Which sensors are used to specify that Pumps and Motors are on or off ?

I mean we cannot depend on the contactor that let the pump/motor on or off because the motor/pump may fail and the contactor is still working !

so how can the PLC/scada specify that the motor or the pump is on or off ?

i hope iam clear enough !

thanks or the replies

1) Use a motor current switch that trips a contact above a minimum amp reading.

2) Use a flow or pressure switch in the discharge line.

3) use a motion detector/proximity switch to sense rotation at the shaft key.

By measuring on the actuated variable.

E-motor: Impulse transmitter on the drive shaft.
Belt conveyor: Impulse transmitter on the tightening pulley.
Pump: Pressure trasnmitter on the outlet side of the pump.
etc..

One that is difficult to measure: The vibrators on a vibrating conveyor. I use a 3-phase monitoring relay for this (one for each motor). It will also detect if a wire has come loose in the terminal box on the motor (not unlikely to happen).

There are several thing you could do but if you want to use
pump to move something, I think by far the best solution is
to have a flowmeter.
Now I know that this is not exactly the most cost effective
solution but to help you more you should provide more detail
on the actual application.
For example:
If you are using pump to maintain level of a tank for example,
you could use pressure sw. or anything else to check the level
of the tank.

check for what the contactor should do.
so if pump check for flow not pressure
if belt check for speed of belt
if car check to see if car is having accident.
i use a lot of inputs to make possible the type of mishap and the alarm.

so if pump check for flow not pressure

Click to expand...

If the flow is blocked, how do you check that the pump is ON ?

Generally I aggree that it is better to check for flow than for pressure, but it depends on the application.

If the flow is blocked, how do you check that the pump is ON ?

Click to expand...

Yes, if nothing is moving in a hydraulic system, there should be no flow to the actuators. And, especially if the pump is a variable displacement (swashplate) pump it will "stroke off" like it is supposed to so as not to be making oil at pressure when it is not needed. Now if the pump is constant volume like a bent axis or gear pump there would then be flow over the relief but how much do you look for? When things are moving, you'll get most of the flow to the system and only a little or none over the relief and vice verca when things are not moving.

Now, maybe you have a hydraulic motor that runs all the time, then, yes, a flow switch or sensor would be great.

If you just have cylinders, rotary actuators and grippers then put switches on all the positions. Time each move and generate an alarm if the move doesn't occur in the expected amount of time (plus a little to avoid nuisance trips).

If you want to check for system leaks, put flow meters on the line to the actuators and the return from the actuators. Check that the flow out equals the flow back in (kind of like a GFCI in the electrical world). I suspect you'd have to play with the settings on this to make it work practically and not nuisance trip, especially if there is an accumulator in the system. You'd probably need timers on both sides (start of motion and end of motion) to account for the accumulator emptying and filling not to mention you couldn't look for an EXACT match on analog signals but just that they are close to one another due to the usual sensor and system built in errors.

Anyway, I think looking at the actual end effectors is best as that is the reason for the system in the first place. If you can wait until a move doesn't complete to flag an error then the position switch/timer method should work for you. If you need to know something is wrong right away you will need to put feedback devices on the moving elements and check those for motion. The type would depend on the motions you are making and the space and budget you have.

These are just some ideas off the top of my head with the limited information we have here. As always, give us more info... get more answer...

Current probes on the motor leads would be the most reliable if all you want to know is if the motor is running. And like was previously stated, you can check the response of the system.

In some instances you might be able to take a straight input off the control power with a relay input card to tell if the starter has power (if the control power is transformed off the same feed as the starter). That in conjunction with the feedback relay off the contactor would be pretty reliable.

Remember you can still have current flow if the motor is decoupled from the pump or running backwards. I was once called because the lube oil pressure switch had failed and the turbine would not start. They wanted me to bypass the switch to get the unit running. They said "see the pump is running". I look at the motor cover and noticed it was running backwards. Apparently an electrican was doing a PM on a starter and some how swapped two phases. So a minimum current threshold should be set or look for process result, such as pressure or flow which I think is a better indication that all is well.

Andy

Whats wrong with using auxillary contacts on the contactor? Isnt that just as reliable..overall? I will state using something to show the shaft is turning seems more reliable.

I think using the aux contacts are fine if it is not critical that the machine being driven by the motor would cause a failure if the driven device wasn't actually spinning. I use both the aux contact and the process result for any critical process. In that way I've verified that the starter is energized and if the resultant process does not occur then I know that something is wrong with the motor/unit such as a tripped overload, sheared coupling, broken belt, clogged fliter, loose pump impeller, etc. If I command the motor/unit to run and I get a process result and no aux contact feedback I know I have a possible problem with my process instrumentation. Of course in a real critical application one should check all the process instrumentation and verify that the devices are in the state that they should be prior to starting the motor. In other words, if the pump is not running but there is pressure, then something is wrong lock out the unit.

Andy

Ayman--

My experience is in water transportation. We pumped water from 6 different silos (25,000 Gals per) into a bottling facility using PID and automated setpoints. We used Flowmeters AND Pressure Transducers in our lines. You can, in certain applications, have pressure w/o flow or flow with little pressure. If you have a Centrifugal pump attached to the discharge piping of a tank, or is below the water level, and a line breaks, you can have flow with little pressure, or pressure upstream of the Transducer. Obviously, you can have a pressurized line with no flow when the water has no escape and the pump is running. We used both transducers since the likelyhood of both sensors failing at the same time is unlikely.

Is your process monitiored by human eyes or PLC/PC?

Rube