Does anyone know how a zero speed monitor works? I know that if the windings of a motor get hot then it will trip out. Does the zero speed monitor say " I'm almost up to speed so don't trip?" any information would be greatly appreciated.

Thanks

 

Generally, a zero speed monitor is just a timer, a one-shot, a monostable multivibrator. It is set to output for say, 1 second. It is retriggerable, so that if it recieves a pulse at least every second it stays on. If it doesn't see a pulse it times out and sets an alarm. It is used with some sort of sensor or switch that is triggered by rotation. When the rotation stops, the timer times out and the alarm is set. In the case of the windings getting hot, either a thermal switch is in series with the timer signal or the overheat is stopping the motor which trips the zero speed. And as you thought, the timer implies that there is some minimum speed that will keep the timer on.

Regards
cww

 

There are several different ways for this to work. Generally, on an AC induction motor with no feedback, a sensor would be added that simply triggers on a revolution. A simple micro-controller makes sure that the sensor is "flickering" fast enough. Obviously, any motor with feedback already has this information, so the extra sensor isn't needed.

Another method is to watch the back-EMF of the motor. Honestly, I don't know much about how that works, but the guys who build motors for a living sure do.

There are probably other methods, each with their own trade-offs.

-James Ingraham
Sage Automation, Inc.

 

Thanks for the replies. So if a motor is trying to turn a load and it's taking a while to get going and the windings are heating up, the zero speed monitor will hold the trip until it gets going and if it doesn't the motor will trip out on winding temperature is that correct?

Thanks

 

Not really, you infer too much.

In a regular motor "bucket" starter arrangement the overload will hold for a while on start. If the current doesn't decrease by the time the overload heaters get hot, the overload trips. Any other logic would be application specific. There are also electronic overload units which perform the same function without the heaters by measuring current vs time and shutting things down if the current doesn't decrease as it does as the motor comes up to speed, The zero speed doesn't necessarily have anything to do with this. I might point out that the great majority of overload trips I've encountered are not an electrical problem. The motor is actually overloaded mechanically.

If you try to get the motor started beyond it's available torque you stand a really good chance of seriously overheating the motor. I've had a case where the motor was replaced twice before I got there and the problem was a shredded conveyor belt rubbing where it couldn't be seen. And they still called it an electrical problem. This is just by way of saying that the trip could be telling you something you shouldn't ignore.

Regards
cww

 

Surprisingly ; )P I agree completely with Curt. It is surprising how people cannot grasp a motor circuit when staring at it from an HMI.

Example: Just because I point at a button (with a mouse or my finger or a physical button), transfers my mechanical pressure to a device. If successful this for example may send an electrical signal to an input in a Plc.

Then logic determines if it is ok to start the motor, this could include interlocks, such as "don't start the belt if the downstream conveyor is not running", or "don't start pump because tank downstream is full".

Then we send an output to the motor bucket which pulls in a relay which must mechanically move. Which the closes contacts that allow current to flow to the motor. This is if things like estops, thermal overloads etc. are ok in the circuit this relay is in, and if the control power is on, and if the motor power is on.

Then we take a set of contacts off this starter and bring them back to the Plc to tell us if the relay picked up (motor running, we think). This is usually tied into fault logic in the Plc "we told it to start but it didn't or we aren't telling it to run and the starter is in". This then goes to the HMI to show color change for running or not, or faulted or not.

Now none of this tactile feedback is really telling us the motor is running, only that the circuit to and through the starter is ok. Belts not gone, shaft not broken. This works if the wire hasn't fallen off the button, the button isn't shot, the HMI code is incorrect, the input card is good, the code is correct and the Plc is working, the output card works, the bucket power is on, the starter works, it's contacts are good, the overloads are ok, the wire back to the Plc is good, the the input is good and power is on, the code is written right , the HMI works and is programmed correctly and on and on........

Now by adding a zero speed switch, I actually know if the device (conveyor belt etc) actually IS moving or not, and not that the starter relay is working. It pulses on movement of an actual part past some point and if set up fail safe lets us know when we tell it to run, that it is moving.

I like Curt have found that usually a motor "kicking out" is usually a mechanical bind etc. but by doing it properly, the HMI tells you that is is a motor fault or a zero speed fault or an overload etc. to help isolate that it was a motor or a physical or an electrical fault.

Wow Curt, we agree.

Dave Ferguson

Sent from my iPad

 

I've often had the same plant blame game - "it's the electrical/control/program problem", even if proven that it is a mechanical issue. Some gray area issues like a photoeye miss aimed due to something running into it is still a controls issue.

When something is not understood (and we work in an area that most take by faith), we are the problem regardless of the root cause.

Early in my career I had a plant person refer to me as "magic fingers" since I often got what he wanted implemented in logic. It just is an example of the image in our field of it being some sort of "magic". Training and education can prevent that but that is expensive and a lot of work so it is rarely done in industry. Please keep asking questions so you understand what is happening and can take advantage of what you learn. As long as you have put in the effort, most of us in the field are happy to expand your understanding.

Russ K.

 

I've generalized that to "Whatever part is least understood _must_ be the problem". When you are part of a team building or working on a big machine, there's lots of finger pointing:^). When the plant manager asked me why I was carrying a huge pipe wrench down the aisle, I told him I was fixing a PLC problem from the last shift.

Regards
cww

 

Zero speed monitors will be different depending on the application - a couple from mining:

1/ Motor drives the head pulley of a conveyor thru belt drive or gearbox. On the tail pulley is a metal target sensed by a proximity switch.

Two timers are used, one for switch open, one for switch closed. If either timer times out this indicates the belt has stopped for any reason so it trips the motor and any other conveyors feeding onto this one.

2/ Motor drives the pulley of a crusher - this pulley is too fast for method 1 so a sensor is set up to detect the pulley spokes as the whip past, this gives a fairly high frequency pulse. The pulse is sent to a switch set to trigger if the frequency drops too low.