Generators in parallel mechanically and electrically 1

[raisinbran]

We have an installation where we may want to run two AC three phase generators from one common driver. The generators will then be connected in parallel electrically. Since they are both driven from a common driver, they are also in parallel mechanically. I have heard that there may be some mechanical torque pulsations (cogging) problems with this type of operation. Is this true? Can someone shed some light on this?
Regards,
Raisinbran

 

[ScottyUK]

You are matched for frequency but not necessarily in phase. You will need to ensure the relative positions of the fields and stator windings are accurate matched otherwise you will have serious circulating current problems.


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

 

[waross]

What Scotty said.
Your best bet is a coupling with a micrometer adjustment. Failing that you may try a taper lock type coupling half WITHOUT a keyway. After alignment you may drill and pin the coupling.
To sync these sets, couple them up and run them. Compare the outputs of the two sets on a dual trace scope. Observe the angular displacement between the two outputs. Adjust the coupling and try again until both traces line up on the scope. Then lock the coupling permanently.
A couple of points that may help;
1: The scope will be showing you electrical degrees, the coupling will be adjusted in mechanical degrees.
At 3600 RPM 1 mechanical degree equals 1 electrical degree.
At 1800 RPM 1 mechanical degree equals 2 electrical degree.
At 1200 RPM 1 mechanical degree equals 3 electrical degree.
2: Do your coarse adjustment by changing connections; reversing one winding connection gives a 180 Degree adjustment, changing to a different phase will give a 120 degree or 240 degree adjustment. By reversing and changing phase together it is possible to get a 60 degree adjustment. For less than 60 degrees you must rotate a coupling on a shaft.
You can do it, good luck.
PS you can not run two sets in parallel from one prime mover with belt drive, unless you use cogged timing belts and then it is not recommended. Really not recommended.

Bill
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"Why not the best?"
Jimmy Carter

 

[itsmoked]

And don't forget that the scope needs to be voltage rated for your task and preferably with fully isolated channels.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Good point on the voltage rating Keith, but you can use a common neutral if a scope with dual isolated channels is not available. It's easy if you have a scope with isolated inputs, but if the scope available has a common input, connect and ground your neutrals to develop a common input point for the scope.

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

 

[ratz1]

Main problem with the generators on common shaft is related to the MW sharing between them (load angle!!). Be sure that the generators are equal electrically and mechanically. Couple them taking care on position and polarity of the rotor poles.
If there are no so big angular differences, the propper position could be achieved by shimming the stators of the two generators on oposite lateral sides in order to null the phase difference.

 

[niallnz]

I have seen this done before on a fishing boat. The two shaft alternators ran on the main engine, and used a fluid coupling. The charateristics of the fluid coupling ensure kW load sharing.
Once one alternator was on the board there is no way to alter the speed of the prime mover to carry out synchronisation on the second alternator in the usual way. Instead there was a choke between the two alternators which was used to limit the outof phase synchronising current when the CB between them was closed. Rough but it worked.

CHeers Niallnz

 

[waross]

This is the reciprocal of the same issues that arise when two synchronous motors are geared to the same gear or shaft.


Bill
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"Why not the best?"
Jimmy Carter