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#1

# One percent unbalance in Voltage leads to unbalance of 10% current in motor

"One percent unbalance in Voltage leads to unbalance of 10% current in motor." plzz explain. this is general statement written in all papers and text books . As per applying Ohms law unbalance is current should be same as unbalance in voltage .
plzz explain
08-08-2014 07:24 AM
Top #2
Induction motor current unbalance can be explained using symmetrical components. When a 3-phase induction motor is supplied by unbalanced voltages, the symmetrical components will consist mainly of positive and negative sequence because the motor does not have a direct neutral connection to ground (e.g., no zero-seq). The neg-seq voltage will produce an air-gap flux that rotates in the opposite direction of the rotor at nearly 2x (200%) the rated shaft speed while the pos-seq voltage produces an air-gap flux at slip speed which is only a few percent of rated shaft speed. Because the current is mostly a function of slip speed, the presence of a small neg-seq voltage produces very large neg-seq current because the slip is nearly 200%. The effective rotor resistance is approximately equal to R/s. For s = 2, the effective rotor resistance becomes very small and a large neg-seq current is produced as a result. The current unbalance can be as high as 6 to 10X the voltage unbalance because of this.
08-08-2014 10:06 AM
Top #3
Thanx for explanation.
08-08-2014 12:20 PM
Top #4
Thanks for the explanations,

As discribed voltage unbalance become a key value to optimize both energy savings and motor reliablity.
Could we have a ratio for example
1% unbalance lead to 5% losses ( in kw) and decrease motor life for 7% (higher temp > lower life !)
the ideal will be to have a global by motor power and a formula to embed in a dedicated Power Meter ?
08-08-2014 02:54 PM
Top #5
No need to analyse 1% of unbalance.