shaft generator

[jalover]

Hi guys, need some help. i got generator rated 315 kw, 400 v, 50 hz. is it normal that when the load is on max, the voltage is seesawing from 380-460 volts. I noticed the KW & ampere meter is fluctuating which i think normal, the frq is almost constant. I wonder why the voltage palying too much , it is only happen when the load is at max, 1st step to 5th step, when the motor runs at step 1 to 4, no voltage varrying, but when we put on the max step which is 5th, the voltage is seesawing. I suspect AVR sensitivity adjustment. thanks in advance.

 

[ScottyUK]

Sounds like the AVR is struggling. Need a bit more information though:

What do you mean "...I noticed the KW & ampere meter is fluctuating which i think normal..."? How big are the fluctuations - are they caused by the load? What is the motor rating?

What type of AVR are you using, also do you have a PMG exciter or fed from the main terminals?


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If we learn from our mistakes I'm getting a great education!

 

[jalover]

Hi Scotty, Thank u for your prompt reply, motor rating is abt 315 kw, the shaft gen is 350 kw., exciter fed from the main terminal, avr is a pcb like, chinese made, no idea what type. the motor use as a thruster, sideway left & right with propeller on it. I wonder why the voltage fluctuated too much bet 380-460 voltage on the max 5th step of the speed control, for 1st -4th no fluctuation. Is is normal that when the load is changing you have this fluctuation of voltage. what is the use of avr then.
thanks again.

 

[ScottyUK]

Wow, you have everything working against you! That's a big motor for the size of generator, you have an AVR whose power source is collapsing at the time the AVR needs it most, and it was made in China! More questions:

How big is the load step from '4' to '5'? I suspect this is probably quite a big step in terms of motor power, maybe as big as 40% to 100% rating and that is what is causing part of the problem. Is the fluctuation a transient which occurs only when you change power or is it a continuous oscillation when at full power?
What adjustments are available on the AVR?
Does the AVR have a quadrature current compensation CT fitted?


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If we learn from our mistakes I'm getting a great education!

 

[waross]

A couple more questions;
How is the motor controlled?
Is this a DC motor?
I usually have AVR stability problems at light or no load, but by all means turn the sensitivity adjustment a little. Quick and easy.
I saw a technician from the shallow end of the gene pool completely dismantle a small alternator and prepare to oven dry the rotor and the stator. I chased him away, reassembled the generator, started it up and gave the stability adjustment a quarter turn. End of problems. I hope that I never again have to go through so much work to adjust the stability of a set.
The point is, check the easy things first.


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

 

[jalover]

thanks warros, motor is ac, scr drive. soft starter. i just wonder why it is only having fluctuation on the step 5 of the controller, the rest ok. during the load test of this gen using water resistance bank, at 100% of the kw rating (350kw) we dont have any problem of voltage fluctuation, volateg is steady at 400v. Is it normal that when the motor is running the nearly max kw rating, kw & ampere & voltage will fluctuate. I think the voltage should remain constant during motor running even at max rating, during starting period, the voltage will vary a little but should be back on the normal value once the motor has come up to speed. but what is happening is during the motor operation, kw, amp & also the volatge have fluctuated too much, voltage even get bigger fluctuation.
hope this explain clear.

 

[waross]

I suspect a problem with the soft start. One approach would be to look at wave forms with a scope. Another way to determine if the problem is in the soft start or the motor is to by pass the soft start with a 500 Amp breaker when the motor is up to speed. The 500 amp breaker should give adequate protection to the running motor and you can turn the soft start off. If 500 Amps is a little light for full load current, go for a 600 Amp breaker.

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

 

[ScottyUK]

Cheap AVRs usually have a problem with highly distorted load currents like the soft start will produce while the ramping motor up or down. This is the reason why generators specc'd for UPS duty tend to be oversized and equipped with a 3-phase averaging AVR. You have a marginally sized generator and an unknown AVR. I'm not surprised that it has problems. We need more information about the AVR, but even with a good AVR I'm not certain that the combination of a small genny and a non-PMG exciter will ever give good performance with a distorted load at roughly 90% of the set's rating.

What's the current limit set to on the soft start? You may find a limiter on the genny is interacting with the soft start creating a load / unload cycle. Back off the current limit on the soft start as far as you dare and see if it eases the problem.


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If we learn from our mistakes I'm getting a great education!

 

[jalover]

another info, the motor is ac variable freq drive, the speed depend on the freq, say 1st step 15hz, 2nd 25hz...and so on, 5th step 49-50 hz. as i said step 1 - 4 no volatge fluctuation, step 5 only gives this fluctuation. Is it normal for the voltage to behave like even the motor is already come up to speed, i think not. during motor operation load is varrying very little. I suspect the AVR stability adjustment is out.
thanks again scotty.

 

[waross]

Are you sure that this is a soft start and not a VFD?
At full load a soft start should be full on and be producing very little distortion.
At full load on a VFD the part cycle conduction characteristics of the rectifier diodes will be wreaking maximum havoc with the AVR.


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

 

[ScottyUK]

Yeah, a VFD isn't much different to a UPS rectifier, except the UPS is typically a thyristor-based controlled rectfier where the drive is a plain bridge. Both types will upset the AVR unless it was carefully specc'd. I still think the genny is marginally sized regardless of the AVR.



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If we learn from our mistakes I'm getting a great education!

 

[jalover]

Thanks again guys. we have 2 other same installation, same eqpt specs, but gives no problem like this. At max step 5, during motor operation, the voltage & freq reads steady, there is only small variation in kw & amp during operation because of the load water resistance on the propeller...the voltage remains as it is ( 400volts )
this means to say, that the installation is matched enough.

 

[edison123]

[highlight]we have 2 other same installation, same eqpt specs, but gives no problem like this.[/highlight]

Swap the AVR's and see whether the problem goes with the AVR or stays with the generator.

 

[waross]

And check for a blown diode in the VFD rectifier.

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