Previous in Forum: Expected life of OIP High voltage transformer bushing   Next in Forum: Continuity testing
Close

Comments Format:






Change Mode
Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







Add Vote
10 comments
Rate Comments: Nested
dar76
Active Contributor

Join Date: May 2011
Posts: 20

Flashing Exciter Field Coil

05/24/2011 12:44 PM

Hello,

I troubleshoot a 180KW caterpillar genset, the problem is no voltage output. I try to flash the exciter using 24Vdc battery with series resistor 3.7 ohms / 7 watts, then start again the engine but still failled. Then I put DMM to measure the output generator phase i got 8vac only. i try again connecting the battery to the exciter field while the engine is running and observe the output voltage, holding it for 10 sec. the voltage rise untill 20vac only and back to 8 vac when the flash voltage is remove. I had measure the flash voltage on the exciter connector its stop rising untill 11.5vdc. The coil resistance of exciter stator is 3 ohms. its a 37vdc 7.7Amp exciter. I have tried new AVR R448 still failled to generate output.

My question is how much output voltage will be generate if the exciter is 11Vdc?

and base on the above situation , is it the main field exciter problem? Can somebody advice me.

Thank you.

dar76
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
loadshare
Power-User
Engineering Fields - Electrical Engineering - New Member Engineering Fields - Mechanical Engineering - New Member

Join Date: Jan 2011
Location: Jakarta
Posts: 162
#1

Re: Flashing Exciter Field Coil

05/24/2011 9:01 PM

Hi Dar76,

According to my experiences, if you tried flashing then still no voltage build, try to check 1. RECTIFIER KIT consist of Dioda and surge suppressor

2. CONTINUITY OF VOLTAGE SENSING from generator to AVR

3. if you use permanent magnet as power source on AVR, you can check the voltage of PERMANENT MAGNET

4. the last thing, you can TEST WINDING INSULATION RESISTANCE, if result of test is very low, try to

5. HEAT SPACE OF GENERATOR, you can put high watt of lamp inside of generator

hopefully can help you....

__________________
 One system
Register to Reply Score 1 for Good Answer
No more "Almost" Good Answers.
dar76
Active Contributor

Join Date: May 2011
Posts: 20
#2
In reply to #1

Re: Flashing Exciter Field Coil

05/25/2011 1:20 AM

hi loadshare,

Thank you for your quick reply. the sensing voltage and power supply for AVR are at the same source tap in T7,T8, & T9, of 12 leads windings, so its 208Vac source I had measure the voltage here is only 4.5Vac, and raise to 7 vac when force to excite using battery, but drop to 4.5vac when the flash volts removed.

I will try it today to heat space of generator, because I have check the item 1 - 4 its in good condition except the winding insulation has little resistance to ground. But when i compared it with the same generator ( Good condition) this windings the one with problem has more higher reading. I'm confuse , anyway i will do today heating it with a 250W street light.
Register to Reply Off Topic (Score 4)
loadshare
Power-User
Engineering Fields - Electrical Engineering - New Member Engineering Fields - Mechanical Engineering - New Member

Join Date: Jan 2011
Location: Jakarta
Posts: 162
#3

Re: Flashing Exciter Field Coil

05/25/2011 1:30 AM

Hi,

Pls concern about enviromental condition, probably high humidity cause failure on generator. After heat the space in gen, recheck again the IR.

Good Luck

__________________
 One system
Register to Reply
LAA_Lucke
Guru

Join Date: Feb 2010
Posts: 1013
Good Answers: 36
#4

Re: Flashing Exciter Field Coil

05/25/2011 2:20 PM

HI,

Flashing the Main Field of the Alternator is done directly on that field, while the engine is at Standstill. Not through the exciter stator (This is a brushless Unit correct?) at standstill.

To do this, you disconnect the wires going from the Rotating rectifier bridge to the Main Field on the Main rotor. Check the Positive and negative poles and connect respectively to the battery for a short time. For precaution, you can use a resistor (3.7Ohm is OK, but very short time since it will develop ~155Watts. ) since the resistance is very small (less than 1 Ohm).

If I understood well, you connected 24VDC + Resistor (3.7Ohms) to the exciter stator while running, and you measured the voltage on the EXCITER Stator terminals and found it to be 11.5V only: Well this is normal since the 24V is in series with 3.7 Ohms and then the 3 ohms of the exciter ==> You practically divided the voltage by 2 (24/2 = 12V).

Check the rectifier Diodes that are on the rotor between the exciter rotor and the main rotating field. Check connections and any sign of burnout etc.

If the exciter windings (Rotor and stator are fine and the rectifier diodes + any surge suppressors are all OK, the AVR should be able to generate the required voltage: Check the current going from the AVR to the exciter stator. Should not exceed the 7.7 A rating. If this is ok then the problem will be in the main windings {Field and /Or stator}.
Register to Reply
dar76
Active Contributor

Join Date: May 2011
Posts: 20
#5
In reply to #4

Re: Flashing Exciter Field Coil

05/25/2011 5:20 PM

hi,

Correct this is a brushless unit, ok will do the flashing in main exciter by tommorow.

The retifier were checked its good, i have done monitoring using DMM in the exciter output of AVR when I start the engine I didn't see voltage output. I suspect the supply voltage is not enough for AVR. Because it is tap in T7, T8, & T9 where the voltage is = to O/P voltage T1, T2, & T3 divide by 2 . now I only getting 8vac at the main output so its only 4vac in the AVR supply.

I dont see surge suppresor in the main field, but there is a 28K/1/2W resistor connected to ground from one of the main field terminal plate. I think its a mark that its a Negative terminal of main field. Am i correct?

what happen if somebody assemble the diode and fixing it in reverse, and polarity of the main field change?

what really is the resistor for?

If it is in positive side then it is a dummy load. If in the negative side then its a ground series resistor. Which is the correct one?
Register to Reply Off Topic (Score 5)
LAA_Lucke
Guru

Join Date: Feb 2010
Posts: 1013
Good Answers: 36
#6
In reply to #5

Re: Flashing Exciter Field Coil

05/26/2011 7:18 AM

1) The connection points to the Main Stator output depends on the AVR required voltage. Therefore, if nobody interfered with the original wiring, then leave it at T7, T8 and T9. Otherwise, you might destroy the AVR (Higher voltage than design??). Normally, with dual voltage alternators, the connection point is on the lower voltage to enable both outputs without interfereing with the AVR connections. Therefore, leave it there.

2) Rotating field rectifier plates: Normally, there are 6 diodes labelled CR1 to CR6 in the drawings. These can be individual Diodes bolted on the heat sinks (3 have the heat sink on the Negative side of the diode and 3 on the Positive side). Also, recently, these Diodes are encapsulated in a module whereby the whole module in bolted. ALSO, There are Two (or One) Surge suppression Diodes: CR7 and CR8. These canbe encapsulated and look like the rectifier diodes (Encapsulted module type) except for the leads. In the Discrete device type (not encapsulated), this item looks like a normal diode but is different and will be placed across from the Negative to the Positive plate. Its Polarity is opposite (Flywheel diode principle) to prevent a short between + and - This is to be checked against being shorted (conducting both ways = damaged).

3) The small resistor: This is a suppression resistor labelled R5 in drawings and is between the + Plate and the ground (Rotor shaft). The job of this resistor is mainly to filter electric noises that can affect electronic equipment. If this is shorting, it could affect the field. Check that its value is OK and that it is on the + Plate of the revolving rectifier assembly. N.B. Since you were asking about the polarity of the rectifier being wrong, it means that maybe these were replaced sometime? If yes, check the polarity.

4) What happens if the rectifier is wrongly assembled and polarity is wrong?: Normally, no problem. It will still produce a voltage output on the main stator outputs. The phases will be in the opposite sequence than expected. But More important is to check that the 3 phase (six diodes) bridge itself is properly assembled!

Also, if the bridge polarity was reversed due to a repair excercise, this could be a cause to kill any remanent magnetism and require a field flashing to regenerate the initial magnetism to start the chain reaction in the opposite direction.

I hope that if all the components are OK, the proper flashing, in the correct direction, will re-generate the remanent magnetism and solve the problem.
Register to Reply Off Topic (Score 5)
dar76
Active Contributor

Join Date: May 2011
Posts: 20
#7
In reply to #6

Re: Flashing Exciter Field Coil

05/26/2011 12:19 PM

hello,

I test run the generator after heating the space of it, and flashing the main field exciter 5 sec only due to low power resistor I use, the resistor voltage is 20vdc so the current flowing trough the exciter is around 5 amperes. The result is still the same no output except for this, I have gain a little higher voltage than before, its around 5.25vac in low voltage tap ( AVR sensing and supply power). The Insulation resistance also getting good reading it is now 5 mega ohms.

For now I still continue heating the space of generator, hoping to gain more residual voltages.

I have checked the rectifier assembly all correct and good condition, but i'm not sure of the polarity of the main field since now I've found the resistor connected to the negative plate of the main field, base on LAA Lucke schematic it is in the wrong place and suppose to be in the positive plate. I suspect that all the rectifier has been removed before and putted back in wrong polarity (but bridging are correct) thats why the resistor is in the negative plate now.

Could this resistor placement can give problems to the excitation? Or may be i need to remove the windings for visual checking and thorough check -up.

I will reverse first the polarity of the main field so the resistor will be in correct positive side and test run again tommorow.

by the way thank you guys for quick reply , this could help me troubleshooting this generator.
Register to Reply
loadshare
Power-User
Engineering Fields - Electrical Engineering - New Member Engineering Fields - Mechanical Engineering - New Member

Join Date: Jan 2011
Location: Jakarta
Posts: 162
#8
In reply to #7

Re: Flashing Exciter Field Coil

05/27/2011 2:20 AM

Hi dar76,

Never trust the new part always good, and the marking on dioda always right.....I have an experience marking dioda different with used

__________________
 One system
Register to Reply
dar76
Active Contributor

Join Date: May 2011
Posts: 20
#9

Re: Flashing Exciter Field Coil

06/07/2011 2:56 PM

Hi to everybody,

I found out the cause on this problem, somebody replaced the surge suppressor diode with a jumper wire, means its shorting the positive and negative terminal.

could anybody know what is the part number of the surge suppressor diode. the exciter volts and amperes for this generator is 36V and 7Amp. I know i can run without it but I want to put new surge suppressor diode for protection.

thank you.

Dar76
Register to Reply
LAA_Lucke
Guru

Join Date: Feb 2010
Posts: 1013
Good Answers: 36
#10
In reply to #9

Re: Flashing Exciter Field Coil

06/07/2011 7:29 PM

Easiest way is to get it from a CAT supplier, referring to the Genset model and serial number etc.

Otherwise, search the web for Transient Surge Supressor Diodes and get one rated to clamp the voltage at ~45V and can dissipate ~700watts. Just guessing the value!
Register to Reply
Register to Reply
Have a Different Question?
10 comments

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
Copy to Clipboard

Users who posted comments:

dar76 (4); LAA_Lucke (3); loadshare (3)

Previous in Forum: Expected life of OIP High voltage transformer bushing   Next in Forum: Continuity testing

Advertisement

Got Something to Say?
Ask a Question
Start a Discussion

Search this Forum
Electrical Engineering:
Go

CR4 Sections
Aerospace
BioMech & BioMed
Chemical & Material Science
Civil Engineering
Communications & Electronics
Education and Engineering Careers
Electrical Engineering
General Discussion
Instrumentation
Manufacturing
Mechanical Engineering
New Technologies & Research
Software & Programming
Sustainable Engineering
Transportation

Site Directories
Forum Directory
Blog Directory
User Group Directory
RSS Feeds

CR4 Rules of Conduct
CR4 FAQ
Site Glossary

Who's Online (74 right now)
Show Members
Advertisement