Imbalances in Voltage & Current in Motors

Kowtham Kumar K

Contents

Introduction Causes of voltage & current imbalances Co-relation between voltage & current imbalances Effects of voltage & current imbalances References

Imbalances: An Introduction

Voltage Imbalances

a. Difference in phase voltages

b. Phase Separation is not 120 degrees

Current Imbalances

c. Difference in phase currents

d. Phase Separation is not 120 degrees

Sources for Voltage Imbalance

Sources Occurrences

UtilityMalfunctioning of Open-Delta regulators

& Open-Delta Transformers

Uneven Single phase load distribution

Facility housing the motor

Malfunctioning of the equipmentMismatched Transformer Taps

“difference in amperes between any two phases at the customer’s peak

load should not be greater than 10 percent or 50 amperes, whichever is

greater.”

Motor Resistive & Inductive unbalances

Effects of Voltage Imbalances

Over-heating of components Intermittent shut-down of motor controllers Load Unbalance Fuses being blown on distribution capacitors Motor Speed & Torque are negatively affected Motor produces excessive noise

Temperature Rise vs. Voltage Imbalance

Voltage Imbalance vs. Efficiency

Standards for Imbalance Measurement

Standards Limits

American National Standards Upto 3%

Pacific Gas & Electric Upto 2.5%

National Equipment Manufacturer’s Association

Upto 1%

Causes of Current Imbalance

Large/Unbalanced distribution of phase voltage Phase to Phase loads

Some instruments simply require single phase at line-to-line voltage (415 V Welder)

Unbalanced number of turns Uneven air gap Different length of wires used Voltage distortions : Harmonics produced by the non-linear

loads Unbalanced three phase loads

Load comprises of both single & three phase equipments

Effects of Current Imbalances

Lead to de-rate motors during unbalanced conditions Causes heat in the motor windings Damages the insulation Torque Pulsations Increased Vibrations Mechanical Stresses Reduced Winding life

Co-relation of the Imbalance Occurrences

A small voltage imbalance causes a larger current imbalance

Voltage Imbalance Increase in Stator & Rotor Losses Increased Current Imbalance

E.g.: Unbalanced voltage of 1% would generally produce unbalanced currents upto 5%

Current unbalance in an adjustable frequency drive can exceed 15 times the voltage unbalance

It should be noted that current imbalance can occur without the voltage imbalance, if problem in the motor circuit exists

References http://www.trind.co.nz/download_file.cfm/Three-Phase_Motor

_Current_Unbalance.pdf?id=60,d

http://216.70.91.212/wp/wp-content/uploads/2012/03/App-Note-PM-VoltageCurrent-Imbalance.pdf

http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_unbalance_rev2.pdf

http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_unbalance_rev2.pdf

http://www.pumped101.com/pm%20part%202.pdf