Motor Protection

Hi guys,

We recently had a rotary air lock jam up and it damaged the reducer, reducer base and keyway. It was a 3HP motor, 460V, 4.2A FLA, SF 1.15.

The overload setting was at 4.8A. The motor under regular load is running at 2.5A. I reduced the overload setting to 3.5A hoping this would cause an overload before damage next time it jams.

I have attached a picture of my available settings, are there any other settings I could change that would protect my equipment better in the event of another jam.

Thanks for any help.

Since the gearbox was damaged, you have to wonder if it is rated for 3 HP input.

How about installing a mechanical overload clutch with indicator?

The problem with what you are attempting here is that the "overload" function is an I2t algorithm, meaning it will not trip INSTANTLY at 4.8A (or 3.5A for that matter), it will still take time, and if the spike in current is really steep, the mechanical damage may be done by the time it reacts.

In the Simocode relay, there is a SEPARATE instantaneous trip setting, I can not longer remember exactly how it's worded, but it is a hard and fast value to which you can assign a finite delay time as well. So for example 10A trip, but only after a 100msec delay, so that motor starting current is ignored.

Check here for some special purpose current/load protection.

I use them on motors for jaws that close on a piece of equipment. The operators used to use the jaws to actually move the equipment, which damaged everything, but usually wouldn't trip the starter overload.

I wire these in series with the starter coil, and set them to just above working current. The instant a damaging overload appears, the starter drops out.

There are several versions, for generic motors, pumps, fans, etc... The work fantastically.

Why set the overload to 4.8A in the first place, when the motor FLA is 4.2A ?

Setting the overload to 3.5A will still mean it will run for sometime, even with a locked rotor. I am guessing something between 2-5 seconds.

The best solution is to install a clutch like already suggested.
If that is not possible, then the least expensive safety I can think of is a simple adjustable CT with digital output.
http://datasheet.octopart.com/EISNNOA50A-Carlo-Gavazzi-datasheet-44585.pdf
edit: Just noticed that I linked to the datasheet for an NPN relay. You probably need a PNP relay.

For the 20A model, simply loop the motor wire 5 times through to increase the sensitivity.
Wire the output signal into the PLC, to make it cut the motor instantly when the signal activates. You have to bypass this at startup.

As jraef points out, using the overload for instantaneous trip is a mistake. Overloads are a time delay device to estimate motor temperature.

You need a device that measures motor shaft TORQUE and sets a hard limit for tripping out. Jraef mentions one and there are others.

Forget the overload for this problem.

But to be clear, your Simocode relay ALREADY has this feature built-in, no need to buy or add anything else. The factory default setting is for this feature to be disabled. You just need to find it in programming and enable it.

Here are a couple of mechanical torque limiting clutches:
http://www.autogard.com/amer/
http://www.bostongear.com/products/clutch/centric.html

Another source for current switches is NK: http://www.nktechnologies.com/current-sensing-switches/

This system was set up by the contractors that put it in. This plant is still going through commissioning. FLA 4.2 can have the overload set as high as 5.25 according to the CEC. They chose 4.8 I'm not sure why. The spike in current would be gradual as it plugs up. The Simocode C uses a current sensing coil that has a rating of 2.4A to 25A. We had the other one plug up last night and it tripped out like it was supposed to at 4.8A. I lowered both now from 4.8 to 3.5. I checked the A draw under normal operations and they are running at 2.2 to 2.4 A. I think I could get a way with 3A setting.
The way these plug up they gradually start to plug up and increase draw on the motor. I do believe lowing the overload setting will work. I checked one that was partly plugged and the amperage increased from 2.4 to 2.9, that's where why I think 3A would be a good number.
Thanks for all the input!
I'll post back in a few weeks, by lowering my settings to 3A I believe we will get overloads earlier into the plug up, so they won't be as bad.
Our long term fix will be to eliminate the plug ups, maybe change the shooting under the rotary and eliminate the bottle neck.

cheers

Gus

Another thing you can do with that Simocode relay is to set up a separate WARNING level that lets the operator know it is STARTING to plug up by programming a different output relay to turn on a light, like a yellow tower light above the hopper.

Thanks jraef, I like that idea. I'm not that familiar with the Simocode setup yet, but I'm getting there. We have 8 or 9 HMI screens in the opearator booth. It might be easier for me to just have a alarm come in on the HMI as a warning....stating to check the rotary for a jam. but not shut them off at that stage, definitely something to look into :0)

I did something similar with a bucket elevator, warning at x amps with an annoying horn. Forced shutdown at y amps.

The annoying horn is key

Electrical components cannot make up for poor mechanical design. The overload is designed to protect the motor and only the motor.

Also, an overload should be set to the motor nameplate FLA (4.2) and not the NEC table FLA (4.8) value. The NEC FLA value is used to size fuses, feeder and branch circuit conductors. The nameplate FLA should be set in the overload. The NEC table value is a "worst case scenario" with a safety factor built into it. It's put in place because motor changeouts are routine enough to where you don't want to size the circuit to one motor's FLA. If someone comes along and changes the motor to one with the same horsepower but is less efficient, now your wiring may not be adequately protected, or you'll get nuisance tripping. Overloads are adjustable, however, so that is why they are set to the Nameplate value and should be checked and re-adjusted if necessary whenever a new motor is installed.

The proper solution here I think would be a mechanical one. Either use a smaller motor or install a breakaway clutch system like others mentioned.

Here in Canada Our Code book states in section 28-306 Rating or trip selection of overload devices for motors is 125% of the full load current rating of a motor having a marked service factor of 1.15 or greater. This is the maximum you are allowed to set your overload device. I understand it may be different for you folks in the USA who follow the NEC. Fuses here in Canada are not sized from the above. We have a separate chart for fuses and yet another chart for circuit breakers.