Motor frequent start alarm

[digitrex]

I have a 13years MV induction motor(8-poles) driving a reciprocating nitrogen compressor. The HP nitrogen are stored in "bullets" for safety purging/inerting system and pressurising cooling water system, both are for plant emergency shutdowns or trips.

Leaks in N2 or cooling water systems lead to HP N2 quickly depleted. During the early years of our plant, the N2 compressor started twice per day. When there is a leak, the compressor may be auto-started 5 to 8 times a day and this frequent starts went on without detection until I accidently discovered it last year from the process pressure trends.

The motor manufacturer specifies on its nameplate max no. of starts of 1000 times per year, which is about 3 times per day or at 8 hours intervals.

My question may be weird but this is what I am thinking now. I am thinking of finding a "frequent start alarm" that can installed on the motor starter. Do you think whether there is such device exist?

 

[edison123]

How about a programmable timer in the motor breaker control circuit that will limit no. of starts to programmed time intervals ?

"Most people stop working when they find a job"

 

[waross]

I was faced with a compressor that was limited to 3 starts per hour. When the other compressor was running in "Base" mode the second compressor would start on demand. I arranged a timer so that once it had started it would continue to run for at least 20 minutes. Then when it stopped, it was ready for an immediate restart if need be, without exceeding three starts per hour. As is common on larger compressors, it was fitted with an unloader, and when not needed could be unloaded instead of being stopped.
Depending on the frequency of your pumping cycles when suppliing a leak, you may be able to use a similar scheme to reduce your starts.
Yours

 

[jraef]

This is a very common application, not at all wierd. Most digital motor protection relays offer a Starts-per-hour and/or a Minimum-time-between-starts function. If you don't already have a good protection relay on that motor, I would suggest adding one, they are cheaper than an emergency rewind! If you have an older realy system with descrete components, you may want to consider upgrading to a modern digital MPR from GE/Multilin, Siemens, Areva, Schneider or a host of other very good products. If you already have one, post it's brand and model, someone here will likely be able to help you to program that function.

The cheapest method would be to search out a specialized timer called a "Short-Cycle" timer. They are designed to do just that function in a round about way.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[mc5w]

A short cycle timer is sometimes hooked up to a central air conditioner to keep it from restarting before the high side pressure has bled off. You should be able to get one at an HVAC supply house.

Jraef's suggestion for a more sophisticated motor protection relay from GE or Schneider would be a better investment considering the size of this motor.

You could also put a gas meter with a pulse output onto the low side of the air compressor or better yet a high pressure gas meter with pulser in between your storage tanks and your process. You could then use a counter that sets off an alarm if nitrogen usage is too high.

 

[digitrex]

Thanks to edison, waross & jraef. However, I have a different idea, I don't want to prevent starting of the motor if the timer is not expired yet. The alarm I am asking for will be used to tell maintenance personnel to search and fix the N2 leak.

The HP Nitrogen system is critical for integrity of the plant, we do not want to leave the N2 pressure to drop below the safety level.

As for jraef's advise, I will look for more information about modern digital MPR to see if it can provide a separate output as an alarm but not to inhibit re-starting of the motor.

 

[Dnicejr]

If this multiple starts per day is a recent trend I would suggest performing a Ultrasound survey to locate your leaks and fix them. Fixing the leaks would get the number of starts of the compressor back to normal, or within manufacturers specifications.

 

[edison123]

May be you could add an auxiliary contact to the motor breaker and connect to a settable counter with an alarm.

"Most people stop working when they find a job"

 

[LionelHutz]

I would use something along the lines of a small programmable relay. The little all-in-one baby PLC's that a bunch of manufacturers now sell. The logic is basically as follows;

A timer free running that gets reset at each start that is set to the minimum time between starts you want to allow.

A counter that counts up at each start. This counter is reset by the above timer reaching it's setpoint.

A logic block that looks for the counter to exceed a set value. This block would toggle an output.

You could then stick in some latching relay logic and a reset button or whatever else you'd want to make it suitable to your application.

Basically, the logic would allow the number of starts per x time to be exceeded until you reach the counter check and then you'd get an output. For example, you could set the timer to 40min and the counter check to 3. Then, the third time in a row that the motor is started with less than 40 minutes between the starts the counter would hit 3 and give you an ouput. If the starts are more than 40 minutes apart then the counter keeps getting reset to 0 and would never reach 3.

Telemecancique Zelio or Moeller Easy are models of these types of relays. You just program them right from the display on the relay.

I believe this will cover what you want. I'm sure there are other ways to do it but I personally think the price and simplicity of these programmable relays is hard to beat.

 

[jraef]

Yes, most MPRs can be programmed so that the starts-per-hour protection function only activates a relay output instead of inhibiting the next start. I am most familiar with the GE/Multilin relays, either the SR469 or the slightly less expensive 369 (only because a lot of features in the 369 are optional, and everything is included in a 469). That is not an endorsement however, I'm sure all or most of the others are equally capable. I should have mentioned this earlier, but what you want to look for is referred to in IEEE device numbers as function 66.

Here are the ones I have used at one time or another, they all can be programmed as I mentioned above.

GE/Multilin link, US
Hint, don't bother with the 239 and 269, they are both obsolete.

Areva Micom P24x relay link

Schweitzer SEL-701 link

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[buzzp]

Isn't the number of starts limited because of the thermal capacity of the motor? Or is this, somehow, compressor related? If you have a decent overload, it should model the thermal capability of the motor. You should be protecting the motor based on this overload. So I don't think your damaging equipment (again depends on the OL your using).
When the system leaks, the compressor starts more often. They have little counters which could be hooked to your aux contacts on your starter/contactor. You can program in the number of counts before an alarm is sounded. Some of them have a RTC so you can cause a reset of the count after X days or hours. They can also be provided with contacts to initiate some other function (alarm light). This would be very cheap to do. However, given the size (and cost) of this motor, it would probably be best to put on some protective relaying outside of the standard stuff like overload and single phase protection as suggested. These relays would give you the functionality you need. If you want a counter or something check out Red Lion Controls or some other manufacturer of process controls.

 

[msherb]

I would suggest using one of the several processor based relays that jraef suggested. I have used several of them in the past, and they all provide the necessary protection you need, and installation can usually be accomplished for under $10,000-$12,000 or so depending on your setup and vendor. Also, the manufacturer of you motor is specifying starts based on thermal capacity of the motor, which is related to the number of starts, but also to other factors of your motor which can't be easily accounted for with an auxiliary contact or similar counter circuit. If the psi system is too important to allow the pressure to drop, then I would say forget protecting the motor, and protect the plant by sacrificing the motor, but track the amount of starts and base your maintenance/rewinds/reconditions on that number.

 

[digitrex]

msherb, you have a good point that sacrificing the motor is a way, similar to what we had done on our Fire Water pump at higher overload setting.

LionelHutz & jraef's ideas is definitely workable.

I will also looking into setting this alarm from the DCS (or the Safeguarding system), where the motor Start & Stop commands are initiated.