List, I am trying to come up with a simple way of determining if a 24VDC motor has stalled. Each PLC system will have 8 motors. The motor's stall rating is 2.5 amps. I would prefer to input the amperage into a 4-20mA input. I could also use a circuit breaker and a DC Discrete input (when the input isn't ON, the breaker must have tripped). Does anyone have a better idea? Thank you, Frank Powell

 

Use a 50mv ammeter shunt tied to a Phoenix Contects Isolator. I use Model MCR-C-UI/UI-DC1. It has a 0 to 60 mv input range (plus several others) and a 4 to 20 ma output (plus several others). The input and output are electrically isolated so the shunt doesn't necessarily have to be in a return (grounded) leg. It works very well and seems to be bullet proof! David McConnell Controls Engineer Boeing/Rocketdyne Stennis Space Center, MS

 

You won't get better advice than already offered. You may want to check Moore Industries for similar equipment. They have a trip unit that will give you a discrete input if you prefer not to do the logic in your PLC.

 

If you something on motor shaft to sense motion such a proximity sensor then have an input making and breaking, then have a plc program wrote to examine output on to motor. Then set-up two timers one with prox input xic the other xio , if timers trip done bit in either prox remaining in on or off position and output on then set stall bit

 

I am familiar with the device listed as well as other products from Phoenix Contact. That solution will work just fine. try their website www.phoenixcon.com

 

A good idea might be to use a proximity switch that is activated by a bolt on the shaft. If the motor is stalled, the prox stayes active or unactive for a certain time. You could also determine broken drives or belts that way. We use this system to check stalled conveyor belts in a curing oven. Regards, Meeuwis Bouw, Superfos Packaging

 

Can you use a proximity switch to detect motion. Use the switch to reset or reload a timer in the PLC. You will need something for the prox to pick up on. What are the RPM's of the motor? You need to be aware of the PLC input response time. I recently did this with a SLC500 and a 900 RPM motor. 1800 RPM would probably still work, but anything over that would require some careful design. Bill Sturm

 

Thanks for the advice. Unfortunately, the motor is in cramped quarters and I can't add a sensor. I am going to measure voltage across a shunt.

 

The technique you describe sounds nice and simple but dont forget to put some back EMF protection, surpression across the motor and at the PLC input. The back EMF generated by the hight current being shut down in the inductive load is likely to be pretty hard the PLC input. Gerald Beaudoin

 

>...something on motor shaft to sense motion such a proximity sensor...< I would love to use a zero speed indicator on the motor shaft, but things are cramped there and I can't fit a sensor in there. Thanks for the suggestions. Frank Powell