Motor Negative Sequence Impedance and Current

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Thread Starter

amritanshup

Can the negative sequence impedance of the motor be lesser than locked rotor impedance of the motor? More importantly, can the sum of positive and negative sequence currents in the rotor during unbalanced supply be greater than locked rotor current for "terminal voltage of lesser than 1 pu".

I ask this because I am interested to know if the hot stall time is still a good indicator of minimum time for motor damage under severe voltage unbalance?
 
Amrit - Both locked-rotor current (as related to the number of starts) and unbalanced-supply voltage affect life expectancy!

However, it's my view that sustained UV operation is more dangerous because the effect is not readily observed!

Can you provide more detail on your comment, "terminal voltage of lesser than 1.0 pu"?

Regards, Phil Corso
 
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Phil,

I am trying to under the motor operation under voltage unbalance in two regions:

Motor start:
During motor start, the motor acts as a transformer, the current induced in the rotor due to positive and negative sequence voltage results in same heating. So as long as the phase voltage of each phase is less than 1 pu, I guess hot stall time is a good indication of minimum time for the motor to sustain damage.

Running motor:
The currents induced in the rotor due to negative sequence voltage are much higher than the positive sequence rotor currents per unit stator current. However, my question is that are the negative sequence rotor currents lesser than locked rotor currents? Also, if the voltage in each phase under voltage unbalance is less than 1 pu and if negative sequence current in the rotor is less than locked rotor current, then can hot stall time may still be used as the minimum time before motor damage occurs.
 
Amrit...

Answer to your 'Motor-Start' question:
No! Unless the supply is unbalanced Neg-Seq current is not present!

Answer to your 'Running Motor' question:
No! Neg-Seq current is not higher in amplitude than Pos-Seq current. Instead, double-frequency eddy-current are made to flow in areas normally free of such effects. I suggest you read:

Http://www.control.com/thread/1026153347 , which uses the "Two-Motor Theory" to explain how Neg-Seq impacts temp-rise!

Phil

 
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Phil,

I understand that negative sequence currents are only present during unbalanced voltages.

Motor start under voltage unbalance: If the slip of the motor is 1 (motor at rest) then the frequency of currents induced by the positive sequence voltage and negative sequence voltage in the rotor of the motor are both at 60Hz/50Hz. Therefore, my understanding is that the heating due to positive and negative sequence currents should have the same effect. Therefore, if the net driving voltage is less than 1 pu, why is that hot stall time not a good indicator for minimum time before which the motor sees damage?

I understand that locked rotor positive/negative sequence resistance of the rotor will be significantly higher than running condition positive sequence resistance due to skin effect (60 Hz currents vs. 2-5 Hz currents during running). Similarly, running condition negative sequence resistance of the rotor is much higher than the running condition positive sequence resistance due to skin effect (120 Hz vs. 2-5 Hz).

However, the difference in rotor resistance between negative sequence running and locked rotor starting should not be much (120 Hz induced currents vs. 60 Hz induced currents). How is that then if negative sequence currents in the rotor are much lesser than locked rotor currents, that hot stall time not a good indicator of minimum time before which the motor sees damage?

Thanks,
Amrit
 
Amrit...

Apples and oranges! Acceleration, or run-up, or starting time, is in the order of seconds! In fact, acceleration of typical-design motors (say < 1,000 Hp, < 6-pole) is under 2 seconds!

On the other hand, unbalanced operation can persist for months or years! The proof is relatively simple. Run the calculation to determine which has greater impact on temp-rise.

Phil
 
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