[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

RE: [TCML] Terry filters (speaker/motor load modeling)

Hi Phil:

It sounds like you have a far better understanding of motor theory than I do (which truthfully is very little); perhaps you can point out where my reasoning leaves the tracks.

My thinking is that in a loaded motor, the lion's share of the power consumed is delivered to the mechanical load.  In a simulation model, this power must leave the model, and the only mechanism to do this is to burn off that power in a resistor to represent the mechanical load.  If the correct model for a motor is predominantly inductive as you suggest, I don't see where the power delivered to the mechanical load is accounted for, as the power dissipated in an inductor is zero.

I agree that an unloaded motor is best modeled as inductive, but that adding a load must add resistive loading.

Thanks, Gary Lau

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of FIFTYGUY@xxxxxxx
> Sent: Wednesday, January 02, 2008 6:06 PM
> To: tesla@xxxxxxxxxx
> Subject: Re: [TCML] Terry filters (speaker/motor load modeling)
> In a message dated 1/2/08 10:12:59 A.M. Eastern Standard Time,
> Gary.Lau@xxxxxx writes:
> >A superconducting motor under a 1 HP load, even though it measures  0 DC
> Ohms and some
> >significant inductance under static (no load) conditions, must be  modeled
> as predominantly resistive
> >to reflect the 1 HP load.  The resistor value would change  depending on the
> magnitude of the
> >mechanical load.  If there were no load, after the rotor  accelerates, the
> resistive component would go
> >away.
> A superconducting motor would be modeled as  predominately *inductance*.
>     A typical squirrel-cage AC induction motor has a  "magnetizing current"
> that it will always carry, at absolutely no load. This  current is
> significantly lower than the DC resistance of the windings would  allow, since a
> lightly-loaded motor is an inductive load. As the load on the  motor increases, the
> current in the rotor increases because the load torque is  causing an increase in
> "slip" between the rotating magnetic field from the  stator windings and the
> actual speed of the rotor. Hence, the magnetic lines of  force cut across the
> rotor faster and more frequently, thus increasing rotor  current. This is
> reflected in the stator current (motors are like  transformers!).
>     Also like transformers, motors have a high inrush  current, especially
> when loaded.
>     I suspect motors might behave even more inductively  than expected, since
> the physical gap between the rotor and stator might cause a  large "leakage"
> inductance?
>     One way to increase the power factor in a facility  is to make sure all
> the motors are running near peak load...
>     FWIW, I'd consider a woofer more of a  permanent-magnet DC motor. And I
> forgot to mention that another factor in woofer  design involves keeping the
> coil over the magnet core. Not only does the coil  need to be over the magnet to
> shed heat to it, but it acts like a "slide-choke"  as it moves off the core.
> Fidelity suffers as this occurs, but you need to move  the voice coil a longer
> distance as frequency lowers to put out the same amount  of sound power.
> Another tradeoff...
>     What could be more Tesla-topical than AC induction  motors?
> -Phil LaBudde
> Center for the Advanced Study of Ballistic  Improbabilities

Tesla mailing list