270KW KATO Generator problem 6

[accordlx]

We rewound the stator on a KATO 270 KW/300 KVA 460v 625amp 100HZ single phase 10 pole synchronous generator that is driven by a six pole 60HZ synchronous motor. The generator puts out the correct voltage and frequency and for all purposes, it functions as it should. The problem is that it now emits a noise that resembles a very loud, high speed jackhammer. Our vibration specialist insist that the noise is electrical and not being caused due to a mechanical issue.
There are 90 slots, 90 coils (30 groups of 3),7 turns per coil, 1-8 coil span, 5 wires in multiple used to make coils-4#15 AWG & 1#14 AWG. I have attached a copy of the internal connection drawing.
KATO will not release any information as it is proprietary. Does anybody have any idea as to what would be the cause?

 

[waross]

Ah, I'm starting to understand what you are doing. Just what, not how. Talk to you tomorrow.

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

 

[edison123]

Ok. Took a look at that winding diagram again. It's a 3 phase winding with 10 circuits all right. The third ring (call it L3) from outside is the unused 3rd phase and the fourth ring (the inner most) is the neutral for all 3 phases.

So, only two phases are loaded, which is a severely unbalanced loading that will overheat the rotor. But why that would create vibration and noise beats me. I am not sure if this was the OEM winding design.

As for EASA's damper winding claim, I think it's hogwash. It's the rotating component that needs the damping (which, I am sure, is taken care of in the rotor with its own damper winding), not the stator.



Muthu

http://www.edison.co.in

 

[electricpete]

Since the stator winding is single phase, it has what I described as a "standing wave" mmf pattern... which can also be decomposed into equal equal forward and reverse rotating fields.

If you tried to describe/analyse it as a 3-phase winding, you could describe those equal forward/reverse components as equal positive and negative sequence fields, which you could also be interpretted as severe unbalance in the three-phase sense.

Yes, that reverse-rotating field component travels at twice rotating speed relative to the rotor and tend to cause some surface currents on the rotor. (It also causes torque pulsations at twice electrical frequency.) This behavior would be the case for any single phase stator winding. I *assume* (point of clarification for OP) that this winding was single phase from the OEM?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

Never been a problem. The overall size and length to diameter ratio of gen sets in the 100KVA and below range is such that single phase loads do not pose a rotor heating problem.
When a standard three phase to single phase reconnection is done by the manufacturer, the kW load is not balanced. With either a zig-zag or a double delta connection "A" phase produces the same kW as "B" phase and "C" phase combined (assuming a balanced single phase load).
each phase develops the same KVA but for unity PF the current is in phase with the voltage on one phase, the current leads the voltage on one phase and the current lags the voltage on the third phase.
The poor mans single phase conversion is to leave a gen-set connected at 120/208 Volts and use two phases at 120/208 Volts and tape up the third phase.
This may cause rotor heating but the rotor is usually able to reject the heat safely.
When a 60 KVA set is reconnected for single phase the rating drops to 40 KVA. The simple sum of the KVA of the individual windings is still 60 KVA but KVA cannot be added without reference to the phase angles. The phase shifts in "B" phase and "C" phase account for the apparent loss of 20 KVA of apparent power.
What's the point? Just that taking single phase power from two phases of a three phase set in this size range does not cause any issues such as the noise issue with this set.

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

 

[edison123]

Link to marathon generators where they mention about changing 3-ph to 1-ph (pages 8 & 9). See if it helps.

http://www.marathonelectric.com/generators/docs/manuals/SB504.pdf

Muthu

http://www.edison.co.in

 

[waross]

Actually this connection is described better by the diagram labeled "LOW (PARALLEL) WYE CONNECTION"
Only L1 and L2 are brought out. All other connections including L3 and L0 are buried.
Note on dedicated single phase windings. These are factory wound and are not common above about 20 KVA at 1800 RPM or 25 KVA at 3600 RPM.

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

 

[accordlx]

Thank you all for your input. I will post the vibration report as soon as I can. Meanwhile, we will be trying a number of things to rule out everything that can be positively ruled out. As soon as I come to any solution I will post the results. Thanks again.

 

[edison123]

Looking at current density (625 Amps and 4 of 15 AWG and 1 of 14 AWG, 10 circuit wye), it works out 7.26 A/sq.mm, which is almost twice that is allowed for such slow speed machines.

Muthu

http://www.edison.co.in

 

[rhatcher]

edison123,
I am not familiar with the idea that the range of allowable amps/circ.mil (or in your case amps/sq.mm) varies according to rotational speed. I have always seen it given as a fixed value without reference to speed. Can you tell me more about this since I am still on the uphill side of the learning curve (and always will be!)?

 

[waross]

1200 RPM is not real slow.

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

 

[wolf39]

accordlx:

The torque transferred through the air gap of a single phase synchronous generator is not constant, like in a three phase machine, but pulsates with twice the rated frequency. The stators of large single phase units therefore are mounted with spring elements between stator housing and foundation.

I assume that you've rewound the stator exactly the way it was wound before. I also assume that you mounted the rewound unit in your test field before delivery. You should check the conditions at site: Condition of foundation and how the single phase generator was coupled with the drive motor. Especially the last point is of paramount importance in order to avoid a coupling failure.

How did the unit behave at site before it was rewound? Ask the owner.

Regards

Wolf

http://www.hydropower-consult.com

 

[wolf39]

accordlx:

Just noticed that my last post was not transmitted completely.

I assume that you've rewound the stator exactly the way it was wound before. I also assume that you mounted the rewound unit in your test field. You should check the conditions at site: Condition of the foundation and how the single phase generator was coupled with the drive motor. Especially the last point is of paramount importance in order to avoid a coupling failure.

Regards

Wolf

http://www.hydropower-consult.com

 

[accordlx]

Update to thread- We performed a mag particle test on all the welds on the generator frame. We found several cracked welds on the supports that hold the stator core in the frame. The support bars were resonating on the aluminum shell that surrounds the frame. After the repair of the welds, we rewound the stator using the same data used for the original rewind. We then tested the unit on our dynomometer and the noise disappeared. Thank to all who contributed to this thread.

 

[waross]

Thanks for the feedback accordlx. You're the one who found the cause of the noise. Star for you.
By the way, i see in the Marathon information on single phase connections, that they suggest 50% KW and KVA for single phase connections. Any information that I have seen from other manufacturers allows 67% KVA loading and often 83% KW loading.

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

 

[Kuldipkenya]

I would think it is the connections on the windings. Have you tried jumpers on the windings. This can help the uneven air gap for sure.

 

[Kuldipkenya]

Sorry I meant Equalizers connections on the windings this helps in the mech balance and reduces the noise