Under Current Sensing

[glenncovington]

Hello all, hope someone has dealt with this before. We have a large 100hp blower that has a nasty habit of having its belts jump off. Years ago I put an undercurrent relay in the system to shut it down. This works fine, but is expensive (approx. $1300 USD from AB). The relay has failed for the second time in 6 years and is need of replacement.

My question: Is there a more economical method to do under current sensing and control. I was thinking a small plc with an Analog input and use the existing CT from the undercurrent relay system. Will this work, or do I need a fancier CT to get the signal stable and in the 4-20ma range? The motor is 100 Hp 460V 3 Phase running at 60 HZ constant speed. All I want to do is drop out an interlock contact if the motor current drops below setpoint.

Thanks for any help.

Glenn

 

[Ken Moore]

An alternative is to put some type of speed sensor on the blower itself. Then, when belts come off, RPM goes down, alarm sounds.

It could be very simple, a prox switch and sensor, count pulses, one pulse per RPM.



Ken

 

[Ken Roach]

A shame you don't have one of those A-B E3 electronic overload relays protecting the motor. Those have 3-phase current monitoring and underload trip functions built in, but you have to be already using DeviceNet to get the data into your controller.

You're right that a Current Transformer isn't enough; that just gets you a 1A or 5A current signal. You also need a Current Transducer to put out a PLC-compatible 4-20 mA or 0-10 V signal.

I like Ken Moore's suggestion of just monitoring the speed of the blower. Most brick PLC's (including the MicroLogix 1000) have a high-speed counter you could use to monitor RPM.

 

[Jim Dungar]

current switches

I have had good luck with this company NK Technologies. Thier devices are very reasonably priced <$200USD

 

[Bitmore]

Jim has the right idea, I have used the NK Technologies current devices for just the purpose you discribed. Actually used two of them, one for low current threshold, and the other for overcurrent.
They come in adjustable sizes so (2) of the same device part number could be used for both functions.

Bitmore

 

[guest]

under current

Glen Time Mark corp. in Tulsa OK makes an inexspensive current
moniter with Hi Low and time built into it. TimeMark maybe one word.
They`re on the Web. If you want fancier HBC Controls 800-879-7918 makes a neet little transducer.
Good Luck
Guest bonkhead

 

[icky812]

I have used CT's from Veris Industries . They work well and are relatively inexpensive. Some of the models have several current ranges. They are also 4-20ma capable.

 

[glenncovington]

Thanks everybody, you have given me a lot of good choices. It looks like there are several makers of current switches for fan control so no need for plc on this one.

BTW this fan is in the bottom of a very nasty pit, that is why it is preferable to control by current at the motor control center versus a prox. on the shaft looking for rotation.

Thanks everybody!!

 

[JesperMP]

I use this little bugger for allmost the same purpose:
[attachment]
EIS datasheet
It simply generates a signal "Current above set level". It is a CT with built-in current limit relay.
And its cheap ! 30 EURO (40 USD) per relay. Off course you need three for each motor.

It comes in two ranges 2-20A and 5-50A, if you have to go above that you must use an intermediary CT and then loop the intermediary CT signal via this relay (make several turns to suit the range of the EIS).

A little tip: the datasheet says there is a 2.5V voltage drop on the output. That would be a problem when putting three in series and sending the signal to a PLC input. But I have found that the voltage drop is more like 0.5V - no problem.

edit: I have corrected the current ranges.

edit again: You only need 1 in your application. I use 3 to sense broken wire.

 

[shooter]

so you would like to know if blower is running:
why not use a small vane and a switch on it. if no airflow give alarm. This way you will detect the real problem of no airflow immediate. even when a ****er blocks the airway.
I would put the switch also direct in the starting sequence so when starting push start button and override the vaneswitch. when vane is down switch off motor.
no fancy **** or anything that breaks and a true alarm.

 

[glenncovington]

Shooter,

If the air supply was clean that would work great, but this is on a material handling fan blowing large wood chips. Any air pressure, or vane switches live a short life in this condition. Plus wiring in the MCC is much easier than the field wiring required for a sensor at the ductwork.

Thanks for the input. I ended up selecting an undercurrent switch from ATC. Link

Thanks again,

Glenn

 

[cnrservices]

How about driving a small belt off the blower pulley, and having that 2nd belt turn a motion detector, or even trip a proxy every 1/4 second(would require large reduction ratio) & if no input for 1/2 second you know something failed on the drive.

 

[Colossalnut]

Lottsa good solutions posted to your prob Glenn and I see you've already made a choice, but I see that no one has addressed the cause. WHY are the belts jumping off? Are the pulleys aligned perfectly? Are the individual belt tensions set properly? How many belts are involved? Is debris lodging between the belts and the pulley? Is the motor level and mounted securely? How 'bout the fan? Are the bearings worn in either the motor or the fan? I'm sure that no one wants to climb down into that nasty pit for any length of time, but if you have to go in to replace belts that frequently why not spend a little more time while you're there and do some investigative troubleshooting. I think that you've come up with a very creative method of sensing the malfunction, but there just may be a way to prevent it from occuring so frequently.

 

[glenncovington]

Colassalnut,

You are correct and I can't say for 100% sure that all is in alignment. This problem of jumping belts might happen once every couple of years. When it happens, there is a big mess with the pipe clogging up from the hog and grinder pushing material into the fan and the fan is not turning. The fan is a 100 hp motor that is dumped across the line (no soft start). This is most likely when the belts jump off (Fan is started once per week, 3rd shift Sunday evening). If the motor is running, the overloads are not tripped, so everything up stream thinks all is okay and starts up. With the undercurrent relay I can catch the jumped belt condition after a startup delay (approx. 30 sec) and shut things down before they clog up.

Like I said this only happens every couple years so most everyone forgets about it while it is working and then when it breaks it is the "biggest problem". With the relay it is there to catch those infrequent events.

Thanks everyone for the ideas. The ATC unit I am getting is about $250 USD vs. $1500 USD for the AB unit I was using. Hopefully it will work well.

Glenn

 

[Colossalnut]

Hi Glenn!
One of my biggest faults is making assumptions when I don't have all the facts. I had assumed that this problem was occuring more frequently than every couple of years. Hahahaha...wish I could be so lucky! I think you've made an excellent decision. I'm not personnally familiar with the product that you've incorporated, but at that price...man, what a deal! Hope it works well for ya...