I have a DC motor that operates from 0-12V DC. It is being used for a rotary welding fixture so I need to be able to adjust the speed.
I have tried different voltage transformers and it runs at various speeds depending on voltage.
I am trying to figure out the easiest way to make an adjustable voltage transformer to use as a control. I tried a potentiometer I got from Radio Shack and fried it. :(
Since you already have step down transformers (and are apparently rectifing the AC to DC), you could buy a variac. A variac is a variable auto-transformer which allows you to vary AC voltage at the turn of a know. One source is McMaster-Carr.
Or from the same radio shack. a variable 0-12 VDC supply rated for the motor current. I would build one myself, but commercial supplies are fairly reasonable from RS or Jameco or the surplus houses. If you need speed regulation, then the motor controller is the way to go.
Depending on the amount of load a "pot" might not be your best choice. You could cheat and get a motor and speed control off of an old wire fed welder. But you need to know your current requirements, voltage is just the begining when it comes to speed control.
I assume its a standard DC controller with Brush commutation?
If so, buy a cheap DC motor controller with adjustable 0-10V command signal, and attach a potentiometer to it. There has got to be gobs of cheap controllers out there. Contact your favorite motion control vendor and tell them what you are looking for and I'm sure they will hook you up.
Short of that, Use a small cheap PLC that has PWM outputs and can drive your motor load. Look up PWM control on the web to find out more if you don't know already. Or if you need high volume design your own circuit with a PWM driver chip, etc.
Due to the nature of welding and the caution of some DC motor control people to avoid use with high capacitive discharge systems (welding), a compressed air motor, flow control, and regulator might do just fine.
What is the running voltage required for your motor and how much current does it need while running? Once it starts the back emf reduces the current, however at start up a motor has a very high start up current because the only thing between the battery terminals is the copper wire of the motor which would be a few hundredths or even tenths of an Ohm. And that is when you probably fried the pot. Also using a pot wastes power and if your motor requires a lot of current your pot heats up. There are so many ways to control the voltage. It depends on the amount of current your motor needs. You fried the pot because it had too small a power rating. If u have a large load then u need to control the current via something like a Buck regulator using a UC3842 IC, however instead of controlling voltage u only control the current. So u ground the voltage loop. This is a very efficient way to do it (approx 90%), however this is a complicated design.
A potentiometer is only useful for adjusting voltage if you can afford to burn up much more power in the pot than in the motor, which is why your pot fried.
The most efficient way to control voltage to a DC motor is to control the duty cycle of a full-voltage signal. Otherwise, get a DC power supply designed for voltage control. Don't try to design one yourself.
Embedded Systems Consulting
The discussion on controlling the speed of a dc motor has been interesting. If we assume the motor is a dc shunt motor, the speed will drop as load torque is added to the motor shaft for some fixed armature voltage. This has been referred to as the "droop" in motor speed with increasing load torque. It is also defined as the percent regulation of the motor which can be calculated as -
% regulation=((rated speed- loaded speed)/rated speed)x 100
Before the advent of servos, a series field could be added to the motor, known as a compound motor. For some fixed load torque, enough series field could be added to eliminate any drop in speed.
The best way to regulate dc motor speed today, is to include a velocity feedback with a servo amplifier. If anyone is interested. I have the proof of velocity regulation for dc or bldc motors. This document can be emailed.
George W. Younkin, PE, IEEE Fellow
Industrial Controls Consulting
A division of Bull's Eye Research, Inc.
N7614 HWY 149
Fond du Lac, WI 54935
Ph: 920: 929-6544
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On September 27, 2004, Scalliwag wrote:
> I have a DC motor that operates from 0-12V DC. It is being used for a rotary welding fixture so I need to be able to adjust the speed.
> I have tried different voltage transformers and it runs at various speeds depending on voltage.
> I am trying to figure out the easiest way to make an adjustable voltage transformer to use as a control. I tried a potentiometer I got from Radio Shack and fried it. :( <
( Complete thread: http://www.control.com/1026201180/index_html )
You don't say what the wattage or current draw of the motor is, but the usual way to do this is with a DC motor drive from Minarik, Bodine, KB Electronics, Extron, Baldor, etc. 12V is pretty low but you can get them. They'll take 120VAC in, put out DC, and have pot input terminals for speed control. Alternatively, if the motor is small enough, you can use an adjustable lab DC power supply like from Beckman, Xantrex, HP, or someone like that. It will come with knobs on the front and meters to
show output voltage and current.
Steve Myres, PE
Also, another way is that if you already have a fast enough control system, you can Pulse width modulate the motor (PWM). You need a transistor output coupled to a Solid State relay module (Phoenix contact has some really tiny ones that work well). If your scan isn't fast enough you will need special hardware or interrupt processing, etc.