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Old 09-15-2012, 09:25 PM   #1
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rookie in need of help ,i have a electrical question


hello all new to this site and electrical,heres my problem i have built a turn table to display my cuctom bicycle .im using a variable speed motor from a scooter its 24vdc rated current 14A output 280W and i want to run it with a 12v 500 cca auto battery i tried to use two resistors20w 8 ohms j txh 10.14 almost the right speed but resistors just got really hot in a minute of running and this thing will run up to 7 hours at a show.so wut i want to know is there a speed control switch i can use(like a dimmer switch)?and like title says rookie lol, any if some help or imput is much appreciated.

Thanks
Mike W.

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Old 09-15-2012, 10:43 PM   #2
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Welcome to the forums, an electrician will be along shortly. Please only post same question once, here. Thank you, Gary

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Old 09-15-2012, 10:43 PM   #3
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Why use any resistors at all?
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Old 09-15-2012, 10:49 PM   #4
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i thought it would work but thats why i wanted to eliminate them and get a speed control switch im also not sure if a home dimmer switch would work?because the motor is dc

Last edited by lilmikew86; 09-15-2012 at 10:51 PM.
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Old 09-15-2012, 11:00 PM   #5
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A standard household dimmer will not work in DC applications. It operates by cutting out portions of the AC sine wave, thus reducing the average voltage without reducing the peak voltage.

You'd burn `em up on DC ...
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Old 09-15-2012, 11:02 PM   #6
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Quote:
Originally Posted by lilmikew86 View Post
i thought it would work but thats why i wanted to eliminate them and get a speed control switch im also not sure if a home dimmer switch would work?because the motor is dc

Do not use the convential home dimmer switch to control any DC motour at all it will not work at all.

There is few specfic DC motour controller you can use but the motour you have I know most will be wired in PM ( pera magenet ) or series connection so the two are most common type of DC motor in that size.

There are few type of motour controller on market some will use the PWM ( pulse width moduation ) or variable restitor one of the two most common but PWM is more common due you can able fine tune the motour speed.

Merci,
Marc
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Old 09-16-2012, 12:30 AM   #7
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ok so then this would work right?


12V-40V 10A 13khz Pulse Width Modulation PWM DC Motor Speed Control Switch

I think I am going to purchase this but is it the right one ?
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Old 09-17-2012, 04:48 AM   #8
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so do i need any thing else besides the PWM?
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Old 09-17-2012, 06:14 AM   #9
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Resistors need to be rated (in watts) for the amount of power dissipated within them, which is not the same as what the motor or light draws. Electrical engineering know how is needed to calculate the wattage needed.

In terms of resistance, the value for the resistor depends on the current draw of the motor or light which also varies as the voltage is reduced for the purpose of speed control or dimming.

Resistors work fine for direct current motors (some limits because undervoltage can make the motor overheat depending on workload) or incandescent lights but they are very energy inefficient.
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Old 09-17-2012, 12:13 PM   #10
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thanks but im trying to get away from the resistors id like to be able to control the speed of the motor slowing and speeding it up
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Old 09-20-2012, 08:05 PM   #11
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does the scooter have a speed controller you could use?

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