Linear Generator Questions

[pgmta4f]

Hello all,

I'm designing a simple linear generator for testing purposes, and I have a couple of questions...

1) I've read in literature that the maximum practical stroke for linear generators is around 3cm, but it didn't elaborate on why. Can anyone shed light on this? It seems that if this were true then things like wave energy generation would be impractical for linear generators.

2) I'm trying to figure out the best way to calculate the resistive force on the shaft in the axial direction as the magnet-containing shaft oscillates inside the coils. Any help in this area is appreciated.

 

[waross]

A Linear Induction Motor will function as an induction generator when the exciting current frequency is dropped below the synchronous speed. (Synchronous speed of a LIM is dependent on the coil spacing and the frequency.)
LIM motors are used to drive some Light Rail Rapid Transit Vehicles and use regeneration (work as induction generators) for deceleration.
Deceleration (working as induction generators) would slow the carriages that I am familiar with almost to a stop. When the regeneration cut off and the brakes were applied the carriages typically stopped within about 0.5 meters.
There is no limit to the maximum length which may be measured in miles or kilometers.
On a small scale you could probably use a VFD with regenerating capability as a control. Your challenge will be to design a linear motor that will match with the VFD. Depending on your load you may be able to tap power off of the braking resistor and avoid the extra expense of a regenerating drive.
For your use it may be better to stack thin coils and push a magnet through them. Rectify the outputs with bridge rectifiers and run them in parallel. That will be, in effect, a number of generators one after the other.
From the strength of the magnet, the speed and the number of turns cut per second you may calculate the EMF.
The EMF applied to the resistance of the active coils and the load will give you the Watts. From the Watts and the speed of movement you may calculate the force.
Estimating the number of active coils may be a little tricky.

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

 

[racookpe1978]

Friction losses should be low, right? So his "theoretical" regeneration losses (energy to accelerate-and-maintain speed - energy recovered from regen braking at the end of the trip) would be primary motor losses (resistance) and air friction, right?

(Well, potential energy changes if he is moving his motor and load uphill or downhill need to be included also I guess.)

 

[waross]

would be primary motor losses (resistance)

I would add "and load resistance losses", but I agree with you.

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