Friday, April 3, 2009

26> Linear Induction Motor Tractor Unit

Here is a simple conceptual drawing of how Linear Inductions Motors (LIM) could be employed in a tractor unit for a gondola style Personal Rapid Transit (PRT) vehicle. Shown is a simplified track encasement, without outside structural support. The 3.2mm (1/8”) gap between is maintained by making internal rails (shown in orange) and the reactor plate (blue/green) both setscrew adjustable. With tight radius turns, the gap will be uneven and perhaps somewhat larger, but this diminished power and efficiency will not materially affect performance. By having two units in tandem closer gaps can be maintained. This design is compatible with the switching protocol illustrated earlier. The LIMS on one side must be turned off during switching, so the pod cannot accelerate strongly through the switching process. There are four LIMs. The proportions shown are consistent with the (Baldor) # LMAC16123D99. (12”x16”) (30.4 x 40.6 cm)

The performance of each are as follows. Each weighs 105 lbs. (48kg) and can produce up to 190 lbs. (845N) of pull (15% duty) and 38lbs. (169N) continuous. That’s a total of 760lbs. (3380N) max total, and 152 lbs. (676N) of continuous pull. The motors (total) weigh 420lbs. (190.5 kg)

The downside is this. These figures are for 60 Hz, 460 volt, 3 phase current. The maximum velocity for these LIMS at 60 Hz. is only about 15 mph. It is unclear how much pull would be sacrificed for additional speed, which is achieved by increasing the frequency of the AC current beyond 60 Hz.

Most past models for PRT have assumed speeds of about 30 mph. (48 kph) That may be fine for short haul downtown environments, but that’s where trolleys, shuttles and even light rail are most competitive. The urban/suburban sprawl typical of many cities (especially American) requires considerably more speed because of the distances involved.


Anonymous said...

Is the switching mechanism based on a worm gear?

Dan said...

Dan the Blogger Responds-
Hi A,
Yes and no. If you mean by "worm gear" a revolving acme threaded rod, then yes. Technically no, because a "worm gear" is the gear that advances slowly while the "worm" rotates. "Screw jack" is a better term. There is even such a thing as a "ball screw jack" with little steel balls that travel in the threads and recirculate through the "nut" part. The unit shown is basically the type shown in my Aug 9 post.(Raytheon design) but the Dec.12 post shows a superior design. (pictures can be enlarged by clicking)It is superior because the latest one's guide wheel could slip into and out of contact, causing noise and wear.

Anonymous said...

Hi Dan,

Thanks for the reference back to the Dec 12 post.

What reliability issues do you think are likely to come up with a motor & gear positioner such as this? Especially given time, use & the introduction foreign materials.

Great blog & posts, btw - terrific to see some discussion of the nuts & bolts.

Dan said...

This is how they move the flaps and tail rudders on jetliners. It is also how they feed steel into the cutter heads (or vice versa) on machine tools, like lathes and mills.
On CNC mills they use an accordian style rubber covering because metal chips and fluid flies everywhere. The motor would probably be a stepper type, fitted with a servo feedback loop to ensure it's not out of position.I imagine that there would be at least two of these units, adding a further layer of reliability.

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