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Looking for some ideas for DC motor control

Greetings All, I'm working on a device to pull wire for welding. The mechanical part is done and now I'm trying to figure out how to do the motor speed control. I thought that a PWM speed control would be best because the motor torque will be close to constant. However, there are only two wires available to turn on the welder, power the motor and turn the motor on. this is because there two wires to the wirefeed gun. These wires are meant to turn on the welder. My idea is to instead use these wires as power wires for the motor. Once power is flowing I could also put a relay coil in series with the motor power wires with the relay doing what the switch in the handle is supposed to do. I can turn on the PWM motor speed controller at the welder. Then the switch in the gun is used to complete the circuit from the PWM to the motor. But since the current will be pulsing I can't see how it could also be used to power the coil in the relay. I'm afraid that if I can get the relay to operate at the lowest average current being delivered to the motor then it will overheat when the motor is being supplied with the highest current. I also don't know if the PWM speed controller will like having the load switched on and off or if the relay coil in series will cause a problem. Any Ideas? Thanks, Eric

Reply to
Eric R Snow
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I am thouroughly confused by your description of the problem. If you are just trying to "steal" the wattage from the welder line, you could use this for the relay as well as the PWM. The output of the PWM does not really interfere with the power source it is using, please explain, or perhaps offer a schematic.

Reply to
Mr. J D

I often have trouble making myself clear. I guess the ability to get a point across clearly is what makes great authors. So I'll try again. The welder cable terminates in the "gun". The welder cable is actually a braided copper tube, like the sheild in some types of coax. Inside the tube is a liner that the welding wire passes through. A shielding gas is also directed through this copper tube. A pair of insulated wires run on the outside of the braided copper. And a rubber insulating sheath surrounds it all. Where the welding cable terminates inside the the gun the two insulated wires are brought out and connect to a trigger switch in the gun. So the braided copper carries the welding current with the welding circuit being completed by a groung cable from the welding power supply. The two wires connected to the trigger switch only turns on the welding current and the drive motor which pushes the welding wire through the welding cable. I want to add a motor to the gun to help pull soft aluminum welding wire through the welding cable. And I am hoping to be able to use the two wires already there to power the pull drive motor. These wire would be disconnected from the welder and connected to the PWM motor power source and a relay. The relay takes the place of the trigger switch. So I will not be "stealing wattage" from the welder power supply. The motor will have it's own power supply which will powered by the 120 volt mains. Thanks, Eric

Reply to
Eric R Snow

Piece of cake - I think. :-)

Use a current sense resistor tied to a comparator. When the trigger is pressed, current flows in the resistor creating a small voltage drop, which the comparator senses and operates the relay. The comparator circuit can be installed at the welder. Put your PWM controller in or on the gun where you mount the feed motor. The trigger will connect the

+12 to the PWM circuit as shown below. The comparator diagram is below this one.

View in Courier font

  • 12 --+ | [1R] | +------> To comparator Vin | o \ < Trigger o | +------------------------------------------+ | | [D1] | | | +------+---+----------+ +-----+ | | | | | | | | | | | | | [1K] | ---------- [D3] [Motor] | | | | 8 | a| | | | +-----|4 | | | | / | | +-----+ | 25K \
Reply to
ehsjr

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