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low speed and high current when voltage dips
2

low speed and high current when voltage dips

low speed and high current when voltage dips

(OP)
A 3.3 KV, 900 KW, 3000 RPM (50 Hz) double cage motor with DOL start driving a centrifugal fan. Whenever the input voltage dips by 5 to 10% (due to starting other motors), this motor speed drops and stator current increases to twice the normal current. The motor does not regain speed nor does the current drops to normal value even after the voltage rises to the normal rated value. Because of high stator current, winding temperature shoots up. Stator winding is healthy. What could be the possible reasons for such a drop in speed and increase in current when the voltage dips (which is unavoidable)? Any method to check the condition of the double cage rotor winding ? Visibly no cracks are seen in the rotor cage windings.

edison123

RE: low speed and high current when voltage dips

If I'm not mistaken this is the identical motor discussed here: Thread237-34769

Yes, it certainly sounds like a rotor bar problem.

In particular, if you shut the motor down and allow it to cool, then it comes up to normal speed fine?  But if you start it and then subject it to another motor start it drops to lower speed/higher current even when the voltage returns to normal?  (I'm quoting what I think you said).

My question is why would the motor go into some kind of stall even when voltage returns to normal when it had no problem starting (under same voltage) in the first place.  Unless there is some other change (load) that you're not telling us about, I can think of only one answer: rotor bar problem due to thermal growth of rotor which doesn't show up until the motor gets up to temperature.  Note it is common for rotor bar problems to take several hours after start to appear.

On-line tests to confirm:
Current signature analysis - look for pole pass sidebands around line frequency. Level of concern is when sideband magnitude approaches/exceeds 1% of line frequency magnitude.

Vib analysis - look for pole pass sidebands around running speed.

Look for oscillation of current meter at pole pass frequency.

Note pole pass frequency is number of poles times difference between running speed and syncronous speed.

Off-line tests were discussed in the thread linked above.  Ultrasonic and dye penetrant and other NDE are sometimes used as well.

RE: low speed and high current when voltage dips

Suggestion: It could also be some interturn short(s) in the stator that are in effect if the stator current increases and stator temperature increases. Do parameters gradually deteriorate or are they about the same?

RE: low speed and high current when voltage dips

It sounds to me like the motor and its driven fan are operating right at the edge of the motor breakdown torque curve.  When the voltage dips only slightly, the motor goes into a stall condition and has insufficient torque to reaccelerate to rated speed.  It probably starts OK with even lower voltage because the fan inlet and/or outlet dampers are likely closed at startup so there is little mechanical load during the start - only the inertia of the motor and fan rotors.

I suggest checking the fan curves against the motor torque curves to see how much margin is there.  I suspect it is very little or none.

RE: low speed and high current when voltage dips

This is normal for a motor current to increase due to dip in applied volts, P = V/I. As Mr jwerthman has mentioned, pl. check the running amps of the motor, it may e very close to rated. The starting times of blowers / fans is higher as compared to pumps. If possible try to pinch inlet/ outlet dampers when such a situation arises, to see if it can pick up speed.

RE: low speed and high current when voltage dips

AK - The post stated that current remains high even after voltage has returned to normal.  

What jwerthman describes is a reasonable explanation.... namely that there may have been differences in the damper configuration between the initial acceleration and the re-acceleration after voltage dip.  In the higher-loaded re-acceleration condition the motor is operating at a speed less than or equal to the speed of breakdwon torque.  

Rotor bar problem is certainly still a valid explanation in my opinion, as well.

The symptoms of jwerthman's scenario would be very high but constant slip, seveal times nameplate slip.  (i.e. if motor nameplate says 3580 for 2-pole motor in 60hz land then nameplate slip speed is 20rpm, but with stalled motor the slip might be 40-60rpm or more and you would find yourself at 3560rpm or lower).  Contributing factors to this scenario in addition to high mechanical load would be low or unbalanced voltages at the motor terminals.

The symptoms of rotor bar problem were given above. These will likely also include higher than normal slip and oscillating speed.

Please let us know any more info.

RE: low speed and high current when voltage dips

(OP)
electricpete

Thanks for your ideas. No appreciable vibrations are noted in the motor during this problem. No current oscillations either. I checked for stator slots to rotor slots configuration for any basic design problems (like designs that produce synchronous and asynchronous torques that result in torque saddling and resultant low speeds). No hidden dragons there either. The probability of defective rotor winding looks to be it. Will come back to all of you after I do some detailed testing on the rotor bars.

jwerthman

your suggestion that torque speed curve may be at the breadown point. Though it is unlikely, will check on that too. Thanks.


To all of you guys, thanks for your time and ideas, which were very good.

edison123

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