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Connection winding salient pole of AC generator to rotating rectifier
2

Connection winding salient pole of AC generator to rotating rectifier

Connection winding salient pole of AC generator to rotating rectifier

(OP)
Dear members,

I have problem on our AC generator (Marelli Generator MJB 355) with salient pole rotor (4 poles). The AC generator was need to repair due to suspressor diode was broken, then we taken out the rotor to repair rotor exciter. We disconnected cables rotor's winding from rotating rectifier and we didn't put tag to 2 cables/conductors.

Are the cable should reconnected to the (-) and (+) terminal of rotating rectifier correctly or we can connect 2 cables without consider (-) and (+) terminals of rotating rectifier?

Many thanks for your help.

Regards,
Barkah

RE: Connection winding salient pole of AC generator to rotating rectifier

If these are the leads I think they are, hook them up and try the machine. If the voltage doesn't build up, reverse the leads and try again.
Some times the rotor will lose its residual magnetism when it is disassembled. If that is the case, the field will have to be flashed and then the connection polarity will not matter.
What size (KVA or kW) and voltage is your machine?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Connection winding salient pole of AC generator to rotating rectifier

(OP)
Dear Bill,

Thanks for your response.

Yes, these are the leads of salient poles rotor which should connected to rotating rectifier. The KVA alternator is 680 kVA and output voltage is 400 VAC.
We are worry to reconnect the lead since we didn't tag the (-) and (+) on these leads before disconnected both leads.

Is "try the machine" means to start the machine then measure the output voltage of the 3 phase generator?
And how to flash the field? What the tools is needed to do flash the field?

I have the picture, unfortunately, I have not know how to attach the picture. I've tried click "Attachment" but it's not work.

Appreciate for your respond. Thank you.

Best regards,
Barkah

RE: Connection winding salient pole of AC generator to rotating rectifier

If you have the PMG option (permanent magnet generator) than the connections are not important. The set will self correct when it is started.
If you have a self excited machine there is enough residual magnetism for the set to "boot-strap" itself. The residual generates some voltage. This is enough for the AVR to develop a little output. This increases the output voltage. With more voltage available the output of the AVR rises and the generator voltage rises. The voltage continues to build until the AVR set point is reached. If the leads are reversed then the first small AVR output tends to act against the residual and prevent the voltage from building. If you are lucky there may still be enough residual to boot-strap when the leads are reconnected in the original order.
Flashing.
If the voltage won't build, then disconnect the AVR and connect a 12 volt automotive battery in place of the AVR. Do not connect the battery and the AVR at the same time. Start the set and watch that the voltage doesn't go to high. Stop the set as soon as you get a stable voltage or if the voltage goes too high. Disconnect the battery and connect the AVR in the same polarity. It should now build voltage properly.
If it won't build up voltage with the AVR report back what voltage was developed with the 12 Volt battery connected.
By the way... Beware of the inductive "kick" when the battery is disconnected. You may want to connect a reverse diode across the field to allow the field to discharge safely through the diode.
Flashing, Plan "B"
This depends on the AVR. Some AVRs use the same terminals for power and sense. If this is the case you are out of luck for plan "B".
If the AVR has separate terminals for power and sense you can power the AVR from an external source for the first start and re-establish the residual magnetism that way. Don't try this if the AVR has common power and sense terminals.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Connection winding salient pole of AC generator to rotating rectifier

Step one:
Start the set. If the voltage builds up no problem. If the voltage does not build up;
Step two:
Stop the set and reverse the field connections. Start the set. If the voltage builds up no problem. If the voltage does not build up;
(Note; If it will be difficult to change the connections, you may skip step two and go directly to step three.)
Step three:
Follow instruction 5.2 in the manual.
5.2. Instructions for manual control of generators
After building voltage with this method, stop the set and reconnect the AVR. Start the set. Voltage should build up normally.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Connection winding salient pole of AC generator to rotating rectifier

(OP)
Dear Bill,

Well noted.
From your explanation, I assume that we can connect both leads to rotating rectifier terminals (-) and (+) with no risk i.e. burns exciter, suppressor diode or rotating rectifier. And if wrong connection only has consequences the output voltage will no build up. Correct me if I'm wrong.

I am asking the question in this forum since we are worry about the risk due to wrong connection of those leads from rotor salient poles to rotating rectifier.

Thank you.

Best regards,
Barkah

RE: Connection winding salient pole of AC generator to rotating rectifier

Hello.
Refer to attachment #7.
The + and - leads from the AVR to the exciter are the ones that are most often interchanged. Generally reversing them will correct the problem and there will still be enough residual for the voltage to build up.
Your issue is a little different.
The AVR supplies the field of the exciter. A brushless exciter generates AC, the AC is rectified by the rotating diodes to DC to power the main field.
I imagine that the connections from the rotating diodes may be difficult to change. Best to follow the instructions to re-polarize the generator with a battery.
Did you check the main field for shorts and grounds??

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Connection winding salient pole of AC generator to rotating rectifier

I have posted a request in the generator forum asking a generator expert (catserveng) to look in on this thread and add any advice.
My concern is not so much the connections but whether there may still be other damage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Connection winding salient pole of AC generator to rotating rectifier

(OP)
Dear Bill.

Appreciate with your respond.

You are correct that the connection from rotating rectifier is not our issue as we didn't disassemble the connection of the rotating rectifier. We disconnected two leads of the main field from (-) and (+) terminal of the rotating rectifier, but unfortunately our technician didn't markup two leads of the main field before disconnected from the output rotating rectifier terminals. That's our issue as Attachment-2 (Schematic_generator_rotor_and_exciter_rotor.png).

That’s why we HAVE NOT reconnected the leads yet BEFORE getting proper advice from expert like you and others, especially whether any risk or NOT if we are wrong to reconnect the leads of the main field to the (-) and (+) output of the rotating rectifier.

If the above issue is already resolved then as your concern is to ensure remain/residual magnetism in repaired exciter whether is enough to build the voltage or not. CMIIW.


Also many thanks in advance to forward the issue to a generator expert in getting additional advice.

Best Regards,
Barkah

RE: Connection winding salient pole of AC generator to rotating rectifier

I have only ran into one issue with swapping the main rotor field leads, in that case it was an older 6 pole machine that when we investigated further had a bad reading on a pole during a pole drop test, we swapped the leads back, unit made voltage, customer ran it about 3 more years.

I would say all of Bill's comments above are right on, did you answer the question as to testing the rotor before going back into service?

About the only thing I would do differently than described is that I always use a 6 volt lantern battery to flash the field. I also usually try a "static flash", with the unit not running first, just touch and hold the temporary leads from your battery to F1(+) and F2(-) and leave there about 5-10 seconds, in most cases is enough to build residual and get a unit up and going. I have used this method pretty consistently on units up to about 4MW in size.

So you started this process because of a blown surge suppressor? Then had to remove the rotor to get at the exciter rotor? Did you determine the cause of the suppressor failing? Usually a suppressor blowing, at least in my experience is due to either a main rotor problem, an event such as a lightening strike, loss of field event, out of phase close or a pole slip. Hardly ever see a surge suppressor fail due to a problem with the pilot exciter windings. Always a first time I guess.

If you have gotten good readings with a megger on the main field, then before going back into service it may be a good idea to do a pole drop test, here is a brief explanation of a simple test, http://www.electrotechnik.net/2010/08/rotor-voltag...

If you have already put the rotor back into the stator it may be hard to get at the front pole jumpers, but even doing a test and getting readings from one end can at least give you an idea of how the rotor is.

Hope that helps, Mike L.

RE: Connection winding salient pole of AC generator to rotating rectifier

Thanks for dropping in Mike.
Barkah, I would follow Mikes advice.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Connection winding salient pole of AC generator to rotating rectifier

(OP)
Dear Mike and Bill,

Thank you for your respond.

Actually, we have not any testing yet after replace suppressor and rewinding the rotor exciter.
As both of you mentioned, before the alternator put back in service, we need to do as follows:
1. Flash the exciter to ensure exciter has enough residual magnetism.
2. Drop test (?) for our salient poles (4 poles.
3. Anything else?

Appreciate with your valuable respond and we will follow your advise.

Thakn you.

BR,
Barkah

RE: Connection winding salient pole of AC generator to rotating rectifier

An insulation resistance test of the main rotor, I usually do a PI test to at least have some kind of record of condition.

RE: Connection winding salient pole of AC generator to rotating rectifier

(OP)
Dear Mike,

Well noted.
We will do insulation test on the main rotor also, especially PI to ensure the condition based on insulation class. CMIIW.

Thank you.

Best Regards,
Barkah

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