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Old 03-15-2013, 11:22 AM   #1
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24vdc Motor speed control - Pot. Question


Hello all,

I'm recently the proud new owner of a 24vdc motor ( 24v / 68.9w / 2.87A full load). I bought a heavy duty NEMA-rated potentiometer to control the speed of this motor. The resistance of the pot. is 10k ohms (I think this is where I went wrong).

The motor doesn't start to turn until the pot. is 7/8 turned and from 7/8 to 1 is all the variable control I seem to get.

I also tried rigging up a pot. from a guitar, but no luck either . I have no idea what the specs were on this one, but I tried it figuring the the resistance of the first one was too high. This pot probably wasn't rated for the load.

So before I go buying a $300 speed controller from McMaster, I'm wondering if I can just buy another pot. with a different resistance? (which is much cheaper)

Also, I prefer not to buy from ebay because this is for an invention and I need some kind of availability/consistency promise. But I'll take any pointing in the right direction.

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Old 03-15-2013, 12:23 PM   #2
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24vdc Motor speed control - Pot. Question


You have two problems: resistance and power dissipation. Your need a variable resistor of about 20-50 ohms probably. It depends on the actual running current of the motor in your application, and the range of control you want. Your resistor needs to handle about 30W minimum. That is a very large rheostat. They exist, but are antiquated and expensive. This method of control is obsolete and extremely inefficient. Nowadays DC motors are controlled with PWM electronic speed controls, and that's what you should be looking for. Something like this:
http://www.suntekstore.com/goods-140...ontroller.html

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Old 03-15-2013, 01:13 PM   #3
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24vdc Motor speed control - Pot. Question


Thanks,

I think I found a solution, and it's not very expensive.

http://www.hobbyengineering.com/H2246.html
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Old 03-16-2013, 04:57 AM   #4
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24vdc Motor speed control - Pot. Question


The kit you linked to is not capable of running on a 24V supply. You could use a 24V supply for the motor with a separate lower voltage supply for the controller, but that's annoying. I'd look for a solution that can run entirely from the motor supply voltage. Something based on a TL494 would be good, since that IC is rated for up to 40V Vcc, and is better suited to this application than a 555 anyway.
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Old 03-16-2013, 10:13 AM   #5
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24vdc Motor speed control - Pot. Question


Proud would be if you had a 90 or 180 volt DC motor. Then a $100.00 controller and a 10k pot would have you in business.
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Old 03-16-2013, 12:09 PM   #6
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24vdc Motor speed control - Pot. Question


Luckily I realized the kit wasn't going to be enough 1 hour after I placed the order, and I cantacted them fast enough to change the item number from H314983 to H01742-01Z

http://www.hobbyengineering.com/H1742.html
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Old 03-17-2013, 03:00 PM   #7
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24vdc Motor speed control - Pot. Question


That's a full H-bridge topology, which is completely unnecessary if you are not reversing the motor. It has four switching devices instead of the single one your application needs. If this is a one-off project then that's fine, but if you intend for this to be produced in some quantity then you'll want a simple and inexpensive solution that's not excessively complex and expensive for the application.

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