What are the standards for three-phase current imbalance in motors?

1. 3-phase voltage imbalance. 2. One phase of the power supply has poor contact, resulting in resistance.

Standard for unbalanced current: When the three-phase power supply is balanced, the difference between any one of the motor’s three-phase no-load currents and the average value of these three phases should not exceed 10% of the average value.

There may be an imbalance in power supply, with one phase carrying a heavier load. In industrial applications, it is sometimes necessary to use one phase combined with the neutral wire to create 220V for other purposes (such as powering control rooms or other 220V devices), which leads to an imbalance among the phases. It is recommended that, in such cases, the power consumption of each 220V device be estimated; by distributing the load evenly across the three phases, the imbalance can be minimized.

This post was last edited by jjli618 on 2010-6-28 at 13:28. When the three-phase power supply is balanced, the deviation of any one of the motor’s three-phase no-load currents from the average value of those three phases should not exceed 10% of the average value. This is taken from the “Technical Specifications for Y2 Series High-Voltage Three-Phase Asynchronous Motors”. When there is an imbalance in the three-phase currents of a three-phase asynchronous motor during operation, possible causes include: (1) An imbalance in the voltages of the three-phase power supply, which leads to an imbalance in the motor’s three-phase currents； (2) Inter-turn short circuit in motor windings ； (3) Winding open circuit (or one or more branches in the winding parallel circuit are open) ； (4) Some coils within the stator winding are connected in reverse ； (5) The number of turns in the three-phase windings of the motor is not equal. The following methods can be used for handling this: (1) Use a voltmeter to measure the voltage of the three-phase power supply; if it is indeed unbalanced, the cause should be identified in order to eliminate it ； (2) For inter-turn short-circuit faults in motor windings, one can first check whether there are any areas at the ends of the windings where the coils have burned or changed color due to high temperatures, or smell any odor of burned insulation. When the location of an inter-turn short circuit cannot be identified by visual inspection, a short-circuit detector can be used for checking. If there is an inter-turn short circuit within the coil, the reading of the ammeter connected in series with the coil circuit of the short-circuit detector will increase significantly ； (3) Open-circuit faults in the windings can be detected by using a multimeter or bridge meter to measure the resistance of each phase; the maximum difference between the resistances of the three phases of the motor’s windings must not exceed 3% of the average value of those resistances ； (4) To check for the fault of reversed connection in some coils of the stator windings, a low-voltage DC voltage can be applied to a certain phase winding, and a compass can be used to check the polarity of each slot along the surface of the core slots. If the direction indicated by the compass for each pole phase group changes sequentially as N, S, N, S, it indicates that the winding connection is correct ； Conversely, it indicates that a certain pole phase group has been reversed ； If the compass, when placed on the surfaces of several adjacent slots within the same pole phase group, shows erratic direction changes, it indicates that there may be individual coils in that pole phase group that are inserted in the wrong direction or connected incorrectly. For incorrectly connected or reversed pole phase groups and coils, corrections should be made in accordance with the wiring method shown in the winding layout diagram or wiring schematic ； (5) For the fault of unequal turns in the three-phase windings, the start and end terminals of each phase can be connected in series, and a voltmeter can be used to measure the voltage drop at various points. First, measure whether the voltages of each phase are equal; then, check whether the voltages of the various pole groups in the abnormal phase are equal; finally, measure whether the voltages of the individual coils within that abnormal pole group are equal. In this way, the coil with an incorrect number of turns can be identified.

Was the current value for the poster measured with a clamp meter or shown on an ammeter? Consider phase loss and check the current transformer.