Alt-BEAM Archive

Message #01618



To: Benjamin Edward Hitchcock beh01@uow.edu.au
From: Steven Bolt sbolt@xs4all.nl
Date: Fri, 19 Mar 1999 07:45:18 +0100 (CET)
Subject: [alt-beam] Re: caps


On Fri, 19 Mar 1999, Benjamin Edward Hitchcock wrote:

> > 1. lets say i have two 2.5V, 1F caps hooked up in series,
---8<---
>
> The energy contained in the first setup = 0.5 * 5^2
> = 0.5 * 25
> = 12.5 Joules

With double the internal resistance.

> > 2. if i hook them up in parallel
---8<---

> The energy contained in the second setup = 2 * 2.5^2
> = 2 * 6.25
> = 12.5 Joules.

With half the internal resistance.

> > 3. if i have a motor that requires 100mA and i hook it up to a large cap
> > how much current will it allow the motor to take? is there a
> > general rule that determines this?
>
> At startup, the motor will take a fair bit of current. You can measure
> this by holding the shaft still, and measuring the current drawn by the
> motor.
> Or you can divide the capacitor volts by the resistance of the motor when
> it's not turning.

Power transferred to the motor is reduced by the internal
resistance of the cap(s), which tends to be significant for very
large capacities. The connection also plays a part; if it is a
transistor, there will be a voltage drop over collector and
emitter. Very small if the transistor is suitable, quite large if
it isn't, or doesn't get enough base current to saturate.

> When the motor is turning, it will actually generate a small voltage which
> opposes the voltage you apply. So when the motor is running, the
> effective voltage across the armature decreases so the current decreases
> as well. Normally pager motors will pull up to 250 mA on startup, and
> this will decrease to about 30 mA once it gets up to speed.

The startup current depends on the voltage used to start the motor.
For typical pager motors, count about 100mA per volt. Running
current depends on voltage, mechanical load and rpms. A good
mechanical match makes a large difference for the efficiency of the
electrical charge to motion conversion.

Best,

Steve

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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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