STANDARD TEST - Phase-to-phase Resistance
What does it tell you?
Phase-to-phase resistance is the measured DC resistance between phases of the stator in an AC motor and between polarities of the armature and field coils in a DC motor.
In AC induction motors, use the phase-to-phase resistance values and resistive imbalances for trending, troubleshooting and quality control. In DC motors, use trending and relative comparison to determine the condition of the phases in the motor and power circuits. This includes comparing readings taken from identical motors operating in similar conditions and comparing current readings against past readings for the same motor.
An increasing resistive imbalance or a changing resistance over time can indicate one or more of the following:
- High resistance connections
- Coil-to-coil, phase-to-phase, or turn-to-turn current leakage paths
- Corroded terminals or connections
- Loose cable terminals or bus bar connections
- Open windings
- Poor crimps or bad soldier joints
- Loose, dirty, or corroded fuse clips or manual disconnect switches
- Undersized conductors (misassemble or improperly engineered)
Why is this important?
The length, size, width, composition, condition, type and temperature of the conductors and connectors determine circuit resistance. When two different conductors are connected, dirt, corrosion or an improper connection increases the circuit resistance. Also, inadequate connections cause heating of the conductor, which increases resistance even more. This could be caused if only a few strands of a conductor or portions of a soldered joint are improperly connected to a terminal or if undersized connectors are used.
In a three-phase motor circuit, the resistance in the conductor paths should be as close to equal as possible. A "resistive imbalance" occurs when the phase have unequal resistance. This produces uneven current flow and excessive heat.
