turn on and off an ac motor with a relay

[Surf_Dragon]

I would like to automate a Bomb Calorimeter. I need turn on a motor during 20 minutes and at 5 minutes activate the ignition.

I would like put a button to turn on the motor also an emergency stop. I'm using a scxi-1325 to send a signal to activate a relay but the problem is how I can send this signal for 20 minutes after I pressed the button and stop when finish and also add a emergency stop? How I can send another signal to activate the ignition when pass 5 minutes? I'm using labview 8.2

Thank you!!!

[RavensFan]

An SCXI-1325 is just a terminal block to mount on the front of another SCXI module. It says it is intended for a SCXI-1124 which is an isolated D/A module.  So I don't understand how you would be using either of these to activate a relay to turn on a motor.  You would need some module with a digital output line to drive a relay.

That said, if you need an emergency stop, wire a E-stop mushroom button inline with that digital out line.  It would be normally closed.  If you hit the button, it would latch open causing the relay to drop out and thus the motor to stop running no matter what is going on in the program.

Everything else in terms of timing the start of the motor relay and the ignition (exactly what are you doing there, is that another relay?) would all be handled in LV with loops and timer functions.

Message Edited by Ravens Fan on 04-25-2007 10:07 PM

[AnalogKid2DigitalMan]

For optimum safety, the normally closed ESTOP pushbutton should go on the contact side (not the coil side) of the relay to remove AC power from the motor.

That way, in case the relay contacts fail in a closed position, your ESTOP will still remove power to the motor.

[RavensFan]

Good point.  In my usual scenario, my digital out line drives the relay which has N/O contacts of the relay in series with the N/C E-stop.  When all contacts are closed (i.e. the digital out line is energizing the relay and the E-stop is NOT pressed) a 24V signal is passed through the wiring to the enable contact on our motor controllers.

It depends on the size of your motor.  If you have a fairly heavy duty motor with a lot of current, you would not want to pass its power through the E-stop button.  It also depends on how many relays you need to have.  If the the digital out line drives a small relay, and that drives something heavier like a motor contactor that has a 120V coil, you could have the E-stop in the 120V circuit (which would be safer than in the low voltage digital line, but certainly don't want it in the high current side of the motor contactor relay.