Linear Electric Motors: Pros & Cons

[studentoftheg]

we were learning about electrical motors in class the other day and my teacher mentioned linear electric motors, which from what he said are basically standard motors 'unravelled'. i was wondering why, if this is the case, you would ever use a standard motor when linear motion was required? why use a 'rotational' motor in combination with camshaft (or equivalent) to convert the rotational motion to linear? if you wanted to change from a rotational motor to a linear motor (when linear motion is required) is it as simple as simply swapping them out and changing the 'drive' shafts? thanks.

 

[Averagesupernova]

Do you have a better proposal to move several hundred pounds in a linear fashion? Linear motors are typically used where hydraulic cylinders would be used except without the hydraulics (naturally). I don't see what is so strange about using a conventional motor to turn a leadscrew. Through gearing all the mechanical advantages work out to what we desire in the end result with torque and speed.
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Typically changing from rotational to linear isn't something the occurs on a regular basis. One is picked at design time and that is the way it stays for the life of the machine.

 

[Dr.D]

Very large linear motors are being developed to launch aircraft off carriers to replace steam powered catapults in the "all electric navy." These will be huge machines, roughly 100 yds long, and deliver 100 of thousands of pounds force. They will also draw immense amounts of power and require extemely complex power electronics to operate them. They are proving to be extremely costly, heavy, and difficult to build. There is a serious question as to whether they will ever be placed in service.

Both linear synchronous and linear induction machines have been investigated. Each has advantages and disadvantages.

One of the fundamental disadvantages with the linear machine is that it is necessarily a pulsed operation. It has a finite stroke, in contrast to the unlimited rotation of a conventional rotating machine. This means that the power draw for a linear machine will necessarily be a pulse, and if it is a big machine, this can be a problem in any setting. It simply cannot operate in steady state.

There are also some rather non-ideal end effects near the end of the stroke that make it less than simply "unrolling" a rotating machine.

 

[dlgoff]

Linear motors can be used for http://en.wikipedia.org/wiki/Linear_motor" .
Welcome to PF.

 

[Dr.D]

Linear machines are also being used for driving various rides at Six Flags, etc.

 

[MATLABdude]

I think there's a little bit of confusion here... There's a linear motor (what's used in a Maglev or in the proposed aircraft carrier launchers), and then there's a linear actuator (which usually uses a rotational motor along with a lead screw or rack and pinion or something similar to translate rotational to linear motion) which usually has a very limited range of motion.

http://en.wikipedia.org/wiki/Linear_motor
http://en.wikipedia.org/wiki/Linear_actuator

As to why you would use a linear motor rather than an electric motor, I'd say that (in a low acceleration) system, you can levitate, and not have any contact or rolling resistance, thus allowing you to increase efficiency, and make a quieter system (you still have to contend with air friction however).

In a high acceleration system, you wouldn't need to carry the fuel / reaction mass needed to accelerate up to your (high) working speed (or any at all, in the case of, say, a mass driver or other similar projectile launcher).

 

[studentoftheg]

Thanks for the replies, I'm having some trouble determing the main differences and abilities of linear synchronous and linear induction motors. The only examples I can seem to find online are discussing rollercoaster rides, I am thinking more of say a mechanical drive system with fixed magnets operating outside a moving shaft, which would this be? Is the only difference whether the current is supplied to the rotor or stator? Where in a linear motor the rotor is a moving platofrm (secondary)? Apologies if I'm getting mixed up here...wikipedia talks of low acceleration and high acceleration which is confusing as it states high acceleration is for rollercoasters and propelling something to high speed and releasing- and it says this is example of LIM, however I would of though that LIM suits my example for, say, moving long distance at constant speed?

 

[mgb_phys]

Linear induction motors normally means those used on trains/rollercoasters with windings on the train and a metal plate on the track. The advantage over regular driven wheels is simplicity - no moving parts or motors/gearboxes, no wheel slip so it works with leaves/snow on the track, less wear on the wheels/lighter wheel track design and potentially faster acceleration. The subway here uses them - although not for a new extention.

Linear actuator motors use magnets inside the moving 'track' this means more power for a smaller winding and you can use the varying magnetic field of the row of fixed magnets as a position sensor to give feedback control.
main drawback I can think of is that compared to a lead screw you don't have any holding torque - you need to use power to hold the actuator still.

 

[studentoftheg]

Thanks, so what would one of these be?

http://www.copleycontrols.com/Motion/Products/Motors/stb.html

Also, for matching up requirements, say you had an electric motor with a torque of 10Nm. Could you divide this by the length of the lead screw (or whatever type of shaft your using to convert rotational to linear) to get the required output force for a linear motor (as shown in tables contained in above link)?

 

[mgb_phys]

I'm guessing those are 'magnets in the shaft' type.
Torque for a leadscrew depends on the thread angle (from memory)

edit. Wiki has a good description http://en.wikipedia.org/wiki/Leadscrew

 

[Dr.D]

Things seem to be getting pretty tangled up in this discussion with the introduction of lead scews and conventional motor drives. This has nothing at all to do with linear motors; lead screw systems are linear actuators of a totally different class.

For a LIM, the forces is proportional to the slip in the airgap. For a LSM, the position of the armature is magnetically locked to that of the stator, so that the force is whatever it has to be (up to the breakdown level) in order to preserve this phase relationship. Exactly the same thing is true in a rotating machine.

 

[dlgoff]

Things seem to be getting pretty tangled up in this discussion ...

Thought I would tangle it up a little more.

Piezoelectric motors make use of the converse piezoelectric effect whereby the material produces acoustic or ultrasonic vibrations in order to produce a linear or rotary motion.

http://en.wikipedia.org/wiki/Piezoelectric_motor"

Regards