Multiparallel circuit connections are used in motors above 5 horsepower. A motor with a high number of parallel circuits, that is, four and eight wye, will show less power loss when one circuit is open. Under normal load the motor will run more slowly and will overheat.Ī microhmmeter is used to identify this problem.
An open circuit will cause the magnetic circuit to be unbalanced. If the motor’s internal connection is multicircuit, it will start but will have reduced power. Its power will drop to about half, and the motor won’t start. An open single-circuit winding will be “ single-phased“. Some causes of slot insulation breakdowns are overheating, conducting contaminants, lightning, age, pressure of a tight coil fit, hot spots caused by lamination damage (from a previous winding failure), and excessive coil movement.Įxcessive coil movement is often caused by thermal growth and/ or coil twisting torque, brought on by reversing (plugging) or a momentary power interruption.Ī wye-connected motor with an open winding will test differently from a delta-connected motor. A solid ground requires rewinding or replacing the motor. Water is the most common cause of a grounded winding. The problem is usually found in a slot, where the slot insulation has broken down.
When a motor is “ grounded“, the winding is shorted either to the laminated core or to the motor’s frame. They become charred quickly from the high circulating current that is transformed into them (Figure 2). Shorted turns in any AC winding are usually visible. Please note that the rotor should be turned during this test to eliminate its effect.
Without the ampere demand of the circulating current, the difference lessens between the amperes of the faulty phase and those of the normal phases.Ī very small difference in resistance is all that is needed to identify the faulty phase. Only the resistance of the wire (turns) within the closed loop is now eliminated from the phase winding.