flash between rotor and stator

[gaux]

we have a permanent magnets rotor, in a syncronous motor, (40 poles), the stator is three phases, and we start it with a v/hz device, in order to use a change into frequency and start it till the nominal spèed, we start with 3 hz and it start to move, but it start to do some noise and between stator and rotor (permanent magnets) there´s some flash... does somebody have some idea why it occurs?

 

[pwrengrds]

It is likely that the stator winding insulation is failed. Try to ohm or megger the windings to ground (with the VDF disconnected).

 

[gaux]

no,isolation was 7 gigas....my thinking is something like condensator effect, i think about the frequency of the chooper for the VDF...maybe this PWM wave see the magnets and the stator with the gape like a condensator, that with the right frequency start to works.....
other of my thinkings, is like i use an open loop system, as the motor is syncronous, maybe i select 2 or 3 hz, on the output from the VDF, but the real speed from the motor is 1 hz or 2,5 hz that means some slip and some voltage induction...
other point, is that the gape is no the same from the magnets to the stator core, for this.... maybe i have some ripple or corona effect.
what do you think about this....

 

[ScottyUK]

How about some details of the motor - is it 20 watts or 2 megawatts, 400V or 11kV, etc?


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

 

[gaux]

10 kw 400 v

 

[waross]

Did you check the insulation with a multimeter or with a 1000 Volt megger?

Bill
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"Why not the best?"
Jimmy Carter

 

[PHovnanian]

VFDs and motors exhibit some strange behavior, resulting in high frequency common mode capacitive coupling between stator windings, rotors and ground. This phenomenon most often results in bearing damage if the rotor is not properly grounded and common mode currents return to ground through the bearings. You might be seeing another symptom of this.

A quick search turned up this thread:

http://www.eng-tips.com/viewthread.cfm?qid=102205

 

[TurbineGen]

gaux, is this a brushless DC motor? What are you using for feedback for your rotor position? If you are blindly giving this machine a frequency without any rotor feedback or positioning the motor is likely not fully synchronized and the motor cannot accelerate to your needed speed in time. The effect is that you would slip poles until the machine makes it up to your synchronous speed. From there you should be able to slowly increase frequency and the motor will catch up. Some BLDC controlers give the motor a "kick start" (low frequency long duartion pulses), and then use the voltage feedback from the rotating machine to sense the rotor position and frequency.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[gaux]

thanks everybody..i´´ll try with a specil VFD for syncronous pm motor... then i´´ll tell you

 

[gaux]

ok, i have tried other VFD, and this is my thinkings, yes this new one, control better the loss of syncronism, and when i was on syncrone speed there´s no flash... but when i change the speed, it start flashing....until i come to a steady situation when i get again syncronism....

i think i need a better VFD, that could see the motor parameters, and with current and voltage know where are the rotor... and all of this in a open loop....driven a syncronous motor....
thanks for your time