Help!single Phase Motor Starting Question

[dennyk1024]

DEAR MEMBERS: I'M NEW TO THE FORUM AND I 'M IN THE HVAC INDUSTRY......I'M SETTING UP A SHOP AND I INSTALLED A 7.5 HP AIR CONPRESSOR. THE MOTOR IS 240 SINGLE PHASE AND THE STARTER IS ALSO SINGLE PHASE. THE STARTER HAS A SIEMENS 42 DF CONTACTOR AS WELL AS AN OVERLOAD HEATER/RELAY. THE START UP WENT WELL AND THE COMPRESSOR STARTED NO PROBLEM. HOWEVER EVERYTIME ANOTHER MACHINE STARTS,,,,,FRACTIONAL HP....THE CONTACTOR DROPS OUT FOR A FRACTION OF A SECOND AND THEN THE MOTOR TRIES TO RESTART. I THOUGHT THAT THE CONTACTOR COIL WAS BAD AND THEN REPLACED IT...SAME PROBLEM. I MEASURED THE VOLTAGE AT THE LINE AND LOAD SIDE OF THE CONTACTOR WHILE THE MOTOR WAS RUNNING AND IT WAS 240 VOLTS, HOWEVER WHEN I READ THE CONTACTOR COIL VOLTAGE I GET 180 VOLTS. AROUND 95 VOLTS FROM EACH TERMINAL TO GROUND. I KNOW THAT THIS VOLTAGE WON'T HOLD THE COIL IN...BUT WHERE IS THE VOLTAGE DROP OCCURING???? THIS REALLY HAS ME PUZZLED...AND I THINK I'M MISSING SOMETHING OBVIOUS.....BUT WHAT?

 

THE MOTOR STARTER IS A FURNAS BRAND...7.5 HP STARTER CAT. NO. 16DF12BGACI 240 V COIL

 

I'M NOT FEEDING THE CONTACTOR WITH AN EXTERNAL CONTACTOR COIL CIRCUIT....I'M USING A PRESSURE SWITCH TO "JUMP" TREMINALS WITHIN THE STARTER TO ENERGIZE THE CONTACTOR COIL.

 

I CAN REALLY USE SOME HELP.....IF YOU CAN dennyk1024_2000@yahoo.com

 

THANKS

DENNY K

[kens]

Hi dennyk, I'm not to sure what part of the world you are in but I guess South America by your time zone and voltage. Is the neutral earthed there? It seems that you have a poor connection somewhere in your control circuit that is causing the low coil voltage so when the other motor starts the further voltage drop is caused which is enough to release the coil. When you read 180 V on your coil is this across the coil terminals or from the phase side to a common neutral point? I think you need to choose a common neutral and begin from the start of the control circuit and measure each point until you see the voltage drop. Ideally you will see no drop until you get to the coil and at that point you will see the full 240V drop. Continue to check on the neutral side as there can be high resistance connections in that part of the circuit also.

 

I dont think that you are missing anythig obvious but its just a matter of methodical point to point testing.

 

Kens

[dennyk1024]

Hi dennyk, I'm not to sure what part of the world you are in but I guess South America by your time zone and voltage. Is the neutral earthed there? It seems that you have a poor connection somewhere in your control circuit that is causing the low coil voltage so when the other motor starts the further voltage drop is caused which is enough to release the coil. When you read 180 V on your coil is this across the coil terminals or from the phase side to a common neutral point? I think you need to choose a common neutral and begin from the start of the control circuit and measure each point until you see the voltage drop. Ideally you will see no drop until you get to the coil and at that point you will see the full 240V drop. Continue to check on the neutral side as there can be high resistance connections in that part of the circuit also.

 

I dont think that you are missing anythig obvious but its just a matter of methodical point to point testing.

 

Kens

 

[dennyk1024]

THANKS FOR THE REPLY......I'M I THE USA @ PITTSBURGH PENNA.

 

I'LL RETRACE MY WIRING.....SOMETIMES IT THE MOST OBVIOUS.....BUT IF ANYONE ELSE HAS A SUGGESTION....PLEASE DO SO. I'M GETTING 180 VOLTS ACROSS THE COIL WHEN THE VOM LEADS ARE TOUCHED TO BOTH SIDES OF THE COIL...WHEN I GO FROM ONE COIL TERMINAL TO GROUND (EARTH) I GET APPROX. 90 VOLTS AT EACH TERMINAL.

 

IN THE US WE USE A NUETRAL TO THE MAIN ENTRANCE AND WE DRIVE A GROUND ROD INTO THE EARTH FOR FAULT CURRENT......I DROVE IN AN EXTRA 8 FOOT ROD AND I'M ALSO USING A COPPER WATER LINE THAT IS BURIED.

 

 

JUST A NOTE....I USED A LUG TO JOIN BOTH THE NEUTRAL FROM THE MOTOR AND THE NEUTRAL FROM THE DISCONNECT TOGETHER IN THE MOTOR STARTER ENCLOSURE AND I BONDED THEM VIA BOLT TO THE MOTOR STARTER ENCLOSURE. THE STARTER ENCLOSURE HAS ALL CONDUIT TO THE SUBPANEL.

 

THIS PROBLEM IS REALLY KICKING MY ASS........

 

IF I HAVE TO...I'LL GO TO A 24V COIL ...BUT I DO NOT WANT TO TAKE THE OVERLOAD HEATER OUT OF THE CIRCUIT

 

THANKS AGAIN

DENNY K

[kens]

Hi Dennyk, I really dont know enough about the US system. I found this on Wikipedia

"Another system commonly seen in North America is to have a delta connected secondary with a centre tap on one of the windings supplying the ground and neutral. This allows for 240 V three phase as well as three different single phase voltages (120 V between two of the phases and the neutral, 208 V between the third phase (known as a wild leg) and neutral and 240 V between any two phases) to be made available from the same supply." Which proceeded to confuse me. I'm not sure which system you are using but if you could draw a circuit diagram I will have a look. The 90V either side of the coil would tend to suggest that there is a problem with the earthing/neutral however it seems that there is a dfferent system in place over there.

Good luck

Kens

 

[AB2005]

Hi Denny k;

 

You said that voltage at contactor's power terminals are 240V both at in and out, then there will be no problem with energizing the coil because the same supply also uses for energizing it. It sounds me that there is wrong connection problem. Have you checked the voltage at power terminal of contactor when contactor drops out? If this voltage also drops out, then there must be a loose connection fault at somewhere. If a loose connection persists in a circuit, then voltage drop problem occurs if a motor or other heavy load starts.

If you can do, please send a circuit diagram which will provide more help for solving your problem.

Please post your question in only small font which is easy to read in short time.

[marke]

Hello Denny k

 

Welcome to the forum.

first, just a small point, try not to post in capital letters, it is a bit like shouting. We will find it easier to read if you use lower case.

 

As far as you question goes, I expect that your starter uses the two phase feed in to power the contactor coil and you are using a pressure switch to open and close the contactor. I would trace the wiring back from each contactor coil terminal with power ON and measure the volt drop across each set of contacts etc that the feed goes through.

I would expect a feed to come from one phase input, through a fuse, then through a stop button, then a start button (with auxiliary contact across it) then through the trip contacts of the overload device and into one side of the coil. The other side of the coil should go through a second fuse to the other phase.

To replace the standard start stop circuit, you would connect a potential free set of contacts from you pressure switch from the first fuse to the overload trip contacts and remove the connections to the buttons.

Then measure the voltage across each fuse, and across the overload trip contacts and then across the pressure switch contacts. Somewhere, there must be around 40 - 60 volts drop and it should be easy to find. It msut be across the trip contacts, or the pressure contacts, or you have the control circuit wrong!!

 

Best regards,

 

[dennyk1024]

Dear Guys....Thanks a million for your help!!!! It was nice to see that so many went out of their way to help me. I tried all of the above but without success. Out of desperation I called about six toll free numbers for Siemens......I muat say that they were great. I ended up on a three way call wih their controls experts because they were stumped too.. After about a half hour of trouble shooting we determined that the fault is in the bi-metal overload. The overload must have a problem with a NC contact that begins to fault causing the contactor coil to lose voltage.....at 175 volts the coil realeases the contactor for a second .....restarts the motor again and the cycle repeats itself. Seimens may even want the starter back for a look-see. This was a tough one!!!!! Again guys...thanks.

 

Denny K