Before any company investigates electrical predictive maintenance (PdM) instrumentation, it should know the strengths of its equipment’s insulation, the voltages its motors are exposed to daily, how a motor typically fails and where these faults typically exist. Only then can you really make a decision as to which electrical PdM equipment is the most appropriate for your operations.
DC Step-Voltage And Surge Testing Of Motors
How a motor typically fails
The motor stator has two main insulating systems that include the ground wall and turn-to-turn insulation. When this insulation is in a good condition it can withstand the normal day-to-day voltage spikes that exist during starting and stopping. Over time, this insulation will deteriorate as a result of mechanical movement of the windings, torque transients, heat, contamination, and other environmental contaminates. Once the dielectric strength of this insulation falls below the incoming voltage spikes, another failure mechanism is introduced: ozone.
Ozone is a very corrosive gas that will quickly deteriorate insulation. Although the motor will continue to run when this failure mechanism is introduced, as it sees continual voltage spikes, the deterioration rate will accelerate. Eventually, the dielectric strength of the insulation will fall below operating voltage or deteriorate to the point that copper wire will touch turn-to-turn. At this point a turn-to-turn or hard welded short has developed.
This DC Test is performed to a voltage that a motor typically sees during starting and stopping. If a motor can’t pass the Step-Voltage and/or surge tests, you can bet on the fact that it is approaching the end of its service life.
The DC voltage is applied to all three phases of the winding and raised slowly to a preprogrammed voltage step level and held for a predetermined time period. It is then raised to the next voltage step and held for the appropriate time period. This process continues until the target test voltage is reached.
Data is logged at the end of each step. This is to ensure the capacitive charge and polarization current is removed and that only real leakage current remains, thus providing a true indication of the ground wall insulation condition. If, at this point, the leakage current (IμA) doubles, insulation weaknesses are indicated and the test should be stopped. If the leakage current (IμA) rises consistently less than double, the motor insulation is in good standing.
The Step-Voltage Test is necessary to ensure that the ground wall insulation and cable can withstand the normal day-to-day voltage spikes the motor typically sees during operation. If a DC Step-Voltage Test is not performed, the operator cannot be assured that the motor will start and operate without failing in service.
To discover all the benefits of choosing Applied Dynamics for voltage tests, or to receive a custom quote for our other services, please contact us directly today. Servicing New York, Connecticut, Massachusetts, New Hampshire, Vermont, Maine and Rhode Island.