7.5kw ST Head Stopped Producing Electricity

[Listerational]

Hello,

I have a 7.5kw ST head that stopped producing electricity.
Two days ago it was working fine.
Yesterday I did not test electricity but I did start generator and ran it for a few minutes. (Note: Yesterday when I started the generator there were two 1kw baseboard heaters plugged into it. Hence, I did start it under load. I did not check to see if the heaters were warming up or not or check the voltage output of the head. I assumed it was producing power).
Today I ran the generator and checked the voltage and noticed that it was not producing power.
Is there something that could have blown when I started the generator under load yesterday?

Pictures of this head can be seen through the following link.  <iframe width="480" height="360" src="http://s1172.photobucket.com/user/Aristotle-Rand/embed/slideshow/"></iframe>

Thank you,

Jim

[mike90045]

did you have ANY loads connected when you shut it down ?  That wipes out the residual Magnetism which is used to "kick-start" the electrical production.  Sounds like you simply need to "re-flash" the winding with a battery and you should be good to go.  Maybe someone has a link handy to the method - I don't, but I've done it before.

[dieselgman]

Does your model ST have an AVR attached? That can make a difference. Your best bet is always to follow procedure and connect loads only after spin-up and shut them off before shut down. Some AVRs will not do so well with the voltage swing that happens otherwise.

dieselgman

[ronmar]

Also if it has an AVR, the unit may have tried to feed too much current to build voltage at low RPM.  This could have opened a field fuse if there is one associated with the AVR.  Most modern AVR's have a frequency below wich they will refuse to excite the field.

I would also visually examine the brushes to make sure they have good contact.

If you do not have an AVR, and your pics look like it does not, flashing the field is fairly easy.  Find the leads comming up from the brushes.  they connect to the bridge rectifyer.  in the pic of your doghouse box, it appears that you have a modern modular style rectifyer just to the left and in front of the connection board in the middle.  It has 4 wires comming off the top?  From the rectifyer, 2 wires go to the brushes.  that is rectified DC. The other 2 come from the "Z" winding that provides the AC excitation energy that gets rectified and sent to the brushes/rotor. 

If the rectifyer is labeled with a + and a - symble where the leads to the brushes are connected, that is easy.  get a 12V battery and some clip leads and connect up + to + and - to -, leaving the last connection disconnected.  Start the engine and get it up to speed.  Once it is running, briefly connect that last connection to the battery power.  This will "flash" the field causing the coils in the rotor to build a magnetic field(some of which will be stored in the iron core).  and excite the generator into output. A voltmeter monitoring the output will show this output excitation. Once it is flashed, that output energy will then be output by the "Z" winding and pass thru the rectifyer causing the output to be maintained.

If your rectifyer is not labeled, you can connect the 12VDC to the other two leads on the bridge rectifyer (the pair running to the "Z" winding)  SInce that input is AC, connecting the DC to this pair is not polarity conscious, and the bridge rectifyer will route the power to the proper brush leads to build the correct polarity field...     

[Listerational]

Thanks for all of the replies to my dilemma.
I was a little nervous I must admit.
I looked and I did not see an AVR.
I will take a look at the brushes and proceed with the process of flashing.
ronmar when I connect power to those conductors do I remove them from the rectifier first? I do not think that I have to but wanted to make sure.

Thanks,
Jim

[ronmar]

You can leave the leads connected.  You are only applying the 12V for a few seconds to kick start the magnetic field, which will feedback thru the "Z" winding , thru the rectifyer and to the brushes to sustain the output.

[mike90045]

You can leave the leads connected.  You are only applying the 12V for a few seconds to kick start the magnetic field, which will feedback thru the "Z" winding , thru the rectifyer and to the brushes to sustain the output.

I used a 12V jump start pack and a taillight bulb to limit the current, just a quick blast of power, close the lids, and crank it up.  (with no loads on it) Give it a minute at normal RPM to build up the field, and you should have power.   But disconnect loads before you shut the engine down, or you will be "flashing" a lot.

[Listerational]

You can leave the leads connected.  You are only applying the 12V for a few seconds to kick start the magnetic field, which will feedback thru the "Z" winding , thru the rectifyer and to the brushes to sustain the output.

I used a 12V jump start pack and a taillight bulb to limit the current, just a quick blast of power, close the lids, and crank it up.  (with no loads on it) Give it a minute at normal RPM to build up the field, and you should have power.   But disconnect loads before you shut the engine down, or you will be "flashing" a lot.

I like the current limiting idea.
Thanks mike and everyone for the great help.

Jim

[Tom]

Another way I've heard to flash a generator, but never had a chance to try, is to plug an old fashioned drill into an outlet, with the generator running pull the trigger and give it a spin by hand. If the drill starts to run you're back in business.

[Listerational]

Another way I've heard to flash a generator, but never had a chance to try, is to plug an old fashioned drill into an outlet, with the generator running pull the trigger and give it a spin by hand. If the drill starts to run you're back in business.

I will give that a try and let you know what happens.

By the way, can someone please explain the theory behind flashing? What exactly is going? I am an electrician so I may be able to grasp your explanation.

Thanks,

Jim

[millman56]

Jim,  field flashing  creates an electromagnet for a split second, this is usually enough to restore the slight residual magnetism in the iron cores, this is  what produces a low voltage at start up,  the excitation system quickly builds this up to full voltage.

Mark.
   

[Tom]

+1 on what millman said, the drill version puts a little bit of power into the system and supposedly starts the process too.

[ronmar]

+2  just making an electromagnetic field that the iron core retains for the next start, providing you do not shut it down inder load.  That load at shutdown can draw the magnetism out of the iron rotor core plates.

Really large generators don't have enough iron core residual magnetism to build enough field for startup due to the size inside the generator.  They typically have a small permanent magnet DC generator off of the end of the shaft, that is wired to the field windings thru a panel switch.  Once the generator is up to speed, the button is pushed and the little generator feeds current to the field to start the generator making output voltage, which the regulator quickly turns around and feeds back into the rotor winding to build to full output.  You are simply doing what really large gensets do but using a battery as the source of the current. 

On an ST you don't really need a  current limiting device.  On my ST-5  The excitation fed back to the rotor is about 70V pk-pk and about 2A under light load.  I have hooked and run a 12V battery directly to feed the field with aligator ended clip test leads.  If I recall correctly, at that voltage level input, the electromagnetic field built by the rotor only built to about 65VAC on the generator output. A ST 7.5 should react similar IMO... 

[millman56]

While waiting 2 months for an AVR consignment to arrive from Rajkot,  I powered  the excitation department on my 3 phase 415 Volt 37 KVA genset with a 24 Volt battery charger on the boost setting  (36 volts or so),   the initial livening up was provided by a quick flash with the 12 volt starting battery,  this worked perfectly with a 30 + AMP load and would start a 7.5 KW fan motor DOL no problem.     
A 75 KVA Dorman set belonging to a friend had a complicated and obsolete AVR which had gone awry,  to get it up and running for him the 24 Volt starting battery was connected to the field via an on/off switch, I think he still runs it like this. 

Mark.

[Listerational]

Hello,

I tried to flash the generator but was unsuccessful. I found the + and - signs on the regulator and connected wires from those terminations to a battery charger briefly while the generator was running. The voltage output was about 70 volts to ground on each phase while the dc power was applied only. As soon as I disconnected the dc power source from the regulator the ac output voltage fell also. The questions that I have been asking myself are 1. Is the regulator working? 2. Should I increase the dc voltage to the regulator from 12-volts to say... 18-volts? Are the brushes conducting electricity?

Any help would be greatly appreciated.

Thanks,

Jim