Re: Looking for some ideas for DC motor control
- From: ehsjr <ehsjr@xxxxxxxxxxxxxxxx>
- Date: Mon, 28 Aug 2006 17:20:25 GMT
Eric R Snow wrote:
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
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 \<---+----|7 555 | |
| / | | | /
| | a| | 3|---[150R]---| TIP31
| [1K] [D2] | | \ e
| | | | | |
| +----+----|6 | |
| | | | | |
| | +----|2 5|----+ | Diodes: 1N4001
|+ | | | | |
[C1] [C2] ---------- [C3] |
|470 | .1uF | .01uF |
| uF | | |
Gnd ---+------+-------------------------+---------+
+ 12V ---+------+-----------------+------------+-----+
a| | | | |
[D1] | +-------------|-----+ [D2] [Relay]
| | | |\ | | a| |
| | | | \| [1meg] +-----+
Vin --------------------[1K]---|- \ | |
| | | | \ | c/
| / | |324 }---+---[100]---| NPN
|+ 5K \ | | / e\
[C1] /<--+--[10K]---|+ / |
|470 \ | /| |
|uF / |/ | |
| | | |
Gnd -----+------+-----------------+------------------+
For the comparator, adjust the pot so that the relay
actuates when the trigger is pressed.
I don't know what the contacts of the relay will carry.
You need to size the relay contacts to be able to
handle the same (or more) current & voltage than the
welder trigger switch. I also don't know if the voltage
that the welder puts in the gun will create transients that
upset the PWM.
Ed
.
- References:
- Looking for some ideas for DC motor control
- From: Eric R Snow
- Looking for some ideas for DC motor control
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