Current Offset due to Impedance Imbalance produces twice line frequency vibration.
An Electric Motor run efficiently when a smooth three phase sine wave of current is drawn into the motor from the load.
When an imbalance occurs the current is distorted and this produces an electrical Vibration. A good explanation of this can be found in the video from Mobius.
A number of sources of this Impedance Imbalance affects Efficiency. They're Self Impedance where the winding has an imbalance, either in the Coil winding or placement, and Mutual Impedance where the Rotor influence determines the current draw.
Mutual Impedance imbalance is dynamic or related to the rotor position, and is a result of Air Gap variations, End Ring defects, Casting Voids, or broken Rotor Bars.
This example is from a motor within 18% Phase Imbalance a common defect from manufacture. As the magnetic flux which is rotating cuts through the rotor bars this creates back EMF in the rotor and current is proportional to slip. When there's is a restriction in that process the current determined by load is distorted.
This plot was taken by measuring the current in high resolution with a scope and Rogowski coil.
The scope meter has a very high resolution and zooming in can display the individual switching from the IGBT's (Output Power Electronics).
When this distortion of current exists losses occur and that in turn elevates operating temperatures. As a result Winding defects propagate and the motor has a shortened service life.
Winding defects are highly correlated to Impedance Imbalance.
3Phi Reliability highly recommend to clients to Acceptance Test motors both New and Overhauled and returning defects to vendors.
The motor test above is motor which was on a long lead time and the client was forced to use, but only lasted eight months. It was recently measured at 30 MegaOhm, corrected for temperature IEEE 43-2000 at 8 MegaOhm. It was caught in time before imminent failure but the issue that concerning is the replacement spare is worse and has been rejected.
What is ironic is the loss in efficiency more than likely during the running time paid for a new motor.
A 3 minute Acceptance Test can weed out these high Impedance Motors, and should be in a purchase specification.
Best Practice Motor Management covers Design, Purchase, Install, and Asset strategies. Any one of the topics has the ability to severely hamper reliability, and worse cost a lot of wasted energy.
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