php?file=offtech_p7.php

for: circle rotor generator (and potentially the motor)

http://www.ship.org.tw/Upload/Electric_Warship_V.pdf

http://www.ship.org.tw/Upload/Electric_Warship_VI.pdf

etc. for more info

 

[DanDel]

I have seen motors for sharpening machines with hollow rotor shafts through which cooling/lubricating fluid is pumped to the cutting surface. These motors were probably at least 50HP, 480V, and used for sharpening long metal blades, such as are used on ice skating rink 'Zamboni' machines.
I don't believe the holes were much larger than 1" in diameter.

 

[HomeMadeSin]

Thanks for the feedback. We've only up to 40HP motors here to play with and I think all are not sized appropriately.

The issue becomes somewhat a "chicken & egg" dilemma. How do you get a motor manufacturer/designer interested in producing something that hasn't been produced before? Standards have to start somewhere. At least right now, I was hoping to modify an existing squirrell cage motor for my testing or feasibility study (empirically) and go on from there. I just wish there was an easy way to "fab" a motor. It's not easy to rapid prototype a motor.