Dol Starting Of Large Industrial Fan

[Cantstandya]

DEAR INTELLIGENT ONES,

FIRST OF ALL, I AM THRILLED TO FIND THIS FORUM AND HOPE TO GAIN SOME NEEDED KNOWLEDGE. I AM A JOURNEYMAN ELECTRICIAN, IBEW. I WORK FOR A CONTRACTOR AT A HEAVY INDUSTRIAL SITE, A CORN PROCESSING FACILITY. WE HAVE BEEN WORKING ON AN EPA PROJECT THAT INCLUDES AN AIR SCRUBBER. THE PROJECT HAS A LARGE FAN (DO NOT HAVE TECHNICAL DATA FOR FAN) FOR MOVING AIR THROUGH A SCRUBBER. THE FAN IS DIRECTLY DRIVEN BY A 3 PHASE, 480V, 125 HP, FLA 144, SF 1.15 MOTOR. MOTOR STARTER IS A DOL SIZE 5, CUTLER-HAMMER. AN ANALOG AIR DAMPER IS INSTALLED IN THIS SYSTEM. NEW CONSTRUCTION INSTALLATION. INSTALLATION IS COMPLETE AND OPERATING, BUT WE HAD STARTING PROBLEMS.

1ST QUESTION- IS THIS FAN AN INERTIAL LOAD? I BELIEVE IT IS.

2ND QUESTION- WHERE/WHAT IS A GOOD SOURCE TO FIND TECHNICAL/ENGINEERING DATA TO PROPERLY BUILD A FAN/MOTOR COMBINATION?

 

OUR STARTING PROBLEMS WERE OVERLOAD HEATERS RELATED. HEATERS SELECTED WERE FH26 (142-155 AMPS) PER C-H HEATER CHART AND MOTOR FLA. MOTOR WAS STARTED WITH DAMPER CLOSED- NO AIR LOAD. MOTOR RAN FOR 15-20 SECONDS UNTIL HEATERS TRIPPED. HEATERS, O.L. RELAY, AND O.L. CURRENT TRANSFORMERS CHECKED GOOD. OUR OVERLOAD EQUIPMENT IS FACTORY WITHOUT TIME DELAY. AMP CLAMP DID NOT REGISTER STARTING CURRENT, BUT AFTER 7 SECONDS +/-, MOTOR RAN AT 70 AMPS.

NEEDLESS TO SAY, WE HAD AN OPINIONATED AUDIENCE. ONE AUDIENCE MEMBER WILL GO TO HIS GRAVE BELIEVING OUR PLC AND START/STOP CONTROL WIRING CAUSED THE HEATERS TO TRIP. TO ALLOW THE MOTOR TO RUN, WE INSTALLED FH31 (225-244 AMPS) HEATERS (A BIT EXTREME, BUT THEY WERE CLOSE AT HAND). MOTOR STARTED AND RAN CONTINUOUSLY AT 70 AMPS CLOSED DAMPER TO 130 AMPS OPEN DAMPER.

BASED ON THE NEC'S OVERLOAD SELECTION IN ARTICLE 430, WE CALCULATED AN AMP VALUE AT 140% OF FLA (201.6 AMPS) WHICH ALLOWED US TO INSTALL FH29 (188-205 AMPS) HEATERS. MOTOR STARTED AND RAN WITH NO PROBLEMS.

ANOTHER INDIVIDUAL ENTERED THE SCENE AND PERFORMED A MOTOR ANALYSIS. HE, AND OUR CONTROL WIRING/HEATER EXPERT, SAID OUR LOAD WAS NOT INERTIAL, BUT THAT IT WAS "A NORMAL LOAD". THE MOTOR ANALYSIS MAN SAID WE WILL NOT UP THE HEATERS. ALTHOUGH, HIS ANALYSIS WAS PERFORMED WITH FH29 HEATERS. FOR 7 SECONDS, HE MEASURED 970-900 AMPS AND THEN DOWN TO 70 AMPS WITH CLOSED DAMPER. HE FELT WE HAD OTHER ELECTRICAL ISSUES, WHICH WE PROVED WE DID NOT. HE HAD US RE-INSTALL FH26 HEATERS. MOTOR STARTED, 20 SECONDS LATER THE HEATERS TRIPPED! (WE WERE SO HAPPY!) MR. ANALYSIS DIRECTED US TO INSTALL FH27 HEATERS AND NOW, SO FAR SO GOOD.

I BELIEVE WE HAVE A NON-LINEAR, INERTIAL LOAD. THE MOTOR ANALYSIS PROVED IT WITH THE SUSTAINED 7 SECOND HIGH START CURRENT THAT FELL DRAMATICALLY AND IT COULD BE SEEN ON THE CURVE IT PRODUCED.

I WANT TO KNOW WHO IS RIGHT. I WANT TO LEARN FROM THIS EXPERIENCE. WHAT SHOULD I HAVE KNOWN TO HAVE PREVENTED THIS SITUATION? HOW CAN I EDUCATE OUR WONDERFUL AUDIENCE? I HAVE LEARNED ALOT FROM THIS, BUT I KNOW THERE IS MORE.

SORRY FOR SUCH A LONG TOPIC, JUST TRYING TO BE INFORMATIVE.

 

I EAGERLY AWAIT FOR INTELLIGENT RESPONSES!

-THE DRAKE-

 

 

 

 

 

 

 

[marke]

Hello Cantstandya

 

Welcome to the forum. - Those upper case characters certainly make the posting hard to read.

 

1.) I would normally consider a fan to be an inertial load. This is evidenced by the time taken to accelerate to full speed and the time taken to slow down to rest.

If you do not have the dampers closed at start, then the shaft torque produced during start is providing both the acceleration torque to accelerate the inertia of the load, and the torque to do work in moving the air.

 

2) I do not know, it is really a case of finding the right people who have the knowledge and skills.

 

From your description, it sounds as though something is connected incorrectly.

As I understand your description, the motor and fan took about 7 seconds to reach full speed and the current dropped to about 70 amps. After about 15 - 20 seconds, the overload tripped. If the overload was thermally too small, it would have tripped during start. To trip some ten odd seconds after the current has dropped to a run current of around 50% rating, suggests that the overloads still saw an overload current rather than a lower current.

 

I would look at the actual heater rating. It will probably be around 5 amps and ensure that the CTs are correct for the application. If the CT ratio is incorrect, then you will have the problem that you are experiencing.

One thought, you are not using an overload relay designed to be used with a star/delta (wye delta) starter are you? if you are then it is designed to measure the winding current rather than the line current and it will do exactly as you are describing.

 

Best regards,

[jraef]

First off, like Marke said, check your CT ratio. For that heater element to fit in that range, you need to be using a 300:5 CT ratio. Also make sure that someone has not installed one or more of the CTs backwards. If your CT has a label "H1" on it, that needs to be oriented towards the source, so that the motor leads are coming from that direction through the CT. Some are marked with just a dot instead of the H1. In other words, the H1 or dot needs to be on the upstream side of all 3 CTs. It's not absolutely critical except that they all need to be the same, and this is the convention. If they are not the same and the secondaries are tied together in a Y as most are when used in OL relays in US equipment, you will get serious accuracy problems.

 

Second; it is not uncommon, and perfectly acceptable, to bypass (shunt) the overload relay during startup, IF it has been demonstrated that a properly selected OL for a motor will not hold in for startup on inertia loads. It is done all the time and allowable per section 430.35 of the NEC. Another trick employed by those unwilling to have the motor unprotected during start is to have a 2nd OL relay in parallel with the higher setting and a contactor that shorts out the first one during startup.

 

The only way the PLC could cause the OL to trip is if the Start command from the PLC is chattering on and off, making the contactor chatter. There is no other direct connection between a PLC and an overload relay, so that statement is a little ridiculous in my opinion and shows that the person who made it is reaching for straws because they have no clue.

 

The only way the "wiring" could cause it would be if perhaps you had used wire that was significantly too small for the application and was causing a severe voltage drop.

 

Voltage drop is another issue to investigate by the way, even if you selected the proper wire. If the power feed cannot handle a DOL starting load and there is a significant (i.e. more than 5%) voltage drop at startup, that could be a major contributing factor and may require the use of a soft starter.

 

Good luck, post back what you find.

 

An by the way, take off the all caps. I know it's a pain in the rear for non-typists (I used to do it too), but it makes your post difficult to read. Humans read not only by the individual letters, but also by the shapes of words. By using all caps you force the reader to look at and process each individual letter one at a time, which we get tired of.