Alt-BEAM Archive
Message #03866
To: Wouter Brok w.j.m.brok@stud.tue.nl
From: "Ben Hitchcock" beh01@uow.edu.au
Date: Sat, 29 May 1999 00:37:35 +0000
Subject: [alt-beam] Re: Fast Bicore, not wasting energy.
oops!
I just did some experiments, which made me change my theory. As I increased
the frequency, the current did go down... but so did the torque output of
the motor.
Seems obvious, doesn't it? Torque is proportional to current, and if you
decrease current, then...
And since the internal resistor is in series with the internal ideal
inductor, then the only way to get more torque is to increase current, which
increases the power lost in the resistor.
Damn! One of these days I'll prove Wilf wrong! I might have to wait until
he's old and senile, but I'll be watching and waiting to pounce.. :-)
I've just about given up on proving Steven wrong, though.
Ben
>Ben, thank you !!!
>
>This is the kind of answer I like.
>I've been thinking about an avaraging-circuit like you think Wilf can dream
>up (I'm sure that the moment he will see the question he has it in his mind
>already), but hoped that wouldn't be necessary. Actually I hoped that there
>would be some optimum frequency for a specific circuit-motor-configuration,
>but what you show is better.
>
>About the block-wave ... you can think it being build out of one sine with
>the block-frequency and a lot of higher-frequency sines to form the edges
>of the blocks (just plain Fourier analasys), so in your explanation those
>higher frequency-sines do not matter, since the higher the frequency the
>better.
>
>Regards,
>
>Wouter Brok.
>
>PS: I do have to give it a bit more thought (slow as I am in those things),
>but your I'm sure you are right and that's the best answer I could get,
>since it is always possible (to a certain extend of course) to increase the
>frequency.
>
>
>>It would depend on the motor involved. If the motor was mostly inductive, I
>>don't think you'd waste much energy at all.
>>
>>Impedance = Ls
>>Where L is the inductance, and s is the laplace transform of the input.
>>
>>For a sinusoidal input (Yes, I know you have a square wave input, but sine
>>waves are easier)
>>s = jw where w (omega) is the frequency in radians per second, and j is the
>>square root of minus one.
>>
>>So the impedance turns out to be fully complex, which means it gives back
>>all the power that you put into it, just 90 degrees out of phase with the
>>input.
>>
>>If the load had some resistance as well, then that changes things.
>>
>>Instead of the impedance being fully inductive, we change the equation thus:
>>
>>Impedance = Ls + R
>>
>>R being resistance.
>>Now here an interesting thing happens. As we increase the frequency (w),
>>the inductive part dominates the impedance more and more, until we have a
>>huge (complex) impedance that is close to 90 degrees out of phase with
>>voltage.
>>
>>Impedance = Ls + R, s being huge, R being the same no matter what frequency
>>you use.
>>
>>So here the L still dominates R.
>>
>>At low frequencies, however, L's contribution is tiny - so the motor's
>>resistance is the dominant factor.
>>
>>The moral of the story is that if you are worried about power consumption,
>>increase your frequency. The inductor will take care of the rest.
>>
>>
>>I suppose Wilf could dream up some sort of circuit, where the output of the
>>oscillator goes through a resistor to a capacitor to ground, and the
>>junction of the resistor and cap follows the 'mean' value of the oscillator.
>> Then you could have two op-amps acting as comparators, with different
>>switch-on levels so that the head would have a 'deadband' in the middle.
>>
>>I think it's easier not to worry too much.
>>
>>Ben
>>
>>>Hello List,
>>>
>>>I've a question for you, of which the answer probably is quite simple and
>>>already known to you, so then I don't have to think about it (pretty lazy
>he).
>>>
>>>Suppose one makes a suspended bicore with quite a fast period of
>>>oscillation (say one kiloHertz) and connects a motor to its outputs.
>>>Instead of the resistor the circuit has two phodiodes and thus has a
>>>duty-cycle dependent on the difference in light falling on those
>>>photodiodes. This basicly is the circuit which has been used by quite a few
>>>of you to make a head point towards the light.
>>>
>>>The problem is that there is an alternating current flowing through the
>>>motor, which, most of the time, doesn't result in motion (except when the
>>>dut-cycle differs from 50%), but is just dissipated, which I find a
>>>terrible waste (I don't need the warmth resulting since I have other
>>>machines which make my room warm for me). What can be done against this?
>>>For example, does it help to put a capacitor across the motor-inputs, or
>>>capacitors between the outputs of the circuit and the inputs of the motor
>>>... or are there ways to store the energy and use it again?
>>>
>>>How do you guys do that ... I mean, loads of you built heads and must have
>>>thought about the problem and solved it (and I don't feel like inventing
>>>the wheele for the second time).
>>>
>>>I'm curious for any reply !!!
>>>
>>>Wouter Brok
>>>
>>>
>>>
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>>
>>
>
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