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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 

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" 
    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

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