CHEAP DC MOTOR - URGENT HELP !

[Mark Kinsler]

>Can some one suggest a small d.c motor that works at 200 to 1000 rpm
>which can be directly driven with a dc power supply of 0-20 volts
>(or lower voltages) with 500ma or less current ? I need this information
>immediately. If you can help, please reply by email and I am not
>
>Precisely what I need is a motor that runs at a constant speed of 225 rpm
>without trouble for long hours with input voltage from a dc power supply
>source that is available in ordinary laboratory.

If you need a very precise speed you're going to need a feedback
circuit. Bearing and brush loads can vary, as can power supply
voltages. 225 rpm is pretty slow for precise speed control under any
circumstances: your voltage would have to be awfully to get that speed
and thus things would be that much less stable. My first suggestion
would be to use a stepping motor in place of your dc motor. These are
quite suitable to low rotational speeds and their speed is regulated
precisely by the driver circuitry. Otherwise, use your Edmund motor and
photocell circuit, but feed back the output of the photocell circuit to
the power supply to keep the speed constant.
Or: Use a belt and pulley arrangement to gear down the speed of a dc
tape recorder motor. These have an electronic speed control built
into them, but they run at around 1200 rpm. MCM Electronics sells these,
1 800 434 6959. There's nothing you can't do with a Mabuchi cassette
motor.
All Electronics Corp, 1 800 826 5432 has a good selection of motors,
both dc and stepper.
M Kinsler

[SARADA]

Hi

Can some one suggest a small d.c motor that works at 200 to 1000 rpm
which can be directly driven with a dc power supply of 0-20 volts
(or lower voltages) with 500ma or less current ? I need this information
immediately. If you can help, please reply by email and I am not

a regular reader of this group. Thanks in advance. I have the edmund
scientific motor that runs irregularly at 2 volts and increases in speed
- God knows why ! I have the photo transistor and other circuit to test
the speed and see the waveform on oscilloscope after a rotating wheel is
attached to the motor shaft. If you can give the manufacturer and tel #
that would be great. Thanks again. Waiting.

Precisely what I need is a motor that runs at a constant speed of 225 rpm
without trouble for long hours with input voltage from a dc power supply
source that is available in ordinary laboratory.

----
Reputation is what people think of you. Character is what you are in fact.
- Swami Sivananda

[Sam Goldwasser]

Without knowing your application, it is hard to make a specific recommendation.
However, one thing is for sure - a basic DC motor PM motor will not hold a very
constant speed. It is very sensitive to load so that minor variations in
bearing friction and other factors will change the speed. If you really
want a constant speed, then an AC synchronous motor may be a better choice.
However, for this to be easy, your desired speed must be a multiple of a
rational number and your powerline frequency since speed is determined by

(((power line freq) * 2 /#poles) * (mechanical gear ratio))*60.

in rpm.

So, for a 60 Hz powerline, a 2 pole motor with a 16:1 speed reducer will work.

Alternatively, you can drive a synchronous or even maybe a stepper motor
from a suitably stable freqeuncy source.

Or, you could phase lock the output of an optical tachometer to a stable
reference and control your DC motor supply voltage.

Or, you could use a motor such as the type used in an audio cassette
player which has a built in electronic or mechanical regulator.

--- sam

[zwge...@gmail.com]

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[jurb...@gmail.com]

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