synchronous motor- exciter field rotation

[slowcentury]

I have a question about the effects of changing the roation of the ac exciter field on a synchronous motor and how to tell which way is right, or if it matters at all...
i could post the nameplate stats, but i feel my test results are more relevant.
with the motor at 100% (360rpm) i have an exciter input amperage of 65, input voltage of 157ac, and a motor voltage of 3950-4000
reversing the exciter field voltage results in an exciter input amperage of 65, input voltage of 173ac, and a motor voltage of 3860-3880
the motor reaches its rated speed in both configurations.
test results on a diferent motor with the same specs show the same correlation
thanks for your time

 

[electricpete]

I assume you are talking about sync motor with brushes.

I wouldn't think swapping the field polarity makes any difference. Is this difference repeatable?


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(2B)+(2B)' ?

 

[electricpete]

Now I think you're talking about brushless.... in that case disregard my last.

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(2B)+(2B)' ?

 

[crshears]

I'm probably missing something, but most of the synchronous motors and/or generators I've dealt with have DC "exciter fields;" what type of equipment are you asking about?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[slowcentury]

this motor has an ac rotating transformer / rectifier assembly to transmit dc to the rotor

 

[slowcentury]

i'm talking about the ac rotation supplied to the rotating transformer. i've been advised that the ac field should be in direct opposition to the mechanical rotation, but i have nothing but my test results and the feel a bit inconclusive

 

[crshears]

Any manufacturer recommendations?

Seems to me that the resultant frequency in the AC windings on the rotor will concatenate [if I'm using the terminology in an acceptable way] either additively or subtractively; and I would ASSume that the manufacturer has designed these windings for sustained operation at only one of the two frequencies...so don't use the wrong one.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[slowcentury]

That's correct and basicaly what the manufacture states in the manual; don't use the wrong one... but i have no idea how to tell what the right one is. both configurations run stably. i'm about to get the power quality meter out and see if there is a diference in power factor, but i feel the drive will compensate excitation voltages to reach the best pf.

 

[crshears]

If mfr says not to use the wrong phase rotation, they should be able to provide you with a diagnostic test allowing you to tell which witch is which

My SWAG [scientific wild-a** guess] would be to use the configuration with the lower AC excitation voltage; less watts being consumed in the rotor circuitry, hence runs cooler, hence unit lasts longer. Reason for my SWAG is that in the first case the concatenation of field strength times speed is additive, therefore a higher machine terminal voltage is being derived. As a consequence less real power is required to be supplied to the field to develop roughly the same voltage.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[slowcentury]

that's roundabout the conclusion that i rationalized. the lower supplied excitaion voltage means the transformer is more being more effective; same rpms, same power factor, assumed same dc rotor voltage... it would have been nice if the manual laid that out, but that's a different post : )
thanks for your time!

 

[LionelHutz]

The only time I've dealt with a synchronous motor that used 3-phase AC for the field, the AC was used to keep the field constant at all motor speeds so the motor could be used at various speeds. So, this would tell me the rotation of the motor doesn't affect the "transformer". I find it a little odd this motor isn't insensitive, but anything is possible.

From what you are observing, I would expect that the first connection is correct. I'm basing this on the higher terminal voltage, which is indicating the motor is acting like a capacitor.

 

[waross]

The correct connection will give a higher frequency on the rotor.

Bill
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"Why not the best?"
Jimmy Carter