Sincronous Motor fault 1

[RaulChavez]

Hi I´m working with a 9600hp 40 poles (180rpm) sicnronous motor with a brushless exciter.

My question is: is possible to drive this motor to a extreme condition of 200% of nominal current and 160rpm by increassing the exciter volt?

This was a real condition, and I´m trying to figure out if the problem is on the brushless exciter or is an overload condition

Thanks

 

[electricpete]

I don't work with sync motors. But I do know they are supposed to run at sync speed.

If sync speed is 180 RPM, then to get to 160 rpm you had to have pole slips. This could be caused by severe mechanical overload (although you'd think there should be protetion to trip the motor in this case) or very low field current. The only way I could see high field voltage causing a problem is if perhaps it resulted in rotor pole damage? (just thinking out loud).

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[waross]

It sounds like either a mechanical overload or UNDER excitation.
respectfully

 

[RaulChavez]

How can a low field current drives the motor to a 200% stator current?, looking the "V-curve" for this motor with a field current arround 0,33A we have a stator current like 270A, also with a field current about 5,38A the stator current go to 4A, with field current = 6,15 the stator current goes to 400A (nominal current)

It seams that a higher field current drive the motor to a higher stator current but I don´t know if is possivel to reach 200%

 

[waross]

The higher the excitation level the more VARs the machine returns to the system. The higher the excitation level, the less likely it is that the poles will slip and let the frequency drop.
Look for mechanical overload to cause both under-speed and high current.
respectfully

 

[ScottyUK]

If the machine is running at 160rpm and the synchronous speed is 180rpm then it is either pole-slipping as ePete said or it has broken synch completely and is running as a badly-designed induction machine. In this case your machine is in trouble because the solid stator forging will see large eddy currents at slip frequency which will cause excessive heating.

Assuming the machine was in synchronism, raising the field current will increase the reactive output of the machine. It is quite possible to reach 200% stator current given a powerful enough exciter and a sufficiently stiff utility connection. A weak utility connection will experience a voltage rise which will limit VAr export; a stiff connection will absorb VAr's without a significant rise in terminal voltage.


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Other times I just let her sleep!

 

[jraef]

I'm with the others on this one. I think if you were running at 160RPM consistently, you were slipping poles or more likely had lost synchronism, in which case you were running on the amortisseur winding and likely causing damage elsewhere.

 

[oldfieldguy]

With your 40-pole motor, the only way you can get the 160 RPM is to change the frequency of the supply.

Anything else is likely to result in some bad odors coming from your motor. The amortisseur winding is not meant to do much besides give starting torque, and therefore does not have thermal characteristics suitable for extended operation. It'll melt down pretty quick.

old field guy